OCS Study 

MMS 2005-009 

 

Coastal Marine Institute 

Interactions Between Migrating Birds 

and Offshore Oil and Gas Platforms 

in the Northern Gulf of Mexico 

Final Report 

Cooperative Agreement

Coastal Marine Institute 

Louisiana State University 

U.S. Department of the Interior

Minerals Management Service 

Gulf of Mexico OCS Region 
U.S. Department of the Interior Cooperative Agreement 

Minerals Management Service Coastal Marine Institute 

Gulf of Mexico OCS Region Louisiana State University 

OCS Study 

 MMS 2005-009 

 

Coastal Marine Institute 

Interactions Between Migrating Birds 

and Offshore Oil and Gas Platforms 

in the Northern Gulf of Mexico 

Final Report 

Editor 

Robert W. Russell 

February 2005 

Prepared under MMS Contract 

1435-01-99-CA-30951-16808 

by 

The Migration Over the Gulf Project 

School of the Coast and Environment 

Louisiana State University 

Baton Rouge, Louisiana 70803 

Published by 
iii

DISCLAIMER 

This report was prepared under contract between the Minerals Management Service (MMS) and 

Louisiana State University. This report has been technically reviewed by the MMS, and it has been 

approved for publication. Approval does not signify that the contents necessarily reflect the views and 

policies of the MMS, nor does mention of trade names or commercial products constitute endorsement or 

recommendation for use. It is, however, exempt from review and compliance with the MMS editorial 

standards. 

REPORT AVAILABILITY 

Extra copies of this report may be obtained from the Public Information Office at the following address: 

U.S. Department of the Interior 

Minerals Management Service 

Gulf of Mexico OCS Region 

Public Information Office (MS 5034) 

1201 Elmwood Park Boulevard 

New Orleans, LA 70123-2394 

Telephone: (504) 736-2519 or 

 (800) 200-GULF 

CITATION 

Suggested citation: 

Russell, R.W. 2005. Interactions between migrating birds and offshore oil and gas platforms in the 

northern Gulf of Mexico: Final Report. U.S. Dept. of the Interior, Minerals Management Service, 

Gulf of Mexico OCS Region, New Orleans, LA. OCS Study MMS 2005-009. 348 pp. 

ABOUT THE COVER 

The cover art depicts a fallout of migrating birds and insects on a platform in the northern Gulf of Mexico 

and is the work of Brian P. Gibbons, a field biologist from Texas who spent nearly a year living offshore 

during this study. 
v 

PREFACE 

This study entitled “Interactions Between Migrating Birds and Offshore Oil and Gas Platforms in the 

Northern Gulf of Mexico,” also known informally as the “Migration Over the Gulf Project” (MOGP), 

provides data and analyses of trans-Gulf bird migration from three years of intensive study on 13 

platforms located in offshore waters in the northern Gulf of Mexico. The study was sponsored by the 

Minerals Management Service (MMS) to provide environmental information to the MMS concerning the 

possible influence of platforms on migrating birds. This volume focuses on broad-scale patterns of 

migration and presents general findings concerning the ecology of migrants on platforms and the 

influences of platforms on trans-Gulf migration. 
vii 

ACKNOWLEDGMENTS 

This report would not have been possible without the contributions of hundreds of people. 

First and foremost, thanks are due to the many dedicated field biologists who lived offshore for weeks or 

months at a time to conduct this study, listed here in descending order of time spent offshore: Rick 

Knight, Brian Gibbons, Mac Myers, Robert Russell, Jon King, Stacy Peterson, Mike Nelson, Karl 

Bardon, Alan Wormington, Dave Patton, Jeff Birdsley, Cameron Cox, Marshall Iliff, Brian Sullivan, 

Brian O’Shea, Dan Christian, John Arvin, Aaron Brees, Rick West, Matthew Holder, Arvind Panjabi, Jim 

Tietz, Laurence Binford, Paul Dickson, Richard Gibbons, Byron Stone, David Muth, Don Norman, and 

Dan Purrington. Thanks are also due to Diane Russell for providing logistical support to the project while 

Robert Russell was offshore. The collective pronouns “we,” “us,” and “our” are used throughout the text 

to acknowledge the important contributions of time, energy, and insight provided by the entire field team. 

A study of this magnitude could never have been conducted without the enthusiastic cooperation of the 

petroleum industry, and we were fortunate to have enjoyed extensive logistical support from British 

Petroleum, ChevronTexaco, ConocoPhillips, ExxonMobil, Newfield Exploration, and Shell Offshore. 

These companies provided long-term housing and large quantities of food to our offshore field personnel, 

as well as regular helicopter transportation to and from shore and administrative support on shore. 

Hundreds of people involved in many facets of the industry provided assistance with all aspects of the 

study, but special thanks go to Russell Putt (British Petroleum), Phil Smith (Shell Offshore), and Vince 

Cottone (ChevronTexaco) for securing access to critical platforms during the expanded phase of the 

study. Although they are too numerous to name individually, we thank the many crewmen on the 

platforms where we worked for their interest, support, and camaraderie. 

Thanks go to Mac Myers for devising the framework for the habitat classification scheme, and to Dave 

Patton, Mac Myers, and Stacy Peterson for assisting with synoptic weather classification. Dave Patton 

went well beyond the call of duty and invested a great deal of his own time downloading NEXRAD 

images for the radar component of the study. Many of the observers contributed photographs from their 

time offshore, and special thanks go to Dave Patton for contributing a large volume of outstanding 

material. Brian Gibbons assisted with enumeration and identification of insects in the light trap samples, 

and Lisa Peri completed the bulk of this laborious work. Victoria M. Bayless kindly made available 

facilities for storing and analyzing the insect samples at the Louisiana State Arthropod Museum. Alexis 

Lugo-Fernandez provided a copy of the Yocke et al. (2000) study, and Mark Yocke granted permission to 

reproduce the diagrammatic weather maps in Figure 5.1. Radar images and weather maps in Chapters 7-9 

are included here courtesy of WSI Corporation, TAP Publishing Company, and Unisys Corporation. 

Special thanks to Brian Gibbons for creating the spectacular artwork that graces the cover of this report. 

This study was originally housed at the LSU Museum of Natural Science, where much of the 

administrative groundwork for the study was completed by Donald Norman, who negotiated access to the 

original platforms used in the study. Special thanks go to Russell Chapman and the staff of the LSU 

School of the Coast and Environment, who agreed to support the study after facilities at the museum 

became unavailable. 

Sidney Gauthreaux wishes to thank Alan Cunningham, Andrew Farnsworth, Steve Lohr, and David 

Mizrahi, who did most of the radar image quantification under his supervision and assisted in the 

preparation of the final subcontractor’s report. Sidney Gauthreaux also thanks Villere Reggio, Jr., who 

was responsible for his involvement in this project, and Carroll Belser, who provided valuable support 

throughout the project. 
viii 

This study was funded by the Minerals Management Service through the Louisiana State University 

Coastal Marine Institute (CMI) program under contracts 14-35-0001-30660 and 

1435-01-99-CA-30951. Many thanks are due to the Minerals Management Service for its exceptional 

support of the study and for its patience during the delays while this report was being completed. We are 

grateful to Larry Rouse for his encouragement and support as Director of the LSU CMI. The Contracting 

Officer’s Technical Representatives (COTRs) for this study were Bob Rogers, Sam Holder, Jeff Childs, 

and Dave Moran. All of the COTRs provided valuable advice and support. 

Robert W. Russell 
ix 

TABLE OF CONTENTS 

Page 

FIGURES...................................................................................................................................................xiii

TABLES ...................................................................................................................................................xvii

Chapter 1 Executive Summary ..................................................................................................................1

PART 1 BACKGROUND ......................................................................................................................7

Chapter 2 Introduction...............................................................................................................................9

2.1 Background and Objectives ........................................................................................................9

2.2 Selection of Study Platforms.....................................................................................................10

2.3 Summary of Field Operations...................................................................................................15

Chapter 3 Field Methods .........................................................................................................................17

3.1 Platform Censuses.....................................................................................................................17

3.2 Flight Censuses .........................................................................................................................17

3.3 Census Schedules......................................................................................................................17

3.4 Opportunistic Work...................................................................................................................19

3.5 Data Recording .........................................................................................................................19

3.5.1 Local Weather ............................................................................................................19

3.5.2 Body Condition of Migrants.......................................................................................20

3.5.3 Foraging Behavior......................................................................................................21

3.5.4 Microhabitat Use ........................................................................................................21

3.5.5 Cause of Death ...........................................................................................................22

3.5.6 Flight Behavior...........................................................................................................23

Chapter 4 Radar Methods ........................................................................................................................25

4.1 Introduction...............................................................................................................................25

4.2 Radar Theory ............................................................................................................................25

4.3 Quantification of NEXRAD Imagery .......................................................................................27

Chapter 5 Synoptic Climatology of the Northern Gulf of Mexico ..........................................................33

5.1 Introduction...............................................................................................................................33

5.2 Methods.....................................................................................................................................33

5.3 Results.......................................................................................................................................35

PART 2 ECOLOGY OF TRANS-GULF BIRD MIGRATION ...........................................................45

Chapter 6 Species Composition of Offshore Migrants............................................................................47

6.1 Ecological Categorization of Species Recorded from Platforms..............................................47

6.2 Summary of Count Data ...........................................................................................................68
x 

Chapter 7 Spring Migration Routes Across the Gulf of Mexico .............................................................85

7.1 Introduction...............................................................................................................................85

7.2 Radar Observations of Landfall on the Northern Gulf Coast....................................................85

7.3 Relationship Between Synoptic Weather and the Location of Landfall ...................................98

7.4 Direct Observations of the Flight Directions of Migrants ........................................................99

7.5 Effects of Synoptic Weather on Patterns of Abundance on Platforms ...................................105

7.6 Interspecific Variation in Migration Routes ...........................................................................110

7.7 Male-Biased Sex Ratios..........................................................................................................114

Chapter 8 Timing and Magnitude of Spring Trans-Gulf Migration and Platform Use .........................119

8.1 Diel Timing of Spring Migration ............................................................................................119

8.2 Radar-Based Estimates of Absolute Bird Migration Traffic...................................................125

8.3 Seasonal Timing of Spring Migration.....................................................................................125

8.3.1 Phenology of the Migration in Aggregate................................................................125

8.3.2 Weather Influences...................................................................................................128

8.3.3 Interspecific Variation in Migration Schedules........................................................135

8.3.4 Sexual Differences in Migration Timing..................................................................143

Chapter 9 Ecology of Fall Trans-Gulf Migration ..................................................................................145

9.1 Introduction.............................................................................................................................145

9.2 Radar Evidence of Migration Routes......................................................................................146

9.3 Direct Observations of the Flight Directions of Migrants ......................................................148

9.4 “Overshooting” of the Gulf of Mexico by Non-Trans-Gulf Migrants ....................................156

9.5 Effects of Synoptic Weather on Patterns of Abundance on Platforms ...................................162

9.6 Interspecific Variation in Migration Routes ...........................................................................162

9.7 Age-Related Variation in Migration Routes ...........................................................................169

9.8 Seasonal Timing......................................................................................................................178

9.9 Interspecific Variation in Migration Schedules ......................................................................178

Chapter 10 Fall Trans-Gulf Migration and Platform Use by Peregrine Falcons .....................................189

10.1 Introduction.............................................................................................................................189

10.2 Seasonal Timing......................................................................................................................189

10.3 Population Structure................................................................................................................189

10.4 Population Size .......................................................................................................................191

10.5 Platform Use ...........................................................................................................................193

10.6 Flight Strategies ......................................................................................................................195

10.7 Possible Selective Influence of Platforms...............................................................................197

Chapter 11 Synoptic Weather and Vagrancy...........................................................................................199

PART 3 SUPPLEMENTAL STUDIES ..............................................................................................209

Chapter 12 “Winter” and “Summer” Migrations.....................................................................................211

12.1 Introduction.............................................................................................................................211

12.2 Results and Discussion............................................................................................................211
xi 

Chapter 13 Insect Dispersal Over the Gulf of Mexico ............................................................................217

13.1 Introduction.............................................................................................................................217

13.2 Methods...................................................................................................................................217

13.2.1 Visual Surveys..........................................................................................................217

13.2.2 Ultraviolet Light Trapping .......................................................................................218

13.2.3 Taxonomic Identification and Sorting of Insect Specimens.....................................219

13.2.4 Computation of Insect Biomass ...............................................................................219

13.3 Magnitude and Taxonomic Composition of the Aerial Plankton ...........................................220

13.4 Seasonal Patterns and Influence of Weather...........................................................................228

13.5 Insects as a Source of Fall Radar Echoes................................................................................229

13.6 Discussion ...............................................................................................................................231

Chapter 14 Evaluation of a Method for Monitoring Audible Fall Bird Migration Traffic Over 

Platforms...............................................................................................................................235

14.1 Introduction.............................................................................................................................235

14.2 Background on Studies of Nocturnal Vocalizations by Migrants...........................................235

14.3 Field Methods .........................................................................................................................235

14.4 Laboratory Analysis................................................................................................................236

14.5 Results.....................................................................................................................................237

14.6 Discussion ...............................................................................................................................242

PART 4 THE IMPACT OF PLATFORMS ON MIGRANT BIRDS ................................................243

Chapter 15 Stopover Ecology of Migrants on Platforms.........................................................................245

15.1 Physiological Condition of Arriving Migrants .......................................................................245

15.1.1 Spring .......................................................................................................................245

15.1.2 Fall............................................................................................................................246

15.2 Foraging Behavior and Success ..............................................................................................247

15.2.1 Spring .......................................................................................................................247

15.2.2 Interspecific Variation..............................................................................................248

15.2.3 Weather Influences...................................................................................................251

15.2.4 Comparison Between Platform Foraging and “Natural” Foraging ..........................252

15.2.5 Energetic Consequences of Foraging on Platforms..................................................254

15.3 Microhabitat Selection ............................................................................................................256

15.3.1 Spring .......................................................................................................................256

15.3.2 Fall............................................................................................................................257

15.3.3 Interspecific Variation..............................................................................................257

15.3.4 Movement Among Microhabitats ............................................................................263

15.3.5 Relationships Between Habitat Selection and Foraging Behavior...........................263

Chapter 16 Nocturnal Circulation Events................................................................................................265

Chapter 17 Sources of Mortality..............................................................................................................299

17.1 Spring......................................................................................................................................299

17.2 Fall ..........................................................................................................................................301

17.3 “Other” Sources of Mortality..................................................................................................303

17.4 Discussion ...............................................................................................................................303
xii 

Chapter 18 Conclusions and Recommendations .....................................................................................305

18.1 Conclusions.............................................................................................................................305

18.1.1 Ecology of Trans-Gulf Bird Migration ....................................................................305

18.1.2 The Impact of Platforms on Migrant Birds ..............................................................308

18.2 Recommendations...................................................................................................................310

18.2.1 Use of Platforms as Ecological Observatories .........................................................310

18.2.2 Management of the Platform Archipelago ...............................................................312

18.2.3 International Outreach..............................................................................................313

LITERATURE CITED .............................................................................................................................315
xiii 

FIGURES 

Page 

Figure 2.1. Map showing the locations of platforms used in this study relative to the distribution of 

all platforms in the northern Gulf .........................................................................................13

Figure 2.2. Relative isolation of the study platforms ..............................................................................14

Figure 4.1. Locations of the Lake Charles (LCH) and New Orleans (LIX) NEXRAD sites ..................28

Figure 4.2. Radar images showing precipitation, strobing, and insect contamination ............................30

Figure 4.3. Pixel counting technique in radar image analysis.................................................................31

Figure 5.1. Seven synoptic types used for classifying large-scale weather patterns in this study...........41

Figure 5.2. Relative frequencies of the different weather types in spring 1998-2000.............................42

Figure 5.3. Relative frequencies of the different weather types in fall 1998-2000 .................................43

Figure 5.4. Relative frequency of the different weather types in spring versus fall................................44

Figure 7.1. Time series of NEXRAD base reflectivity images showing first arrival of trans-Gulf 

migrants off Brownsville on April 10, 2000 .........................................................................87

Figure 7.2. Composite NEXRAD images showing base reflectivity along the Gulf Coast on the 

mornings of April 15 and 16, 2000 .......................................................................................88

Figure 7.3. Time series of NEXRAD base reflectivity composites on April 15, 2000 ...........................89

Figure 7.4. NEXRAD base velocity images corresponding to the reflectivity images for April 15 in 

Figure 7.2 ..............................................................................................................................90

Figure 7.5. NEXRAD images from the Houston site (HGX) showing the arrival of migrating birds 

on the Upper Texas Coast from the SSW at 17:00 Z (12:00 CDT) on April 21, 1999.........91

Figure 7.6. Composite NEXRAD image showing base reflectivity along the Gulf Coast on the 

morning of March 29, 2000, at 16:00 Z (10:00 CST)...........................................................92

Figure 7.7. Eastward displacement of trans-Gulf migration by a cold front on April 24, 2000..............93

Figure 7.8. NEXRAD images from the Houston site (HGX) showing the arrival of migrating birds 

on the Upper Texas Coast from the SSE at 01:21 Z on May 14, 2000 (20:21 CDT on 

May 13).................................................................................................................................95

Figure 7.9. Examples of radar-observed evening fallouts of trans-Gulf migrants along the Texas 

coast ......................................................................................................................................96

Figure 7.10. Example of radar-observed nocturnal fallout along the Florida Keys of trans-Caribbean 

migrants from Cuba, and “leakage” of some Cuban migrants into the Gulf of Mexico .......97

Figure 7.11. Patterns of abundance of long-distance migrant landbirds (aerial insectivores and 

neotropical migrants) on platforms in spring 1998 in relation to synoptic weather type....106

Figure 7.12. Patterns of abundance of long-distance migrant landbirds (aerial insectivores and 

neotropical migrants) on platforms in spring 1999 in relation to synoptic weather type....107

Figure 7.13. Patterns of abundance of long-distance migrant landbirds (aerial insectivores and 

neotropical migrants) on platforms in spring 2000 in relation to synoptic weather type....108

Figure 7.14. Patterns of abundance of medium-distance migrants on platforms in spring 2000 in 

relation to synoptic weather type ........................................................................................109
xiv 

Figure 7.15. Frequency distribution of mean species-specific longitudinal positions in spring 2000 ....113

Figure 8.1. Time series showing the progressive arrival of trans-Gulf migrants on the northern 

Gulf Coast on May 13, 1998...............................................................................................120

Figure 8.2. Phenology of radar-observed spring migration in 1998 and 1999......................................126

Figure 8.3. Phenology of platform-observed spring migration in 1998-2000.......................................127

Figure 8.4. Frequency distribution of species-specific peak arrival times in spring 2000 ....................142

Figure 9.1. NEXRAD base reflectivity image from the Mobile site (MOB) showing a large 

movement of migrating birds and insects departing southward from the northeastern 

Gulf Coast shortly after nightfall on September 30, 1998 ..................................................147

Figure 9.2. Time series of NEXRAD base reflectivity images from the Houston site (HGX) 

showing a large movement of migrating birds and insects departing the Upper Texas 

Coast after nightfall on October 7, 1998.............................................................................149

Figure 9.3. NEXRAD base reflectivity images from the Corpus Christi site (CRP) showing a large 

movement of presumed migrating birds traveling southwestward off the central Texas 

coast on the night of September 23-24, 1998......................................................................150

Figure 9.4. Time series of NEXRAD images showing the development of an “exploding flower” at 

Brownsville (BRO) after nightfall on November 8, 2000...................................................151

Figure 9.5. NEXRAD images from the Brownsville site (BRO) showing a large movement of 

migrating birds arriving on the lower Texas coast on the morning of September 30, 1998152

Figure 9.6. NEXRAD base reflectivity image from the Brownsville site (BRO) showing a fallout of 

migrants along the Tamaulipas coast on the morning of November 9, 2000 (14:22 Z) .....153

Figure 9.7. NEXRAD base reflectivity image from the Key West site (BYX) showing a large flight 

of migrants approaching Cuba from the NNW on the morning of October 8, 2000 

(15:29 Z) .............................................................................................................................154

Figure 9.8. Relationship between lateral wind strength and the orientation of nocturnal migration at 

VK786 during fall 1999 ......................................................................................................157

Figure 9.9. Frequency distribution of flight tracks from “moonwatching” at MI622 on the night of 

September 12-13, 2000 .......................................................................................................158

Figure 9.10. Distributions of vanishing bearings of Ruby-crowned Kinglets, Marsh Wrens, and 

House Wrens.......................................................................................................................161

Figure 9.11. Comparison of vanishing bearings of medium-distance migrants and short-distance 

migrants...............................................................................................................................162

Figure 9.12. Patterns of abundance of long-distance migrant landbirds (aerial insectivores and 

neotropical migrants) on platforms in fall 1999 in relation to synoptic weather type ........163

Figure 9.13. Patterns of abundance of medium-distance migrants on platforms in fall 1999 in relation 

to synoptic weather type .....................................................................................................164

Figure 9.14. Patterns of abundance of short-distance migrants on platforms in fall 1999 in relation to 

synoptic weather type..........................................................................................................165

Figure 9.15. Frequency distribution of mean species-specific longitudinal positions in fall 1999 .........168

Figure 9.16. Patterns of abundance and age ratios of Bay-breasted Warblers and Prairie Warblers 

across the Gulf during fall 1999..........................................................................................170
xv 

Figure 9.17. Patterns of abundance and age ratios of Chestnut-sided Warblers and Blackburnian 

Warblers across the Gulf during fall 1999 ..........................................................................171

Figure 9.18. Patterns of abundance and age ratios of Black-and-white Warblers and Northern Parulas 

across the Gulf during fall 1999..........................................................................................172

Figure 9.19. Patterns of abundance and age ratios of American Redstarts and Kentucky Warblers 

across the Gulf during fall 1999..........................................................................................173

Figure 9.20. Patterns of abundance and age ratios of Prothonotary Warblers and Hooded Warblers 

across the Gulf during fall 1999..........................................................................................174

Figure 9.21. Patterns of abundance and age ratios of Yellow Warblers and Magnolia Warblers across 

the Gulf during fall 1999.....................................................................................................175

Figure 9.22. Patterns of abundance and age ratios of Common Yellowthroats and Black-throated 

Green Warblers across the Gulf during fall 1999 ...............................................................176

Figure 9.23. Patterns of abundance and age ratios of Mourning Warblers and Wilson’s Warblers 

across the Gulf during fall 1999..........................................................................................177

Figure 9.24. Phenology of platform-observed fall migration in 1998 and 1999 .....................................179

Figure 9.25. Frequency distribution of species-specific peak fall arrival times in 1999.........................187

Figure 10.1. Frequency distribution of the dates of initial detection of all 372 Peregrine Falcons 

recorded during the fall 1999 season ..................................................................................190

Figure 10.2. Longitudinal trend in sex ratios of Peregrine Falcons in fall 1999 .....................................191

Figure 10.3. Numbers of Peregrines undertaking stopovers on each of the 10 study platforms in fall 

1999.....................................................................................................................................192

Figure 10.4. Comparison of body masses of prey taken during the fall by male and female Peregrine 

Falcons ................................................................................................................................195

Figure 10.5. Vanishing bearings of Peregrine Falcons in fall 1999 ........................................................196

Figure 10.6. Comparison of arrival times of Peregrines and a representative trans-Gulf migrant 

passerine in fall 1999 ..........................................................................................................197

Figure 11.1. Comparative distribution of occurrences of Caribbean vagrants and western vagrants in 

spring 1998..........................................................................................................................201

Figure 11.2. Comparative distribution of occurrences of Caribbean vagrants and western vagrants in 

spring 1999..........................................................................................................................202

Figure 11.3. Comparative distribution of occurrences of Caribbean vagrants and western vagrants in 

spring 2000..........................................................................................................................203

Figure 11.4. Comparative distribution of occurrences of Caribbean vagrants and western vagrants in 

fall 1998 ..............................................................................................................................206

Figure 11.5. Comparative distribution of occurrences of Caribbean vagrants and western vagrants in 

fall 1999 ..............................................................................................................................207

Figure 11.6. Comparison of vanishing bearings of Western vagrants and Caribbean vagrants in 

spring and fall .....................................................................................................................208

Figure 13.1. Comparison of total abundance and biomass of insects sampled by light traps on 

offshore platforms during spring and fall............................................................................226
xvi 

Figure 13.2. Comparison of taxonomic composition of insect assemblages sampled by light traps 

on offshore platforms during spring and fall.......................................................................227

Figure 13.3. Intraseasonal variation in abundance and biomass of insects sampled by light traps on 

offshore platforms in fall 1999............................................................................................228

Figure 13.4. Intraseasonal variation in abundance and biomass of insects sampled by light traps on 

offshore platforms in spring 2000.......................................................................................229

Figure 13.5. Comparison of light-trap loads from SP89 with NEXRAD radar echoes from the LIX 

site in fall 1999....................................................................................................................230

Figure 14.1. Total calls per hour recorded at VK786 during fall 1999 ...................................................241

Figure 15.1. Case studies of the foraging performance of two warblers on EW826 on May 7, 1999 ....255
xvii 

TABLES 

Page 

Table 2.1. Platforms used in this study ..................................................................................................12

Table 2.2. Selected characteristics of the study platforms .....................................................................14

Table 2.3. Summary of field effort (inclusive dates of field work)........................................................16

Table 3.1. Schedule of daily field operations in the spring....................................................................18

Table 3.2. Schedule of daily field operations in the fall ........................................................................19

Table 3.3. Beaufort scale........................................................................................................................20

Table 5.1. Synoptic weather types during spring migration, 1998-2000 ...............................................36

Table 5.2. Synoptic weather types during fall migration, 1998-2000....................................................38

Table 6.1. Waterfowl recorded from the study platforms ......................................................................48

Table 6.2. Herons recorded from the study platforms............................................................................48

Table 6.3. Diurnal raptors recorded from the study platforms...............................................................49

Table 6.4. Shorebirds recorded from the study platforms......................................................................49

Table 6.5. Neotropical migrant landbirds recorded from the study platforms .......................................51

Table 6.6. Medium-distance migrants recorded from the study platforms ............................................55

Table 6.7. Short-distance migrants recorded from the study platforms .................................................57

Table 6.8. Caribbean vagrants recorded from the study platforms ........................................................59

Table 6.9. Western vagrants recorded from the study platforms ...........................................................61

Table 6.10. Aerial insectivores recorded from the study platforms .........................................................66

Table 6.11. Species whose expected trans-Gulf status was unclear because of unknown migratory 

status, the existence of widespread resident populations, or other factors............................66

Table 6.12. Summary of platform count data grouped by season, year, and ecological group................69

Table 6.13. Number of species and recognizable forms recorded in each season in relation to status 

(stopover versus flyby) and ecological group .......................................................................84

Table 7.1. Spatial patterns of spring trans-Gulf migration in relation to synoptic weather ...................98

Table 7.2. Vanishing bearings of spring migrant ducks in relation to synoptic weather and position 

in the Gulf ...........................................................................................................................100

Table 7.3. Vanishing bearings of spring migrant herons in relation to synoptic weather and position 

in the Gulf ...........................................................................................................................101

Table 7.4. Vanishing bearings of spring migrant shorebirds in relation to synoptic weather and 

position in the Gulf .............................................................................................................102

Table 7.5. Vanishing bearings of spring migrant aerial insectivores in relation to synoptic weather 

and position in the Gulf.......................................................................................................103

Table 7.6. Vanishing bearings of spring neotropical migrants in relation to synoptic weather and 

position in the Gulf .............................................................................................................104

Table 7.7. Comparative longitudinal distributions of spring trans-Gulf migrants ...............................111
xviii 

Table 7.8. Comparisons of geographic distribution and abundance between ECH and BH days for 

selected species ...................................................................................................................114

Table 7.9. Sex ratios of sexually dichromatic species during spring 2000 ..........................................115

Table 7.10. Geographic variation in sex ratio of sexually dichromatic species in spring 2000.............117

Table 8-1a. Spring radar-observed trans-Gulf migration time of initial arrival on the northern Gulf 

Coast (CST) ........................................................................................................................122

Table 8-1b. Spring radar-observed trans-Gulf migration time of the peak in number of birds arriving 

on the northern Gulf Coast (CST).......................................................................................123

Table 8-1c. Spring radar-observed trans-Gulf migration duration of arrival period (hr) on the 

northern Gulf Coast.............................................................................................................124

Table 8.2. Influence of synoptic weather on the occurrence of radar-observed trans-Gulf migration 

in spring 1998 and 1999......................................................................................................128

Table 8.3. Comparison of daily platform counts and radar-observed migration traffic in relation to 

synoptic weather in spring 1998 .........................................................................................129

Table 8.4. Comparison of daily platform counts and radar-observed migration traffic in relation to 

synoptic weather in spring 1999 .........................................................................................132

Table 8.5. Species-specific timing of spring trans-Gulf migration in 2000.........................................135

Table 8.6. Sexual differences in migration timing in spring 2000.......................................................143

Table 9.1. Summary of case studies of nocturnal flight direction at VK786 in fall 1999....................155

Table 9.2. Mean vanishing bearings of medium-distance migrants (MDM) and short-distance 

migrants (SDM) ..................................................................................................................160

Table 9.3. Comparative longitudinal distributions of fall trans-Gulf migrants....................................166

Table 9.4. Species-specific timing of fall trans-Gulf migration in 1999..............................................180

Table 10.1. Avian prey species taken by Peregrine Falcons using offshore platforms during fall 

migration .............................................................................................................................193

Table 11.1. Spring vagrants recorded in 1998-2000 ..............................................................................200

Table 11.2. Fall vagrants recorded in 1998 and 1999 ............................................................................204

Table 12.1. “Winter” observations at GC18: November 18-December 16, 1999, and January 6-

February 29, 2000 ...............................................................................................................212

Table 12.2. “Summer” observations at GC18: June 1-August 1, 2000..................................................213

Table 13.1. Logarithmic abundance scale for visual surveys of insects. ...............................................218

Table 13.2. Dates of light trap deployment on the platforms.................................................................218

Table 13.3. Allometric equations used for estimating insect body mass from body length...................219

Table 13.4. Taxonomic composition of the aerial plankton sampled by light traps ..............................220

Table 14.1. Summary of hourly and nightly numbers of calls recorded over VK786 

during fall 1999...................................................................................................................237

Table 14.2. Species composition of calling migrants recorded over VK786 during fall 1999 ..............239

Table 14.3. Some nocturnal call note group classifications...................................................................240
xix 

Table 15.1. Initial body condition of newly arrived small neotropical migrants in relation to 

synoptic weather type during spring 2000 ..........................................................................245

Table 15.2. Initial body condition of newly arrived large neotropical migrants in relation to synoptic 

weather type during spring 2000.........................................................................................246

Table 15.3. Initial body condition of newly arrived small neotropical migrants in relation to synoptic 

weather type during fall 1999 .............................................................................................246

Table 15.4. Initial body condition of newly arrived large neotropical migrants in relation to synoptic 

weather type during fall 1999 .............................................................................................247

Table 15.5. Comparative foraging status of the different species groups in fall 1999...........................249

Table 15.6. Species-specific foraging status in fall 1999.......................................................................249

Table 15.7. Relationship of foraging status to weather in fall 1999 ......................................................252

Table 15.8. Relationship of foraging success to weather in fall 1999....................................................252

Table 15.9. Comparison of foraging behavior of fall migrants on platforms to foraging behavior of 

the same species in coastal chenieres..................................................................................253

Table 15.10. Comparison of foraging rates of fall migrants on platforms to foraging rates of the same 

species in coastal chenieres.................................................................................................254

Table 15.11. Microhabitat use by migrants on platforms during spring 2000 .........................................257

Table 15.12. Microhabitat use by migrants on platforms during fall 1999..............................................257

Table 15.13. Interspecific variation in microhabitat use by migrants in spring 2000..............................258

Table 15.14. Interspecific variation in microhabitat use by migrants in fall 1999...................................260

Table 15.15. Species that differed between spring and fall in preferred microhabitat.............................262

Table 15.16. Movement among platform microhabitats by small neotropical migrants in fall 1999 ......263

Table 15.17. Microhabitat use by small neotropical migrants in fall 1999 in relation 

to foraging status.................................................................................................................264

Table 15.18. Foraging success in relation to microhabitat use by small neotropical migrants 

in fall 1999 ..........................................................................................................................264

Table 15.19. Foraging success in relation to microhabitat use by medium-distance migrants 

in fall 1999 ..........................................................................................................................264

Table 16.1. Summary of all nocturnal circulation events detected during spring 2000.........................268

Table 17.1. Sources of mortality in spring 1998....................................................................................299

Table 17.2. Sources of mortality in spring 1999....................................................................................299

Table 17.3. Sources of mortality in spring 2000....................................................................................300

Table 17.4. Sources of mortality in fall 1998.........................................................................................301

Table 17.5. Sources of mortality in fall 1999.........................................................................................302

Table 17.6. “Other” sources of mortality during the study ....................................................................303
1 

Chapter 1 

 

Executive Summary 

Robert W. Russell 

The Gulf of Mexico is a major ecological barrier confronted by hundreds of millions of migrating birds 

each spring and fall. Trans-Gulf migrations evolved in the absence of natural islands that could serve as 

stopover sites; thus, the installation of an artificial archipelago of nearly 4000 oil and gas production 

platforms in the northern Gulf over the past five decades has introduced a novel and potentially important 

component into the en route environment of trans-Gulf bird migrants. From 1998-2000, my research 

group at LSU studied the ecology of trans-Gulf migration and the influence of platforms on migrants 

using a team of field biologists stationed on an array of platforms across the northern Gulf. This study 

was funded by the Minerals Management Service (MMS) through a cooperative agreement with the 

Louisiana State University Coastal Marine Institute, with extensive support provided by British 

Petroleum, ChevronTexaco, ConocoPhillips, ExxonMobil, Newfield Exploration, and Shell Offshore. 

The objectives of this study were to quantify spring and fall trans-Gulf migrations and to evaluate the 

influence of offshore platforms on trans-Gulf migrating birds. In particular, this study sought to address 

the following questions: 1) Which species are trans-Gulf migrants? 2) Are there specific migration 

routes across the Gulf of Mexico? 3) When do migrants use platforms for stopovers, and how does the 

timing of platform use relate to the seasonal and diel timing of trans-Gulf migration as well as weather? 

4) How many individual migrants use platforms for stopovers, and how are the numbers of migrants using 

platforms related to total trans-Gulf migration traffic aloft? 5) What is the condition of migrating birds 

that stop on platforms, and what factors determine how long they stay? 6) During stopovers, do migrants 

use platforms in predictable ways? 7) How many migrants that stop on platforms depart successfully 

versus die there, and why do some birds die? 

We sought to select platforms for study that were representative of the population of platforms at large, 

with respect to both structure and geography. We staffed as many as 10 platforms in a 

given season, stretching from North Padre Island 975 in the extreme western end of the Gulf to Viosca 

Knoll 786 at the eastern boundary of the Central Planning Area. All platforms used in this study were 

major fixed-leg platforms, with the exception of one compliant tower. Standardized field work was 

conducted on five platforms from mid-March to mid-May in spring 1998 and 1999 and from mid-August 

to mid-November in fall 1998. The study was subsequently expanded considerably in temporal and 

geographical scope; ten platforms were staffed from early August through mid-November in fall 1999, 

and nine platforms were staffed from early March to late May in spring 2000. In addition to the standard 

spring and fall field programs, one platform was staffed year-round in 1999-2000.

The basic field protocol consisted of a standardized, time-designated “platform census” conducted by an 

observer while walking around the platform on a prescribed route, with the goal of locating, counting, and 

identifying all living birds on the platform at different times throughout the day. When a migrant was 

detected, it was identified to species and (when possible) age and sex, and details of its behavior and 

apparent physiological condition were recorded. This repetitive and highly standardized monitoring 

scheme permitted us to develop detailed longitudinal case histories for individuals that undertook 

extended stopovers on the study platforms. In addition to the censuses of birds stopping over on the 

platforms, visual surveys of the airspace around platforms were used to assess the volume of flyby 

migration traffic and to quantify the flight behavior of trans-Gulf migrants. 
2 

An important adjunct to our field work on the platforms was the remote observation of migration over the 

Gulf using land-based radars. Radar operates by emitting a beam of radio waves into the atmosphere and 

measuring the amount of energy reflected back to the radar unit by any “targets” in the beam’s path. The 

strength of the radar reflection is partly a function of the targets’ size and density, and theoretical and 

empirical models of radar cross section are available which permit one to estimate the density of migrants 

based on radar reflectivity. The National Weather Service currently operates a national network of Sband Doppler weather surveillance radars known as NEXRAD (Next Generation Radar), including 10 

radar sites that provide a nearly complete observational network around the northern Gulf Coast from 

Brownsville, Texas, to Key West, Florida. Radar reflectivity can be converted into migration traffic rates 

using theoretical and empirical models of radar cross section provided that one has information 

concerning the approximate size distribution of the radar scatterers, such as is available from the platform 

observations. 

Migration is profoundly influenced by the weather. To understand the influence of weather on trans-Gulf 

migration and platform use, we developed a synoptic climatology to relate large-scale weather patterns 

over the Gulf to variability in trans-Gulf migration and platform use. Our synoptic typecasting scheme 

was a slight modification of systems previously developed for the northern Gulf Coast, and recognized 

the following eight synoptic-scale weather types, which are hereafter indicated by their abbreviations in 

boldface for easy recognition: 1) Gulf Front (GF); 2) East Coast Low (ELOW); 3) Midwest Continental 

High (MCH); 4) Eastern Continental High (ECH); 5) Bermuda High (BH); 6) Gulf High (GH); 7) 

Tropical Low (TLOW); and 8) Not Determined (ND). Weather over the Gulf during this study usually 

followed a predictable cycle: As the center of an anticyclone drifted eastward across the continent and out 

over the Atlantic Ocean, winds over the northern Gulf veered from NE (MCH) to E (ECH) and 

eventually to SE (BH). Eventually a cyclone moved eastward and a front passed over the Gulf (GF) 

bringing winds from the NW. Following frontal passage, winds over the Gulf were dominated by the 

departing cyclone (ELOW) until a new anticyclone approached and the cycle repeated. Occasionally the 

cycle was interrupted when the anticyclone drifted out over the Gulf (GH), usually resulting in light 

winds over the northern Gulf. During the summer and fall, tropical weather systems (TLOW) often 

developed in the Gulf or entered the Gulf from the east, usually bringing strong winds and foul weather. 

Prior to this study, the conventional wisdom had been that spring trans-Gulf migration involves a roughly 

straight-line, shortest-distance flight from the Yucatan Peninsula to the upper Gulf Coast. Our results 

support parts of this scenario but also indicate that the situation is considerably more complex. 

Backtracking from radar images and arrival times on platforms indicates that most spring migrants initiate 

their flights from the Yucatan Peninsula and/or the northern coast of the Isthmus of Tehuantepec. Radar 

and direct observational evidence indicates that most trans-Gulf migration takes place over the western 

Gulf and suggests that the route of migrants is curvilinear and divergent, veering from a probable mean 

heading of northwest at points of origin, to north off the south Texas coast, to northeast off the Upper 

Texas Coast and Louisiana. Large flights are usually associated with Eastern Continental High (ECH) or 

Bermuda High (BH) synoptic weather patterns, in which winds similarly veer clockwise around the 

western Gulf. We therefore suggest that the route of trans-Gulf migrants is influenced by the availability 

of tailwinds, with migrants attempting to minimize the time or energy expenditure required for crossing. 

This hypothesis is strengthened by the finding that centers of offshore abundance as well as areas of 

eventual landfall varied in concert with synoptic weather. On ECH days when winds typically had a 

stronger westward component over the southern Gulf and often maintained a westward component over 

the northern Gulf, migrants were most abundant on platforms in the far western Gulf and landfall was 

usually along the Texas coast. In contrast, on BH days, when winds had a weaker westerly component 

over the southern Gulf and usually an eastward component over the northern Gulf, peak offshore 

abundance shifted eastward and landfall was more likely to take place farther east along the northern Gulf 
3 

Coast, occasionally as far as the Florida Panhandle. All available evidence indicates that the main 

migration stream is at least partially “steered” by synoptic-scale winds. 

In addition to being subject to geographic displacement via steering by synoptic winds, the migration 

stream itself showed evidence of having a complex geographic structure. In at least several species of 

warblers, females apparently take a more direct route across the Gulf, and males tend to take a more 

westerly route. Species with different goals prefer to depart under different synoptic weather types. 

The diel timing of spring trans-Gulf migration followed a predictable pattern that was evident both in 

radar imagery and from direct visual observations on the platforms. Spring migration over the northern 

Gulf began between early morning and early afternoon, peaked 3-4 h after first detection, and continued 

until 7-12 h after first detection. Patterns of diel timing varied geographically and were related to 

weather, again consistent with a strong synoptic steering influence on migration routes across the Gulf. 

The bulk of spring trans-Gulf migration detected by radar occurred between March 25 and May 24, but 

very large flights (>25 million migrants) occurred only in the 3-week period from April 22 to May 13. 

Waterfowl and herons peaked by early April. Shorebirds had widely varying migration schedules, with 

different species peaking as early as mid-March and as late as the end of May. Landbird migrants showed 

peaks throughout the season, but a majority of species peaked in the second half of April. Theoretical 

analyses of radar data yielded estimated total seasonal estimates of 316 million trans-Gulf migrants in 

spring 1998 and 147 million trans-Gulf migrants in spring 1999. In both years, about two thirds of all 

migrants made landfall west of South Marsh Island. 

Radar-observed spring migration was characterized by a series of pulses and tended to be “all-ornothing”, i.e., either significant trans-Gulf migration was evident on radar or else it was essentially 

entirely absent. Dramatic hiatuses in radar-observed migration were always associated with strong cold 

fronts that penetrated deep into Mexico and set up persistent northerly winds over most of the Gulf. 

Conversely, radar-observed migration peaks were almost strictly associated with ECH and BH days. 

Fall trans-Gulf migration was more difficult to study because the extensive presence of aerial insects 

precluded quantitative interpretation of radar imagery. In addition, one of the two field seasons was 

partly compromised by prolonged absences from the platforms due to obligatory evacuations in response 

to developing tropical weather systems. Nevertheless, we argue that the heaviest trans-Gulf migration 

traffic in fall originates from the stretch of the northern Gulf Coast running eastward from Alabama. 

Although we were constrained from sampling much of this area by the absence of platforms in the eastern 

Gulf during the study period, our contention is supported by observed longitudinal trends in abundance 

and age ratios. Southbound “fall” migrants were observed as early as May 20 and as late as January, but 

the vast majority of the migration occurred from mid-August to early November. There seemed to be 

several phases in the fall migration. During the early fall, migration by long-distance migrants appeared 

to be obligate and was not strongly influenced by weather. Later in the fall, major trans-Gulf movements 

of shorter-distance migrants were generally associated with cold fronts and northerly winds. Direct 

observations at the eastern-most platform indicated that the direction of flight was most often due south 

but varied from south-southwest to south-southeast. As with spring, variation in the direction of travel 

was clearly influenced by wind. 

We also detected considerable fall migration over the far western Gulf, where flight direction usually had 

a westerly component. The western-Gulf route was used by a high proportion of juveniles, and appeared 

to represent a risk-averse migration strategy favoring a shorter, less risky overwater flight leg at the 

expense of a more circuitous overall migration route. We suspect that many of the adults traveling over 

the western Gulf were individuals that reached the northeastern Gulf Coast with inadequate fat stores for a 

direct trans-Gulf and worked their way westward along the coast, perhaps stopping over along the way. 
4 

One of the interesting features of the fall migration offshore was the frequent occurrence of a variety of 

species that do not typically winter south of the northern Gulf Coast. These species were evidently 

mostly “overshoots” that inadvertently traveled past their intended destinations and found themselves 

unexpectedly over water at first light, or else circum-Gulf migrants that inadvertently drifted eastward 

over the Gulf during nocturnal flight. Accordingly, these species were often observed flying north or 

west during daylight hours, presumably trying to get back to land. 

The year-round observations on one platform indicated that northbound (“spring”) trans-Gulf migration 

spans late January to early June, and southbound (“fall”) trans-Gulf migration and overshooting spans 

early July to early December. Surprisingly, we found that northbound and southbound migrations 

overlapped temporally at the extremes: The latest southbound migrant recorded during the study 

(Common Snipe) occurred on 28 January, and the earliest northbound migrant (Purple Martin) occurred 

the following day. Southbound migration of Purple Martins began in late April and southbound shorebird 

migration began in late May, well before the latest northbound migrant was recorded on 8 June (Northern 

Waterthrush). Overwater movements during the brief interim periods between spring and fall migrations 

(mid-December to mid-January, mid-June to early July) seemed to be dominated by herons traveling 

along an east-west axis. The nature of these movements is currently unknown. 

Death of migrants by starvation was fairly common in the spring. Dead birds in spring lacked any trace 

of fat and had conspicuously protruding keels, indicating that they had begun to catabolize nonfat dry 

body components prior to arrival on the platforms. Water consumption by trans-Gulf migrants was very 

rare, indicating that water is not a limiting factor to trans-Gulf migrants. 

Platforms have three primary proximate impacts on migrant birds: 1) they provide habitat for resting and 

refueling; 2) they induce nocturnal circulations; and 3) they result in some mortality through collisions. 

Platforms appeared to be suitable stopover habitats for most species, and most of the migrants that 

stopped over on platforms probably benefited from their stay, particularly in spring. Many of these 

migrants were able to feed successfully, and some appeared to achieve rates of mass gain that exceeded 

what is typical in terrestrial habitats. Even the individuals that do not feed probably benefit 

physiologically from the availability of the platforms. Migrants may be affected by sources of fatigue 

other than total depletion of fat stores, such as excessive accumulation of lactic acid, failure of the nervemuscle junction, or upset of central nervous coordination. These types of fatigue may be eliminated by 

simple rest. Many of the migrants that rested quietly on the platforms for hours to days were probably 

recovering from such sources of fatigue. 

Migrants used platforms in highly nonrandom ways and selected specific platform microhabitats (i.e., 

used alternative microhabitats nonrandomly), much in the same way that they select specific habitats 

during terrestrial stopovers. Preferred platform microhabitats were species-specific and generally 

consistent between spring and fall. 

Platforms may facilitate the evolution of trans-Gulf migration strategies in certain species by providing 

“steppingstones” that allow incipient migrants to cross the Gulf successfully via a series of shorter flights. 

Cattle Egrets colonized eastern North America only in the last half-century, but have already become one 

of the most common species on platforms. White-winged Doves and Eurasian Collared-Doves are 

rapidly evolving trans-Gulf migration strategies in concert with population explosions and major range 

expansions into the southeastern United States. 

Peregrine Falcons are perhaps the most striking beneficiaries of platforms. This species, which formerly 

was near extinction, underwent a dramatic population recovery that was temporally coincident with the 
5 

period of fastest expansion of the platform archipelago in the Gulf. The majority of juveniles in the North 

American population of this species now uses oil platforms in the northern Gulf during the fall for resting 

and hunting. Their behavior and the similarity of ecological circumstances to the Mediterranean Sea, 

where a related species has evolved a strategy of breeding on islands during the fall when abundant transMediterranean migrant landbird prey are available for provisioning young, suggests that Peregrine 

Falcons might eventually establish a breeding population on the Gulf platform archipelago. 

Migrants sometimes arrived at certain platforms shortly after nightfall and proceeded to circle those 

platforms for variable periods ranging from minutes to hours. These circulations clearly occurred because 

nocturnal migrants were attracted to platform lights, and tended to occur on overcast nights. It is believed 

that circulations are maintained when birds get inside the cone of light surrounding the platform and are 

reluctant to leave, seemingly becoming trapped by the surrounding “wall of darkness” and the loss of 

visual cues to the horizon. Circulations put birds at risk for collision with the platform or with each other, 

and result in non-useful expenditure of energy. 

Collisions with platforms were most common in fall because most migrants were aloft over the platforms 

during hours of darkness in that season. Available information suggests that the platform archipelago 

may cause roughly 200,000 collision deaths per year, which is negligible compared to other 

anthropogenic sources of mortality. However, several lines of evidence suggest that future development 

of the eastern Gulf of Mexico may result in a disproportionately large increase in collision mortality in 

fall trans-Gulf neotropical migrants. 

We provide six specific recommendations for the Minerals Management Service and other parties 

concerned with trans-Gulf migration and the impact of offshore oil and gas activities on birds: 

1. Attention should be paid to the possibility of developing and maintaining a network 

of decommissioned platforms as permanent “observatories” for long-term ecological 

research. In addition to facilitating the long-term monitoring of migratory bird 

populations, such observatories would permit studies of seabirds, insects, fishes, 

meteorology, oceanography, and other subjects. Economic feasibility of such a 

project would be made possible by cost-sharing among a wide variety of agencies 

and organizations. 

2. We suggest that the Minerals Management Service should consider implementing an 

ongoing platform monitoring program in the eastern Gulf of Mexico as that area is 

developed by the petroleum industry. Our findings suggest that the heaviest transGulf migration in fall emanates from the stretch of the northern Gulf Coast running 

eastward from Alabama, and that neotropical migrants over the eastern Gulf may be 

particularly vulnerable to collisions with platforms. The observer program operated 

by the National Marine Fisheries Service to monitor catch and bycatch in commercial 

fisheries may serve as a useful model for developing an analogous program to 

monitor mortality (= “bycatch”) on eastern Gulf platforms. 

3. The impact of nocturnal circulation events on both spring and fall trans-Gulf 

migrants remains poorly known, and this phenomenon should be examined in a 

focused observational study using night-vision optics and thermal imaging 

equipment. The goals of such a study should be to quantify in greater detail the 

dimensions of the circulation phenomenon, to try to determine why some platforms 

often induce circulations and others never do, to assess the rate of turnover during 

major circulation events, and to model the energetic impacts on migrants. 
6 

4. If fall collision mortality in the eastern Gulf proves to be significant or if results from 

the study of circulations suggest that the adverse impacts of this phenomenon should 

be addressed, experiments should be undertaken to evaluate the role of different color 

schemes and lighting regimes in the attraction of migrants to platforms. Simple 

changes in light signatures have resulted in dramatic reductions in avian attraction 

and mortality at tall lighted structures on land, and would presumably be equally 

effective at sea. 

5. Production of a colorful informational brochure about trans-Gulf migration for 

distribution to offshore workers and other people involved in the industry would be a 

useful way to promote a wider awareness of the ecological importance of the Gulf, 

and may be an incentive to platform workers to help maintain a safe environment for 

avian visitors to platforms. 

6. Biologists interested in the ecology and conservation of trans-Gulf migrants should 

initiate outreach efforts to involve international colleagues in the development of a 

network for information exchange concerning events in all geographic sectors of the 

Gulf, since trans-Gulf migration occurs over waters subject to hydrocarbon 

development governed by other nations (Mexico, Cuba). 
PART 1 

 

BACKGROUND 
9 

Chapter 2 

 

Introduction 

Robert W. Russell 

2.1 BACKGROUND AND OBJECTIVES

Each spring, vast numbers of landbirds migrate northward across the Gulf of Mexico en route to breeding 

habitats in the United States and Canada from their wintering quarters in the neotropics. Following a 

short breeding season in the north, most of these birds return southward across the Gulf, their numbers 

then augmented by offspring produced over the summer. From the standpoint of sheer numbers, these 

annual trans-Gulf migrations constitute one of the great wildlife events of the world. Nevertheless, many 

basic aspects of the migrations have remained poorly known until now; indeed, until the 1950s, the very 

idea that birds routinely and “intentionally” migrate across the Gulf was treated with skepticism by some 

respected scientists (e.g., Williams 1945, 1947, 1952). 

Scholarly studies of the trans-Gulf flights — based largely on indirect evidence and coastal observations 

— were initiated in the 1940s by George Lowery (1945, 1946, 1951) at Louisiana State University. 

Those early studies, together with subsequent work by Lowery’s students and colleagues (e.g., Buskirk 

1968; Hebrard 1971; Gauthreaux 1971, 1972; Able 1972), have until now remained the basis for our 

understanding of trans-Gulf migration. 

Radar studies have indicated that the flight pathway of the majority of trans-Gulf migrants in spring is 

directed toward the coasts of Louisiana and Texas (Gauthreaux 1970, 1971, 1992), and thus over Gulf 

waters in which are located the majority of offshore oil and gas production facilities. Offshore platforms, 

which house production equipment and living quarters for personnel, have played a central role in the 

development of oil and gas resources in the Gulf of Mexico. The history of offshore platforms in the Gulf 

has been short but dynamic. The first offshore platform was installed in the Gulf of Mexico in 1947. The 

first multi-platform complex was installed in 1960. By 1974, 800 platforms had been installed in the 

Gulf. As the number of platforms grew, so did the geographic extent of their distribution offshore. Fixed 

platform installation depth reached 30 m in 1955, 60 m in 1962, and 300 m in 1978. Production began in 

waters exceeding 600 m in 1984, and in waters exceeding 1500 m in 1997. The thousands of platforms 

now located on the continental shelf of the northern Gulf of Mexico make up the largest artificial island 

system in the world. 

One of the most important components of birds’ migration strategies is their use of local habitats for 

resting and refueling while en route. In light of the absence of natural islands or other terrestrial habitats 

during crossings of the Gulf of Mexico, it seems inevitable that the installation of thousands of artificial 

islands in the northern Gulf must affect migrants in some fashion. However, to date, no systematic 

studies had examined the influence of Gulf platforms on trans-Gulf migrating birds. 

Beginning in spring 1998, interactions between migrating birds and offshore platforms were investigated 

by personnel at Louisiana State University, initially based at the Museum of Natural Science and later at 

the School of the Coast and Environment. This study, funded by the Minerals Management Service 

(MMS) through a cooperative agreement with the LSU Coastal Marine Institute, had the support of 

several major oil companies (British Petroleum, ChevronTexaco, ConocoPhillips, ExxonMobil, Newfield 

Exploration, and Shell Offshore). 
10 

The MMS is a bureau within the Department of the Interior that supports marine environmental studies as 

part of its mission to gather technical information for management decisions concerning mineral 

resources on the continental shelf. The long-term survival of migratory birds is of considerable concern 

to the Department of the Interior. Long-term monitoring of populations in the eastern United States has 

suggested that major declines of some forest-dwelling species may be underway (e.g., Robbins et al. 

1989; Terborgh 1989; Askins et al. 1990; Hill and Hagan 1991; Peterjohn et al. 1995). Because 

neotropical migrants comprise a large percentage of these breeding birds, a widespread decline in their 

numbers could seriously reduce the biodiversity of eastern and central North America. Most neotropical 

migrants travel across the Gulf of Mexico during the spring and/or fall, so documenting the possible 

influences of offshore platforms on their migrations is an important prerequisite for the development of 

long-term conservation strategies (Faaborg 2002). 

The objectives of this study were to quantify spring and fall trans-Gulf migrations and to evaluate the 

influence of offshore platforms on trans-Gulf migrating birds. In particular, this study sought to address 

the following broad questions: 

1. Which species are trans-Gulf migrants? 

2. Are there specific migration routes across the Gulf of Mexico? 

3. When do migrants use platforms for stopovers, and how does the timing of platform 

use relate to the seasonal and diel timing of trans-Gulf migration as well as weather? 

4. How many individual migrants use platforms for stopovers, and how are the numbers 

of migrants using platforms related to total trans-Gulf migration traffic aloft? 

5. What is the condition of migrating birds that stop on platforms, and what factors 

determine how long they stay? 

6. During stopovers, do migrants use platforms in predictable ways? 

7. How many migrants that stop on platforms depart successfully versus die there, and 

why do some birds die? 

2.2 SELECTION OF STUDY PLATFORMS

We sought to select platforms for study that were representative of the population of platforms at large, 

with respect to both structure and geography. 

A production platform consists of two major components: an underwater support structure and an abovewater production system. The production system comprises a deck or series of decks accommodating 

production equipment, control buildings, and living quarters. The most commonly used type of 

production platform in the northern Gulf of Mexico is the fixed-leg platform, which is supported by a 

rigid metal jacket that is attached to the sea floor with piles. All platforms used in this study (Table 2.1, 

Figure 2.1) are fixed-leg platforms except Viosca Knoll 786 (“Petronius”), which is a compliant tower. 

Compliant towers differ from fixed platforms in that the underwater support structure is not a rigid jacket 

but rather a narrow, flexible tower that can move horizontally and thereby withstand stronger forces of 

wind and current that are of greater concern in deep water. 

Platforms may be categorized into “major structures” and “minor structures.” Major structures include 

traditional fixed-leg platforms as well as more recent innovations such as compliant towers, tension leg 

platforms, and spars. Minor structures are defined here to comprise caissons and well protectors. All 

platforms used in this study were necessarily major structures; we were unable to include minor structures 
11 

in the study because of the lack of housing facilities and other support on those structures. Opportunistic 

visits to minor structures suggested that they are also used by migrating birds and that most of the 

findings reported here are also applicable to minor structures, albeit on a smaller scale. 

At the conclusion of this study, there were 3881 production platforms in the U.S. Gulf of Mexico, 

including 2125 major structures (1762 in the Central Planning Area and 363 in the Western Planning 

Area) and 1756 minor structures (1591 in the Central Planning Area and 165 in the Western Planning 

Area). The selection of platforms used in this study covered the entire longitudinal range of the platform 

archipelago (Figure 2.1). 

A number of characteristics of platforms were considered to be potentially important factors influencing 

their use by migrants, but because of the relatively small number of platforms we were able to sample, it 

was not possible to incorporate all factors into a rigorously blocked study design. Consequently, it is 

necessary to consider the possible impact of these factors on a platform-specific basis. 

One potentially important factor is the distance of a platform to the nearest shore. We expected that tired 

spring migrants might be more likely to use the first platforms they encounter during northbound flight 

across the Gulf, i.e., platforms farthest from the northern Gulf Coast. Similarly, we expected that spring 

migrants might be less likely to use platforms very close to shore since natural habitats would often be 

visible to an airborne migrant over such platforms. Our study platforms vary in distance to nearest shore 

from 8-204 km. Land is visible to the human eye in appropriate atmospheric conditions from only three 

of our platforms (PL10, MI622, SP89). 
12 

Table 2.1. 

Platforms used in this study 

 Block/Platform Location Water Depth 

Host Company Abbreviation Name (lat N, long W) (feet) Ye

Shell PN975 North Padre Island 975-A 26°49’58”, 

96°56’24” 

 156 

BP Amoco MI622 Matagorda Island 622-C 28°06’06”, 

96°22’52” 

 83 

Newfield HI561 High Island 561-A 27°58’54”, 

94°30’20” 

 255 

Texaco GB189 Garden Banks 189-A 27°46’43’’, 

93°18’34’’ 

 720 

Exxon VR265 Vermilion 265-A 28°30’45”, 

92°27’04” 

 165 

Newfield SM147 South Marsh Island 147-A 28°13’40”, 

92°00’60” 

 235 

Phillips SM66 South Marsh Island 66-C 28°38’48”, 

91°56’15” 

129 

Mobil GC18 Green Canyon 18-A 27°56’37”, 

91°01’45” 

 750 

Mobil PL10 South Pelto 10-B 28°56’53”, 

90°43’25” 

 35 

BP Amoco EW826 Ewing Bank 826-A 28°09’48”, 

90°21’31” 

 483 

Exxon SP89 South Pass 89-B 28°40’50”, 

89°23’15” 

400 

Texaco VK786 Viosca Knoll 786 (“Petronius”) 29°13’44”, 

87°46’55” 

 1754 
13 

North

Padre

Island

975

Matagorda

Island 622

High Island 561

Garden

Banks

189

Vermilion

265

South

Marsh

Island

147

South

Marsh

Island

66

Green

Canyon

18

South

Pelto

10

Ewing 

Bank 

826

South

Pass

89

Main 

Pass 

252

“Bud”

Viosca

Knoll 

786 

“Petronius”

Figure 2.1. Map showing the locations of platforms used in this study (large red circles) relative to the 

distribution of all platforms in the northern Gulf (small yellow circles). 

Another potentially important factor is the density of neighboring platforms in waters surrounding a study 

platform. The zone of airspace that is closer to a particular platform than to any neighboring platform 

may be termed the platform’s “hinterland” (cf. Cairns 1989). The hinterland is important because it may 

determine the source population of airborne migrants from which a platform may draw. The extent of the 

hinterlands of our study platforms varies greatly, with some platforms being extremely isolated and others 

being embedded within extensive platform “metropolises” (Figure 2.2). 

Another potentially important factor is whether a platform is part of a complex. A “platform complex” is 

defined here as a group of platform structures connected by walkways. Three of our study platforms 

(PL10, VR265, MI622) are actually multi-platform complexes. 

Another potentially important factor is whether a platform flares gas (i.e., has an active flame). Flaring is 

potentially important because some previous reports have suggested that nocturnal migrants may be 

attracted to and/or burned in gas flares (e.g., Davies 1940; Sage 1979; Bjorges 1987). Only two of our 

platforms (SP89, VR265) routinely flared gas during the study. 

Distance from shore, extent of isolation, status as a complex, and occurrence of flaring on our study 

platforms are summarized in Table 2.2. 
14 

0

10

20

30

40

50

60

70

80

10 15 20 25 30

Range from focal platform

Other platforms within range

PN975

VK786

GC18

EW826

GB189

HI561

SP89

MI622

PL10

SM147

SM66

VR265

Figure 2.2. Relative isolation of the study platforms. The figure shows the number of 

platforms located within a given radius of each platform as a function of 

radius. 

Table 2.2. 

Selected characteristics of the study platforms 

The table shows the distance to the nearest shoreline, degree of isolation of the 

platform (categorized in three arbitrary levels; see Figure 2.2), whether the platform 

is actually a complex of platforms, and whether the platform flared gas (i.e., had an 

active flame) during the study. 

Platform Distance to 

Nearest Shore 

(km) 

Isolation 

Category 

Platform 

Complex? 

Flaring? 

VK786 145 isolated no no 

SP89 23 moderate no yes 

EW826 87 isolated no no 

GC18 154 isolated no no 

PL10 8 crowded yes no 

SM66 92 crowded no no 

SM147 127 crowded1 no no 

VR265 116 crowded yes yes 

GB189 204 isolated no no 

HI561 134 moderate no no 

MI622 24 crowded yes no 

PN975 48 isolated no no 

1

However, there are virtually no platforms to the south of SM147.
15 

2.3 SUMMARY OF FIELD OPERATIONS

Standardized field work was conducted during five migration seasons from spring 1998 through spring 

2000 (Table 2.3). In spring 1998, five platforms were staffed continuously from mid-March through midMay with no interruptions. SM147 was substituted for SM66 beginning in fall 1998 due to extensive 

maintenance activity on SM66. In fall 1998, five platforms were staffed from mid-August through midNovember, but much of September was unfortunately lost to hurricane evacuations. GC18 was 

substituted for PL10 beginning in spring 1999 due to the sale of the latter by Mobil to Vastar and the 

unwillingness of Vastar to permit us to continue working on PL10. In spring 1999, five platforms were 

staffed from late March through early May, with interruption only at GC18 for less than a week due to a 

contingency on the platform. 

The study was expanded considerably in temporal and geographical scope beginning in fall 1999, when 

ten platforms were staffed from early August through mid-November. A few interruptions of <10 days 

resulted from hurricane evacuations, availability of personnel, and a family emergency. VK786 was 

unavailable in spring 2000 due to construction activity, and nine platforms were staffed from early March 

through late May, with only three interruptions of less than a week caused by illness, a family emergency, 

and a staffing limitation. 

In addition to the standard field program, GC18 was staffed during the winter of 1999-2000 

(November 18-December 16, January 6-February 29) and during the summer of 2000 (June 1-August 1). 

During the fall of 2000, specialized studies of migratory orientation and foraging behavior were 

conducted on PN975, MI622, and Main Pass 252-A (MP252, also known as “Bud”; 29°21’39”N, 

87°53’09”W).
16 

Table 2.3. 

 

Summary of field effort (inclusive dates of field work) 

Platforms are listed in order from east to west. 

Platform Spring 1998 Fall 1998 Spring 1999 Fall 1999 Spring 2000 

VK786 --- --- --- Aug 11-Oct 14, 

Oct 16-Nov 16 

--- 

SP89 --- --- --- Aug 4-Oct 20, 

Oct 27-Nov 17 

Mar 7-May 31 

EW826 Mar 15-May 14 Aug 13-31, 

Sept 15-16, 

Sept 22-25, 

Oct 1-Nov 9 

Mar 18-May 13 Aug 5-Sept 19, 

Sept 23-Nov 18 

Mar 9-May 25 

PL10 Mar 16-May 15 Aug 14-31, 

Sept 7-8, 15-16, 

Sept 22-24, 

Oct 2-Nov 1 

--- --- --- 

GC18 --- --- Mar 18-20, 

Mar 26-May 13 

Aug 4-Nov 17 Mar 7-May 31 

SM66 Mar 13-May 14 --- --- --- --- 

SM147 --- Aug 14-31, Sept 5-

17, 

Sept 21-25, 

Oct 1-Nov 13 

Mar 18-May 12 Aug 4-Sept 8, 

Sept 18-Nov 16 

Mar 8-May 10, 

May 17-31 

VR265 Mar 13-May 12 Aug 14-Sep 1, 

Sept 4-16, 

Sept 21-26, 

Oct 1-Nov 17 

Mar 18-May 13 Aug 4-Nov 16 Mar 7-May 29 

GB189 Mar 13-May 15 Aug 15-31, 

Sept 3-8,13-16, 

Sept 21-24, 

Oct 1-Nov 14 

Mar 19-May 12 Aug 4-21, 

Aug 24-Nov 17 

Mar 7-May 27 

HI561 --- --- --- Aug 5-Sept 8, 

Sept 15-Oct 21, 

Oct 28-Nov 17 

Mar 9-May 30 

MI622 --- --- --- Aug 4-20, 

Aug 25-Nov 17 

Mar 9-17, 

Mar 20-May 31 

PN975 --- --- --- Aug 6-21, 

Aug 25-Sept 15, 

Sept 25-Nov 17 

Mar 9-31, 

Apr 7-May 31 

Total 

platformdays 

310 363 277 998 738 
17 

Chapter 3 

 

Field Methods 

Robert W. Russell 

3.1 PLATFORM CENSUSES

The basic field protocol consisted of a standardized, time-designated census conducted by an observer 

while walking around the platform on a prescribed route. The goal of this “platform census” (PC) was to 

locate, count, and identify all living birds on the platform at different times during the day. The platform 

census was repeated identically throughout the day from pre-dawn to post-sunset, allowing us to 

determine when birds arrived and when they left. When we detected a migrant, we identified it to species 

and when possible age and sex; we assessed its likely body condition based on factors such as alertness, 

wariness, vigor, and apparent thermal stress; and we recorded details of its behavior. 

This repetitive and highly standardized monitoring scheme permitted us to develop detailed longitudinal 

case histories for nearly all individuals that undertook extended stopovers on the study platforms. 

Whenever possible, we attempted to track the stopover of each bird, relying on distinctive plumage 

characteristics, individual behavioral idiosyncrasies, and sheer hard work when many birds of a given 

species were present. Nevertheless, it was sometimes impossible to keep track of individual birds when 

more than two individuals of a given species were present on the platform simultaneously; consequently, 

our data on stopover durations are slightly biased toward birds that arrived on the platforms during 

periods of relatively low migration traffic. 

3.2 FLIGHT CENSUSES

The platform censuses were intended to provide information on platform use by migrants. Because not 

all incoming migrants landed on platforms, it was important to obtain independent information on 

incoming migration traffic. Thus, in addition to the platform censuses, we conducted several timedesignated surveys from a fixed point of the airspace over and around the platform. These flight censuses 

were conducted from a south-facing site during the spring and from a north-facing site during the fall. 

During flight censuses, observers attempted to locate, count, identify, and follow all visible flying 

migrants. When we detected a bird in flight, we recorded its flight altitude, ascertained whether or not it 

approached or attempted to use the platform, and recorded its vanishing bearing (see below for details). 

3.3 CENSUS SCHEDULES

In spring, the platform census was repeated identically six times at equally spaced intervals throughout 

the day from pre-dawn to post-sunset (Table 3.1). The start of PC0 was about nine hours after the 

expected time of departure of migrants from the Yucatan Peninsula (taken to be the midpoint between the 

end of evening civil twilight and the end of nautical twilight; Hebrard 1971), and the conclusion of PC5 

was about 25 hours after the expected time of departure from the Yucatan. Flight censuses were 

conducted in late morning and late afternoon to discern between early and late flights; the midday flight 

census (FCm) was added to the schedule starting in spring 1999. The seawatch census (SEA) was 

conducted as part of an ancillary study to quantify seabird occurrence in waters surrounding platforms, 

but data on trans-Gulf migrants were recorded in a fashion identical to the flight censuses. In spring 2000 

on GB189 only, we conducted an additional “circulation census” for an hour beginning at the end of 
18 

evening astronomical twilight to attempt to quantify the nocturnal circulation phenomenon 

(described in Chapter 16). 

Table 3.1. 

 

Schedule of daily field operations in the spring 

Census Designation Time (CST) 

Platform census 0 (PC0): 04:00-05:00 

Platform census 1(PC1): 07:00-08:00 

Seawatch census (SEA): 08:00-08:30 

Flight census 1 (FC1): 09:30-10:00 

Platform census 2 (PC2): 10:00-11:00 

Midday flight census (FCm): 12:30-13:00 

Platform census 3 (PC3): 13:00-14:00 

Flight census 2 (FC2): 15:30-16:00 

Platform census 4 (PC4): 16:00-17:00 

Platform census 5 (PC5): 19:00-20:00 

The schedule during the fall (Table 3.2) was modified to account for the expectation that most trans-Gulf 

migrants would be aloft over the platforms at night, and the consequent expectation that rapid changes in 

ambient light conditions would be important events for migrants. Because of the longer duration of the 

fall field season and an expectation that transitions in lighting would figure more importantly during the 

fall, the fall schedule was tied directly to astronomical benchmarks. Thus, the precise clock timing of 

censuses varied from platform to platform and over the course of the season. The first platform census 

(PC1) ended at the beginning of morning civil twilight and was intended to quantify the use of platforms 

at night, since departures were unexpected before morning civil twilight. Subsequent censuses were 

keyed to sunrise or sunset to facilitate meaningful comparisons among platforms and over the long 

season. Platform census 4 (PC4) had to be conducted at a fixed time in order to permit observers on all 

platforms to attend dinner over the entire course of the season. Flight censuses were conducted beginning 

one hour after sunrise, three hours after sunrise, and five hours after sunrise to attempt to quantify the 

timing of arrival of diurnal migrants that were expected to depart the northern Gulf Coast at first light. A 

final flight census was conducted at sunset to attempt to document diurnal migrants that might descend to 

the platforms at nightfall. 
19 

Table 3.2. 

 

Schedule of daily field operations in the fall 

BMCT = beginning of morning civil twilight; SR = sunrise; 

SS=sunset. 

Census Designation Time (CST) 

Predawn census (PC1): BMCT-1 → BMCT 

Sunrise census (PC2): SR → SR+1 

Flight census 1 (FC1): SR+1 → SR+1.5 

Seabird census (SEA): SR+1.5 → SR+2 

Platform census 3 (PC3): SR+2 → SR+3 

Flight census 2 (FC2): SR+3 → SR+3.5 

Flight census 3 (FC3): SR+5 → SR+5.5 

Platform census 4 (PC4): 14:00-15:00 

Sunset census (PC5): SS-1 → SS 

Flight census 4 (FC4): SS → SS+0.5 

Night census (PC6): SS+0.5 → SS+1.5 

3.4 OPPORTUNISTIC WORK

Flight directional information and general behavioral notes were also recorded opportunistically 

throughout the day between formal platform censuses. 

In addition to censusing live migrants, we salvaged dead birds under a U.S. Fish and Wildlife Service 

permit to the Louisiana State University Museum of Natural Science. Specimens were frozen on the 

platforms until transferred ashore for preparation as voucher specimens for the Museum’s permanent 

collections. Tissue samples of selected species were obtained and deposited in the Museum’s genetic 

resources collection, where available to qualified researchers for molecular systematics studies. 

Additional tissue samples were provided to Mr. Donald Norman for comparative ecotoxicological studies 

of organochlorine residues in spring versus fall migrants. 

3.5 DATA RECORDING

3.5.1 Local Weather 

Local ambient weather was characterized at the beginning of each census on each platform using the 

following schemes. 

Overall weather was assigned to one of the following categories: 

CP = convective precipitation (e.g., thunderstorms, heavy rain, intermittent heavy rain) 

SP = stratiform precipitation (e.g., continuous drizzle with low cloud deck) 

F = fog 

S = clear (< 25% cloud cover) 

PC = partly cloudy (25% ≤ cloud cover < 75%) 

O = overcast (≥ 75% cloud cover) 
20 

Visibility was estimated on a logarithmic scale: 

4 = >10 km 

3 = 1-10 km 

2 = 100 m - 1 km 

1 = 10-100 m 

0 = <10 m 

When an accurate anemometer was available, precise measurements of wind speed were taken. 

Otherwise, wind speed was assessed using the Beaufort scale (Table 3.3). 

Table 3.3. 

 

Beaufort scale 

Beaufort 

Force 

Wind Speed 

(knots) 

Wind Speed 

km·h-1 

Description of Wind Sea State 

0 <1 <1 Calm Sea like a mirror 

1 1-3 1-5 Light air Ripples only, no foam crests 

2 4-6 6-11 Light breeze Small wavelets (0.2 m); crests do not break, and 

have a glassy appearance 

3 7-10 12-19 Gentle breeze Large wavelets (0.6 m); crests begin to break, 

perhaps scattered white horses. 

4 11-16 20-29 Moderate breeze Small waves (1 m), fairly frequent white horses 

5 17-21 30-39 Fresh breeze Moderate waves (1.8 m), many white horses 

6 22-27 40-50 Strong breeze Large waves (3 m), white foam crests; probably 

some spray 

7 28-33 51-61 Near gale Mounting sea (4 m) with white foam blown in 

streaks downwind 

8 34-40 62-74 Gale Moderately high waves (5.5 m), crests break 

into spindrift 

9 41-47 76-87 Strong gale High waves (7 m), dense foam, visibility 

affected 

10 48-55 88-102 Storm Very high waves (9 m), heavy sea roll, visibility 

impaired; surface generally white 

11 56-63 103-118 Violent storm Exceptionally high waves (11 m), visibility poor

12 64+ 119+ Hurricane Massive waves (14 m), air filled with foam and 

spray, visibility bad 

3.5.2 Body Condition of Migrants 

The apparent physical condition of a migrant during each encounter was assessed using the following 

scheme: 

Migrants were classified as “alive and vigorous” (AV) if they appeared healthy, wary, and vigorous, and 

exhibited behaviors typical for the species. As a rule of thumb, a bird was assigned to this category if it 
21 

flushed (flew away) before being approached to within 3 m, or if it spent a good deal of time 

(>10%) flying around the platform. 

Migrants were classified as “alive but tired” (AT) if they were obviously tired. Birds observed sleeping 

during the day were usually assigned to this category, as were active birds that appeared moderately 

disheveled from physical exertion. Birds in this category usually flew readily and easily before being 

approached to within 3 m (if they were awake). 

Migrants were classified as “alive but exhausted” (AE) if they exhibited signs of energetic distress, such 

as lethargy, extreme feather-fluffing, or a noticeably disheveled appearance. As a rule of thumb, a bird 

was assigned to this category if it could be approached while awake to within 3 m before flying or 

hopping away. 

Migrants were classified as “moribund” (M) if they were alive but could be picked up without resistance, 

or if they otherwise appeared to be injured or severely emaciated. 

3.5.3 Foraging Behavior 

Upon each observation, an individual migrant was classified into one of three categories according to 

foraging behavior: 

0 = Did not appear to be looking for food 

1 = Appeared to be looking actively for food 

2 = Successfully obtained food 

If a migrant was ever observed foraging during the course of its stopover, then for the purposes of 

statistical analysis its foraging status was considered to be positive (i.e., it foraged); if the migrant was 

never observed foraging, then its foraging status was considered to be negative (i.e., it did not forage). 

We classified foraging success using similar reasoning. If a migrant was ever observed to obtain food, 

then it was classified as a successful forager; if it foraged but was never observed to obtain food, then it 

was classified as an unsuccessful forager. 

 

Drinking behavior was categorized similarly. If a migrant was ever observed drinking during the course 

of its stopover, then its drinking status was positive; otherwise, its drinking status was negative. 

3.5.4 Microhabitat Use 

Based on initial observations that migrants appeared to be selecting specific “habitats” on the platforms, 

we expanded the observational program to include microhabitat use beginning in fall 1999. Microhabitat 

use was assessed using the following scheme, which incorporates both physical structure and how the 

birds use the structure: 

Exposed Perch (EP): Pipe rails, antennas, communication towers, flare booms, building roofs, helipad 

edges, cranes, edges or peaks of open equipment such as machinery or tanks. Birds used EP habitats to 

rest and sleep on and to forage from. 

Open Deck (OD): Ordinary steel decks, woods decks, flat building roofs, and helipad decks. Birds used 

OD habitats to rest, sleep, and forage on. Birds were assigned to this habitat if they were spending most 

of their time on the deck; for example, a bird that flew down from a railing to grab a moth from the deck 

would be assigned EP habitat, not OD. 
22 

Grating (GG): Open grating composing decks, sections of decks, and stairs, and sometimes 

functioning as both deck and ceiling. Birds used GG habitats to rest and sleep on and to forage in, 

under, and over. On parts of the platforms squares of grating are interspersed with squares of open deck. 

If a bird were just hopping around indiscriminately, it was generally assigned OD habitat; however, if it 

engaged in probing in and around the grating, then it was assigned GG habitat. 

Low Cover (LC): Piles of grating, pipe, or rope; spools of wire, cable, or hose; boxes, machinery, tanks, 

and piping. Substrates composing LC habitat may extend from the deck up to a substantial height (~5 m), 

but this habitat specifically excludes the beams and large pipes running underneath but close to other 

floors or decks (which constitute HC habitat). Birds used LC habitats to rest, sleep, and forage in. Birds 

often loitered around low cover but hopped out onto the deck as well. If they seemed to be centered in the 

cover, occasionally venturing out from it, then they were assigned LC habitat; if they were in the cover 

primarily when disturbed, then they were assigned OD habitat. 

High Cover (HC): Beams and pipes running just below overhead decks and ceilings. Birds used HC 

habitats primarily for resting and sleeping, and sometimes for foraging. Most birds assigned HC habitat 

were sitting on the lips of high beams. 

Trapped (TR): Overhead beams or grating, sheds, warehouses, and breezeways. Birds assigned to TR 

“habitat” were trapped and unable to escape, at least temporarily; they generally showed distress, trying to 

escape or resting between efforts to escape. On some platforms, the well bay was especially likely to trap 

birds during darkness. 

We recorded the habitat of an individual migrant when it was first detected on the platform (“initial 

habitat”) and when it was observed for the second time (“second habitat”). In addition, we assigned a 

“usual habitat” to longer-staying individuals; this was a subjective assessment based on repeated 

experience with an individual over the course of its stopover.

3.5.5 Cause of Death 

When a dead migrant was discovered or a death was observed directly, the cause of death was assigned to 

one of the following categories: 

Starvation: Starvation victims were evident by the protruding keel and vestigial quantities of breast 

muscle. 

Predation: The principal predators of migrants were Peregrine Falcons (Falco peregrinus), Merlins 

(Falco columbarius), American Kestrels (Falco sparverius), Sharp-shinned Hawks (Accipiter striatus), 

Loggerhead Shrikes (Lanius ludovicianus), and Laughing Gulls (Larus atricilla). 

Collision: Collision victims were typically found below tall structures such as flare booms, 

communication towers, cranes, drill derricks, tanks, helipads, and platform legs. Indications of trauma 

were usually evident (external bleeding, blood in mouth, feathers missing, broken neck). 

Drowning: Exhausted migrants in flight were sometimes observed splashing in the water and drowning 

as a result. 

Oiling: Birds discovered dead with >10% of plumage fouled by oil were assigned to this category, 

although the precise cause of death (ingestion, insulation failure, etc.) was usually impossible to 

determine. 
23 

Burning: Birds burned in gas flares had obviously singed plumage. 

Air Intake Fans: Birds attempting to pass through compressor fans and generator fans were often 

decapitated and either became lodged within the fans or else fell directly below the fans. 

Entrapment: Birds sometimes got their legs or toes caught in tight spaces between grating squares and 

subsequently died of exposure or starvation. Birds landing in a strong wind evidently could get blown 

over and have their head caught in grating slats, and subsequently die from cervical dislocation or 

exposure. 

Poisoning: Birds observed consuming toxic chemicals and later found dead with no other evident cause 

of death were assumed to have been poisoned. 

Workers: Platform workers sometimes inadvertently killed tired or naïve birds by sitting on them, 

kicking them, or stepping on them. 

Unknown: Occasional fat individuals were found dead with no signs of trauma, oiling, or other problems. 

Some of these individuals probably died from ingestion of toxic materials, but we were unable to assign a 

cause of death with confidence. 

3.5.6 Flight Behavior 

General flight behavior of birds passing by the platform without landing (“flybys”) was characterized as 

follows: 

F = Flew by without approaching or attempting to land on the platform. 

A = Approached the platform by deviating from a straight course, but did not land. 

CR = “Cruising”. This category was applicable for individuals that never landed but that 

associated with the platform in a more persistent way than could accommodated 

under the “Approached” category. CR was generally used only for aerial 

insectivores (swallows, swifts, nighthawks) and raptors that appeared to be foraging 

around the platforms. 

Flight altitude (above sea level) was estimated using the following scale: 

4 = >500 m (difficult to see with binoculars) 

3 = >100 m (easily overlooked with naked eye) 

2 = 10-100 m 

1 = 1-10 m 

0 = skimming the sea surface (<1 m) 

Vanishing bearings of flybys were recorded on a 16-point scale (N, NNE, NE, ENE, etc.). To minimize 

parallax error, we followed all flybys for as long as possible with binoculars, and recorded the vanishing 

bearings when the birds disappeared from view. 

In addition to the flybys, we recorded flight directions of all migrants observed arriving on or departing 

from the platforms. Arrival bearings were recorded only for birds arriving directly from over the water 

and initially detected far enough out that a bearing could be determined with a reasonable degree of 

confidence.
25 

 Chapter 4 

 

Radar Methods 

Robert W. Russell and Sidney A. Gauthreaux, Jr. 

4.1 INTRODUCTION 

 (by Robert W. Russell) 

An important adjunct to our field work on the platforms was the remote observation of migration over the 

Gulf using land-based radars. Radar (RAdio Detection And Ranging) operates by transmitting a radio 

signal into the atmosphere. If there are objects in the path of the radio beam, the beam is scattered and 

part of the energy is reflected back to the radar site, where a wide-dish antenna captures the signals and 

sends them through a receiver for processing. The amount of energy returned to the receiver is partly a 

function of the targets’ size and density. Theoretical and empirical models of radar cross section are 

available which permit one to estimate the density of migrants based on returned energy (Wilson et al. 

1994; Russell and Wilson 1997). Thus, radar can ideally provide both large-scale distributional 

information as well as quantitative measures of the local abundance of bird and insect migrants. Doppler 

radars have the additional capability of being able to determine the radial component of the velocity of 

targets by measuring frequency shifts in the reflected signal caused by the targets’ motion. 

The National Weather Service operates a national network of modern WSR-88D Doppler weather 

surveillance radars known as NEXRAD (Next Generation Radar), which operate at a wavelength of 10.5 

cm (S-band) with a beamwidth of 0.96E and a peak power output of 750 kW. Currently there are 10 

NEXRAD radars in operation at sites around the Gulf Coast, providing a nearly complete observational 

network stretching from Brownsville, Texas, to Key West, Florida. We opportunistically accessed highresolution, near-real-time radar imagery via the internet at <http://www.intellicast.com> and at 

<http://www.weathertap.com>. The near-real-time imagery was used to focus observational effort in the 

field and to determine large-scale geographic patterns of migration. In addition, the raw data products 

from two NEXRAD sites were downloaded to optical disk under an MMS subcontract to Dr. Sidney 

Gauthreaux at Clemson University (see Chapter 4.3). These raw data were used to analyze the timing of 

migration and to estimate the abundance of migrants involved in the trans-Gulf migrations. 

4.2 RADAR THEORY 

 (by Robert W. Russell) 

Radar operates by emitting a beam of radio waves into the atmosphere and measuring the amount of 

power reflected back to the radar unit by any “targets” in the beam’s path. The radar equation expresses 

average returned power (Pr) as a function of distance to the targets (r) and the total reflectivity of the 

targets: 

_ C · |K|2

 · Z 

Pr = -------------, (4.1) 

r

2

where C is a constant that depends on characteristics of the radar unit (wavelength [λ], peak transmitted 

power, antenna gain, beam width, and pulse length), K is a parameter dependent on the complex index of 

refraction, and Z is the “reflectivity factor.” The value of |K|2

 is 0.93 for water. Assuming that the radar 

targets are spherical particles with diameter Di such that Di /λ . 0.1 (i.e., under the “Rayleigh 
26 

approximation”), Z is defined as the summation over a unit volume of the particle diameters raised 

to the sixth power: 

Z = Σ Di

6

. (4.2) 

vol

If the particle size distribution is known, then by measuring Pr and r, one can use equations 4.1 and 4.2 to 

calculate the target density. 

When the Rayleigh approximation is not applicable or if it is not known whether it is applicable, Z is 

written as Ze (the effective reflectivity factor) and the size of a target must be characterized instead by its 

radar cross section. Radar cross section is a measure of a target’s backscattering efficiency, and therefore 

describes the size of a target as “seen” by radar. More formally, radar cross section is defined as “the area 

of a [hypothetical] target that reflects back isotropically and would have caused the same return power as 

the original [i.e., actual] target” (Levanon 1988). Ze can be expressed in terms of the volume reflectivity 

(η): 

η · λ4

Ze = ----------, (4.3) 

π5 · |K|2

where η is defined as the summation of the radar cross sections (σi ) of all the scatterers in a unit volume 

of space scanned by radar: 

η = Σ σi. (4.4) 

vol 

The reflectivity factor Ze has units of mm6 @m-3. By convention, reflectivity factors are usually reported in 

the logarithmic form (dBZe), where 

dBZe = 10 · log 10 Ze. (4.5) 

For the NEXRAD radars used in this study (λ = 10.5 cm), equation 4.3 can be rewritten as: 

η = 23.4 · Ze, (4.6) 

where η has units of cm2 @km-3. The volume density of bird migration can be calculated directly from 

equation 4.6 if the radar cross section of the bird targets is known, and the flux of migration 

can then be calculated from the measured air speed (S) of the migrants. Assuming an average cross 

section of σ0 (units = cm2

), migration flux can be estimated by: 

43.3 · Ze · S 

flux = -----------------, (4.7) 

σ0

where S is given in knots (the standard unit reported in NEXRAD data) and flux has units of birds 

km-2 @h-1. 

Unfortunately, the radar cross section of a real target generally does not coincide with its actual physical 

area. Cross sections of targets with regular geometric forms can be calculated from theoretical equations 

(Ruck et al. 1970). In practice, however, cross sections of biological organisms (which have very 
27 

complex morphologies) must be determined empirically, and relatively few such empirical 

determinations have been made. 

Russell and Wilson (1997) analyzed all available literature data on radar cross sections of arthropods and 

birds (almost all obtained with X-band radars) and found a biphasic relationship, with species having 

body length #10 mm (microinsects) showing a stronger pattern of mass-dependence of their cross 

sections than longer species (macroinsects and birds). The best-fitting model relating X-band radar cross 

section to body length (L, units = mm) and to wet body mass (M, units = g) was: 

σ = e10.12 M3.12 (L # 10) (4.8) 

σ = e0.13 M0.40 (L > 10) (4.9) 

To be useful for analysis, the X-band cross sections predicted by this empirical model must be scaled to 

S-band cross sections as would be observed by NEXRAD radars. Complicating matters, the relationship 

between radar cross section and actual target size varies with the radar wavelength (λ), with the form of 

this relationship depending on the ratio of λ to target size. In the Rayleigh scattering region (L/λ . 0.1), 

cross section is inversely proportional to the fourth power of radar wavelength: 

σ % λ- 4. (4.10) 

This relationship obtains because targets that are small compared to the radar wavelength scatter energy 

nearly isotropically. In contrast, larger targets scatter energy in a more directive pattern, often scattering 

considerably more energy in the forward direction rather than back toward the radar. This is called Mie 

scattering and results in a complicated radar return that shows large fluctuations and no simple 

wavelength dependence (Skolnik 1962; Konrad et al. 1968). 

In the Rayleigh region, scattering cross section varies approximately with the square of the target’s mass. 

Microinsects approximated this pattern of mass-dependence and can thus be characterized as Rayleigh 

scatterers. However, longer organisms show a much weaker pattern of mass-dependence and clearly fall 

into the Mie scattering region, so Rayleigh scaling cannot be applied to birds. Instead, wavelengthdependency of birds’ cross sections was estimated to be 

σ % λ1.21 (4.11) 

from a model fit to published multiwavelength cross section data on Boat-tailed Grackles (Quiscalus 

major), House Sparrows (Passer domesticus), and a Rock Dove (Columba livia). Combining equations 

4.9 and 4.11 yields the following equation for estimating S-band radar cross sections of birds: 

σs = 4.8 · M0.40 (4.12) 

When information is available on the species composition of a migratory flight, equations 4.7 and 4.12 

can be used to estimate migration traffic rates. 

4.3 QUANTIFICATION OF NEXRAD IMAGERY

 (by Sidney A. Gauthreaux, Jr., and Robert W. Russell) 

Intensive collection of weather and radar data for the project began on March 1, 1998. Radar data came 

from NEXRAD stations at Slidell (LIX) and Lake Charles (LCH), Louisiana (Figure 4.1). Once every 

hour a base reflectivity product and a base velocity product were downloaded to a computer in the 
28 

Clemson University Radar Ornithology Laboratory (CUROL). The base reflectivity product is an 

image showing the distribution and the reflectivity (measured in dBZe) of targets in the atmosphere 

within 230 km of the radar. The base velocity product is an image showing the radial velocity (the 

component of a target’s velocity traveling along the radar beam and measured in knots) of the same 

targets. The radar data files were provided by a NEXRAD Information Distribution Service (NIDS). The 

files were downloaded by modem from Unisys, Inc., during 1998 (from March 1 at 00:00 UTC through 

May 31 at 23:00 UTC and from August 15 at 00:00 UTC through November 15 at 23:00 UTC) and via 

satellite from Marta Systems, Inc., in 1999 (March 1 at 00:00 UTC through May 31 at 23:00 UTC and 

from August 15 at 00:00 through November 15 at 23:00 UTC). All files were archived on CDs and are 

stored at CUROL. 

Figure 4.1. Locations of the Lake Charles (LCH) and New Orleans (LIX) NEXRAD sites. The circles 

delimit the normal surveillance range of 230 km. 

Because data on winds aloft are critically important for the discrimination of different types of targets 

(e.g., insects, dust and smoke, birds), we downloaded skew-t text files for LCH and LIX via the internet 

twice each day. The data were gathered by radiosondes launched by the National Weather Service at 

23:00 and 11:00 UTC, and the data for each height above the station include the barometric pressure, 

wind direction and speed, temperature, and dew point temperature. 

To identify targets most likely to have been migrating birds in base reflectivity images, we used a 

Netscape plug-in that opens Level III data (NIDS files) and examined the patterns of echoes in the 

images. Unlike weather echoes, echoes from birds, bats, and insects frequently produce a disk-like or an 

annular pattern of echoes with the radar site at the center. Although the patterns may be saturated and 

extend out to a range of 185 km when heavy movements are underway, the patterns are usually stippled, 

indicating that not all pulse volumes in a radar scan contain targets. To confirm that echoes were from 
29 

migrating birds, we used the Netscape plug-in to open base velocity files and we filtered out pulse 

volumes with radial velocities within 15 knots of balloon-measured wind velocity for the same 

altitudes. Because most insect movements rarely exceed velocities of 15 knots above wind speed, when 

targets were moving in excess of 15 knots above following winds, or moving against a head wind, we 

concluded that the targets were likely birds. 

Precipitation, beam bending from superrefraction, strobing, and chaff confound the determination of 

echoes from birds by masking echoes or disrupting the echo pattern (Figure 4.2). Insects and other slow 

moving aerial biota in the atmosphere also present a serious problem, contributing to pulse volume 

reflectivity. Slow targets like insects have low base velocities or drift with the wind at or near ambient 

wind speed (Figure 4.2). In cases when these issues arose, we noted such conditions in the database. 

Once we classified an image as depicting birds, we used a pixel counting program (FullPixelSearch) to 

quantify pixels of different reflectivity (dBZe). We first drew a 50-by-50 pixel square over an area of 

maximum reflectivity within the echo pattern produced by birds and within a range of 55 to 110 km 

(Figure 4.3). We then counted the number of pixels in each reflectivity category. In cases when weak 

strobes, scattered precipitation, or insects (or a combination of the three) occurred, we placed the square 

within an area that maximized the reflectivity from birds and minimized the contamination. We followed 

a similar procedure for counting pixels in radial velocity images by placing the square in the area of 

greatest velocity. Because radial velocity measures the component of target speed along a radius from the 

radar site, the values within the box represents the true ground speed of a field of uniformly moving 

targets. We recorded in the database the coordinates of the top, left corner of every square because we 

placed the square in different positions on different reflectivity and velocity images. Because pixel values 

are provided in dBZe ranges rather than as precise values of Ze, we converted pixel counts into average 

reflectivity factors using back-transformed range midpoints and the following equations: 

Clear-air operating mode: 

Ze = (1/2,500) · {[4.41 · (4-8 dBZe count)] + [11.08 · (8-12 dBZe count)] 

+ [27.83 · (12-16 dBZe count)] + [69.91 · (16-20 dBZe count)] 

+ [175.59 · (20-24 dBZe count)] + [441.07 · (24-28 dBZe count)] 

+ [1,107.93 · (28-32 dBZe count)]} 

Precipitation operating mode: 

Ze = (1/2,500) · [6.58 · (5-10 dBZe count)] + [20.81 · (10-15 dBZe count)] 

+ [65.81 · (15-20 dBZe count)] + [208.11 · (20-25 dBZe count)] 

+ [658.11 · (25-30 dBZe count)] + [2,081.14 · (30-35 dBZe count)] 

Similarly, we converted pixel counts into average ground speed values using the following equation, 

arbitrarily selecting a value of 70 knots for the 64+ knot range: 

S = (1) 3count) · {[15 · (10-20 knot count)] + [23 · (20-26 knot count)] 

+ [31 · (26-36 knot count)] + [43 · (36-50 knot count)] 

+ [57 · (50-64 knot count)] + [70 · (64+ knot count)]} 

where 3count represents the total count of pulse volumes (out of 2,500 possible) that were determined to 

have been dominated by birds (i.e., radial velocities exceeded 15 knots above balloon-measured wind 

velocity). 
30 

Figure 4.2. Radar images showing precipitation, strobing, and insect contamination. The 

reflectivity image from LIX on September 9, 1999 at 02:34 UTC (top) is 

contaminated by strobes (visible around the perimeter as linear spikes) and 

precipitation (visible most clearly in the far west as blocky green and yellow 

patterns). The corresponding base velocity image (bottom) suggests return 

from insects (uniform grayish region representing low velocity). 

Contamination of these types obscures echoes from birds. 
31 

 

Figure 4.3. Pixel counting technique in radar image analysis. When an image contains bird echoes, the 

number of pixels for each dBZe value within a 50-by-50 pixel square (delimited by a black 

and white line to the SW of the station) is counted using the FullPixelSearch program. The 

square is always drawn on the image within a 55-110 km range. The program displays the 

number of pixels when the cursor is placed over one of the bars in the bar graph. 
33 

Chapter 5 

 

Synoptic Climatology of the Northern Gulf of Mexico 

Robert W. Russell 

5.1 INTRODUCTION

To understand the influence of weather on trans-Gulf migration and platform use, we sought to relate dayto-day variability in observed migration to large-scale weather patterns over the Gulf via a synoptic 

climatology. In synoptic climatological analyses, one attempts to identify a relatively small number of 

representative weather patterns that are capable of characterizing the observed daily variability in some 

phenomenon of interest, and then relates these types to variation in some environmental parameter or 

process of interest (Barry and Perry 1973; Yarnal 1993). Construction of a synoptic climatology of 

migration involves the categorization of atmospheric circulation types relevant to migrating birds and the 

determination of the climatological frequency of occurrence of different types of migration events. 

The method for classifying daily weather patterns into discrete types may be either manual or automated. 

In manual weather-typing schemes, the investigator subjectively classifies the continuum of atmospheric 

circulation patterns into a reduced set of characteristic weather types, usually on the basis of isobaric 

patterns. Automated procedures usually use clustering algorithms with measured surface variables as 

input (the “air-mass approach”). Manual typing schemes are necessarily subjective but have the 

important advantage of being flexible to the needs of a particular investigation. Automated typing 

schemes have the advantages of being objective and of classifying the weather itself, but are relatively 

insensitive to atmospheric circulation patterns that may affect processes operating over large scales. 

A synoptic climatology for the northern Gulf Coast has previously been developed by Muller (1977; see 

also Muller and Wax 1977, Muller 1979) and modified by Yocke et al. (2000). We revised these earlier 

schemes slightly to be relevant to migrating birds. 

5.2 METHODS

We recognized the following eight synoptic-scale weather types, which are hereafter indicated by their 

abbreviations in boldface for easy recognition: 

Gulf Front (GF) — This type subsumed Yocke et al.’s “Gulf Front or Trough N/S” and “Gulf Front or 

Trough E/W” types, which correspond respectively to Muller’s “Pacific High” and “Frontal Overrunning” 

types. On days characterized by this type, cyclonic circulation around a deep surface low over the 

Mississippi Valley brings mild and dry air following the cold front across the northern Gulf. An eastwest or northeast-southwest oriented front or trough is located in the northern Gulf region within about 

100 km of the coastline. Winds in the northern Gulf are variable, but generally have a northerly 

component on the northern or western side of the front and a southerly component on the southern or 

eastern side. Frequently waves develop along the front over the western Gulf, and then sweep 

northeastward bringing heavy clouds and precipitation to the Gulf Coast. This pattern is often 

accompanied by other patterns. In this study, if winds on both sides of the front were from the same 

direction (indicating a weak front) and the wind directions were consistent with the other accompanying 

synoptic pattern, then the GF pattern was ignored and the other accompanying pattern was chosen. 
34 

East Coast Low (ELOW) — This new type described by Yocke et al. is similar to GF except that 

the low-pressure system has moved east of the Mississippi River and the front has correspondingly 

swept over the Gulf, through Florida, and into the Atlantic. From the standpoint of our study region in 

the northern Gulf, prevailing weather is often very similar between GF and ELOW days. However, on 

GF days, the front has not penetrated to the Yucatan Peninsula and so in spring GF is likely to be 

conducive to the initiation of northward trans-Gulf migration. In contrast, winds over the Yucatan on 

ELOW days will generally be unfavorable for the initiation of spring trans-Gulf migration. 

Midwest Continental High (MCH) — This type corresponds to Muller’s “Continental High” type. On 

MCH days, winds over the northern Gulf are dominated by anticyclonic flow around a high-pressure 

system centered west of the Mississippi River, over or east of the Rocky Mountains, and north of the 

Texas/Mexico border. Surface winds are from the northeast, and the region is dominated by fair weather 

associated with the core of the anticyclone. 

Eastern Continental High (ECH) — This type devised by Yocke et al. subsumes Muller’s “Coastal 

Return” type as well as some situations that would be classified under Muller’s “Continental High” type. 

On ECH days, winds over the northern Gulf are dominated by anticyclonic flow around a high-pressure 

system located east of the Mississippi River and west of the eastern seaboard, somewhere between the 

Gulf Coast and southern Canada. Surface winds may be from the east or southeast (eastern areas) or from 

the south (western areas). 

Bermuda High (BH) — This type is a subdivision of Muller’s “Coastal Return” type (with the remainder 

of the Coastal Return type falling under ECH). It is very similar to the ECH type, but the high-pressure 

system is centered over the Atlantic Ocean. A ridge of tropical air extends westward from the Atlantic 

over the southeastern states, and surface winds in the northern Gulf may be from the southeast or south. 

Gulf High (GH) -- This type corresponds to Mueller’s type of the same name. On GH days. high 

pressure is centered over the Gulf or over the immediate Gulf Coast and usually associated with a weak 

pressure gradient and weak or nonexistent winds. We subsumed the Yocke et. al. (2000) “No Gradient” 

type into the GH type. 

Tropical Low (TLOW) — This type corresponds to Mueller’s “Gulf Tropical Disturbances.” On TLOW

days, a hurricane, tropical storm, or tropical depression exists in the Gulf region and dominates its 

weather. Winds reflect the strong cyclonic flow around the low pressure center and are locally highly 

variable, depending upon the precise position of the system. 

Not Determined (ND) — This default type comprised complex weather situations that did not fall clearly 

into one of the seven other types. 

We did not use the “Midwest Low” type recognized by Yocke et al., which corresponds to Muller’s “Gulf 

Return” type. In a Midwest Low scenario, a low-pressure system is located east of the Rockies with a 

north-south or northeast-southwest oriented front approaching the Gulf. Surface winds are dominated by 

cyclonic flow around the Low, with a strong return flow of maritime tropical air developing over the 

northern Gulf in advance of the front. Wind direction is generally from the SW (western Gulf) or S 

(farther east). We did not formally recognize this type because it was very infrequent during this study, 

and easily subsumed into the ECH or BH types. 

On GF days during the fall, we also assigned a weather type behind the frontal boundary (i.e., as if the 

front were absent). The reason for this was that migrating birds departing the northern Gulf Coast on GF

days generally travel in the same direction as the frontal boundary (which is not the case during the 
35 

spring), and primarily experience post-frontal conditions on GF days. This secondary type (usually 

MCH or ELOW) was used in all analyses of fall migration, unless otherwise noted. 

Each day during “spring” (March-May) and “fall” (August-November) in 1998, 1999, and 2000 was 

manually classified into one of the eight synoptic weather types based on surface pressure contours and 

wind flow patterns at 12:00 UTC (06:00 CST). Sources of information used to assign daily weather types 

included: 1) daily synoptic maps archived by Unisys at <http://weather.unisys.com/archive/sfc_map/>; 2) 

daily surface synoptic charts published by NOAA (Daily Weather Maps: Weekly Series); and when the 

previous two data sources were unavailable, inconsistent, or inconclusive, 3) local weather data recorded 

on the platforms. Pressure patterns and the configuration of major high- and low-pressure systems were 

used as the principal decision criteria. In cases where more than one pattern was present, we chose the 

feature with the greater influence on winds in the northern Gulf. If it was difficult to determine which 

feature had a greater influence on winds in the northern Gulf, we favored the upstream feature (generally 

the feature to the west) rather than the downstream feature (generally to the east). All classifications were 

made without reference to any bird migration data. 

5.3 RESULTS

The weather types assigned to each day during each migration season throughout the study are presented 

in Table 5.1 (“spring”: March-May) and Table 5.2 (“fall”: August-November). 

Inspection of Tables 5.1 and 5.2 reveals that weather over the Gulf usually follows a predictable cycle 

(Figure 5.1). As the center of an anticyclone drifts eastward across the continent and out over the Atlantic 

Ocean, winds over the northern Gulf veer from NE (MCH) to E (ECH) and eventually to SE (BH). 

Eventually a cyclone moves eastward and a front passes over the Gulf (GF) bringing winds from the NW. 

Following frontal passage, winds over the Gulf are dominated by the departing cyclone (ELOW) until a 

new anticyclone approaches and the cycle repeats. Occasionally the cycle is interrupted when the 

anticyclone drifts out over the Gulf (GH), usually resulting in light winds over the northern Gulf. During 

the summer and fall, tropical weather systems may develop in the Gulf or enter the Gulf from the east 

(TLOW), usually bringing strong winds and foul weather. 

The relative frequencies of the different weather types in each spring are depicted in Figure 5.2. 

Conditions were rarely unfavorable for northward trans-Gulf departures in May. However, favorability of 

departure conditions earlier in the spring varied dramatically from year to year, with unfavorable 

conditions (ELOW and MCH) more than twice as frequent in 1998 than in 1999 and 2000. Conditions 

conducive to departure but likely to induce fallouts in the northern Gulf of Mexico (GF) were common in 

March in 1999 and 2000; were common in April in 1998 and 2000; and were rare in May in all years. 

May tended to be dominated by a single weather pattern, which was different in each of the three years: 

GH in 1998, ECH in 1999, and BH in 2000. 

The relative frequencies of the different weather types in each fall are depicted in Figure 5.3. The most 

striking features in fall were the strong predominance of the GH type in August and the tremendous 

variation in frequency of TLOW from year to year. TLOW was the most common weather pattern 

during the first half of the fall migration season in 1998, but was uncommon in 1999 and was rare in 

2000. ECH was the most common weather type during October and, in two years of three, in September. 

November was characterized by a more even distribution of weather types, with no pattern persisting 

across years. 

Synoptic weather conditions are compared between spring and fall in Figure 5.4. Overall, spring was 

characterized by a greater predominance of weather types most favorable for trans-Gulf flight in the 

seasonally appropriate direction (BH, ECH) than was fall (MCH, ELOW). 
36 

Table 5.1. 

 

Synoptic weather types during spring migration, 

1998-2000 

 Synoptic Type 

Date 1998 1999 2000 

1-Mar ELOW GH BH 

2-Mar ELOW GH GF 

3-Mar MCH GF GH 

4-Mar ECH ECH GF 

5-Mar BH ECH GF 

6-Mar ECH BH ECH 

7-Mar BH GF ECH 

8-Mar GF ECH ECH 

9-Mar ELOW GF BH 

10-Mar MCH GF BH 

11-Mar MCH GF GF 

12-Mar MCH GF GF 

13-Mar ECH GF ECH 

14-Mar ECH ELOW ECH 

15-Mar ECH ELOW ECH 

16-Mar ECH ECH BH 

17-Mar ECH BH GF 

18-Mar ECH BH GF 

19-Mar GF GH GF 

20-Mar ELOW GF GF 

21-Mar ELOW GF ECH 

22-Mar MCH MCH ECH 

23-Mar GH GH ECH 

24-Mar GH BH ECH 

25-Mar ECH GF ECH 

26-Mar BH GF GH 

27-Mar BH ECH GF 

28-Mar BH ECH GF 

29-Mar BH ECH BH 

30-Mar BH GF GF 

31-Mar GF ND GF 

1-Apr GF BH ECH 

2-Apr BH BH BH 

3-Apr GF BH GF 

4-Apr GF ECH GF 

5-Apr MCH BH MCH 

6-Apr ECH GH BH 

7-Apr BH ECH BH 

8-Apr BH BH GF 

9-Apr GF BH MCH 

10-Apr ELOW BH GH 

11-Apr MCH GH BH 

12-Apr ECH GH GF 
37 

Table 5.1. 

 

Synoptic weather types during spring migration, 

1998-2000 

 Synoptic Type 

Date 1998 1999 2000 

13-Apr BH ECH GF 

14-Apr BH ECH GF 

15-Apr BH GF GH 

16-Apr BH ELOW ECH 

17-Apr GF ELOW GH 

18-Apr GF MCH GH 

19-Apr GF GH ECH 

20-Apr MCH GH BH 

21-Apr MCH GH GF 

22-Apr ELOW BH GF 

23-Apr ELOW BH ECH 

24-Apr GH GH GF 

25-Apr BH GH GF 

26-Apr BH BH MCH 

27-Apr BH BH GH 

28-Apr GF GF GH 

29-Apr GF ECH GF 

30-Apr GF MCH ECH 

1-May GH MCH ECH 

2-May GH ECH BH 

3-May BH ECH ND 

4-May GF ECH ECH 

5-May ECH BH ECH 

6-May BH GF BH 

7-May BH GF BH 

8-May BH GF BH 

9-May BH ECH BH 

10-May GF ECH BH 

11-May GF ECH BH 

12-May ECH ECH BH 

13-May ECH GF GF 

14-May ECH BH MCH 

15-May GH BH MCH 

16-May GH ECH GH 

17-May GH ECH BH 

18-May GH ECH BH 

19-May GH MCH BH 

20-May GH ECH BH 

21-May GH ECH BH 

22-May GH ECH GH 

23-May GH GH GH 

24-May GH GH BH 

25-May ECH GH BH 
38 

Table 5.1. 

 

Synoptic weather types during spring migration, 

1998-2000 

 Synoptic Type 

Date 1998 1999 2000 

26-May ECH GH BH 

27-May ND GH BH 

28-May GH GH GH 

29-May GH ECH GH 

30-May GH ECH GH 

31-May GH ECH ECH 

Table 5.2. 

 

Synoptic weather types during fall migration, 1998-2000 

 Synoptic Type 

Date 1998 1999 2000 

1-Aug GH GH GH 

2-Aug GH GH GH 

3-Aug ND GH GH 

4-Aug ND TLOW GH 

5-Aug ND TLOW GH 

6-Aug GH TLOW GH 

7-Aug TLOW ND GH 

8-Aug TLOW GH GH 

9-Aug GH GH GH 

10-Aug GH GH GH 

11-Aug GH GH GH 

12-Aug GH GH GH 

13-Aug GH GH GF/MCH 

14-Aug GH GH TLOW 

15-Aug GH TLOW GH 

16-Aug GH TLOW GH 

17-Aug GH GH GH 

18-Aug GH GH GH 

19-Aug ECH GH GH 

20-Aug TLOW GH GH 

21-Aug TLOW GH GH 

22-Aug TLOW TLOW ECH 

23-Aug ECH TLOW ECH 

24-Aug ECH GH ECH 

25-Aug ECH GH GH 

26-Aug GH GH GH 

27-Aug GH GH GH 

28-Aug GH GH GH 

29-Aug TLOW GH GH 

30-Aug TLOW GH GH 
39 

Table 5.2. 

 

Synoptic weather types during fall migration, 1998-2000 

 Synoptic Type 

Date 1998 1999 2000 

31-Aug TLOW ECH GH 

1-Sep TLOW ECH GH 

2-Sep TLOW ECH ND 

3-Sep TLOW GH ND 

4-Sep GH GH GH 

5-Sep ECH GH ND 

6-Sep ECH GH GF/ECH 

7-Sep ECH GH GF/ECH 

8-Sep TLOW GH ECH 

9-Sep TLOW GF/ELOW ECH 

10-Sep TLOW GF/MCH ECH 

11-Sep TLOW ECH ECH 

12-Sep TLOW ECH ECH 

13-Sep TLOW ECH ECH 

14-Sep ECH MCH ECH 

15-Sep ECH MCH ND 

16-Sep ECH MCH MCH 

17-Sep TLOW MCH TLOW 

18-Sep TLOW ECH TLOW 

19-Sep TLOW TLOW ECH 

20-Sep TLOW TLOW BH 

21-Sep ND TLOW TLOW 

22-Sep GH MCH TLOW 

23-Sep MCH MCH BH 

24-Sep ECH GH GH 

25-Sep TLOW ECH GF/ELOW 

26-Sep TLOW ECH GF/ELOW 

27-Sep TLOW ECH MCH 

28-Sep TLOW ECH MCH 

29-Sep TLOW ECH ECH 

30-Sep ELOW MCH ECH 

1-Oct GH MCH ECH 

2-Oct GH ECH ECH 

3-Oct ECH ECH ECH 

4-Oct BH ECH ECH 

5-Oct BH MCH ND 

6-Oct ECH TLOW GH 

7-Oct GF/ELOW TLOW GF/MCH 

8-Oct GF/MCH ECH MCH 

9-Oct MCH BH MCH 

10-Oct MCH ELOW MCH 

11-Oct ECH MCH MCH 

12-Oct ECH ECH ECH 

13-Oct MCH ECH ECH 
40 

Table 5.2. 

 

Synoptic weather types during fall migration, 1998-2000 

 Synoptic Type 

Date 1998 1999 2000 

14-Oct MCH TLOW ECH 

15-Oct ECH TLOW ECH 

16-Oct ECH TLOW ECH 

17-Oct ECH GH GH 

18-Oct BH MCH GF/MCH 

19-Oct GF/MCH MCH MCH 

20-Oct MCH MCH ECH 

21-Oct MCH MCH ECH 

22-Oct MCH MCH ECH 

23-Oct MCH ELOW ECH 

24-Oct ECH MCH ECH 

25-Oct ECH ECH ECH 

26-Oct ECH ECH ECH 

27-Oct ECH GH ECH 

28-Oct ECH ECH GH 

29-Oct ECH ECH GH 

30-Oct GH ECH GH 

31-Oct GH GF/ELOW GH 

1-Nov GH GF/ELOW GH 

2-Nov GF/ELOW ELOW ECH 

3-Nov GF/ELOW ELOW ECH 

4-Nov MCH ECH GH 

5-Nov MCH ECH GH 

6-Nov MCH ECH ND 

7-Nov ECH MCH GF/MCH 

8-Nov ECH MCH GF/ELOW 

9-Nov BH ECH GF/ELOW 

10-Nov BH ECH ELOW 

11-Nov GF/MCH ND MCH 

12-Nov ECH MCH GH 

13-Nov ECH ECH GF/ELOW 

14-Nov ND GH GF/ELOW 

15-Nov GF/ELOW ELOW MCH 

16-Nov ELOW ELOW ECH 

17-Nov ELOW MCH GF/MCH 

18-Nov ECH ECH GF/MCH 

19-Nov ECH ECH TLOW 

20-Nov ECH ND GF/ELOW 

21-Nov GF/MCH ND MCH 

22-Nov ECH ECH GH 

23-Nov BH ECH ECH 

24-Nov ND GF/MCH GF/ELOW 

25-Nov GH GF/MCH GF/ELOW 

26-Nov GH ELOW GH 
41 

Table 5.2. 

 

Synoptic weather types during fall migration, 1998-2000 

 Synoptic Type 

Date 1998 1999 2000 

27-Nov GH GH GH 

28-Nov ECH GH GH 

29-Nov ECH MCH GH 

30-Nov BH MCH MCH 

GF Gulf Front

TLOW Tropical Low

MCH 

Midwest 

Continental 

High

ECH

Eastern 

Continental 

High

Bermuda 

High

BH

Gulf 

High

GH

Eastern

Low

ELOW

GF Gulf Front

TLOW Tropical Low

MCH 

Midwest 

Continental 

High

ECH

Eastern 

Continental 

High

Bermuda 

High

BH

Gulf 

High

GH

Eastern

Low

ELOW

Figure 5.1. Seven synoptic types used for classifying large-scale weather patterns in this 

study. The arrows indicate typical synoptic weather transitions in the study 

area. The diagrammatic weather maps are modified from Yocke et al. (2000) 

with permission. 
42 

0

5

10

15

20

ELOW/MCH

GF

GH

ECH

BH

0

5

10

15

20

0

5

10

15

20

March April May

Number of Days

1998

1999

2000

Figure 5.2. Relative frequencies of the different weather types in spring 1998-2000. Weather types that 

are consistently favorable for northbound migrants are colored green, and those that are 

consistently unfavorable are red. 
43 

0

5

10

15

20

25

30

Aug Sept Oct Nov

0

5

10

15

20

250

5

10

15

20

TLOW

BH

GH

GF

ECH

MCH/ELOW

Number of Days

1998

1999

2000

Figure 5.3. Relative frequencies of the different weather types in fall 1998-2000. Weather types that are 

consistently favorable for southbound migrants are colored green, and those that are 

consistently unfavorable are red.
44 

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

BH

GF

ECH

MCH/ELOW

GH

ND

TLOW

Synoptic Type

Proportion of Days

Spring

Fall

Figure 5.4. Relative frequency of the different weather types in spring versus fall. Days 

from all years (1998-2000) are combined for each season. 
PART 2 

 

ECOLOGY OF 

TRANS-GULF BIRD MIGRATION 
47 

Chapter 6 

 

Species Composition of Offshore Migrants 

Robert W. Russell 

6.1 ECOLOGICAL CATEGORIZATION OF SPECIES RECORDED FROM PLATFORMS

Many unexpected species were recorded during the study, and it was clear that multiple factors were 

responsible for the presence of different species on Gulf platforms. In order to attempt to isolate the 

factors responsible for their presence, all species recorded during the study were first classified into 

groups based on biogeography, migratory behavior, and other ecological characteristics. 

All ducks and geese (family Anatidae) were included together in one group hereafter referred to as 

“waterfowl” (Table 6.1). Most waterfowl species are nocturnal migrants that travel in flocks, and most 

were not expected to be trans-Gulf migrants on the basis of their known winter distributions. Gregarious 

wading birds including herons, egrets, ibises, and spoonbills (families Ardeidae, Threskiornithidae) were 

considered together in one group hereafter referred to as “herons” (Table 6.2). Most heron species are 

nocturnal migrants that travel in flocks, and most were expected to be trans-Gulf migrants on the basis of 

their known winter distributions. Bitterns were excluded from the “herons” group because of their 

solitary behavior. Falconiform raptors (families Accipitridae, Falconidae) are all diurnal migrants, but 

different species exhibit widely varying water-crossing behaviors. Diurnal raptors are generally treated 

here on a species-specific basis (Table 6.3). Shorebirds (families Charadriidae, Recurvirostridae, 

Scolopacidae) migrate by both day and night, generally traveling in flocks, and all species were included 

together in one group (Table 6.4). 

Neotropical migrant “landbird” species with wintering ranges extending routinely into Central America 

beyond the Isthmus of Tehuantepec were considered to be potential trans-Gulf migrants on the basis of 

distribution, and were included together in one group hereafter referred to as “neotropical migrant 

landbirds” (Table 6.5). This group included landbirds as well as solitary marsh birds (Pied-billed Grebe, 

bitterns, gallinules, rails); members of this group are nocturnal migrants that travel alone. For some 

analyses, this group was further subdivided into small neotropical migrant landbirds (body mass <21 g) 

and large neotropical migrant landbirds (body mass >21 g), in part to accommodate the fact that flight 

speed increases with body mass (Pennycuick 1989). 

The group referred to as “medium-distance migrants” included species that routinely winter along the 

northern Gulf Coast and at least as far south as southern Tamaulipas, but not beyond southern Veracruz, 

and not on the Yucatan Peninsula, except as a vagrant (Table 6.6). Medium-distance migrant species 

were so identified because their winter distributions suggest that some portion of the population might 

travel over the Gulf, but only over the western Gulf. The group referred to as “short-distance migrants” 

included species that have no known regular wintering populations south of northern Tamaulipas and are 

unrecorded elsewhere from Atlantic Slope of Mexico (Table 6.7). Short-distance migrants were expected 

(a priori) not to be “intentional” trans-Gulf migrants. 
48 

Table 6.1. 

 

Waterfowl recorded from the study platforms 

Species are listed in taxonomic order according to AOU (1998). Body mass data 

are from Sibley (2000). 

Species Body Mass (g) 

Fulvous Whistling-Duck (Dendrocygna bicolor) 670 

Black-bellied Whistling-Duck (Dendrocygna autumnalis) 830 

Greater White-fronted Goose (Anser albifrons) 2,200 

Snow Goose (Chen caerulescens) 2,420 

Ross’s Goose (Chen rossii) 1,250 

Gadwall (Anas strepera) 910 

American Wigeon (Anas americana) 720 

Mallard (Anas platyrhynchos) 1,100 

Blue-winged Teal (Anas discors) 380 

Cinnamon Teal (Anas cyanoptera) 400 

Northern Shoveler (Anas clypeata) 610 

Northern Pintail (Anas acuta) 800 

Green-winged Teal (Anas crecca) 350 

Redhead (Aythya americana) 1,050 

Ring-necked Duck (Aythya collaris) 700 

Greater Scaup (Aythya marila) 1,050 

Lesser Scaup (Aythya affinis) 830 

Surf Scoter (Melanitta perspicillata) 950 

White-winged Scoter (Melanitta fusca) 1,670 

Hooded Merganser (Lophodytes cucullatus) 620 

Red-breasted Merganser (Mergus serrator) 1,060 

Table 6.2. 

 

Herons recorded from the study platforms 

Listing order and source of body mass data as in Table 6.1. 

Species Body Mass (g) 

American Bittern (Botaurus lentiginosus) 700 

Great Blue Heron (Ardea herodias) 2,400 

Great Egret (Ardea alba) 870 

Snowy Egret (Egretta thula) 360 

Little Blue Heron (Egretta caerulea) 340 

Tricolored Heron (Egretta tricolor) 380 

Reddish Egret (Egretta rufescens) 450 

Cattle Egret (Bubulcus ibis) 340 

Green Heron (Butorides virescens) 210 

Black-crowned Night-Heron (Nycticorax nycticorax) 870 

Yellow-crowned Night-Heron (Nyctanassa violacea) 690 

White Ibis (Eudocimus albus) 900 

White-faced Ibis (Plegadis chihi) 610 

Roseate Spoonbill (Ajaia ajaja) 1,500 
49 

Table 6.3. 

 

Diurnal raptors recorded from the study platforms 

Listing order and source of body mass data as in Table 6.1. 

Species Body Mass (g) 

Osprey (Pandion haliaetus) 1,600 

Swallow-tailed Kite (Elanoides forficatus) 420 

Mississippi Kite (Ictinia mississippiensis) 280 

Northern Harrier (Circus cyaneus) 420 

Sharp-shinned Hawk (Accipiter striatus) 140 

Cooper’s Hawk (Accipiter cooperii) 450 

Broad-winged Hawk (Buteo platypterus) 390 

American Kestrel (Falco sparverius) 117 

Merlin (Falco columbarius) 190 

Aplomado Falcon (Falco femoralis) 335 

Peregrine Falcon (Falco peregrinus) 720 

Table 6.4. 

 

Shorebirds recorded from the study platforms 

Listing order and source of body mass data as in Table 6.1. 

Species Body Mass (g) 

Black-bellied Plover (Pluvialis squatarola) 240 

American Golden-Plover (Pluvialis dominica) 145 

Wilson’s Plover (Charadrius wilsonia) 60 

Semipalmated Plover (Charadrius semipalmatus) 45 

Killdeer (Charadrius vociferus) 95 

Black-necked Stilt (Himantopus mexicanus) 160 

American Avocet (Recurvirostra americana) 315 

Greater Yellowlegs (Tringa melanoleuca) 160 

Lesser Yellowlegs (Tringa flavipes) 80 

Solitary Sandpiper (Tringa solitaria) 50 

Willet (Catoptrophorus semipalmatus) 215 

Spotted Sandpiper (Actitis macularia) 40 

Upland Sandpiper (Bartramia longicauda) 170 

Whimbrel (Numenius phaeopus) 390 

Long-billed Curlew (Numenius americanus) 590 

Hudsonian Godwit (Limosa haemastica) 300 

Marbled Godwit (Limosa fedoa) 370 

Ruddy Turnstone (Arenaria interpres) 110 

Red Knot (Calidris canutus) 135 

Sanderling (Calidris alba) 60 

Semipalmated Sandpiper (Calidris pusilla) 25 

Western Sandpiper (Calidris mauri) 26 

Least Sandpiper (Calidris minutilla) 20 

White-rumped Sandpiper (Calidris fuscicollis) 42 
50 

Table 6.4. 

 

Shorebirds recorded from the study platforms 

Listing order and source of body mass data as in Table 6.1. 

Species Body Mass (g) 

Baird’s Sandpiper (Calidris bairdii) 38 

Pectoral Sandpiper (Calidris melanotos) 73 

Dunlin (Calidris alpine) 60 

Stilt Sandpiper (Calidris himantopus) 58 

Buff-breasted Sandpiper (Tryngites subruficollis) 63 

Long-billed Dowitcher (Limnodromus scolopaceus) 115 

Common Snipe (Gallinago gallinago) 105 

Wilson’s Phalarope (Phalaropus tricolor) 60 

Red-necked Phalarope (Phalaropus lobatus) 35 
51 

Table 6.5. 

 

Neotropical migrant landbirds recorded from the study platforms 

This group includes those species and recognizable subspecies of landbirds and marsh birds that have wintering 

ranges routinely extending into Central America (beyond the Isthmus of Tehuantepec). “Range” provides relevant 

details on each species’ known winter distribution (Win), summer breeding distribution (Sum), and/or migration 

route (Mig). Except where otherwise noted, all species are considered to be widespread during migration along the 

Atlantic Slope from Tamaulipas to n. Honduras. Listing order and source of body mass data as in Table 6.1. Range 

data are from Howell and Webb (1995), Rising (1996), Dunn and Garrett (1997), and Sibley (2000). 

Species Body Mass (g) Range 

Pied-billed Grebe (Podilymbus podiceps) 450 Win: s. USA-? 

Least Bittern (Ixobrychus exilis) 80 Win: s. Texas-Columbia; also s. Florida. 

Black Rail (Laterallus jamaicensis) 30 Win: unclear due to extremely secretive 

behavior and existence of several disjunct 

populations; probably s. USA, w. West 

Indies, and possibly e. Mexico and n. 

Central America. 

Sora (Porzana carolina) 75 Win: s. USA-n. South America. 

Purple Gallinule (Porphyrula martinica) 235 Win: Veracruz-South America. 

Common Moorhen (Gallinula chloropus) 315 Win: s. USA-? 

American Coot (Fulica americana) 650 Win: s. USA-n. Columbia. 

Mourning Dove (Zenaida macroura) 120 Win: n. USA-s. Central America. 

Black-billed Cuckoo (Coccyzus erythropthalmus) 52 Win: South America. 

Yellow-billed Cuckoo (C. americanus) 65 Win: South America. 

Common Nighthawk (Chordeiles minor) 62 Win: South America. 

Chuck-will’s-widow (Caprimulgus carolinensis) 120 Win: n. Veracruz-n. Central America, 

except not on Yucatan Peninsula 

Whip-poor-will (C. vociferus) 54 Win: s. USA-w. Panama, except not on 

Yucatan Peninsula. Mig: widespread, 

except unrecorded on Yucatan Peninsula 

Belted Kingfisher (Ceryle alcyon) 150 Win: n. USA-n. South America. 

Yellow-bellied Sapsucker (Sphyrapicus varius) 50 Win: s. USA-Panama. 

Eastern Wood-Pewee (Contopus virens) 14 Win: South America. 

Yellow-bellied Flycatcher (Empidonax flaviventris) 11.5 Win: Veracruz-Panama. 

Acadian Flycatcher (E. virescens) 13 Win: e. Nicaragua-Ecuador. Fall Mig:

from Yucatan Peninsula eastward. 

Alder Flycatcher (E. alnorum) 13.5 Win: South America. Mig: widespread, 

except unrecorded on Yucatan Peninsula 

Willow Flycatcher (E. traillii) 13.5 Win: Honduras-Panama. Mig: widespread, 

except unrecorded on Yucatan Peninsula 

Least Flycatcher (E. minimus) 10.3 Win: s. Tamaulipas-Costa Rica. 

Great Crested Flycatcher (Myiarchus crinitus) 34 Win: s. Veracruz-n. South America. 

Eastern Kingbird (Tyrannus tyrannus) 40 Win: South America. 
52 

Table 6.5. 

 

Neotropical migrant landbirds recorded from the study platforms 

This group includes those species and recognizable subspecies of landbirds and marsh birds that have wintering 

ranges routinely extending into Central America (beyond the Isthmus of Tehuantepec). “Range” provides relevant 

details on each species’ known winter distribution (Win), summer breeding distribution (Sum), and/or migration 

route (Mig). Except where otherwise noted, all species are considered to be widespread during migration along the 

Atlantic Slope from Tamaulipas to n. Honduras. Listing order and source of body mass data as in Table 6.1. Range 

data are from Howell and Webb (1995), Rising (1996), Dunn and Garrett (1997), and Sibley (2000). 

Species Body Mass (g) Range 

Scissor-tailed Flycatcher (T. forficatus) 43 Sum: east to sw. Missouri, w. Arkansas, w. 

Louisiana, coastal Texas, n. Tamaulipas. 

Win: s. Veracruz-w. Campeche; a few in 

s. Florida; vagrant on Yucatan Peninsula 

and in Belize. 

White-eyed Vireo (Vireo griseus) 11.5 Win: se. USA-n. Nicaragua. 

Yellow-throated Vireo (V. flavifrons) 18 Win: s. Veracruz-n. South America. 

Warbling Vireo (V. gilvus) 12 Win: w. Mexico-s. Honduras; absent from 

Yucatan Peninsula. Mig: Tamaulipas-s. 

Veracruz; unrecorded on Yucatan 

Peninsula 

Philadelphia Vireo (V. philadelphicus) 12 Win: s. Veracruz-n. Columbia. 

Red-eyed Vireo (V. olivaceus) 17 Win: South America. 

Blue-gray Gnatcatcher (Polioptila caerulea) 6 Win: s. USA-Honduras. 

Veery (Catharus fuscescens) 31 Win: Columbia-Brazil. Mig: from s. 

Veracruz eastward. 

Gray-cheeked Thrush (C. minimus) 32 Win: Columbia-Brazil. Mig: from s. 

Veracruz eastward. 

Swainson’s Thrush (C. ustulatus) 31 Win: s. Veracruz-n. Argentina, except not 

on Yucatan Peninsula Mig: widespread, 

but less common on Yucatan Peninsula 

Wood Thrush (Hylocichla mustelina) 47 Win: s. Veracruz-n. Columbia. 

Gray Catbird (Dumetella carolinensis) 37 Win: se. USA-Panama. 

Blue-winged Warbler (Vermivora pinus) 8.5 Win: S Veracruz-Costa Rica. 

Golden-winged Warbler (V. chrysoptera) 8.8 Win: s. Mexico (Chiapas)-n. South 

America; absent from Yucatan Peninsula 

Tennessee Warbler (V. peregrina) 10 Win: s. Veracruz-n. South America, except 

not on Yucatan Peninsula; most abundant 

Costa Rica-nw. Columbia. 

Nashville Warbler (V. ruficapilla) 8.7 Win: s. Texas-Guatemala; absent from 

Yucatan Peninsula. Mig: Tamaulipas-s. 

Veracruz; vagrant on Yucatan Peninsula; 

rare in se. USA. 

Northern Parula (Parula americana) 8.6 Win: primarily West Indies, where 

common; also fairly common s. 

Veracruz-n. Honduras. 
53 

Table 6.5. 

 

Neotropical migrant landbirds recorded from the study platforms 

This group includes those species and recognizable subspecies of landbirds and marsh birds that have wintering 

ranges routinely extending into Central America (beyond the Isthmus of Tehuantepec). “Range” provides relevant 

details on each species’ known winter distribution (Win), summer breeding distribution (Sum), and/or migration 

route (Mig). Except where otherwise noted, all species are considered to be widespread during migration along the 

Atlantic Slope from Tamaulipas to n. Honduras. Listing order and source of body mass data as in Table 6.1. Range 

data are from Howell and Webb (1995), Rising (1996), Dunn and Garrett (1997), and Sibley (2000). 

Species Body Mass (g) Range 

Yellow Warbler (Dendroica petechia) 9.5 Win: n. Veracruz-n. South America. 

Chestnut-sided Warbler (D. pensylvanica) 9.6 Win: s. Veracruz-e. Panama, primarily from 

Honduras eastward; absent from Yucatan 

Peninsula. 

Magnolia Warbler (D. magnolia) 8.7 Win: n. Veracruz-w. Panama. 

“Myrtle” Warbler (D. coronata coronata) 12.3 Win: s. USA-Honduras; Greater Antilles. 

Black-throated Green Warbler (D. virens) 8.8 Win: s. Tamaulipas-cen. Panama. 

Blackburnian Warbler (D. fusca) 9.8 Win: n. South America. Mig: widespread, 

except uncommon on Yucatan Peninsula.

Yellow-throated Warbler (D. dominica albilora) 9.4 Win: s. Texas-Nicaragua. 

Bay-breasted Warbler (D. castanea) 12.5 Win: Panama-nw. South America. Fall 

Mig: mainly from Yucatan Peninsula 

eastward. 

Cerulean Warbler (D. cerulea) 9.3 Win: n. South America. Mig: widespread 

from Veracruz southward. 

Black-and-white Warbler (Mniotilta varia) 10.7 Win: s. Texas-nw. South America; s. 

Florida; West Indies. 

American Redstart (Setophaga ruticilla) 8.3 Win: n. Veracruz-n. South America; West 

Indies. 

Prothonotary Warbler (Protonotaria citrea) 16 Win: Yucatan Peninsula (rare) and n. 

Honduras (scarce) to nw. South America. 

Mig: mainly from Yucatan Peninsula 

eastward. 

Worm-eating Warbler (Helmitheros vermivora) 13 Win: s. Veracruz-central Panama; West 

Indies. 

Swainson’s Warbler (Limnothlypis swainsonii) 19 Win: primarily in West Indies, especially 

Greater Antilles; also Yucatan Peninsula, 

Belize. 

Ovenbird (Seiurus aurocapillus) 19.5 Win: s. Tamaulipas-cen. Panama; s. 

Florida; West Indies. 

Northern Waterthrush (S. noveboracensis) 18 Win: s. Tamaulipas-n. South America; West 

Indies. 

Louisiana Waterthrush (S. motacilla) 20.5 Win: s. Tamaulipas-Panama, except not on 

Yucatan Peninsula; West Indies. 

Kentucky Warbler (Oporornis formosus) 14 Win: s. Veracruz to w. Panama. 
54 

Table 6.5. 

 

Neotropical migrant landbirds recorded from the study platforms 

This group includes those species and recognizable subspecies of landbirds and marsh birds that have wintering 

ranges routinely extending into Central America (beyond the Isthmus of Tehuantepec). “Range” provides relevant 

details on each species’ known winter distribution (Win), summer breeding distribution (Sum), and/or migration 

route (Mig). Except where otherwise noted, all species are considered to be widespread during migration along the 

Atlantic Slope from Tamaulipas to n. Honduras. Listing order and source of body mass data as in Table 6.1. Range 

data are from Howell and Webb (1995), Rising (1996), Dunn and Garrett (1997), and Sibley (2000). 

Species Body Mass (g) Range 

Mourning Warbler (O. philadelphia) 12.5 Win: s. Nicaragua-nw. South America. 

Mig: widespread, except rare on Yucatan 

Peninsula; rare in se. USA. 

Common Yellowthroat (Geothlypis trichas) 10 Win: s. USA-cen. Panama; West Indies. 

Hooded Warbler (Wilsonia citrina) 10.5 Win: s. Veracruz-Nicaragua. 

Wilson’s Warbler (W. pusilla) 7.7 Win: mainly n. Tamaulipas-Costa Rica; 

also regular but uncommon se. Louisianas. Texas. Mig: widespread, except rare to 

uncommon on Yucatan Peninsula; rare in 

se. USA. 

Canada Warbler (W. canadensis) 10.3 Win: n. South America. Mig: widespread, 

except rare on Yucatan Peninsula; scarce 

in se. USA. 

Yellow-breasted Chat (Icteria virens) 25 Win: s. Tamaulipas-w. Panama. 

Summer Tanager (Piranga rubra) 29 Win: n. Veracruz-n. South America. 

Scarlet Tanager (P. olivacea) 28 Win: Columbia-Bolivia. Mig: mainly from 

Yucatan Peninsula eastward. 

Savannah Sparrow (Passerculus sandwichensis) 20 Win: s. USA-Belize; w. West Indies 

(Bahamas, Cuba, Isle of Pines, Cayman 

Is., Swan Is.) 

Rose-breasted Grosbeak (Pheucticus ludovicianus) 45 Win: s. Veracruz-Peru. 

Blue Grosbeak (Guiraca caerulea) 28 Win: s. Tamaulipas-Costa Rica. 

Indigo Bunting (Passerina cyanea) 14.5 Win: n. Veracruz-Panama. 

Painted Bunting (P. ciris) 15.5 Win: s. Tamaulipas-cen. Panama. 

Dickcissel (Spiza americana) 27 Win: s. Veracruz-n. South America, except 

not on Yucatan Peninsula. Mig:

widespread, but uncommon on Yucatan 

Peninsula. 

Orchard Oriole (Icterus spurius) 19 Win: s. Veracruz-n. South America. 

Baltimore Oriole (I. galbula) 33 Win: s. Tamaulipas-n. South America. 

Mig: widespread, but uncommon on 

Yucatan Peninsula. 
55 

Table 6.6. 

 

Medium-distance migrants recorded from the study platforms 

This group includes those species and recognizable subspecies of landbirds and marsh birds that routinely winter 

along the northern Gulf Coast and at least as far south as s. Tamaulipas, but not beyond s. Veracruz (and not on the 

Yucatan Peninsula, except as a vagrant). “Winter range” provides relevant details on each species’ known winter 

distribution along the Gulf Coast. Listing order and data sources as in Tables 6.1 and 6.5. 

Species Body Mass (g) Winter Range 

Virginia Rail (Rallus limicola) 85 s. Florida-n. Veracruz; unrecorded on 

Yucatan Peninsula. 

“Yellow-shafted” Flicker (Colaptes auratus auratus) 130 s. Florida-s. Tamaulipas; unrecorded on 

Yucatan Peninsula. 

Eastern Phoebe (Sayornis phoebe) 20 s. Florida-s. Veracruz; vagrant to Yucatan 

Peninsula. 

Loggerhead Shrike (Lanius ludovicianus) 48 s. Florida-s. Veracruz; unrecorded on 

Yucatan Peninsula. 

Blue-headed Vireo (Vireo solitarius) 16 s. Florida-s. Veracruz; vagrant on Yucatan 

Peninsula. 

House Wren (Troglodytes aedon) 11 s. Florida-s. Veracruz; unrecorded on 

Yucatan Peninsula. 

Sedge Wren (Cistothorus platensis) 9 s. Florida-n. Veracruz; unrecorded on 

Yucatan Peninsula. 

Marsh Wren (C. palustris) 11 s. Florida-n. Veracruz; unrecorded on 

Yucatan Peninsula. 

Hermit Thrush (Catharus guttatus) 31 cen. Florida-s. Tamaulipas; rare in s. 

Florida; vagrant to s. Veracruz; 

unrecorded on Yucatan Peninsula. 

American Robin (Turdus migratorius) 77 s. Florida-n. Veracruz; vagrant to s. 

Veracruz, Yucatan Peninsula, Belize. 

Northern Mockingbird (Mimus polyglottos) 49 s. Florida-s. Veracruz; unrecorded on 

Yucatan Peninsula. 

American Pipit (Anthus rubescens) 21 s. Florida-s. Veracruz; vagrant to Tabasco, 

Yucatan Peninsula. 

Orange-crowned Warbler (Vermivora celata) 9 s. Florida-s. Veracruz; vagrant on n. 

Yucatan Peninsula. 

Chipping Sparrow (Spizella passerina) 12 cen. Florida-s. Tamaulipas; rare in s. 

Florida; unrecorded on Yucatan 

Peninsula. 

Clay-colored Sparrow (S. pallida) 12 cen. Texas-s. Tamaulipas; vagrant farther 

east along n. Gulf Coast; rare in 

Veracruz; vagrant on n. Yucatan 

Peninsula. 

Vesper Sparrow (Pooecetes gramineus) 26 cen. Florida-s. Tamaulipas; rare in s. 

Florida; vagrant on n. Yucatan 

Peninsula. 
56 

Table 6.6. 

 

Medium-distance migrants recorded from the study platforms 

This group includes those species and recognizable subspecies of landbirds and marsh birds that routinely winter 

along the northern Gulf Coast and at least as far south as s. Tamaulipas, but not beyond s. Veracruz (and not on the 

Yucatan Peninsula, except as a vagrant). “Winter range” provides relevant details on each species’ known winter 

distribution along the Gulf Coast. Listing order and data sources as in Tables 6.1 and 6.5. 

Species Body Mass (g) Winter Range 

Lark Sparrow (Chondestes grammacus) 29 e. Texas-n. Veracruz; rare farther east 

along n. Gulf Coast; vagrant on n. 

Yucatan Peninsula. 

Grasshopper Sparrow (Ammodramus savannarum) 17 s. Florida-Veracruz; rare to uncommon on 

Yucatan Peninsula; also locally in w. 

West Indies (Bahamas, Cuba, Isle of 

Pines, Cayman Is., Swan Is.). 

Lincoln’s Sparrow (Melospiza lincolnii) 17 s. Florida-s. Veracruz; vagrant on Yucatan 

Peninsula, in Belize, and in West 

Indies. 

Swamp Sparrow (M. georgiana) 17 s. Florida-n. Veracruz; irregular and less 

common from Texas south; unrecorded 

on Yucatan Peninsula. 

White-crowned Sparrow (Zonotrichia leucophrys) 29 nw. Florida-s. Tamaulipas; rare on Florida 

Peninsula; vagrant on n.Yucatan 

Peninsula, in Belize, and in West 

Indies. 

Western Meadowlark (Sturnella neglecta) 97 e. Louisiana-n. Veracruz; unrecorded on 

Yucatan Peninsula. 

Brewer’s Blackbird (Euphagus cyanocephalus) 63 Alabama-n. Veracruz; unrecorded on 

Yucatan Peninsula. 

Brown-headed Cowbird (Molothrus ater) 44 s. Florida-s. Veracruz; unrecorded on 

Yucatan Peninsula. 

Pine Siskin (Carduelis pinus) 15 cen. Florida-s. Tamaulipas; unrecorded on 

Yucatan Peninsula. 

American Goldfinch (C. tristis) 13 s. Florida-n. Veracruz; unrecorded on 

Yucatan Peninsula. 
57 

Table 6.7. 

 

Short-distance migrants recorded from the study platforms 

This group includes those species and recognizable subspecies of landbirds and marsh birds with no known regular 

wintering populations south of n. Tamaulipas. All are unrecorded from Atlantic Slope of Mexico (including the 

Yucatan Peninsula) unless otherwise indicated. “Winter range” provides relevant details on each species’ known 

winter distribution along the Gulf Coast. Listing order and data sources as in Tables 6.1 and 6.5. 

Species Body Mass (g) Winter Range 

Yellow Rail (Coturnicops noveboracensis) 50 cen. Florida-cen. Texas. 

Clapper Rail (Rallus longirostris) 290 s. Florida-n. Tamaulipas (disjunct 

population in coastal Yucatan Peninsula 

thought to be resident). 

King Rail (Rallus elegans) 360 s. Florida-s. Texas; disjunct resident 

population in coastal Veracruz, but 

migrants from USA populations 

unrecorded in Mexico. 

Long-eared Owl (Asio otus) 260 Virginia-Kentucky-Arkansas-cen. Texas; 

irregular and rare vagrant south to Gulf 

Coast from s. Florida-s. Texas. 

Short-eared Owl (A. flammeus) 350 South Carolina-Tennessee-Arkansas-cen. 

Texas; a few regularly winter south to 

Gulf Coast from s. Florida-cen. Texas; 

irregular and rare south to n. Veracruz. 

Red-headed Woodpecker (Melanerpes 

erythrocephalus) 

72 cen. Florida-cen. Texas; rare in s. Florida, 

s. Texas. 

Downy Woodpecker (Picoides pubescens) 27 s. Florida-cen. Texas. 

Red-breasted Nuthatch (Sitta canadensis) 10 n. Florida-e. Texas; irregular in s. Florida, 

s. Texas; vagrant to Nuevo Leon. 

White-breasted Nuthatch (S. carolinensis) 21 s. Georgia —ne. Texas. 

Carolina Wren (Thryothorus ludovicianus) 21 usually considered to be nonmigratory; 

resident to s. Tamaulipas; unrecorded on 

Yucatan Peninsula. 

Winter Wren (Troglodytes troglodytes) 9 nw. Florida-cen. Texas; vagrant to 

Tamaulipas. 

Golden-crowned Kinglet (Regulus satrapa) 6 n. Florida-s. Texas; irregularly rare to 

uncommon in s. Florida, n. Tamaulipas. 

Ruby-crowned Kinglet (R. calendula) 6.5 s. Florida-n. Tamaulipas; vagrant to s. 

Veracruz, Yucatan Peninsula. 

Brown Thrasher (Toxostoma rufum) 69 s. Florida-cen. Texas; vagrant to 

Tamaulipas. 

European Starling (Sturnus vulgaris) 82 s. Florida-n. Tamaulipas; vagrant to s. 

Veracruz, n. Yucatan Peninsula. 

Pine Warbler (Dendroica pinus) 12 cen. Florida-cen. Texas; vagrant to n. 

Tamaulipas. 

“Yellow” Palm Warbler (D. palmarum hypochrysea) 10.3 n. Florida-se. Louisiana; one record in 

Quintana Roo. 
58 

Table 6.7. 

 

Short-distance migrants recorded from the study platforms 

This group includes those species and recognizable subspecies of landbirds and marsh birds with no known regular 

wintering populations south of n. Tamaulipas. All are unrecorded from Atlantic Slope of Mexico (including the 

Yucatan Peninsula) unless otherwise indicated. “Winter range” provides relevant details on each species’ known 

winter distribution along the Gulf Coast. Listing order and data sources as in Tables 6.1 and 6.5. 

Species Body Mass (g) Winter Range 

Eastern Towhee (Pipilo erythrophthalmus) 40 s. Florida to s. Texas. 

American Tree Sparrow (Spizella arborea) 20 Virginia-Tennessee-nw. Arkansas-n. Texas.

Field Sparrow (Spizella pusilla) 12.5 s. Florida-n. Tamaulipas. 

Henslow’s Sparrow (Ammodramus henslowii) 13 n. Florida-cen. Texas. 

LeConte’s Sparrow (A. leconteii) 13 n. Florida-s. Texas. 

Nelson’s Sharp-tailed Sparrow (A. nelsoni) 17 cen. Florida-s. Texas; vagrant to n. 

Tamaulipas. 

Song Sparrow (Melospiza melodia) 20 n. Florida-cen. Texas; rare in s. Florida; 

uncommon in s. Texas; rare to 

uncommon in n. Nuevo Leon, but 

otherwise unrecorded on Atlantic Slope 

of Mexico. 

White-throated Sparrow (Zonotrichia albicollis) 26 cen. Florida to n. Tamaulipas; uncommon 

in s. Florida. 

“Slate-colored” Junco (Junco hyemalis hyemalis) 19 n. Florida to cen. Texas; rare in s. Florida; 

irregularly rare to uncommon to s. 

Tamaulipas; vagrant to s. Veracruz. 

Common Grackle (Quiscalus quiscula) 115 s. Florida-cen. Texas. 

Boat-tailed Grackle (Q. major) 168 s. Florida-cen. Texas. 

The group referred to as “Caribbean vagrants” included those species that normally breed, winter, or 

migrate in the Caribbean area and were unexpected (a priori) to occur in the study area except as rare 

vagrants (Table 6.8). The group referred to as “western vagrants” comprised species that normally breed 

and winter no farther east than central coastal Texas and were unexpected (a priori) to occur in the study 

area except as rare vagrants (Table 6.9). 

Swifts and swallows are obligate aerial insectivores and all are diurnal migrants; this group is referred to 

as “aerial insectivores” (Table 6.10). 

Ruby-throated Hummingbirds (Archilochus colubris; body mass = 3.2 g) are diurnal migrants with a 

variety of unique ecological and physiological characteristics, and are treated separately here. 

Finally, species whose expected trans-Gulf status was unclear because of unknown migratory status, the 

existence of widespread resident populations, or other factors are listed in Table 6.11. 
59 

Table 6.8. 

 

Caribbean vagrants recorded from the study platforms 

This group includes those species and recognizable subspecies that normally breed, winter, or migrate in the 

Caribbean area and were unexpected a priori to occur in the study area except as rare vagrants. “Range” provides 

relevant details on each species’ known winter distribution (Win), summer breeding distribution (Sum), and/or 

migration route (Mig). Listing order and data sources as in Tables 6.1 and 6.5. 

Species Body Mass (g) Range 

Black Swift (Cypseloides niger) 45 Caribbean population considered to be 

resident. 

Cuban Pewee ? Unrecorded in Mexico. 

Gray Kingbird (Tyrannus dominicensis) 44 Win: Caribbean to n. South America. 

Mig: rare on e. coast of Yucatan 

Peninsula. 

Loggerhead Kingbird (T. caudifasciatus) ? Unrecorded in Mexico. 

Black-whiskered Vireo (Vireo altiloquus) 18 Sum: resident along peninsular Florida 

coast. Win: South America. Mig: 

vagrant in Quintana Roo, Belize. 

“Caribbean” Cave Swallow (Petrochelidon fulva 

citata) 

15 Win: Breeding population on n. Yucatan 

Peninsula thought to be nomadic in 

winter. Mig: unknown. 

Cape May Warbler (Dendroica tigrina) 11 Win: primarily West Indies; uncommon 

along Caribbean coast (mainly offshore 

islands) from Quintana Roo to n. 

Honduras. Mig: from e. coast of 

Yucatan Peninsula eastward, primarily 

via Florida. 

Black-throated Blue Warbler (D. caerulescens) 10.2 Win: primarily West Indies; rareuncommon along Caribbean coast 

(mainly offshore islands) from 

Quintana Roo to nw. South America. 

Mig: from e. coast of Yucatan 

Peninsula eastward, primarily via 

Florida. 

Prairie Warbler (D. discolor) 7.7 Win: mainly West Indies; rare-uncommon 

along Caribbean coast (mainly offshore 

islands) from Quintana Roo to n. 

Honduras; also s. Florida. Mig: from e. 

coast of Yucatan Peninsula eastward, 

primarily via Florida. 

“Western” Palm Warbler (D. palmarum palmarum) 10.3 Win: se. USA (west to Louisiana), West 

Indies, and along Caribbean coast from 

n. Yucatan Peninsula to Costa Rica. 

Mig: from n. Yucatan Peninsula east. 
60 

Table 6.8. 

 

Caribbean vagrants recorded from the study platforms 

This group includes those species and recognizable subspecies that normally breed, winter, or migrate in the 

Caribbean area and were unexpected a priori to occur in the study area except as rare vagrants. “Range” provides 

relevant details on each species’ known winter distribution (Win), summer breeding distribution (Sum), and/or 

migration route (Mig). Listing order and data sources as in Tables 6.1 and 6.5. 

Species Body Mass (g) Range 

Blackpoll Warbler (D. striata) 13 Win: South America. Spring mig: 

probably mainly over w. Caribbean 

toward Florida, overflying the islands; 

rare in ne. Yucatan Peninsula. Fall 

mig: probably entirely overwater from 

Atlantic Coast to n. South America; 

rare in ne. Yucatan Peninsula. 

Connecticut Warbler (Oporornis agilis) 15 Win: South America. Spring mig: 

probably across Caribbean to Florida; 

one record in Belize. Fall mig: 

probably across Atlantic Ocean from e. 

USA to n. South America. 

Bobolink (Dolichonyx oryzivorus) 43 Win: s. South America. Mig: mainly 

across Caribbean; irregularly 

uncommon-fairly common in spring 

and rare-uncommon in fall from ne. 

Yucatan Peninsula to Honduras Bay 

Islands. 
61 

Table 6.9. 

 

Western vagrants recorded from the study platforms 

This group includes those species and recognizable subspecies that normally breed and winter no farther east than 

central coastal Texas and were unexpected a priori to occur in the study area except as rare vagrants. All are 

unrecorded from the Yucatan Peninsula unless otherwise indicated. “Range” provides relevant details on each 

species’ known winter distribution (Win), summer breeding distribution (Sum), and/or migration route (Mig). 

Listing order and data sources as in Tables 6.1 and 6.5. 

Species Body Mass( g) Range 

Band-tailed Pigeon (Columba fasciata) 360 Sum: east to cen. Colorado, w. Texas, and 

throughout interior Central America. 

Win: mostly resident within summer 

range, but populations in sw. USA 

move elsewhere in winter. 

Groove-billed Ani (Crotophaga sulcirostris) 85 Sum: s. Texas-South America. Win: 

mostly resident within summer range; a 

few east to Upper Texas Coast. 

Flammulated Owl (Otus flammeolus) 60 Sum: east to cen. Colorado, se. New 

Mexico, w. Texas, cen. Mexico. Win: 

highlands of cen. Mexico-Guatemala; 

unrecorded on Atlantic slope of 

Mexico. 

Burrowing Owl (Athene cunicularia) 155 Sum: east to e. North Dakota, cen. Kansas, 

cen. Texas, cen. Mexico; also a disjunct 

resident population in Florida. Win: 

cen. Texas-s. Veracruz, and on Pacific 

Slope into Honduras; vagrant on 

Yucatan Peninsula. 

Lesser Nighthawk (Chordeiles acutipennis) 50 Sum: east to se. New Mexico, cen. Texas 

coast, Tamaulipas; a disjunct 

population on n. Yucatan Peninsula. 

Win: north to n. Veracruz, including 

Yucatan Peninsula. 

Black-chinned Hummingbird (Archilochus 

alexandri) 

3.3 Sum: east to w. Montana, w. Colorado, se. 

New Mexico, cen. Texas, n. Nuevo 

Leon. Win: primarily on Pacific Slope 

of Mexico. 

Rufous Hummingbird (Selasphorus rufus) 3.4 Sum: east to sw. Alberta, w. Montana, e. 

Idaho. Win: primarily on Pacific Slope 

of Mexico; in recent years, range has 

expanded dramatically into se. USA 

due to artificial selection by 

hummingbird feeding enthusiasts. 

Ringed Kingfisher (Ceryle torquata) 315 Sum: s. Texas-South America, but not on 

n. Yucatan Peninsula. Win: mainly in 

summer range. 
62 

Table 6.9. 

 

Western vagrants recorded from the study platforms 

This group includes those species and recognizable subspecies that normally breed and winter no farther east than 

central coastal Texas and were unexpected a priori to occur in the study area except as rare vagrants. All are 

unrecorded from the Yucatan Peninsula unless otherwise indicated. “Range” provides relevant details on each 

species’ known winter distribution (Win), summer breeding distribution (Sum), and/or migration route (Mig). 

Listing order and data sources as in Tables 6.1 and 6.5. 

Species Body Mass( g) Range 

Western Wood-Pewee (Contopus sordidulus) 13 Sum: east to sw. Manitoba, w. North 

Dakota, w. Nebraska, e. New Mexico, 

w. Texas, Coahuila, and through 

interior to Honduras. Win: South 

America. 

Say’s Phoebe (Sayornis saya) 21 Sum: east to s. Saskatchewan, w. South 

Dakota, w. Kansas, e. New Mexico, w. 

Texas, Coahuila, cen. Mexico. Win: 

east to cen. Texas coast, Tamaulipas, 

and in interior to Oaxaca. 

Vermilion Flycatcher (Pyrocephalus rubinus) 14.5 Sum: east to e. New Mexico, cen. Texas, 

Nuevo Leon, s. Veracruz, and 

disjunctly to Nicaragua (including on 

Yucatan Peninsula). Win: in s. portion 

of breeding range and eastward to cen. 

Texas coast, Tamaulipas, n. Veracruz. 

Ash-throated Flycatcher (Myiarchus cinerascens) 27 Sum: east to sw. Wyoming, se. Colorado, 

w. Oklahoma, cen. Texas, w. Nuevo 

Leon, and in interior to cen. Mexico. 

Win: mostly on Pacific Slope south to 

Nicaragua; on Atlantic Slope in 

Tamaulipas and Veracruz. 

Brown-crested Flycatcher (Myiarchus tyrannulus) 44 Sum: s. Texas-South America, including on 

Yucatan Peninsula. Win: north to 

Veracruz, including on Yucatan 

Peninsula. 

Tropical Kingbird (Tyrannus melancholicus) 40 Sum: s. Texas-South America, including on 

Yucatan Peninsula. Win: generally 

considered to be resident through most 

of summer range. 

Couch’s Kingbird (Tyrannus couchii) 43 Sum: cen. Texas coast-Belize, including 

Yucatan Peninsula. Win: s. TexasBelize, including Yucatan Peninsula. 

Western Kingbird (Tyrannus verticalis) 40 Sum: east to w. Wisconsin, w. Iowa, e. 

Kansas, cen. Oklahoma, cen. Texas. 

Win: mostly on Pacific Slope and in 

interior from cen. Mexico to Costa 

Rica; a few in s. Florida. 
63 

Table 6.9. 

 

Western vagrants recorded from the study platforms 

This group includes those species and recognizable subspecies that normally breed and winter no farther east than 

central coastal Texas and were unexpected a priori to occur in the study area except as rare vagrants. All are 

unrecorded from the Yucatan Peninsula unless otherwise indicated. “Range” provides relevant details on each 

species’ known winter distribution (Win), summer breeding distribution (Sum), and/or migration route (Mig). 

Listing order and data sources as in Tables 6.1 and 6.5. 

Species Body Mass( g) Range 

Bell’s Vireo (Vireo bellii) 8.5 Sum: east to Illinois, Missouri, e. Texas, 

and in interior to cen. Mexico. Win: 

mainly on Pacific Slope of Mexico to 

El Salvador. 

Violet-green Swallow (Tachycineta thalassina) 14 Sum: east to e. Montana, cen. Colorado, w. 

Texas, and in interior to cen. Mexico. 

Win: Pacific Slope and interior of 

Mexico to El Salvador. 

“Mexican” Cave Swallow (Petrochelidon fulva 

fulva?) 

15 Sum: east to cen. Texas, w. Tamaulipas, 

with disjunct populations along the 

fringe of an expanding population to e. 

Texas. Win: poorly known; probably 

mainly in s. portion of summer range. 

Sage Thrasher (Oreoscoptes montanus) 43 Sum: east to se. Montana, cen. Colorado, 

nw. New Mexico. Win: east to cen. 

Texas, n. Tamaulipas. 

Long-billed Thrasher (Toxostoma longirostre) 70 Sum: s. Texas-n. Veracruz. Win: usually 

considered permanent resident within 

summer range. 

Sprague’s Pipit (Anthus spragueii) 25 Sum: east to s. Manitoba, e. North Dakota. 

Win: east to w. Louisiana, coastal 

Texas, Tamaulipas, n. Veracruz. 

“Audubon’s” Warbler (Dendroica coronata 

auduboni) 

12.3 Sum: east to w. South Dakota, cen. 

Colorado, extreme w. Texas. Win: east 

to w. Texas, s. Tamaulipas, and 

through interior highlands to w. 

Honduras; vagrant in s. Veracruz; 

unrecorded on Yucatan Peninsula. 

Black-throated Gray Warbler (Dendroica nigrescens) 8.4 Sum: east to nw. Wyoming, se. Colorado, 

se. New Mexico. Win: mainly on 

Pacific Slope and in interior, east to 

San Luis PotosR and Oaxaca; rarely to 

lower Rio Grande Valley in s. Texas; 

vagrant in s. Veracruz; unrecorded on 

Yucatan Peninsula. 

Townsend’s Warbler (Dendroica townsendi) 8.8 Sum: east to sw. Alberta, w. Montana, cen. 

Idaho. Win: interior highlands from s. 

Nuevo Leon to w. Nicaragua; vagrant 

in s. Veracruz; unrecorded on Yucatan 

Peninsula. 
64 

Table 6.9. 

 

Western vagrants recorded from the study platforms 

This group includes those species and recognizable subspecies that normally breed and winter no farther east than 

central coastal Texas and were unexpected a priori to occur in the study area except as rare vagrants. All are 

unrecorded from the Yucatan Peninsula unless otherwise indicated. “Range” provides relevant details on each 

species’ known winter distribution (Win), summer breeding distribution (Sum), and/or migration route (Mig). 

Listing order and data sources as in Tables 6.1 and 6.5. 

Species Body Mass( g) Range 

Hermit Warbler (Dendroica occidentalis) 9.2 Sum: east to Cascades and Sierra Nevada 

from cen. Washington to cen. 

California. Win: interior highlands 

from s. Nuevo Leon to w. Nicaragua; 

vagrant in s. Veracruz; unrecorded on 

Yucatan Peninsula. 

MacGillivray’s Warbler (Oporornis tolmiei) 10.5 Sum: east to sw. Alberta, w. Montana, w. 

South Dakota (Black Hills), cen. 

Colorado, cen. New Mexico, s. Nuevo 

Leon. Win: mainly in highlands from 

s. Nuevo Leon to w. Panama; vagrant 

in s. Veracruz; unrecorded on Yucatan 

Peninsula. 

Western Tanager (Piranga ludoviciana) 28 Sum: east to w. Saskatchewan, e. 

Wyoming, cen. New Mexico, w. Texas. 

Win: mainly on Pacific Slope and in 

interior from s. Mexico-w. Panama; 

less common on Atlantic Slope from s. 

Tamaulipas to Guatemala; vagrant on 

Yucatan Peninsula. 

Green-tailed Towhee (Pipilo chlorurus) 29 Sum: east to cen. Wyoming, cen. Colorado, 

nw. New Mexico. Win: east to cen. 

coastal Texas, n. Tamaulipas. 

Spotted Towhee (Pipilo maculatus) 40 Sum: east to cen. North Dakota, cen. 

Nebraska, cen. Colorado, New Mexico, 

and in highlands of w. Texas and 

Mexico south to Oaxaca. Win: east to 

e. Kansas, e. Oklahoma, cen. Coastal 

Texas, n. Tamaulipas. 

Cassin’s Sparrow (Aimophila cassinii) 19 Sum: east to cen. Kansas, cen. Oklahoma, 

cen. coastal Texas (Corpus Christi 

area), s. Tamaulipas. Win: primarily 

within summer range, north to s. Texas; 

unrecorded on Atlantic Slope south of 

Tamaulipas. 

“Gray-headed” Junco (Junco hyemalis caniceps) ? Sum: east to cen. Colorado, cen. New 

Mexico, extreme w. Texas. Win: 

primarily at lower elevations within 

breeding range; also southeast to 

Chihuahua, n. Durango; unrecorded on 

Atlantic slope of Mexico. 
65 

Table 6.9. 

 

Western vagrants recorded from the study platforms 

This group includes those species and recognizable subspecies that normally breed and winter no farther east than 

central coastal Texas and were unexpected a priori to occur in the study area except as rare vagrants. All are 

unrecorded from the Yucatan Peninsula unless otherwise indicated. “Range” provides relevant details on each 

species’ known winter distribution (Win), summer breeding distribution (Sum), and/or migration route (Mig). 

Listing order and data sources as in Tables 6.1 and 6.5. 

Species Body Mass( g) Range 

“Pink-sided” Junco (Junco hyemalis mearnsi) ? Sum: east to cen. Montana, nw. Wyoming. 

Win: east to e.. Colorado, e. New 

Mexico, sw. Texas, cen. Chihuahua 

Chestnut-collared Longspur (Calcarius ornatus) 19 Sum: east to e. North Dakota, e. South 

Dakota, se. Colorado. Win: east to 

cen. Oklahoma, cen. Texas, w. 

Coahuila, and on Mexican Plateau to 

Zacatecas. 

Pyrrhuloxia (Cardinalis sinuatus) 36 Sum: east to cen. interior Texas, s. coastal 

Texas, s. Tamaulipas. Win: usually 

resident within summer range. 

Lazuli Bunting (Passerina amoena) 15.5 Sum: east to cen. North Dakota, cen. 

Nebraska, cen. Oklahoma. Win: 

Pacific Slope of Mexico and interior to 

Oaxaca. 

Varied Bunting (P. versicolor) 12 Sum: east to w. Texas, extreme s. Texas, s. 

Tamaulipas. Win: n. Tamaulipas-s. 

Veracruz. 

Yellow-headed Blackbird (Xanthocephalus 

xanthocephalus) 

65 Sum: east to Wisconsin, n. Illinois, n. 

Nebraska, n. New Mexico. Win: east 

to w. Texas, Tamaulipas, n. Veracruz; 

vagrant to Yucatan Peninsula. 

Bronzed Cowbird (Molothrus aeneus) 62 Sum: cen. coastal Texas-cen. Panama. 

Win: north to n. Tamaulipas. 

Bullock’s Oriole (Icterus bullockii) 36 Sum: east to w. South Dakota, w. 

Nebraska, w. Texas, and in highlands 

to cen. Mexico. Win: s. Tamaulipas-s. 

Veracruz, and in interior to Guatemala.

Lesser Goldfinch (Carduelis psaltria) 9.5 Sum: east to se. Colorado, e. New Mexico, 

s. Texas, Nuevo Leon, and in interior to 

Oaxaca; also a disjunct year-round 

resident population on n. Yucatan 

Peninsula. Win: east to cen. Texas 

coast, Tamaulipas, n. Veracruz. 
66 

Table 6.10. 

 

Aerial insectivores recorded from the study platforms 

“Winter range” provides relevant details on each species’ known winter distribution. Listing order and data sources 

as in Tables 6.1 and 6.5. 

Species Body Mass (g) Winter Range 

Chimney Swift (Chaetura pelagica) 23 South America. 

Purple Martin (Progne subis) 56 South America. 

Tree Swallow (Tachycineta bicolor) 20 s. USA-n. Honduras. 

Northern Rough-winged Swallow (Stelgidopteryx 

serripennis) 

16 s. Texas-Panama; also s. Florida. 

Bank Swallow (Riparia riparia) 13.5 South America. 

Cliff Swallow (Petrochelidon pyrrhonota) 21 South America. 

Barn Swallow (Hirundo rustica) 19 s. Veracruz-s. South America, but not on 

Yucatan Peninsula. 

Table 6.11. 

 

Species whose expected trans-Gulf status was unclear because of unknown migratory status, the existence of 

widespread resident populations, or other factors 

Listing order and data sources as in Tables 6.1 and 6.5. 

Species Body Mass (g) Range 

Rock Dove (Columba livia) 270 All individuals recorded offshore appeared to 

be racing pigeons. 

Eurasian Collared-Dove (Streptopelia decaocto) 200 Species is currently colonizing se. USA; 

migratory status unknown. 

White-winged Dove (Zenaida asiatica) 150 Status complex; year-round resident from e. 

Texas-n. Veracruz and on Yucatan 

Peninsula; additional wintering populations 

along Gulf Coast from w. FloridaLouisiana and cen. Veracruz-Tabasco; also 

a permanent resident population in s. 

Florida thought to be derived from 

introduced stock. This species is 

undergoing a dramatic population increase 

and range expansion with increasingly 

frequent extralimital breeding and 

migration records, but details of the 

expansion remain unclear. 

Inca dove (Columbina inca) 47 Considered to be a permanent resident from 

sw. Louisiana-cen. Tabasco, but species 

has undergone a range expansion with 

population in Louisiana established 

recently. 
67 

Table 6.11. 

 

Species whose expected trans-Gulf status was unclear because of unknown migratory status, the existence of 

widespread resident populations, or other factors 

Listing order and data sources as in Tables 6.1 and 6.5. 

Species Body Mass (g) Range 

Common Ground-Dove (C. passerina) 30 Considered to be a permanent resident along 

Gulf coast from s. Florida-s, Veracruz; 

disjunct resident population on Yucatan 

Peninsula. 

Barn Owl (Tyto alba) 460 Resident throughout much of USA and Central 

America; northern part of population 

known to migrate long distances, but 

winter range of migratory individuals 

unclear due to widespread presence of 

year-round residents. 

Cedar Waxwing (Bombycilla cedrorum) 32 Winters throughout s. USA; irregularly rareuncommon in winter on Yucatan Peninsula 

and along entire Atlantic coast of Mexico, 

but presumed not to be a trans-Gulf 

migrant because of its status as a strictly 

diurnal migrant. 

Red-winged Blackbird (Agelaius phoeniceus) 52 Resident throughout USA and along entire 

Atlantic coast of Mexico to nw. Honduras; 

northern part of population known to be 

migratory, but winter range of migratory 

individuals unclear due to widespread 

presence of year-round residents. 

Eastern Meadowlark (Sturnella magna) 90 Resident throughout much of USA and along 

Atlantic coast of Mexico to Honduras; 

northern part of population known to be 

migratory, but winter range of migratory 

individuals unclear due to widespread 

presence of year-round residents. 

Great-tailed Grackle (Quiscalus mexicanus) 148 Year-round resident from cen. Louisiana 

through most of Mexico; population has 

recently been expanding northward and 

eastward; northern part of population 

known to be migratory, but winter range of 

migratory individuals unclear due to 

widespread presence of year-round 

residents. 
68 

6.2 SUMMARY OF COUNT DATA

A complete enumeration of all migrant birds recorded on or observed from the study platforms is 

presented in Table 6.12, with data grouped by season, year, and ecological group. 

Altogether, 119,810 individuals of 279 species were recorded offshore during the study. During the 

spring, 82,281 individuals were recorded, 14% as stopovers and the balance as flybys. During the fall, 

37,529 individuals were recorded, 30% as stopovers and the balance as flybys. The significantly larger 

proportion of fall migrants recorded as stopovers (G = 3,850, P << 0.0001) was due to the fact that fall 

migration over the northern Gulf proceeded mostly at night, when the detectability of migrants was 

relatively low, whereas the opposite was the case during the spring. Table 6.13 shows the number of 

species and recognizable forms recorded in each season in relation to status (stopover versus flyby) and 

ecological category. 
69 

Table 6.12. 

Summary of platform count data grouped by season, year, and ecological group 

Species are listed in descending order of abundance within each ecological group. 

SPRING FALL 

1998 1999 2000 1998 1

Species Group Stop Fly Stop Fly Stop Fly Stop Fly StoBlue-winged Teal waterfowl 0 334 0 728 0 4,305 2 28 6 unidentified duck waterfowl 0 179 0 97 0 619 0 112 0 Lesser Scaup waterfowl 0 0 0 16 0 242 0 0 0 Greater White-fronted Goose waterfowl 0 0 0 0 0 15 0 0 0 unidentified scoter waterfowl 0 185 0 0 0 0 0 0 0 Snow Goose waterfowl 0 0 0 0 0 0 0 1 0 Northern Pintail waterfowl 0 40 0 0 0 18 0 0 0 Northern Shoveler waterfowl 0 0 0 0 0 90 0 0 0 Green-winged Teal waterfowl 0 0 0 25 0 7 0 1 1 American Wigeon waterfowl 0 0 0 0 0 21 0 8 0 Fulvous Whistling-Duck waterfowl 0 0 0 0 0 64 0 3 0 unidentified goose waterfowl 0 0 0 0 0 0 0 0 0 Black-bellied Whistling-Duck waterfowl 0 0 0 0 0 0 0 0 0 Cinnamon Teal waterfowl 0 22 0 0 0 1 0 0 0 Gadwall waterfowl 0 0 0 0 0 0 0 0 0 Greater Scaup waterfowl 0 0 0 16 0 0 0 0 0 Redhead waterfowl 0 0 0 0 0 1 0 0 0 Red-breasted Merganser waterfowl 0 0 0 0 0 0 0 8 0 White-winged Scoter waterfowl 0 9 0 0 0 0 0 0 0 Ring-necked Duck waterfowl 0 0 0 0 0 5 0 0 0 Hooded Merganser waterfowl 0 0 0 0 0 0 0 0 0 
70 

Table 6.12. 

Summary of platform count data grouped by season, year, and ecological group 

Species are listed in descending order of abundance within each ecological group. 

SPRING FALL 

1998 1999 2000 1998 1

Species Group Stop Fly Stop Fly Stop Fly Stop Fly StoMallard waterfowl 0 0 0 0 0 0 0 0 0 Ross’s Goose waterfowl 0 0 0 0 0 0 0 0 0 Surf Scoter waterfowl 0 0 0 0 0 0 0 0 0 Cattle Egret heron 234 823 337 1,470 601 5,056 81 358 200 unidentified small white egret heron 0 85 0 567 1 3,618 0 682 1 Little Blue Heron heron 5 171 1 559 11 1,855 1 67 20 unidentified heron heron 0 79 0 497 1 1,168 0 192 0 Great Egret heron 1 71 8 284 21 1,288 3 299 1 Great Blue Heron heron 3 88 1 197 19 644 3 834 14 Green Heron heron 71 206 114 76 204 1,062 25 12 50 Snowy Egret heron 6 95 4 264 17 361 0 76 1 Yellow-crowned Night-Heron heron 7 32 12 55 14 422 0 13 4 White Ibis heron 0 3 2 90 4 12 3 96 1 Tricolored Heron heron 1 31 2 56 3 214 0 15 0 unidentified Plegadis ibis heron 0 0 0 0 0 0 0 1 0 Black-crowned Night-Heron heron 1 2 0 11 0 35 0 0 0 Roseate Spoonbill heron 0 0 0 0 0 0 0 27 0 White-faced Ibis heron 0 0 0 0 0 27 0 0 0 Reddish Egret heron 2 10 1 2 0 6 0 0 0 American Bittern heron 2 0 0 4 2 1 0 0 2 Peregrine Falcon diurnal raptor 31 19 50 33 63 50 96 61 280 
71 

Table 6.12. 

Summary of platform count data grouped by season, year, and ecological group 

Species are listed in descending order of abundance within each ecological group. 

SPRING FALL 

1998 1999 2000 1998 1

Species Group Stop Fly Stop Fly Stop Fly Stop Fly StoAmerican Kestrel diurnal raptor 5 5 0 1 12 6 21 2 61 Osprey diurnal raptor 6 23 3 16 4 31 3 16 8 Merlin diurnal raptor 5 18 6 3 5 14 2 6 21 unidentified small falcon diurnal raptor 0 0 0 4 0 2 0 4 1 Sharp-shinned Hawk diurnal raptor 0 0 3 1 1 1 3 3 0 Mississippi Kite diurnal raptor 0 12 0 0 0 0 0 0 0 Swallow-tailed Kite diurnal raptor 0 1 0 5 0 5 0 0 0 Northern Harrier diurnal raptor 0 0 0 0 0 0 0 0 0 Cooper’s Hawk diurnal raptor 0 0 0 0 0 0 0 1 0 Aplomado Falcon diurnal raptor 0 0 0 0 1 0 0 0 0 Broad-winged Hawk diurnal raptor 0 0 0 0 0 0 0 0 1 unidentified shorebird shorebird 2 227 0 294 1 1,093 0 187 11 Upland Sandpiper shorebird 0 18 0 17 0 113 0 5 3 American Golden-Plover shorebird 0 57 0 232 5 76 0 0 0 unidentified “peep” shorebird 0 12 0 19 0 262 0 0 0 Pectoral Sandpiper shorebird 0 17 0 33 0 233 0 0 0 Lesser Yellowlegs shorebird 0 1 0 0 1 242 0 5 0 Greater Yellowlegs shorebird 0 2 0 1 0 123 0 1 0 Black-necked Stilt shorebird 0 0 0 0 0 117 0 0 0 Whimbrel shorebird 0 1 0 39 0 66 0 0 0 White-rumped Sandpiper shorebird 0 0 0 15 0 79 0 0 0 
72 

Table 6.12. 

Summary of platform count data grouped by season, year, and ecological group 

Species are listed in descending order of abundance within each ecological group. 

SPRING FALL 

1998 1999 2000 1998 1

Species Group Stop Fly Stop Fly Stop Fly Stop Fly StoLeast Sandpiper shorebird 2 17 0 0 5 38 0 5 1 unidentified dowitcher shorebird 0 7 0 0 0 39 0 0 0 Hudsonian Godwit shorebird 0 50 0 0 0 1 0 0 0 Killdeer shorebird 0 0 0 0 0 31 0 1 0 Willet shorebird 1 8 1 4 0 23 0 1 0 Buff-breasted Sandpiper shorebird 0 4 0 6 0 27 0 1 0 Solitary Sandpiper shorebird 0 0 0 0 0 11 0 4 1 Spotted Sandpiper shorebird 4 2 4 1 6 2 0 0 2 Sanderling shorebird 2 1 1 0 1 4 0 1 2 Black-bellied Plover shorebird 0 0 0 0 0 20 0 0 0 Long-billed Curlew shorebird 0 20 0 0 0 0 0 0 0 Common Snipe shorebird 0 0 0 0 1 1 0 0 2 Marbled Godwit shorebird 0 10 0 0 0 7 0 0 0 Ruddy Turnstone shorebird 0 0 0 0 1 2 0 5 0 Baird’s Sandpiper shorebird 1 0 0 0 0 10 1 0 0 Dunlin shorebird 0 15 0 0 0 0 0 0 0 Semipalmated Plover shorebird 0 0 0 1 0 0 0 0 1 Stilt Sandpiper shorebird 0 0 0 0 0 8 0 3 0 Red-necked Phalarope shorebird 0 6 0 0 0 0 0 0 1 Semipalmated Sandpiper shorebird 1 4 2 0 0 0 0 0 0 Western Sandpiper shorebird 0 3 1 0 1 0 0 0 1 
73 

Table 6.12. 

Summary of platform count data grouped by season, year, and ecological group 

Species are listed in descending order of abundance within each ecological group. 

SPRING FALL 

1998 1999 2000 1998 1

Species Group Stop Fly Stop Fly Stop Fly Stop Fly StoLong-billed Dowitcher shorebird 0 0 0 0 0 0 0 0 0 American Avocet shorebird 0 0 0 0 0 0 0 0 0 Wilson’s Phalarope shorebird 0 1 0 0 0 1 0 0 0 Red Knot shorebird 0 0 0 0 0 0 0 1 0 Wilson’s Plover shorebird 0 0 0 0 1 0 0 0 0 unidentified warbler neotropical migrant (sm) 24 310 14 88 49 1,906 99 253 106 Magnolia Warbler neotropical migrant (sm) 79 18 82 9 325 1,309 79 5 581 Common Yellowthroat neotropical migrant (sm) 129 4 78 9 402 112 124 5 458 Mourning Dove neotropical migrant (lg) 35 17 3 2 72 45 85 14 701 Eastern Kingbird neotropical migrant (lg) 16 23 5 202 7 783 10 7 32 Bay-breasted Warbler neotropical migrant (sm) 784 57 7 4 151 130 0 0 60 unidentified Catharus thrush neotropical migrant (lg) 2 10 1 2 6 572 1 11 3 Gray Catbird neotropical migrant (lg) 236 21 43 11 248 306 34 2 64 Indigo Bunting neotropical migrant (sm) 68 13 12 5 90 359 8 17 322 Common Nighthawk neotropical migrant (lg) 5 30 0 85 4 627 1 56 4 Prothonotary Warbler neotropical migrant (sm) 24 5 34 9 14 19 215 27 196 American Redstart neotropical migrant (sm) 45 2 26 5 89 31 81 6 404 Ovenbird neotropical migrant (sm) 272 13 18 4 93 54 66 8 185 Yellow Warbler neotropical migrant (sm) 41 2 4 0 40 23 144 15 222 Hooded Warbler neotropical migrant (sm) 41 6 41 16 156 36 67 3 121 Black-throated Green Warbler neotropical migrant (sm) 24 0 5 1 48 15 21 1 371 
74 

Table 6.12. 

Summary of platform count data grouped by season, year, and ecological group 

Species are listed in descending order of abundance within each ecological group. 

SPRING FALL 

1998 1999 2000 1998 1

Species Group Stop Fly Stop Fly Stop Fly Stop Fly StoBlack-and-white Warbler neotropical migrant (sm) 84 5 16 5 161 95 39 6 112 Northern Parula neotropical migrant (sm) 41 3 39 5 63 13 60 0 199 Tennessee Warbler neotropical migrant (sm) 137 4 16 1 107 21 18 3 148 Dickcissel neotropical migrant (lg) 5 1 0 22 14 98 6 11 73 “Myrtle” Warbler neotropical migrant (sm) 24 1 31 7 24 8 38 6 145 Orchard Oriole neotropical migrant (lg) 41 3 32 10 99 63 2 0 37 Sora neotropical migrant (lg) 35 9 11 1 41 16 0 10 38 Yellow-billed Cuckoo neotropical migrant (lg) 70 35 20 11 53 38 5 2 16 Red-eyed Vireo neotropical migrant (sm) 99 3 13 0 35 39 16 8 37 Baltimore Oriole neotropical migrant (lg) 6 6 3 0 43 165 3 1 31 Yellow-breasted Chat neotropical migrant (lg) 2 1 2 0 14 5 17 0 172 Northern Waterthrush neotropical migrant (sm) 54 12 13 0 27 2 35 6 50 Savannah Sparrow neotropical migrant (sm) 9 0 0 1 59 6 7 0 101 Chestnut-sided Warbler neotropical migrant (sm) 50 2 9 1 33 20 9 0 61 Kentucky Warbler neotropical migrant (sm) 23 5 13 1 51 16 26 4 31 Blackburnian Warbler neotropical migrant (sm) 8 1 3 2 49 37 14 2 51 Scissor-tailed Flycatcher neotropical migrant (lg) 5 3 0 1 13 27 1 1 43 Rose-breasted Grosbeak neotropical migrant (lg) 44 4 2 0 6 41 0 1 14 Scarlet Tanager neotropical migrant (lg) 55 18 9 0 14 28 2 0 18 unidentified cuckoo neotropical migrant (lg) 0 0 0 22 0 108 0 0 0 Chuck-will’s-widow neotropical migrant (lg) 8 1 14 1 20 3 12 2 57 
75 

Table 6.12. 

Summary of platform count data grouped by season, year, and ecological group 

Species are listed in descending order of abundance within each ecological group. 

SPRING FALL 

1998 1999 2000 1998 1

Species Group Stop Fly Stop Fly Stop Fly Stop Fly StoBelted Kingfisher neotropical migrant (lg) 9 1 16 0 46 12 13 1 22 Eastern Wood-Pewee neotropical migrant (sm) 11 2 3 0 31 18 16 4 40 Purple Gallinule neotropical migrant (lg) 19 5 12 2 19 8 0 10 7 White-eyed Vireo neotropical migrant (sm) 10 0 6 0 20 2 5 5 42 Swainson’s Thrush neotropical migrant (lg) 23 5 1 1 3 12 3 3 26 Worm-eating Warbler neotropical migrant (sm) 9 1 9 3 17 1 10 3 25 Summer Tanager neotropical migrant (lg) 18 3 5 3 12 21 6 3 18 Least Flycatcher neotropical migrant (sm) 6 0 0 0 5 0 5 0 65 unidentified Empidonax flycatcher 

 neotropical migrant (sm) 6 0 0 0 2 6 19 1 28 

Yellow-throated Warbler neotropical migrant (sm) 2 1 8 0 22 2 9 1 27 Nashville Warbler neotropical migrant (sm) 6 0 1 0 12 0 2 0 48 Great Crested Flycatcher neotropical migrant (lg) 1 0 0 0 4 2 5 0 44 Least Bittern neotropical migrant (lg) 7 0 6 0 9 2 3 13 4 Cerulean Warbler neotropical migrant (sm) 3 0 1 0 8 3 7 0 16 Acadian Flycatcher neotropical migrant (sm) 5 2 2 0 11 0 11 2 23 Wood Thrush neotropical migrant (lg) 11 6 9 6 8 6 0 0 0 Louisiana Waterthrush neotropical migrant (sm) 6 0 5 0 33 5 2 0 5 Blue Grosbeak neotropical migrant (lg) 7 0 2 0 6 6 6 0 19 Wilson’s Warbler neotropical migrant (sm) 0 0 0 0 0 1 12 0 36 unidentified flycatcher neotropical migrant (sm) 0 0 0 0 1 34 0 1 3 
76 

Table 6.12. 

Summary of platform count data grouped by season, year, and ecological group 

Species are listed in descending order of abundance within each ecological group. 

SPRING FALL 

1998 1999 2000 1998 1

Species Group Stop Fly Stop Fly Stop Fly Stop Fly StoPied-billed Grebe neotropical migrant (lg) 0 0 0 0 0 6 0 1 9 “Traill’s” Flycatcher neotropical migrant (sm) 0 0 0 0 3 0 4 0 31 Blue-winged Warbler neotropical migrant (sm) 3 0 1 0 18 5 1 0 8 American Coot neotropical migrant (lg) 2 1 2 0 1 15 0 0 3 Mourning Warbler neotropical migrant (sm) 0 0 0 0 1 0 5 0 25 Yellow-throated Vireo neotropical migrant (sm) 1 2 1 0 15 2 1 1 7 Painted Bunting neotropical migrant (sm) 4 0 4 2 8 6 3 0 4 Gray-cheeked Thrush neotropical migrant (lg) 16 6 2 0 0 0 0 0 3 Swainson’s Warbler neotropical migrant (sm) 0 0 7 0 5 1 3 1 8 Blue-gray Gnatcatcher neotropical migrant (sm) 1 0 0 0 3 3 1 1 14 Canada Warbler neotropical migrant (sm) 0 0 0 0 2 0 4 0 18 Veery neotropical migrant (lg) 7 3 0 0 2 3 2 1 3 Black-billed Cuckoo neotropical migrant (lg) 2 2 0 1 3 5 0 0 2 Common Moorhen neotropical migrant (lg) 4 1 0 0 5 3 1 0 1 Yellow-bellied Sapsucker neotropical migrant (lg) 1 1 2 2 6 0 1 1 2 unidentified tanager neotropical migrant (lg) 0 0 0 0 1 10 0 0 0 Philadelphia Vireo neotropical migrant (sm) 0 0 0 0 5 3 3 0 2 Golden-winged Warbler neotropical migrant (sm) 0 0 0 0 7 3 1 1 1 Warbling Vireo neotropical migrant (sm) 0 0 0 0 4 0 0 0 6 Whip-Poor-Will neotropical migrant (lg) 3 0 0 0 0 0 2 0 3 unidentified oriole neotropical migrant (lg) 0 0 0 0 0 4 0 0 0 
77 

Table 6.12. 

Summary of platform count data grouped by season, year, and ecological group 

Species are listed in descending order of abundance within each ecological group. 

SPRING FALL 

1998 1999 2000 1998 1

Species Group Stop Fly Stop Fly Stop Fly Stop Fly StoYellow-bellied Flycatcher neotropical migrant (sm) 0 0 0 0 0 0 0 0 7 hybrid warbler neotropical migrant (sm) 0 0 0 0 1 0 0 0 1 Black Rail neotropical migrant (lg) 0 0 0 0 0 1 0 0 0 Alder Flycatcher neotropical migrant (sm) 0 0 0 0 1 0 0 0 0 Willow Flycatcher neotropical migrant (sm) 0 0 0 0 0 0 0 0 0 Brown-headed Cowbird medium-distance migrant 61 116 8 0 101 54 1 0 24 Lincoln’s Sparrow medium-distance migrant 2 0 0 0 86 5 7 0 210 House Wren medium-distance migrant 0 0 0 0 2 0 33 1 204 Marsh Wren medium-distance migrant 0 0 0 0 1 0 50 4 157 Grasshopper Sparrow medium-distance migrant 3 1 0 1 52 3 3 0 80 Clay-colored Sparrow medium-distance migrant 1 0 0 0 33 0 3 0 51 Orange-crowned Warbler medium-distance migrant 2 0 0 0 1 0 4 0 60 Northern Mockingbird medium-distance migrant 7 0 4 0 11 1 7 0 34 Chipping Sparrow medium-distance migrant 0 0 0 0 20 1 4 0 39 Swamp Sparrow medium-distance migrant 0 0 0 0 0 0 8 0 41 Eastern Phoebe medium-distance migrant 0 0 0 0 1 0 1 1 40 Western Meadowlark medium-distance migrant 0 0 0 0 2 0 2 0 36 Lark Sparrow medium-distance migrant 3 0 0 0 16 0 1 0 18 unidentified wren medium-distance migrant 0 0 0 0 1 0 0 0 3 Vesper Sparrow medium-distance migrant 3 1 0 0 5 4 1 0 9 Sedge Wren medium-distance migrant 0 0 0 0 0 0 4 0 17 
78 

Table 6.12. 

Summary of platform count data grouped by season, year, and ecological group 

Species are listed in descending order of abundance within each ecological group. 

SPRING FALL 

1998 1999 2000 1998 1

Species Group Stop Fly Stop Fly Stop Fly Stop Fly StoAmerican Robin medium-distance migrant 0 0 0 0 1 3 2 0 6 White-crowned Sparrow medium-distance migrant 0 0 0 0 0 0 1 0 10 “Yellow-shafted” Flicker medium-distance migrant 0 0 0 0 0 0 2 1 5 American Pipit medium-distance migrant 1 0 0 0 0 2 0 0 4 Blue-headed Vireo medium-distance migrant 0 0 0 0 4 0 1 0 5 Virginia Rail medium-distance migrant 0 0 0 0 1 0 0 0 8 Hermit Thrush medium-distance migrant 0 0 0 0 0 0 1 0 4 Loggerhead Shrike medium-distance migrant 0 0 0 0 0 1 0 0 7 Brewer’s Blackbird medium-distance migrant 1 0 0 0 0 1 0 0 3 Pine Siskin medium-distance migrant 0 0 0 0 0 0 1 0 0 American Goldfinch medium-distance migrant 0 0 0 0 0 0 0 0 0 Ruby-crowned Kinglet short-distance migrant 0 0 0 0 2 0 38 1 115 Brown Thrasher short-distance migrant 1 0 0 0 4 0 15 1 50 White-throated Sparrow short-distance migrant 1 0 0 0 1 0 2 0 25 Song Sparrow short-distance migrant 0 0 0 0 1 0 1 0 25 Golden-crowned Kinglet short-distance migrant 0 0 0 0 0 0 7 2 8 Winter Wren short-distance migrant 0 0 0 0 0 0 4 0 10 Pine Warbler short-distance migrant 0 0 0 0 0 0 0 0 13 Field Sparrow short-distance migrant 0 0 0 0 0 0 0 0 8 Clapper Rail short-distance migrant 0 0 0 0 0 1 0 0 6 “Slate-colored” Junco short-distance migrant 0 0 0 0 1 0 0 0 4 
79 

Table 6.12. 

Summary of platform count data grouped by season, year, and ecological group 

Species are listed in descending order of abundance within each ecological group. 

SPRING FALL 

1998 1999 2000 1998 1

Species Group Stop Fly Stop Fly Stop Fly Stop Fly StoCommon Grackle short-distance migrant 0 0 0 0 0 1 0 0 3 Leconte’s Sparrow short-distance migrant 0 0 0 0 0 0 0 0 2 Long-eared Owl short-distance migrant 0 0 0 0 0 0 0 0 4 Red-headed Woodpecker short-distance migrant 0 0 0 0 0 0 0 0 2 Eastern Towhee short-distance migrant 0 0 0 0 0 1 0 0 1 Short-eared Owl short-distance migrant 0 0 0 0 1 1 0 0 0 Downy Woodpecker short-distance migrant 0 0 0 0 0 0 0 0 2 King Rail short-distance migrant 0 0 0 0 0 0 0 0 2 Red-breasted Nuthatch short-distance migrant 0 0 0 0 0 0 0 0 2 Short-eared/Long-eared Owl short-distance migrant 0 0 0 0 0 0 0 0 0 “Yellow” Palm Warbler short-distance migrant 0 0 0 1 0 0 1 0 0 Carolina Wren short-distance migrant 0 0 0 0 1 0 0 0 0 American Tree Sparrow short-distance migrant 1 0 0 0 0 0 0 0 0 Boat-tailed Grackle short-distance migrant 0 0 0 0 0 0 0 0 1 Eurasian Starling short-distance migrant 0 0 0 0 1 0 0 0 0 Henslow’s Sparrow short-distance migrant 0 0 0 0 0 0 0 0 1 White-breasted Nuthatch short-distance migrant 0 0 0 0 0 0 0 0 1 Yellow Rail short-distance migrant 0 0 0 0 0 0 0 0 1 Blackpoll Warbler Caribbean vagrant 47 4 38 8 47 11 0 0 4 “Western” Palm Warbler Caribbean vagrant 13 0 8 0 9 0 41 1 67 Bobolink Caribbean vagrant 11 0 11 9 9 29 2 0 5 
80 

Table 6.12. 

Summary of platform count data grouped by season, year, and ecological group 

Species are listed in descending order of abundance within each ecological group. 

SPRING FALL 

1998 1999 2000 1998 1

Species Group Stop Fly Stop Fly Stop Fly Stop Fly StoPrairie Warbler Caribbean vagrant 6 0 2 0 4 1 12 2 25 Black-throated Blue Warbler Caribbean vagrant 2 0 1 1 2 0 7 2 19 Cape May Warbler Caribbean vagrant 3 1 1 0 3 2 1 0 5 Gray Kingbird Caribbean vagrant 1 0 1 0 1 0 2 1 0 Black Swift Caribbean vagrant 0 0 0 0 0 0 0 0 0 Black-whiskered Vireo Caribbean vagrant 1 0 0 0 1 0 0 0 0 “Caribbean” Cave Swallow Caribbean vagrant 0 0 1 0 0 0 0 0 0 Connecticut Warbler Caribbean vagrant 1 0 0 0 0 0 0 0 0 Cuban Pewee Caribbean vagrant 0 0 0 0 0 0 0 0 1 Loggerhead Kingbird Caribbean vagrant 0 0 0 0 0 0 0 0 1 Lesser Nighthawk western vagrant 1 4 2 0 10 28 1 1 1 Western Kingbird western vagrant 7 1 1 1 1 2 1 0 1 Yellow-headed Blackbird western vagrant 4 0 1 0 4 0 1 0 6 Pyrrhuloxia western vagrant 0 0 0 0 0 0 0 0 6 Burrowing Owl western vagrant 0 0 0 0 2 0 0 0 6 “Mexican” Cave Swallow western vagrant 0 0 0 0 0 0 1 0 0 Ash-throated Flycatcher western vagrant 2 0 0 0 2 0 0 0 1 “Audubon’s” Warbler western vagrant 0 0 0 0 2 0 0 0 3 Cassin’s Sparrow western vagrant 0 0 0 0 2 0 0 0 3 Tropical/Couch’s Kingbird western vagrant 0 1 1 0 0 0 0 0 2 Brown-crested Flycatcher western vagrant 2 0 0 0 1 0 0 0 0 
81 

Table 6.12. 

Summary of platform count data grouped by season, year, and ecological group 

Species are listed in descending order of abundance within each ecological group. 

SPRING FALL 

1998 1999 2000 1998 1

Species Group Stop Fly Stop Fly Stop Fly Stop Fly StoBronzed Cowbird western vagrant 0 1 0 0 0 0 0 0 2 Couch’s Kingbird western vagrant 1 0 0 0 1 0 0 0 1 Groove-billed Ani western vagrant 0 0 0 0 0 0 0 0 3 Green-tailed Towhee western vagrant 0 0 0 0 0 0 0 0 3 Long-billed Thrasher western vagrant 0 0 0 0 0 0 0 0 3 MacGillivray’s Warbler western vagrant 0 0 0 0 0 0 0 0 3 Western Tanager western vagrant 2 0 0 0 0 0 0 1 0 Bell’s Vireo western vagrant 0 0 0 0 0 0 0 0 2 Bullock’s Oriole western vagrant 0 0 0 0 0 0 0 0 2 Flammulated Owl western vagrant 0 0 0 0 0 0 0 0 2 “Gray-headed” Junco western vagrant 0 0 0 0 0 0 0 0 2 Townsend’s Warbler western vagrant 1 0 0 0 0 0 0 0 1 Tropical Kingbird western vagrant 1 0 1 0 0 0 0 0 0 Vermilion Flycatcher western vagrant 0 0 0 0 0 0 0 0 2 Western Wood-Pewee western vagrant 0 0 0 0 0 0 0 0 2 Band-tailed Pigeon western vagrant 0 0 0 0 1 0 0 0 0 Black-chinned Hummingbird western vagrant 0 0 0 0 0 0 0 0 1 Black-throated Gray Warbler western vagrant 0 0 1 0 0 0 0 0 0 Chestnut-collared Longspur western vagrant 0 0 0 0 0 0 0 0 1 Hermit Warbler western vagrant 0 0 1 0 0 0 0 0 0 Lazuli Bunting western vagrant 0 0 0 0 1 0 0 0 0 
82 

Table 6.12. 

Summary of platform count data grouped by season, year, and ecological group 

Species are listed in descending order of abundance within each ecological group. 

SPRING FALL 

1998 1999 2000 1998 1

Species Group Stop Fly Stop Fly Stop Fly Stop Fly StoLesser Goldfinch western vagrant 0 0 0 0 0 0 0 0 1 “Pink-sided” Junco western vagrant 0 0 0 0 0 0 0 0 1 Ringed Kingfisher western vagrant 0 0 0 0 0 0 0 0 1 Rufous Hummingbird western vagrant 0 0 0 0 0 0 0 0 1 Say’s Phoebe western vagrant 0 0 0 0 0 0 0 0 1 Sage Thrasher western vagrant 0 0 0 0 0 0 0 0 1 Sprague’s Pipit western vagrant 0 0 0 0 0 0 0 0 1 Spotted Towhee western vagrant 0 0 0 0 0 0 0 0 1 Varied Bunting western vagrant 0 0 0 0 0 1 0 0 0 Violet-green Swallow western vagrant 0 0 0 0 0 1 0 0 0 Barn Swallow aerial insectivore 540 313 308 211 265 759 63 301 141 Purple Martin aerial insectivore 138 72 12 110 277 152 12 59 25 Tree Swallow aerial insectivore 37 38 19 47 95 622 0 6 1 unidentified swallow aerial insectivore 1 21 3 121 5 99 0 32 4 Bank Swallow aerial insectivore 32 37 19 11 12 189 5 1 21 Chimney Swift aerial insectivore 20 95 1 67 11 160 0 11 1 Cliff Swallow aerial insectivore 31 16 6 14 19 54 1 9 46 Northern Rough-winged Swallow aerial insectivore 4 3 8 16 13 7 3 5 5 Ruby-throated Hummingbird 80 14 53 15 57 61 24 11 93 White-winged Dove miscellaneous 65 22 30 1 106 20 52 2 222 Common Ground-Dove miscellaneous 0 0 0 0 0 0 0 0 27 
83 

Table 6.12. 

Summary of platform count data grouped by season, year, and ecological group 

Species are listed in descending order of abundance within each ecological group. 

SPRING FALL 

1998 1999 2000 1998 1

Species Group Stop Fly Stop Fly Stop Fly Stop Fly StoEastern Meadowlark miscellaneous 1 0 0 0 3 2 1 0 22 Red-winged Blackbird miscellaneous 2 0 0 0 1 0 0 0 6 Eurasian Collared-Dove miscellaneous 0 0 0 0 0 0 0 0 8 Rock Dove miscellaneous 2 1 1 0 1 0 0 0 1 Barn Owl miscellaneous 0 0 0 0 0 0 0 0 5 Inca Dove miscellaneous 0 0 0 0 0 1 0 0 2 Cedar Waxwing miscellaneous 0 0 0 0 0 0 1 0 0 Great-tailed Grackle miscellaneous 0 0 0 0 0 0 0 0 1 unidentified passerine unidentified 71 1,304 37 5,589 70 17,358 21 337 137 unidentified bird unidentified 0 8 5 579 1 597 0 72 10 unidentified sparrow unidentified 0 0 0 0 7 57 2 1 16 unidentified meadowlark unidentified 0 0 0 0 4 2 1 1 60 unidentified dove unidentified 0 0 0 5 1 13 0 0 3 unidentified nighthawk unidentified 0 0 0 0 0 5 0 1 0 unidentified hummingbird unidentified 0 0 0 0 0 0 0 3 0 Great-tailed/Boat-tailed Grackle unidentified 0 0 0 0 0 0 0 0 1 
84 

Table 6.13. 

 

Number of species and recognizable forms recorded in each season in relation to status (stopover versus 

flyby) and ecological group 

 SPRING FALL 

 1998 1999 2000 1998 1999 

Group Stop Fly Stop Fly Stop Fly Stop Fly Stop Fly TOTAL 

Waterfowl 0 4 0 4 0 11 1 6 2 17 21 

Herons 11 11 10 12 10 13 6 11 9 13 15 

Diurnal Raptors 4 6 4 6 6 6 5 6 5 7 11 

Shorebirds 6 20 5 10 9 24 1 12 9 18 33 

Neotropical Migrants 62 48 52 35 67 63 61 44 71 62 74 

Medium-distance Migrants 10 3 2 1 16 10 21 4 24 21 26 

Short-distance Migrants 3 0 0 1 8 4 7 3 22 9 27 

Caribbean Vagrants 9 2 8 3 8 4 6 4 8 5 13 

Western Vagrants 9 4 6 1 11 4 4 2 31 4 41 

Aerial Insectivores 7 7 7 7 7 7 5 7 7 7 7 

Ruby-throated Hummingbird 1 1 1 1 1 1 1 1 1 1 1 

Miscellaneous 4 2 2 1 4 3 3 1 9 4 10 

Unidentified 0 0 0 0 0 0 0 1 0 0 0 

TOTAL 126 108 97 82 147 150 121 102 198 168 279 
85 

Chapter 7 

 

Spring Migration Routes Across the Gulf of Mexico 

Robert W. Russell 

7.1 INTRODUCTION

Little progress has been made in quantifying the geography of spring trans-Gulf migration since Cooke 

(1905) first postulated a spring trans-Gulf route from Mexico to northeastern Texas. 

We used five methods to deduce spring migration routes across the Gulf of Mexico. First, we inspected 

daily time series of radar images from the ten NEXRAD sites around the northern Gulf. Visual inspection 

of base reflectivity images permitted us to determine directly where landfall occurred (i.e., where the 

trans-Gulf route terminated), and inspection of radial velocity images permitted us to determine the 

direction from which the migrants were arriving. 

Second, we examined the relationship between synoptic weather patterns and the location of radarobserved landfall along the northern Gulf Coast. Because migrating birds select favorable winds for 

departing on long-distance flights, the form of the weather-landfall relationship could potentially provide 

information on the departure points of migrants as well as their destinations. 

Third, we analyzed the orientation of migrants observed in direct flight over the Gulf. These observations 

permitted us to determine migratory flight directions outside of the range of the land-based radars. 

Fourth, we looked for east-west gradients in the abundance of migrants stopping over on platforms. Our 

expectation was that, all else being equal, there should be a positive correlation between the abundance of 

migrants in the source population aloft and the numbers of migrants dropping out to use platforms below 

and, therefore, that relative abundance on platforms should delineate migratory corridors (Ralph 1981). 

Fifth, we looked for species-specific patterns of abundance across the Gulf, and attempted to determine 

whether these patterns were invariant with respect to synoptic weather. 

At the conclusion of the chapter, we document extraordinarily male-biased sex ratios in a variety of transGulf migrants and examine the possibility of sexual differences in overwater migration routes. 

7.2 RADAR OBSERVATIONS OF LANDFALL ON THE NORTHERN GULF COAST

Visual inspection of spring radar images suggested a number of robust patterns, which are described 

below with graphical examples and quantified more rigorously in subsequent sections. 

The first trans-Gulf migrants in the study area were usually detected off Brownsville around 10:00-12:00 

Z (e.g., Figure 7.1), with large volumes of offshore migration traffic subsequently occurring between 

southern Texas and southwest Louisiana, and peak instantaneous densities of migrants usually occurring 

in the airspace off southern and/or central Texas (e.g., Figure 7.2). The typical pattern over the course of 

the day was for peak instantaneous densities to diminish while shifting eastward toward southwest 

Louisiana. Figure 7.3 shows an example from April 15, 2000. By 15:00 Z, dense migration traffic (12-20 

dBZe) was aloft over waters offshore from central and southern Texas, but migrants were just beginning 

to enter the airspace off the Upper Texas Coast and were entirely absent off Louisiana. Four hours later, 
86 

migration traffic was still heaviest off central Texas, where it was nonetheless reduced considerably 

in magnitude (to ~4-8 dBZe, implying roughly an order of magnitude decline), but migration traffic 

off the Upper Texas Coast had increased to ~0-4 dBZe. Another four hours later, migration traffic off 

central Texas had further diminished to ~0-4 dBZe, traffic was heaviest off the Upper Texas Coast where 

it had increased to ~4-8 dBZe, and light migration traffic (~0-4 dBZe) was evident off southwest 

Louisiana. 

Base velocity images generally indicated that the direction of travel was due north off southern and 

central Texas, and to the northeast off the Upper Texas Coast (e.g., Figures 7.4, 7.5). These NEXRAD 

velocity data, together with the pattern of progressively later arrival clockwise around the northern Gulf 

Coast (e.g., Figure 7.3), suggest that the migration pathway of many trans-Gulf migrants is curvilinear, 

veering eastward as the northern Gulf Coast is approached. The discrepancy between peak instantaneous 

densities of migrants off the southern and central Texas coast (higher) versus off the Upper Texas Coast 

and southwest Louisiana (lower) suggests that there is a divergence of flight directions within the main 

migration stream as the northern Gulf Coast is approached. In other words, some proportion of the 

migrants begins heading inland along the central Texas coast (typically in Calhoun County, where the 

orientation of the Texas coastline changes from N-S to SW-NE). This scenario would result in a pattern 

of declining densities of migrants clockwise around the northern Gulf Coast, which is consistent with 

observations. 

The departure points of trans-Gulf migrants can be estimated using time series of radar reflectivity and 

velocity images. For example, the base velocity images for April 15 corresponding to Figure 7.2 

indicated that the migrants were traveling at ground speeds of 15-25 m · s-1. Nocturnal migrants generally 

initiate migratory flight around a half hour after sunset, which was at about 19:00 CDT along the southern 

Gulf coast on April 14. Thus, the fastest migrants (~25 m · s-1) departing at 19:30 CDT on April 14 had 

been flying for about 11 hours when they arrived off Brownsville on April 15 (Figure 7.1), and had thus 

covered about 1000 km. These calculations suggest that the migrants could have departed from the 

southern Gulf coast between the Isthmus of Tehuantepec and the northern Yucatan Peninsula. Analogous 

calculations on other days yielded similar conclusions. 

Large radar-observed migration events were progressively less common to the east of southwest 

Louisiana, and were rare as far east as the Florida Panhandle. Significant events along the northeastern 

Gulf Coast were confined to BH days when winds had a westerly component over the northern Gulf (e.g., 

Figure 7.6) and to GF days when trans-Gulf migrants were displaced eastward by an advancing cold front 

(e.g., Figure 7.7). 
87 

11:30 Z

(06:30 CDT)

11:40 Z

(06:40 CDT)

11:50 Z

(06:50 CDT)

12:00 Z

(07:00 CDT)

12:10 Z

(07:10 CDT)

12:20 Z

(07:20 CDT)

Figure 7.1. Time series of NEXRAD base reflectivity images showing first 

arrival of trans-Gulf migrants off Brownsville on April 10, 2000. 

Synoptic weather for this flight was classified as GH. All radar 

scans were obtained in clear-air mode at an elevation of 0.5E. 

Images are courtesy of TAP Publishing Company. 
88 

Base Reflectivity (dBZe)

15 April 2000

~15Z (10:00 CDT)

Ba

~1

Figure 7.2. Composite NEXRAD images showing base reflectivity along the Gulf Coast on the mornings of April 15 and 16, 200

was classified as GH for April 15 and as ECH for April 16. All radar scans were obtained in clear-air mode at an elevare courtesy of TAP Publishing Company. 
89 

~15Z (10:00 CDT)

~19Z (14:00 CDT)

~23Z (18:00 CDT)

Figure 7.3. Time series of NEXRAD base reflectivity composites 

on April 15, 2000. All radar scans were obtained in 

clear-air mode at an elevation of 0.5E. Images are 

courtesy of TAP Publishing Company. 
90 

A

B

C

Figure 7.4. NEXRAD base velocity images corresponding to the 

reflectivity images for April 15 in Figure 7.2. A. 

Houston NEXRAD (HGX). B. Corpus Christi 

NEXRAD (CRP). C. Brownsville NEXRAD (BRO). 

Yellows and browns indicates movement away from 

the radar site and greens and blues indicate movement 

toward the radar. Radar scans were obtained in clearair mode at an elevation of 0.5E. Images are courtesy 

of TAP Publishing Company. 
91 

Figure 7.5. NEXRAD images from the Houston site (HGX) showing the arrival of migrating birds on the Upper Texas Coast from t

CDT) on April 21, 1999. Synoptic weather for this flight was classified as GH. Left: Reflectivity image showing the di

“half-donut” shape of the radar echoes is due to the fact that the radar beam scans at a slight upward angle (0.5E) and thu

high-altitude migrants at some distance from the radar site. Right: Corresponding velocity image showing direction of mmovement away from the radar site and green indicates movement toward the radar. The tube-like features offshore ma

Air Force meteorological experiments. Radar scans were obtained in precipitation mode at an elevation of 0.5E. Images Corporation. 
92 

Figure 7.6. Composite NEXRAD image showing base reflectivity along the Gulf Coast on the morning of March 29, 2000, at 16:0

Synoptic weather for this flight was classified as BH. All radar scans were obtained in precipitation mode at an elevati

are courtesy of TAP Publishing Company. 
93 

A

B

C D

Figure 7.7. Eastward displacement of trans-Gulf migration by a cold front on April 24, 2000. A. 

Synoptic weather map showing surface wind vectors and position of cold front at 

20:00 Z. Image is courtesy of Unisys Corporation. B. Composite NEXRAD image 

(MOB and EVX) showing base reflectivity along the northeastern Gulf Coast at 19:30 

Z (14:30 CDT). C. NEXRAD image from Lake Charles (LCH) showing the absence 

of reflectivity off Louisiana at 19:30 Z (14:30 CDT). D. NEXRAD base velocity 

image from Red Bay (EVX) corresponding to the reflectivity image in B. Darker 

blues indicate faster movement toward the radar. Radar scans were obtained in 

precipitation mode (B, D) and clear-air mode (C) at an elevation of 0.5°. All radar 

images are courtesy of TAP Publishing Company. 
94 

When winds across the northwestern Gulf were unfavorable for trans-Gulf migrants — but not so 

unfavorable as to induce a large-scale displacement of the migration stream (e.g., Figure 7.7), the 

usual pattern of clockwise travel was absent. Most of the migrants that did undertake the crossing on 

such days apparently opted for a more direct route toward the Texas coast, where they could be seen in 

radar images arriving from the SSE (e.g., Figure 7.8) instead of continuing on toward the NNE or NE 

(Figures 7.4, 7.5). 

On most days, radar imagery clearly showed that most or all trans-Gulf migrants proceeded variable 

distances inland upon landfall along the northern Gulf Coast. However, on some occasions, large 

numbers of migrants “fell out” along the immediate coastline. These fallouts yielded distinctive radar 

signatures (e.g., Figure 7.9) and corresponded to reports of excellent birdwatching along the coast. 

Although not a focus of this study, another important phenomenon noted regularly in radar imagery was 

the nightly migration of birds from Cuba to the Florida Keys and the south Florida peninsula. On some 

occasions, this Cuban flight “leaked” into the Gulf of Mexico (e.g., Figure 7.10) and could have been the 

source for some of the “Caribbean vagrants” recorded on the platforms (Table 6.8, Chapter 11). 
95 

Figure 7.8. NEXRAD images from the Houston site (HGX) showing the arrival of migrating birds on the Upper Texas Coast from 

on May 14, 2000 (20:21 CDT on May 13). Synoptic weather for this flight was classified as GF. Left: Base reflectivi

distribution of birds. Right: Corresponding base velocity image showing direction of movement. Yellows and oranges away from the radar site and greens indicate movement toward the radar. Radar scans were obtained in precipitation mof 0.5E. Images are courtesy of TAP Publishing Company.
96 

A

B

Figure 7.9. Examples of radar-observed evening fallouts of trans-Gulf 

migrants along the Texas coast. A. Base reflectivity image from 

Houston NEXRAD (HGX) for March 26, 2000, at 00:51 Z (March 

25 at 18:53 CST). Synoptic weather was classified as ECH for 

March 25. B. Base reflectivity image from Corpus Christi 

NEXRAD (CRP) for April 9, 2000, at 00:44 Z (April 8 at 19:47 

CDT). Synoptic weather was classified as GF for April 8. Both 

radar scans were obtained in clear-air mode at an elevation of 0.5E. 

Images are courtesy of TAP Publishing Company. 
97 

Figure 7.10. Example of radar-observed nocturnal fallout along the Florida Keys of trans-Caribbean 

migrants from Cuba, and “leakage” of some Cuban migrants into the Gulf of Mexico. Base 

reflectivity image is from Key West NEXRAD (BYX) on April 18, 2000, at 08:23 Z (04:23 

EDT). Radar scan was obtained in clear-air mode at an elevation of 0.5E. Image is courtesy 

of TAP Publishing Company. 
98 

7.3 RELATIONSHIP BETWEEN SYNOPTIC WEATHER AND THE LOCATION OF 

LANDFALL

Inspection of daily radar images suggested that the portion of the northern Gulf Coast on which migrants 

made landfall varied dramatically from day to day (Chapter 7.2). To test the hypothesis that this variation 

was caused by synoptic weather, we tabulated the frequency of occurrence of landfall along different 

portions of the coast under different synoptic conditions (Table 7.1). This tabulation suggested that area 

of landfall was significantly related to synoptic weather pattern (with the two eastern geographic bins 

aggregated to avoid sparse cells: G = 11.0, df = 4, P = 0.027). Under synoptic weather conditions when 

winds over the Gulf typically had a strong westward component (ECH, MCH) or were weak (GH), transGulf migrants usually made landfall along the western Gulf Coast. Under synoptic weather conditions 

when winds over the southern Gulf typically had a weaker westward component and winds over the 

northern Gulf often had an eastward component (BH), landfall was more frequent along the Upper Texas 

Coast and from Louisiana to the Florida Panhandle. On days when a cold front intruded over the Gulf 

(GF), radar-observed landfall tended to occur along the extreme portions of the study area: south Texas, 

southeastern Louisiana to Florida, or both. 

There are several possible explanations for these patterns. First, the day-to-day shifts may be a function 

of the entire population of trans-Gulf migrants being “steered” by synoptic-scale winds. Second, the 

shifts may be caused by different species (or different individuals with a species) having different 

preferred flight routes, with each species (or individual) departing selectively on days when winds are 

more favorable with respect to its preferred route. These alternative causal explanations are examined 

further in Chapter 7.6. 

Table 7.1. 

 

Spatial patterns of spring trans-Gulf migration in relation to synoptic weather 

The “stretch of coast with largest landfall” was defined as the region with the highest 

sustained radar-observed density of migrants (measured by dBZe) over the course of the day. 

The “Lower Texas Coast-Central Texas Coast” stretch was defined as the region covered by 

the Brownsville (BRO) and Corpus Christi (CRP) NEXRAD sites, north to Matagorda 

County, Texas. The “Upper Texas Coast-Southwestern Louisiana” stretch was defined as the 

region covered by the Houston (HGX) and Lake Charles (LCH) NEXRAD sites, from 

Brazoria County, Texas, east to Vermilion Parish, Louisiana. The “Southeastern LouisianaFlorida Panhandle” stretch was defined as the region covered by the New Orleans (LIX), 

Mobile (MOB), and Eglin Air Force Base (EVX) NEXRAD sites, east of Marsh Island, 

Louisiana. “Bimodal” indicates that two disjunct peaks of migration traffic were evident in 

southern Texas and the eastern area, with radar-observed migration absent or reduced in the 

intervening area. All data are from 2000. 

 Stretch of Coast with Largest Landfall 

Synoptic 

Weather 

Type 

Lower Texas CoastCentral Texas Coast Upper Texas CoastSouthwestern Louisiana Southeastern LouisianaFlorida Panhandle 

MCH 5 0 0 

ECH 11 2 0 

GH 12 2 1 

BH 9 7 3 

GF 8 3 3 
99 

7.4 DIRECT OBSERVATIONS OF THE FLIGHT DIRECTIONS OF MIGRANTS

Flight directions of migrants detected visually from platforms varied among migrant categories, 

platforms, and weather types. 

Ducks generally flew to the NNW, with little apparent influence of platform location or weather type 

(Table 7.2). 

Flight behavior of herons was quite variable (Table 7.3), but they generally flew to the N, with little 

obvious influence of platform location or weather type. One exception to this generalization was that 

herons consistently flew to the west of due N on GF days in the western Gulf (i.e., at PN975, MI622, and 

HI561). 

Samples sizes were small for shorebirds, but available data indicated that they generally flew to the N or 

NNW (Table 7.4). The one exception, as with herons, was that shorebirds consistently flew to the NW on 

GF days in the far western Gulf. 

Aerial insectivores showed a clear pattern of more easterly flight directions farther east in the Gulf (Table 

7.5). At PN975, they generally flew toward the NNW except on BH days, when they flew to the N. At 

MI622, they flew to the N on ECH days, to the NNW on GF days, and to the E on BH days. In the 

central portion of the study area, they generally flew to the N or NNW regardless of weather type. In the 

far eastern portion of the study area, they tended to fly east of due north. 

Neotropical migrant landbirds also showed a clear pattern of more easterly flight directions farther east in 

the Gulf (Table 7.6). Vanishing bearings averaged NNW at PN975 and MI622, N in the central portion 

of the study area, and NNE toward the far east at SP89. Neotropical migrants also showed several clear 

patterns with respect to synoptic weather type. Flight directions were more eastward on BH days than on 

ECH days, except at PN975 and MI622 where the opposite pattern held. There was also an obvious 

pattern of more westerly flight directions (to the NW) in the western Gulf on GF days. 

Insufficient data were available for a detailed analysis of the influence of geography and weather on the 

orientation of medium-distance migrants. However, the pooled sample of medium-distance migrants 

from all platforms and all weather types had a mean vanishing bearing to the NW (0 = 320°; 95% CI = 

307°-333°; r = 0.79; Rayleigh P < 0.0001; N = 34). 

In general, most data are consistent with the hypothesis of a veering and diverging migration route across 

the Gulf for long-distance migrant landbirds (but not ducks, herons, or shorebirds), with flight directions 

more eastward at eastern platforms. The data are also consistent with the hypothesis that migration routes 

of landbirds (but not ducks, herons, or shorebirds) are flexible and subject to steering by synoptic-scale 

winds, with flight directions more eastward on BH days than on ECH days. 

Several anomalies stand out and invite explanation. All taxa showed evidence of more westerly flight 

directions (i.e., to NW) on GF days in the western Gulf. This pattern suggests that, upon encountering a 

cold front, some migrants abandon the curvilinear flight route and instead opt for a more immediate 

landfall (e.g., Figure 7.8). Oddly, at PN975 and MI622, vanishing bearings were more westerly on BH

days than on ECH days — despite the expectation that the main migration stream should occur farther 

east on BH days than on ECH days (Figure 7.6, Table 7.1). This anomaly suggests that on BH days, 

migrants occurring in the far western Gulf may be lower-quality individuals (i.e., on the periphery of the 

migration stream), which would be more likely to be fat-depleted and seeking immediate landfall. 
100 

Table 7.2. 

 

Vanishing bearings of spring migrant ducks in relation to synoptic weather and position in the Gulf 

All data are from spring 2000. The direction of movement is given by the direction of the mean vector for each 

sample (0) along with a 95% confidence interval for 0. The length of the mean vector (r) is a measure of how 

tightly clustered the sample is around the mean direction. P indicates the significance level from a Rayleigh test 

(Ho = uniformity versus H1 = unimodality). Sample size is given by N, which represents the number of 

independent observations and not the total number of individuals involved (e.g., a flock of five migrants counts 

for only one observation, not five). 

 PN975 MI622 HI561 GB189 VR265 SM147 GC18 EW826 SP89 

ECH 0 345° na na na 356° na 319° na na 

95% CI 339°-352° na na na 351°-2° na 276°-1° na na 

r 0.94 na na na 0.99 na 0.84 na na 

P <0.0001 na na 0.20 <0.0001 na 0.020 na na 

N 31 0 0 3 12 0 5 0 1 

 

GH 0 338° na na na 354° na 302° na 345° 

95% CI na na na na 350°-358° na 285°-318° na 322°-8° 

 r 1.00 na na na 0.99 na 0.92 na 0.98 

 P 0.007 na 0.14 na <0.0001 na <0.0001 na 0.039 

 N 4 0 2 0 27 0 11 1 3 

 

BH 0 347° na 343° na 343° na 319° na 342° 

95% CI 343°-352° na 314°-12° na 335°-350° na 302°-337° na 281°-42°

r 0.98 na 0.95 na 0.95 na 0.91 na 0.79 

P <0.0001 na 0.015 na <0.0001 na <0.0001 na 0.074 

N 25 0 4 0 23 1 10 0 4 

 

GF 0 338° na na 353° 356° 359° 322° na 351° 

95% CI 336°-340° na na 330°-15° 351°-2° 328°-30° 299°-344° na 335°-6° 

r 0.99 na na 0.93 0.98 0.91 0.87 na 0.96 

P <0.0001 0.20 na 0.001 <0.0001 0.007 <0.0001 na <0.0001 

N 48 2 1 6 17 5 10 1 7 
101 

Table 7.3. 

Vanishing bearings of spring migrant herons in relation to synoptic weather and position in the Gulf 

All data are from spring 2000. See Table 7.2 for explanation of notation.  PN975 MI622 HI561 GB189 VR265 SM147 GC18 EWECH 0 347° 14° 39° 7° 1° 11° 29° 

95% CI 341°-354° 3°-25° 33°-45° 344°-29° 357°-5° 355°-26° 16°-41° 342

r 0.91 0.87 0.96 0.81 0.96 0.83 0.76 0

P <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 0.

N 56 27 31 13 63 20 43 

GH 0 353° na 354° 341° 2° 14° 354° 3

95% CI 346°-1° na 338°-10° 319°-3° 358°-6° 357°-31° 343°-4° 341°

r 0.98 na 0.93 0.85 0.97 0.92 0.97 0

P <0.0001 0.15 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.

N 10 2 10 11 59 10 10 

BH 0 1° 14° 4° 357° 3° 5° 352° 3

95% CI 351°-12° 2°-27° 354°-14° 335°-19° 356°-10° 342°-27° 330°-15° 34

r 0.86641 0.79856 0.93617 0.78351 0.93004 0.81096 0.79706 0.9

P <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.

N 34 37 17 15 39 13 14 

GF 0 330° 352° 345° 8° 356° 6° 356° 3

95% CI 324°-336° 343°-2° 337°-353° 4°-11° 354°-358° 354°-18° 352°-359° 348

r 0.89 0.86 0.79 0.86 0.95 0.77 0.93 0

P <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.

N 92 42 86 289 288 45 123 

MCH 0 354° 30° 16° 28° 357° 348° 23°  95% CI 344°-5° 20°-40° 7°-26° 8°-48° 346°-9° 332°-5° 332°-73°  r 0.96 0.90 0.95 0.85 0.91 0.94 0.71  P <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 0.042  N 14 29 15 13 18 8 6 
102 

Table 7.4. 

 

Vanishing bearings of spring migrant shorebirds in relation to 

synoptic weather and position in the Gulf 

All data are from spring 2000. See Table 7.2 for explanation of 

notation. 

 PN975 

& MI622 

HI561 

& GB189

VR265 

& SM147

GC18 

& EW826 

& SP89 

ECH 0 355° 0° na na 

95% CI 339°-11° 346°-14° na na 

r 0.80 0.96 na na 

P <0.0001 <0.0001 na na 

N 21 8 1 1 

 

GH 0 na 351° 349° 326° 

95% CI na 337°-5° 322°-16° 309°-342° 

r na 0.98 0.91 0.94 

P 0.14 0.002 0.002 <0.0001 

N 2 5 6 8 

 

BH 0 348° 342° 342° 7° 

95% CI 318°-17° 303°-21° 321°-3° 344°-31° 

r 0.58 0.68 0.96 0.98 

P 0.002 0.031 0.003 0.039 

N 17 7 5 3 

 

GF 0 324° 357° 351° 355° 

 95% CI 316°-332° 343°-10° 344°-358° 349°-1° 

 r 0.87 0.86 0.93 0.95 

 P <0.0001 <0.0001 <0.0001 <0.0001 

 N 54 22 38 34 
103 

Table 7.5. 

 

Vanishing bearings of spring migrant aerial insectivores in relation to synoptic weather and position in the Gulf 

All data are from spring 2000. See Table 7.2 for explanation of notation. 

 PN975 MI622 HI561 GB189 VR265 SM147 GC18 EW826 SP89 

ECH 0 330° 353° 18° na 353° 348° 8° na 13° 

95% CI 294°-6° 325°-22° 337°-58° na 343°-4° 334°-1° 332°-44° na 358°-28°

r 0.68 0.84 0.63 na 0.95 0.94 0.76 na 0.89 

P 0.019 0.001 0.035 0.15 <0.0001 <0.0001 0.006 na <0.0001

N 8 8 8 2 15 11 8 1 15 

 

GH 0 330° na 347° 7° 0° 346° 346° 5° 6° 

95% CI 283°-17° na 324°-9° 337°-37° na 313°-20° 330°-3° 327°-43° 311°-61°

r 0.93 na 0.95 0.88 1.00 0.75 0.94 0.91 0.74 

P 0.061 0.14 0.004 0.004 <0.0001 0.003 <0.0001 0.024 0.056 

N 3 2 5 6 11 9 8 4 5 

 

BH 0 353° 83° 0° 351° 353° na 14° 353° 9° 

95% CI 343°-3° 56°-111° na 306°-37° 341°-6° na 351°-38° 307°-38° 347°-32°

r 0.91 0.53 1.00 0.70 0.95 1.00 0.87 0.88 0.95 

P <0.0001 <0.001 0.001 0.026 <0.0001 0.14 <0.001 0.034 0.004 

N 24 25 5 7 10 2 9 4 5 

 

GF 0 342° 344° 340° 356° 0° 358° 354° 349° 10° 

 95% CI 328°-357° 305°-24° 325°-356° 345°-7° 348°-12° 331°-24° 350°-358° 331°-6° 353°-27°

 r 0.86 0.64 0.94 0.87 0.88 0.81 0.99 0.98 0.91 

 P <0.0001 0.031 <0.0001 <0.0001 <0.0001 <0.001 <0.0001 0.010 <0.0001

 N 17 8 9 28 21 10 23 4 11 
104 

Table 7.6. 

Vanishing bearings of spring neotropical migrants in relation to synoptic weather and position in the Gulf 

All data are from spring 2000. See Table 7.2 for explanation of notation.  PN975 MI622 HI561 GB189 VR265 SM147 GC18 ECH 0 342° 354° 315° 344° 350° na 338° 

95% CI 334°-350° 346°-3° 300°-331° 341°-347° 335°-4° na 334°-342° 

r 0.95 0.88 0.75 0.98 0.92 0.59 0.97 

P <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 0.18 <0.0001 

N 21 44 29 47 13 5 46 

GH 0 324° 344° 358° 345° 354° 4° 352° 

95% CI 290°-357° 303°-25° 354°-2° 339°-352° 343°-5° 351°-18° 348°-356° 

r 0.86 0.79 0.99 0.94 0.90 0.86 0.98 

P 0.00564 0.01575 0 0 0 0 0 

N 6 6 21 37 23 21 38 

BH 0 334° 337° 354° 351° 352° na 0° 

95% CI 330°-339° 331°-343° 350°-358° 339°-4° 348°-356° na 355°-5° 

r 0.89 0.86 0.97 0.76 0.94 0.61 0.93 

P <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 0.100 <0.0001 

N 129 106 37 43 86 6 90 

GF 0 315° 304° 329° 334° 346° 12° 356°  95% CI 303°-328° 295°-314° 317°-342° 328°-340° 339°-352° 3°-22° 355°-357°  r 0.80 0.79 0.76 0.80 0.86 0.70 0.97  P <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001  N 37 58 45 141 95 92 454 
105 

7.5 EFFECTS OF SYNOPTIC WEATHER ON PATTERNS OF ABUNDANCE ON 

PLATFORMS

Patterns of abundance across the platforms were compared among weather types. In spring 1998, 

migrants were most abundant on the westernmost platform studied (GB189) under all weather conditions 

(Figure 7.11). Patterns of abundance were less consistent in spring 1999, but migrants were still most 

abundant at GB189 under all weather conditions favorable for trans-Gulf migration, i.e., BH, ECH, and 

GH (Figure 7.12). 

In spring 2000, when we operated on an extended array of platforms across the Gulf, patterns of 

abundance were clearly related to weather type (Figure 7.13). On ECH days, migrants were most 

abundant in the far western Gulf, i.e., on HI561 and PN975. However, on BH days, peak abundance 

occurred on GB189. This apparent eastward shift in the center of abundance on BH days (or westward 

shift on ECH days) is consistent with inferences from radar observations (Chapters 7.2, 7.3) as well as 

from direct observations of orientation (Chapter 7.4). All of this evidence, considered together, indicates 

that the main migration stream is at least partially “steered” by synoptic-scale winds. 

Abundance was rather uniform across platforms on GH days, which is not surprising in light of the weak 

winds and lack of steering influence on such days. On GF days, abundance was greatest in the far 

western Gulf, especially at PN975. This pattern is consistent with the generally greater abundance of 

migrants over the western Gulf, together with the radar evidence and direct visual observations indicating 

that migrants turn toward the Texas coast when encountering a cold front (Figures 7.8, 7.9; Table 7.6). 

Medium-distance migrants (Table 6.6) were most abundant in the far western Gulf on PN975 and MI622 

under all weather conditions (Figure 7.14). Importantly, these species were fairly common on ECH days, 

i.e., under weather conditions favorable for trans-Gulf movement and inconsistent with offshore drift. 

This fact, together with the consistently NW vanishing bearings of medium-distance migrants (Chapter 

7.4), suggests that some of these species may undertake an overwater route over the far western Gulf from 

the southernmost portions of their wintering range in Veracruz to the south Texas coast. However, it is 

not possible to disprove alternatives to the hypothesis that they are “intentional” overwater migrants and, 

indeed, it seems likely that at least some of the medium-distance migrants found themselves over water 

after drifting offshore during nocturnal circum-Gulf flights around the western Gulf Coast, and were 

simply attempting to regain land in the direction of the normal migration route. 
106 

BH ECH

GH GF

ELOW & MCH

184

38 43

43

117

125

19 35

28

84

37

15 9

14

19

48

23 21

21

30

2079

162 140

130

164

Figure 7.11. Patterns of abundance of long-distance migrant landbirds (aerial insectivores and 

neotropical migrants) on platforms in spring 1998 in relation to synoptic weather type. 

Numbers shown for a given weather type indicate total numbers of individuals 

initiating stopovers on each platform across all days characterized by that weather type. 
107 

BH ECH

GH GF

ELOW & MCH

30

8

12

14

29

143

31

22

55

51

37

14

20

21

36

48

37

37

100

90

29

19

13

53

43

Figure 7.12. Patterns of abundance of long-distance migrant landbirds (aerial insectivores and neotropical 

migrants) on platforms in spring 1999 in relation to synoptic weather type. Numbers shown 

for a given weather type indicate total numbers of individuals initiating stopovers on each 

platform across all days characterized by that weather type. 
108 

BH ECH

GH GF

ELOW & MCH

149

159 42 247

34

24

40

38

23

146

99 189

116

26

14

25

27

9

594

241 174 212

191

134

160

89

65

9

32 11 9

33

10

25 8

12

33

47 30 23

19

27

34

28

8

Figure 7.13. Patterns of abundance of long-distance migrant landbirds (aerial insectivores and 

neotropical migrants) on platforms in spring 2000 in relation to synoptic weather type. 

Numbers shown for a given weather type indicate total numbers of individuals initiating 

stopovers on each platform across all days characterized by that weather type. 
109 

BH ECH

GH GF

ELOW & MCH

16

43 0 0

1

0

0

1

0

13

32 5 1

0

0

0

0

0

69

59 7 9

1

2

2

3

0

3

3 0 0

0

0

0

0

0

3

7 0 0

1

0

0

0

0

Figure 7.14. Patterns of abundance of medium-distance migrants on platforms in spring 2000 in relation to 

synoptic weather type. Numbers shown for a given weather type indicate total numbers of 

individuals initiating stopovers on each platform across all days characterized by that weather 

type. 
110 

7.6 INTERSPECIFIC VARIATION IN MIGRATION ROUTES

There were dramatic differences among species in patterns of abundance across the Gulf (Table 7.7), 

implicating species-specific migration routes. The frequency distribution of mean species-specific 

longitudinal positions is shown in Figure 7.15. Of special note is the fact that six of the eight species with 

the most western distributions were medium-distance migrants. Herons, falcons, and certain larger 

Neotropical migrants (Sora, Purple Gallinule, Chuck-Will’s-Widow, and Belted Kingfisher) had more 

easterly distributions, implying a more direct route across the Gulf, which would be consistent with their 

larger body sizes (and concomitantly faster flight and reduced reliance on wind assistance). Otherwise, 

the distribution of mean species-specific positions for Neotropical migrants is tightly clustered over the 

western Gulf, peaking around the longitude of HI561. Only two species had mean positions east of 

SM147, with none as far east as GC18. 

An important question is whether day-to-day shifts in the position of the migration stream (and resulting 

location of landfall; Chapters 7.2, 7.3) in response to varying synoptic weather are the result of individual 

migrants employing flexible tactics or differential wind selection by different populations of migrants. 

In the first case (“flexible tactics”), an individual’s route and destination along the northern Gulf Coast 

would be determined in part by the synoptic weather scenario prevailing at the time when the individual 

completed refueling at the point of origin and became ready to depart. As a result, the composition and 

overall size of the migration stream would be expected to be similar or identical under different synoptic 

weather scenarios, but the abundance of migrants at a given location would vary as a result of the varying 

location of the migration stream. 

In the second case, individuals or species with differences in preferred route or point of landfall would 

await synoptic conditions appropriate to their preferences, and the composition of the migration stream 

would consequently vary dramatically under different weather scenarios. This potential phenomenon was 

first formally proposed by Evans (1966) and Nisbet and Drury (1967), and was later dubbed “pseudodrift” 

by Alerstam (1978). 

One way to distinguish between these two possibilities (“flexible tactics” versus “pseudodrift”) is to 

compare a species’ distribution and abundance between different synoptic weather scenarios. In the case 

of flexible tactics, species-specific geographic patterns should vary in response to varying steering winds, 

but the overall abundance of the species should not. The opposite predictions can be made for 

pseudodrift: species-specific geographic patterns should NOT vary in response to steering winds, but 

overall abundance should. 

We tested these predictions using data from ECH and BH days in spring 2000 for all species for which 

adequate data were available. The results are shown in Table 7.8. All species occurred farther to the east 

on BH days than on ECH days, as expected under the “flexible tactics” hypothesis. Single-species tests 

were significant only for Black-and-white Warblers and Hooded Warblers, but the fact that all seven 

comparisons yielded results in accordance with the predicted direction under the “flexible tactics” 

hypothesis was significant in itself (sign test, P = 0.008). 

Proportionately more individuals of all species were recorded throughout the study area on ECH days 

than on BH days, and the difference was significant for all species except Common Yellowthroats. This 

suggests that these species preferred to depart on ECH days (when steering winds are more likely to 

result in landfall on the Texas coast than farther east), and is consistent with the hypothesis of differential 

wind selection or “pseudodrift.” 
111 

The results are thus partly consistent with hypotheses of both flexible tactics and differential wind 

selection, and suggest that a blend of both phenomena may account for the observed patterns of 

spatiotemporal dynamics and response to wind. Further evaluation of the behavioral mechanisms 

underlying the observed patterns would require more focused study. 

Table 7.7. 

 

Comparative longitudinal distributions of spring trans-Gulf migrants 

Numbers shown indicate numbers of individuals undertaking stopovers on each platform during spring 2000. 

Species are listed in order from most westerly distribution at top to most easterly distribution at bottom. Species 

with <5 stopovers are not included 

 PN975 MI622 HI561 GB189 VR265 SM147 GC18 EW826 SP89 Total 

Chipping Sparrow 13 7 0 0 0 0 0 0 0 20 

Clay-colored Sparrow 15 16 1 1 0 0 0 0 0 33 

Blue-winged Warbler 10 7 0 0 1 0 0 0 0 18 

Brown-headed Cowbird 54 40 2 3 0 0 0 2 0 101 

Grasshopper Sparrow 11 37 1 1 1 0 0 1 0 52 

Yellow-breasted Chat 1 12 0 1 0 0 0 0 0 14 

Lincoln's Sparrow 16 59 4 3 1 2 0 1 0 86 

Lark Sparrow 3 11 0 1 1 0 0 0 0 16 

Dickcissel 6 4 3 1 0 0 0 0 0 14 

Magnolia Warbler 214 23 33 17 10 13 6 6 3 325 

Tree Swallow 63 4 7 15 0 1 2 3 0 95 

Mourning Dove 21 37 0 8 0 1 2 2 1 72 

Northern Mockingbird 1 7 1 0 1 0 1 0 0 11 

Vesper Sparrow 2 1 0 2 0 0 0 0 0 5 

Savannah Sparrow 15 27 2 4 2 4 2 2 1 59 

Blue Grosbeak 1 3 0 1 1 0 0 0 0 6 

Common Yellowthroat 175 101 17 26 28 11 15 18 11 402 

Painted Bunting 0 5 1 0 2 0 0 0 0 8 

Blackburnian Warbler 27 1 7 5 0 5 1 0 3 49 

Nashville Warbler 5 1 1 3 2 0 0 0 0 12 

Baltimore Oriole 17 12 0 2 3 3 0 5 1 43 

Black-and-white Warbler 68 14 40 6 10 5 10 4 4 161 

Northern Waterthrush 8 5 6 3 2 0 2 1 0 27 

Black-throated Green Warbler 26 2 2 6 0 2 5 3 2 48 

Philadelphia Vireo 2 0 2 0 0 0 1 0 0 5 

Northern Parula 19 12 10 6 7 1 4 2 2 63 

Indigo Bunting 23 25 7 7 9 8 4 1 6 90 

White-winged Dove 29 27 6 16 2 5 18 1 2 106 

Worm-eating Warbler 5 2 4 1 2 1 1 1 0 17 

Hooded Warbler 36 36 26 13 10 10 13 6 6 156 

Yellow-throated Warbler 2 5 8 3 2 0 1 1 0 22 

Kentucky Warbler 16 9 5 4 4 5 3 3 2 51 

Eastern Wood-Pewee 3 11 2 6 4 3 2 0 0 31 

White-eyed Vireo 1 8 4 0 3 3 0 0 1 20 
112 

Table 7.7. 

 

Comparative longitudinal distributions of spring trans-Gulf migrants 

Numbers shown indicate numbers of individuals undertaking stopovers on each platform during spring 2000. 

Species are listed in order from most westerly distribution at top to most easterly distribution at bottom. Species 

with <5 stopovers are not included 

 PN975 MI622 HI561 GB189 VR265 SM147 GC18 EW826 SP89 Total 

Common Moorhen 0 1 2 2 0 0 0 0 0 5 

Myrtle Warbler 4 8 3 2 1 0 3 0 3 24 

Scissor-tailed Flycatcher 4 3 0 0 1 1 4 0 0 13 

Lesser Nighthawk 1 2 2 3 1 0 1 0 0 10 

Yellow Warbler 9 6 10 2 4 2 2 3 2 40 

Louisiana Waterthrush 5 3 17 1 0 1 2 2 2 33 

Ovenbird 30 11 8 4 12 9 8 3 8 93 

Cliff Swallow 4 3 0 8 0 1 1 2 0 19 

Yellow-throated Vireo 0 1 12 0 0 0 1 0 1 15 

Orchard Oriole 10 33 11 6 12 7 4 13 3 99 

Purple Martin 27 21 70 129 8 9 9 2 2 277 

Chimney Swift 4 0 0 2 1 3 1 0 0 11 

Golden-winged Warbler 1 1 1 1 1 2 0 0 0 7 

Chestnut-sided Warbler 11 2 4 3 2 3 4 0 4 33 

Tennessee Warbler 24 8 25 11 9 10 12 3 5 107 

Merlin 2 0 0 1 0 0 2 0 0 5 

American Kestrel 1 4 1 2 1 0 1 0 2 12 

Great Blue Heron 3 2 7 0 0 2 2 3 0 19 

Eastern Kingbird 2 0 0 2 2 0 1 0 0 7 

Snowy Egret 5 0 4 1 1 0 4 2 0 17 

Least Bittern 0 3 1 1 2 1 0 0 1 9 

American Redstart 17 12 12 14 8 3 8 7 8 89 

Northern Rough-winged 

Swallow 

1 2 2 3 0 4 0 1 0 13 

Peregrine Falcon 8 13 4 13 4 4 8 5 4 63 

Little Blue Heron 1 0 0 9 1 0 0 0 0 11 

Yellow-bellied Sapsucker 1 1 1 0 0 1 1 1 0 6 

Bobolink 0 2 1 4 0 0 1 0 1 9 

Sora 1 8 12 0 7 2 6 4 1 41 

Belted Kingfisher 4 10 3 7 3 2 11 5 1 46 

Rose-breasted Grosbeak 0 2 0 0 2 0 2 0 0 6 

Bay-breasted Warbler 2 1 7 126 1 12 1 0 1 151 

Yellow-crowned Night-Heron 3 0 3 1 0 1 6 0 0 14 

Great Egret 4 4 1 2 0 1 3 2 4 21 

Wood Thrush 0 3 0 0 2 0 2 0 1 8 

Barn Swallow 37 9 32 82 3 22 27 47 6 265 

Ruby-throated Hummingbird 6 4 4 9 14 11 5 2 2 57 

Summer Tanager 0 2 2 2 3 0 1 1 1 12 
113 

Table 7.7. 

 

Comparative longitudinal distributions of spring trans-Gulf migrants 

Numbers shown indicate numbers of individuals undertaking stopovers on each platform during spring 2000. 

Species are listed in order from most westerly distribution at top to most easterly distribution at bottom. Species 

with <5 stopovers are not included 

 PN975 MI622 HI561 GB189 VR265 SM147 GC18 EW826 SP89 Total 

Cattle Egret 33 58 28 245 48 24 86 55 24 601 

Yellow-billed Cuckoo 1 12 3 6 8 5 8 7 3 53 

Red-eyed Vireo 2 6 1 2 3 5 13 0 2 34 

Bank Swallow 2 0 0 3 1 1 4 1 0 12 

Green Heron 16 39 3 13 35 15 25 35 23 204 

Gray Catbird 12 26 12 23 87 19 42 17 10 248 

Swainson's Warbler 1 0 0 1 0 0 3 0 0 5 

Chuck-Will's-Widow 2 1 1 3 1 1 9 2 0 20 

Blackpoll Warbler 4 1 4 7 7 4 12 4 4 47 

Acadian Flycatcher 0 2 0 1 2 2 1 2 1 11 

Purple Gallinule 0 1 1 3 7 2 2 2 1 19 

Scarlet Tanager 0 1 0 6 1 1 1 2 2 14 

Prothonotary Warbler 0 3 0 2 1 0 2 3 3 14 

Western Palm Warbler 0 1 0 1 2 1 2 1 1 9 

Cerulean Warbler 0 0 1 2 1 0 3 0 1 8 

Least Flycatcher 0 0 0 2 0 1 1 0 1 5 

PN 975 HI561 GB189 GC 18

Neotropical migrant landbirds (excluding )

Herons

Peregrine Falcon, Merlin, American Kestrel

Sora, Purple Gallinule, Chuck-Will’s-Widow, Belted Kingfisher

Medium-distance migrants

MI622 SM147

Figure 7.15. Frequency distribution of mean species-specific 

longitudinal positions in spring 2000. Each symbol 

indicates the mean position calculated from the entire 

sample of stopovers (i.e., all weather types) for one of the 

species listed in Table 7.7. Longitude is indicated by 

reference to the relative positions of selected study 

platforms. 
114 

Table 7.8. 

 

Comparisons of geographic distribution and abundance between ECH and BH days for selected species 

 “Median platform” indicates the median longitudinal position of individuals from the combined sample of ECH

and BH days. “Proportion east of median platform” compares the proportion of individuals recorded east of the 

median platform between ECH and BH days; total sample sizes for the two weather types are indicated in 

parentheses. P values indicate significance levels from single degree-of-freedom log-likelihood ratio tests for 

independence between geographic distribution and synoptic weather type. “Abundance ratio” compares the total 

abundance of a species across all platforms on ECH days versus BH days. The expected ratio was calculated for 

April and May, when most individuals of these species were recorded (9 ECH days and 22 BH days, i.e., 9 ) 22 = 

0.41). P values indicate significance levels from single degree-of-freedom chi-square tests for deviations between 

observed and expected abundances under the null expectation that total abundance was unrelated to synoptic 

weather. 

 Median Proportion East of Median Platform (N) Abundance Ratio (ECH:BH) 

Species Platform ECH days BH days G P Observed Expected P2 P 

Common 

Yellowthroat 

MI622 0.24 (41) 0.27 (95) 0.1 0.72 0.43 0.41 0.1 0.77 

Black-andwhite 

Warbler 

HI561 0.04 (50) 0.85 (13) 36.2 <0.0001 3.85 0.41 77.6 <0.0001 

White-winged 

Dove 

MI622 0.40 (25) 0.56 (25) 1.3 0.26 1.00 0.41 10.7 0.001 

Hooded 

Warbler 

HI561 0.15 (26) 0.56 (18) 8.0 0.005 1.44 0.41 19.3 <0.0001 

Purple Martin HI561 0.44 (132) 0.50 (46) 0.5 0.48 2.87 0.41 176.3 <0.0001 

Barn Swallow GB189 0.24 (86) 0.35 (48) 1.8 0.18 1.79 0.41 80.5 <0.0001 

Gray Catbird GB189 0.37 (19) 0.52 (25) 1.0 0.32 0.76 0.41 4.3 0.038 

7.7 MALE-BIASED SEX RATIOS

Over the course of the study it became obvious that we were seeing considerably more males than 

females in sexually dichromatic species that could be identified to sex in the field. Sex ratios for these 

species are summarized in Table 7.9, which shows that 28 out of 31 species (90%) had male-biased sex 

ratios. 

Fisher (1930) seems to have been the first to have taken an interest in the evolution of sex ratio. He 

showed that natural selection should favor the maintenance of 1:1 sex ratios because of the advantage of 

investing in offspring of the rarer sex. An influential hypothesis during the early years of behavioral 

ecology proposed that natural selection should result in the ability of animals to manipulate the sex ratio 

of their offspring adaptively in accordance with environmental circumstances (Trivers and Willard 1973). 

However, to date, there has been little evidence of offspring sex-ratio manipulation or skewed sex ratios 

in nature, except in association with atypical ecological circumstances such as cooperative breeding (e.g., 

Komdeur et al. 1997), polygyny (e.g., Westerdahl et al. 2000), etc. In fact, most longer-term studies of 

passerines (reviewed by Wheelwright and Seabury 2003) find stable 1:1 offspring sex ratios in support of 

Fisherian theory. 
115 

In light of the theoretical and empirical evidence indicating that population-level sex ratios should 

be close to 1.0, the extreme male bias we observed begs explanation. 

First, it is possible that the extreme sex ratios we observed are accurate representations of population 

structure. A considerable body of evidence has accumulated indicating that in several species of 

neotropical migrant warblers, males and females are spatially segregated on the wintering grounds (e.g., 

Lynch et al. 1985; Wunderle 1992; Parrish and Sherry 1994) and that this sexual segregation is due to 

behavioral dominance of males (Morton et al. 1987; Morton 1990; Stutchbury 1994; Marra 2000). 

Recently, Marra and Holmes (2001) showed that such dominance behavior and the resulting differential 

habitat segregation has consequences for the physical condition and survival of excluded individuals. 

Thus, it is possible that sexual segregation on the wintering grounds may cause reduced overwinter 

survivorship of females, resulting in skewed sex ratios during spring migration and on the breeding 

grounds. In fact, it is well known that breeding populations of many neotropical migrant landbirds 

contain large pools of unmated “floater” males (e.g., Rappole et al. 1977). 

Table 7.9. 

 

Sex ratios of sexually dichromatic species during spring 2000 

Numbers given are the totals from all nine study platforms. Only species 

with total N ≥ 10 are included. Species are listed in descending order of male 

bias. 

% & %%:&&

Magnolia Warbler 1,128 38 29.7 

Prothonotary Warbler 17 1 17.0 

Chestnut-sided Warbler 41 3 13.7 

Kentucky Warbler 36 4 9.0 

Blackburnian Warbler 68 11 6.2 

Baltimore Oriole 157 33 4.8 

Indigo Bunting 219 47 4.7 

Yellow-throated Warbler 9 2 4.5 

Golden-winged Warbler 8 2 4.0 

Yellow Warbler 38 11 3.5 

Brown-headed Cowbird 82 26 3.2 

Northern Parula 47 15 3.1 

Blue Grosbeak 9 3 3.0 

Black-throated Green Warbler 44 15 2.9 

Hooded Warbler 143 49 2.9 

Blue-winged Warbler 11 4 2.8 

Common Yellowthroat 328 133 2.5 

Scarlet Tanager 29 13 2.2 

Blackpoll Warbler 36 19 1.9 

Black-and-white Warbler 113 60 1.9 

Bay-breasted Warbler 98 53 1.8 

Tennessee Warbler 51 28 1.8 
116 

Table 7.9. 

 

Sex ratios of sexually dichromatic species during spring 2000 

Numbers given are the totals from all nine study platforms. Only species 

with total N ≥ 10 are included. Species are listed in descending order of male 

bias. 

% & %%:&&

Cerulean Warbler 7 4 1.8 

Rose-breasted Grosbeak 28 17 1.6 

Summer Tanager 20 13 1.5 

American Redstart 60 46 1.3 

Purple Martin 147 133 1.1a 

Orchard Oriole 67 62 1.1 

Ruby-throated Hummingbird 36 48 0.8 

Belted Kingfisher 17 33 0.5 

Myrtle Warbler 2 9 0.2 

a

 Excludes 59 individuals that were either females or second-year males. 

Second, it is possible that the sex ratios we observed were biased by differential propensities of males 

versus females to drop out of the airspace and be detected by the platform observers. Such a difference 

could result if females are more efficient fliers than males. Indeed, it seems reasonable to expect that 

competitive disadvantages on the wintering grounds must be balanced by some sort of compensatory 

survival advantage in another aspect of their life history. In one of the most well-studied cases of such a 

balance between competition and compensation, Carpenter et al. (1993a,b) found that female Rufous 

Hummingbirds are competitively disadvantaged along their migration corridor because they are 

behaviorally subordinate to males, and less likely to obtain high-quality flower territories with which to 

replenish migratory energy stores. However, females have a variety of exploitative advantages over 

males, including more energetically efficient flight and superior skills in thieving nectar. The females’ 

exploitative advantages appeared to compensate completely for their competitive disadvantages, since 

there were no demonstrable differences in overall migration performance between the sexes. 

Unfortunately, analogous information concerning possible compensatory advantages of female trans-Gulf 

migrants is not available. Elkins (1988) suggested that females appear to be more resilient to the effects 

of sudden adverse weather changes encountered during migration, but did not elaborate. 

Finally, it is possible that the skewed sex ratios we observed might result from some sort of differential 

route selection by the sexes. To evaluate this possibility, we examined geographic patterns of sex ratio. 

The results (Table 7.10) indicate that sexual segregation of migration route does occur in some species. 

Magnolia Warblers, Bay-breasted Warblers, Black-and-white Warblers, and American Redstarts showed 

a pattern of increasing male bias toward the west. This pattern could result if females take a more direct 

route across the Gulf, and would therefore be consistent with the idea that females may compensate for 

their competitive disadvantages in part via more efficient migratory flight. However, the data do not 

support the idea that differential route selection can account for the highly skewed sex ratios we observed; 

the only portion of the migratory population that we did not sample was a possible circum-Gulf 

component, and the observed trends suggest that such a component (if it exists) is likely to be even more 

extremely male-biased. 
117 

Curiously, all of the non-warblers (Orchard Oriole, Baltimore Oriole, Indigo Bunting) showed a 

pattern of male bias increasing toward the east. This pattern could be consistent with differential 

route selection as an explanation for male-biased sex ratios over the Gulf if females in these species are 

more likely to undertake a circum-Gulf migration. Unfortunately, relevant data concerning circum-Gulf 

migration are not available. 

Table 7.10. 

 

Geographic variation in sex ratio of sexually dichromatic species in spring 2000 

Sex ratios are computed from pooled samples from three longitudinal strata: “West” = 

PN975 + MI622 + HI561; “Central” = GB189 + VR265 + SM147; “East” = GC18 + 

EW826 + SP89. Significance levels are from G-tests (df = 2) of the null hypothesis that 

sex ratio was independent of region. Data are shown only for species for which 

meaningful G-tests could be conducted. Species are listed in order starting with those 

showing a trend toward stronger male bias in the west, and concluding with species 

showing stronger male bias toward the east. 

 Sex Ratio (%%:&&) 

West Central East N G P 

Magnolia Warbler 38.85 8.50 6.43 1,166 

Bay-breasted Warbler 7.50 1.64 1.00 151 

Black-and-white Warbler 2.09 1.63 1.11 173 

American Redstart 1.65 1.17 0.93 106 

Common Yellowthroat 2.39 2.42 3.00 461 

Hooded Warbler 2.93 2.17 4.14 192 

Orchard Oriole 0.94 1.07 1.40 129 

Baltimore Oriole 2.42 8.20 14.50 190 

Indigo Bunting 2.79 2.15 36.33 266 
119 

Chapter 8 

 

Timing and Magnitude of Spring Trans-Gulf Migration and Platform Use 

Robert W. Russell and Sidney A. Gauthreaux, Jr. 

8.1. DIEL TIMING OF SPRING MIGRATION

The diel timing of spring trans-Gulf migration followed a predictable pattern that was evident both in 

radar imagery and from direct visual observations on the platforms. Figure 8.1 shows a typical 

progression of radar images over the course of a spring day. On this day, the first migrants began arriving 

on the northern Gulf Coast at around 10 AM local time; migration was extremely heavy by the afternoon, 

and was essentially over by 9 PM (the final image primarily depicts nocturnal insect migrants). On this 

particular day, roughly 25 million migrant birds were estimated to have arrived on the northern Gulf 

coast, most having departed from the Yucatan Peninsula the night before. The pattern of migration 

indicated in the radar imagery was mirrored in our observations on the platforms: landing of migrants on 

platforms began at around 10 AM, peaked in the afternoon, and was finished by early evening. 

Although this pattern of diel timing seemed remarkably consistent, there were minor differences from day 

to day that appeared to be related to seasonal and weather factors. An analysis of covariance of radarobserved arrival time (with ELOW and MCH days excluded due to insufficient samples) showed that at 

Lake Charles (LCH) in spring 1998, the arrival of incoming trans-Gulf migrants began significantly later 

as the season progressed, but arrival time did not vary among days of different weather type. Time of 

peak trans-Gulf migration traffic detected at Lake Charles also occurred significantly later as the season 

progressed, and differences among weather types were nearly significant. Duration of radar-observed 

trans-Gulf migration at Lake Charles averaged 8.5 h and did not vary with date or among weather types. 

An analysis of covariance (with ELOW and MCH days excluded due to insufficient samples) of radarobserved arrival time showed that at New Orleans in spring 1998, TGM arrival time did not vary with 

date, but did vary significantly and dramatically among days of different weather type. Similarly, time of 

peak TGM at New Orleans did not vary with date, but varied significantly and dramatically among 

weather types. Duration of TGM at New Orleans did not vary with date but varied significantly and 

dramatically among weather types. 
120 

-28 -24 -28 -16 -12 -8 0 4 8 -4 12 16 20 

24

Radar Reflectivity Factor (dBZe)

9 AM 10 AM 11 AM 1 PM

4 PM 5 PM 8 PM 9 PM

Figure 8.1. Time series showing the progressive arrival of trans-Gulf migrants on the northern Gulf Coast on May 13, 1998. Sy

this flight was classified as ECH. Times indicated are in local time (CDT). Radar scans were obtained in clear-air 

elevation of 0.5E. Images are courtesy of WSI Corporation. 
121 

On BH days in spring 1998 when TGM was detected by radar at both Lake Charles and New 

Orleans, TGM started significantly earlier at New Orleans, peaked significantly earlier at New 

Orleans, and lasted longer at New Orleans. Paired samples were of insufficient size for statistical testing 

on ECH (n = 7) and GH (n = 6) days. 

Analysis of covariance (with ELOW and MCH days excluded due to insufficient samples) of radarobserved arrival time showed that at Lake Charles in spring 1999, TGM started significantly later as the 

season progressed, but arrival time did not vary among days of different weather type. Time of peak 

TGM also occurred significantly later as the season progressed, and differences among weather types 

were nearly significant. Duration of TGM at Lake Charles averaged 8.2 h and did not vary with date or 

among weather types. 

Analysis of covariance (with ELOW and MCH days excluded due to insufficient samples) of radarobserved arrival time showed that at New Orleans in spring 1999, TGM started significantly earlier as the 

season progressed but arrival time did not vary among days of different weather type. Similarly, time of 

peak TGM at New Orleans started significantly earlier as the season progressed and arrival time did not 

vary among days of different weather type. Duration of TGM at New Orleans averaged 7.7 h and did not 

vary with date or among weather types. 

On BH days in spring 1999 when TGM was detected by radar at both Lake Charles and New Orleans, 

there were no differences between the two in TGM arrival time, TGM peak, or TGM duration. On ECH

days in spring 1999 when TGM was detected at both Lake Charles and New Orleans, TGM arrival was 

significantly earlier at Lake Charles. However, there was no significant difference in time of peak TGM. 

Duration of TGM duration was longer at Lake Charles and the difference was nearly significant. Paired 

samples were of insufficient size for statistical testing on GH days (n = 8). 

In summary, radar-observed migration over the northern Gulf began between early morning and early 

afternoon, peaked 3-4 h after first detection, and continued until 7-12 h after first detection. Variability in 

diel timing was related in a complex way to location, weather, year, and time of year. These relationships 

are summarized in Table 8.1a-c. 

Time of first daily arrival of TGM was unrelated to weather type in three out of four comparisons. In the 

case where earliest TGM detection was related to weather type (New Orleans 1998), first arrival was 

earliest on BH days (12:22 UTC) and averaged more than five hours later on ECH and GH days (17:33-

18:04 UTC). Time of peak TGM was related to weather type in three out of four comparisons, but these 

differences were confounded with geographic differences and not consistent between years. At Lake 

Charles, TGM peaked earliest on GF and GH days in 1998 but on ECH and BH days in 1999. At New 

Orleans, TGM peaked 4-6 h earlier on BH and GF days than on ECH and GH days in 1998, but no effect 

of weather type was detected in 1999. In 1998, both first arrival and peak TGM occurred 2-3 h earlier at 

New Orleans than at Lake Charles on BH days, and peak TGM occurred 2-5 h earlier at Lake Charles on 

ECH, GH, and GF days. Diel timing also averaged earlier at New Orleans on BH days in 1999, though 

this difference could not be demonstrated statistically. On ECH days in 1999, first arrival occurred 1.5 h 

earlier at Lake Charles than at New Orleans, but there was no detectable difference in time of TGM peak. 

In summary, to the extent that differences were detected, TGM tended to occur earliest at New Orleans on 

BH days and earliest at Lake Charles on ECH, GH, and GF days. 

The pattern of TGM tending to arrive earlier at New Orleans than at Lake Charles on BH days, but earlier 

at Lake Charles on ECH, GH, and GF days, is consistent with a strong synoptic steering influence on 

migration routes across the Gulf. On BH days, migrants traveling with the geostrophic flow should 

follow a straighter, more direct route to the northern Gulf Coast from the Yucatan Peninsula and adjacent 

Mexican shores. In contrast, migrants exploiting the geostrophic flow on ECH and GH days would 
122 

follow a more curvilinear route, with the western flank skirting the Texas coast and the northern 

vanguard approaching from a more westerly bearing. 

 

Table 8-1a. 

 

Spring radar-observed trans-Gulf migration time of initial arrival on the northern Gulf Coast (CST) 

 1998 1999 

 Lake Charles New Orleans Lake Charles New Orleans 

Seasonal Trend Significant, 

Progressively 

later 

Not Significant Significant, 

Progressively 

later 

Significant, 

Progressively 

Earlier 

 Beta = 0.0027 --- Beta = 0.0022 Beta = 0.0020 

 F 1,45 = 10.91 F1,40 = 0.62 F1,52 = 6.92 F1,42 = 4.46 

 P = 0.0019 P = 0.44 P = 0.011 P = 0.041 

 

Variation among 

synoptic weather 

types 

Not Significant Significant Not Significant Not Significant 

 F 3,45 = 1.10 F3,40 = 9.24 F 3, 52 = 1.48 F3,42 = 1.26 

 P = 0.36 P = 0.0001 P =0.23 P = 0.30 

 

Overall x (CST) 9:06 AM + 0:25 

SE 

--- 10:37 AM + 0:25 

SE 

9:48 AM 

Bermuda High (CST) --- 6:22 AM --- --- 

East Continental High 

(CST) 

--- 11:33 AM --- --- 

Gulf High (CST) --- 12:04 PM --- --- 

Gulf Front (CST) --- 7:41 AM --- --- 

 

Paired comparison on 

Bermuda High Days 

Significant 

Lake Charles (8:02 AM CST) > New 

Orleans (6:29 AM CST) 

Not significant 

Lake Charles (11:02 AM CST) = New 

Orleans 

 T = 2.38 T = 0.86 

 DF = 16 DF = 16 

 P = 0.03 P = 0.40 

 

Paired comparison on 

East Continental High 

days 

Not measured Significant 

Lake Charles (11:02 CST) > New 

Orleans (10:36 AM CST) 

 T = 7.43 

 DF = 11 

 P = 0.0001 
123 

Table 8-1b. 

 

Spring radar-observed trans-Gulf migration time of the peak in 

number of birds arriving on the northern Gulf Coast (CST) 

 1998 1999 

 Lake Charles New Orleans Lake Charles New Orleans 

Seasonal trend Significant Not significant Significant Significant 

 Beta = 0.0033 --- Beta = 0.0025 Beta = 0.0023 

 F1,45 = 7.58 F1,40 = 0.81 F1,50 = 9.63 F1,41 = 5.35 

 P = 0.0085 P = 0.37 P = 0.0031 P = 0.026 

 

Variation among synoptic 

weather types 

Significant Significant Nearly 

Significant 

Not Significant

 F3,45 = 2.77 F3,40 = 5.88 F3,50 = 2.58 F3,41 = 1.22 

 P = 0.052 P = 0.002 P =0.064 P = 0.31 

 

Overall x (CST) --- --- --- 12:27 PM 

Bermuda High (CST) 12:58 PM 10:08 AM 1:51 PM --- 

East Continental High (CST) 1:36 PM 3:28 AM 1:10 PM --- 

Gulf High (CST) 11:25 AM 4:08 PM 3:59 PM --- 

Gulf Front (CST) 8:55 AM 11:03AM 3:32 PM --- 

 

Paired comparison on Bermuda 

High Days 

Significant Not Significant 

 T = 2.99 T = 0.41 

 DF = 16 DF = 10 

 P = 0.009 P = 0.69 

 Lake Charles (12:20 PM CST) > New 

Orleans (10:26 PM CST) 

Lake Charles (12:41 PM CST) = 

New Orleans 

 

Paired comparison on East 

Continental High days 

Not measured T = 0.75 

Lake Charles (12:47 PM CST) = 

New Orleans 

 DF = 10 

 P = 0.055 
124 

Table 8-1c. 

 

Spring radar-observed trans-Gulf migration duration of arrival period (hr) on the northern Gulf Coast 

 1998 1999 

 Lake Charles New Orleans Lake Charles New Orleans 

Seasonal Trend Not significant Not Significant Significant Not Significant 

 F1,45 = 0.64 F1,40 = 1.84 F1,52 = 6.92 F3,41 = 1.05 

 P = 0.13 P = 0.18 P = 0.011 P = 0.31 

 

Variation among synoptic 

weather types 

Not significant Significant Not significant Not significant 

 F3,.45 = 2.01 F3,40 6.08 F3,52 = 0.74 F3,41 = 0.49 

 P = 0.13 P = 0.0016 P = 0.53 P = 0.69 

 

Overall x (hr) 8.5 hr + 0.42 SE 8.2 hr + 0.41 SE 7.7 hr + 0.42 SE 

Bermuda High (hr) 11.7 hr 

East Continental High (hr) 8.0 hr 

Gulf High (hr) 6.7 hr 

Gulf Front (hr) 9.3 hr 

 

Paired comparison on 

Bermuda High Days 

Significant Not Significant 

 T = 2.61 T = 0.86 

 DF = 16 DF = 16 

 P = 0.019 0.40 

 Lake Charles (9.9 hr) < New Orleans 

(11.8 hr) 

Lake Charles (8.7 hr) = New Orleans 

 

Paired comparison on East 

Continental High days 

Not tested Not significant 

 T = 7.43 

 ` DF = 11 

 P = 0.055 

 Lake Charles (9.2 hr) > New Orleans 

(7.2 hr) 

The radar-observed pattern in daily duration of TGM (i.e., 2 h longer at New Orleans than at Lake 

Charles on BH days, and 2 h longer at Lake Charles than at New Orleans on ECH days) mirrored the 

patterns of TGM arrival (i.e., longer daily durations were associated with earlier arrivals), suggesting that 

radar-observed duration is usually bounded by nightfall (presumably because migrants descend in altitude 

at nightfall). This finding indicates that duration of radar-observed TGM is probably not a useful 

comparative parameter, and more importantly, that a portion of spring TGM is probably missed by radar, 

even on favorable weather days. 

Lake Charles showed a consistent pattern in seasonal trend of diel timing, with both TGM arrival and 

peak TGM occurring later as the season progressed in both years. This pattern can be explained simply as 

a result of increasing daylength during the spring. As spring progresses, nightfall on the Yucatan 

Peninsula or other departure points occurs progressively later. Therefore, migrants departing shortly after 
125 

nightfall should arrive over the northern Gulf later on the following day as the season progresses, all 

else being equal. The findings from Lake Charles were thus consistent with the null expectation. 

However, diel migration timing at New Orleans was earlier as the season progressed in 1999, 

independent of any influences of synoptic weather type. A seasonal trend toward shorter trans-Gulf 

transit times is particularly unexpected because a number of studies have shown that late migrants tend to 

be lower-quality individuals, which should be less efficient in transiting the Gulf. One possible 

explanation for this discrepancy is that the late-season flights recorded by New Orleans NEXRAD may be 

dominated by shorebirds, which fly faster than landbirds and rarely stop on platforms. 

Most comparisons indicated that the diel timing of TGM over the northern Gulf averaged 1-3 h later in 

1999 than in 1998, even after accounting for synoptic weather type. The reason for this difference is not 

clear, though the most obvious candidate explanation would be persistently stronger winds aloft in 1998. 

Regardless of the causality, an important implication of substantial interannual variation in transit times is 

that the condition of arriving migrants on the northern Gulf Coast should be expected to vary among years 

(due to the very high energetic cost of powered flight). 

8.2. RADAR-BASED ESTIMATES OF ABSOLUTE BIRD MIGRATION TRAFFIC

To estimate the total daily number of trans-Gulf migrants detected by radar during spring 1998 and spring 

1999, we necessarily made a number of assumptions. We assumed that changes in species composition 

over the course of the season can be characterized by the daily variation in mean body mass of the pool of 

migrants stopping over on and flying past our study platforms. Daily average radar cross section was then 

estimated using equation 4.12, and estimated migration fluxes were calculated using equation 4.7 and the 

actual NEXRAD data. Migrants were assumed to travel uniformly in an altitudinal band from 300-1300 

m, and to arrive across a 900-km swath from the central Texas coast to around Mobile Bay, Alabama. 

The NEXRAD results from LCH were assumed to be applicable uniformly to the western 500 km of the 

arrival front, and the LIX results were assumed to be applicable uniformly to the eastern 400 km (roughly 

from South Marsh Island eastward). Radar cross section calculations for the western area were based on 

observations from GB189 and VR265, and calculations for the eastern area were based on SM66 (1998), 

SM147 (1999), PL10 (1998), GC18 (1999), and EW826. In cases where trans-Gulf migration was 

observed by radar before or after our field seasons, we based radar cross section calculations on the 

average of the first two days or last two days of the field season. 

These calculations yielded total seasonal estimates of 316 million trans-Gulf migrants in spring 1998 

(68% arriving west of South Marsh Island) and 147 million trans-Gulf migrants in spring 1999 (67% 

arriving west of South Marsh Island). 

8.3. SEASONAL TIMING OF SPRING MIGRATION

8.3.1 Phenology of the Migration in Aggregate 

The volume of trans-Gulf migration varied dramatically from day to day, both in terms of radar estimates 

and actual numbers observed on platforms. 

“Small” trans-Gulf flights (>1 million migrants estimated from radar) occurred between March 25 and 

May 24. “Moderate” flights (>5 million migrants) occurred between April 2 and May 15. “Megaflights” 

(>25 million migrants) occurred between April 22 and May 13. 

Radar-observed migration was characterized by a series of pulses and tended to be “all-or-nothing”, i.e., 

either significant trans-Gulf migration was evident on radar or else it was essentially entirely absent 

(Figure 8.2). Interestingly, the peaks of the pulses seemed to conform to a smooth pattern in each year. 

In 1999, migration peaks seemed to be approximately Gaussian and symmetric around the peak date of 
126 

April 22. In contrast, radar-observed migration peaked much later in 1998 and the overall pattern 

was highly skewed to the left. 

Patterns of migrant abundance detected from platforms showed less of the “all-or-nothing” character than 

did radar-observed migration, but at the same time showed more extreme peaking (Figure 8.3). 

March

10 20 1 10 20 1 10 20 30

April May

0

5

10

15

20

25

30

0

5

10

15

20

25

30

Daily Trans-Gulf Migration Traffic (millions of birds)

1998

1999

BH

BH

BH

BH

BH

BH

ECH

ECH

ECH ECH

1000

10000

100000

1000000

10000000

100000000

1000

10000

100000

1000000

10000000

100000000

BH GF GF BH BH

ECH

ECH ECH GH

ECH

ECH BH GF BH GH GH

Figure 8.2. Phenology of radar-observed spring migration in 1998 and 1999. The inset figures show the same data 

(x+1)-transformed on a logarithmic scale for perspective. Synoptic weather types are indicated above 

intraseasonal migration peaks. 
127 

0

500

1000

1500

2000

2500

3500

0

200

400

1000

1200

1800

2000

Birds Observed Per Day

1998

1999

ECH BH

BH

GH BH

GF

ECH

GF ECH

1

10

100

1000

1

10

100

1000

GF

ECH

GF

GH

GH

GH

GF

GF

BH

GF

BH GF

GF

ECH GH BH BH

GF

ECH

March

10 20 1 10 20 1 10 20 30

April May

1

10

100

1000

10000

2000

GF

GF

GF

ECH

BH BH

BH

GH

BH

ECH

GF GF MCH

GF ECH GF

0

2000

4000

6000

8000

12,000

10,000

3000

600

800

1400

1600

Figure 8.3. Phenology of platform-observed spring migration in 1998-2000. The inset figures show the same 

data (x+1)-transformed on a logarithmic scale for perspective. Synoptic weather types are 

indicated above intraseasonal migration peaks, and inside a box on days with rain. 
128 

8.3.2 Weather Influences 

Trans-Gulf migration was often completely absent on radar for prolonged periods, even around the 

expected seasonal peak of migration (Figure 8.2: e.g., April 9-12, April 19-24, and April 29-May 1 in 

1998, and April 15-19 and April 28-May 1 in 1999). These dramatic hiatuses were always associated 

with strong cold fronts that penetrated deep into Mexico and set up persistent northerly winds over most 

of the Gulf. Conversely, radar-observed migration peaks were almost strictly associated with ECH and 

BH days, though minor peaks sometimes occurred on GF days in March-early April and on GH days in 

late May (Figure 8.2). 

The influence of synoptic weather on the occurrence of radar-observed migration was similar but not 

identical between the Lake Charles and New Orleans NEXRAD sites (Table 8.2). Trans-Gulf migration 

was almost always detected by NEXRAD on BH days. In spring 1998, TGM was detected at Lake 

Charles on all BH days except March 7 and April 2, and at New Orleans on all BH days except March 5, 

March 7, March 26, and April 7. In spring 1999, TGM was detected on all BH days at both Lake Charles 

and New Orleans. 

There was an interesting difference between Lake Charles and New Orleans in the frequency of radar 

detection of TGM on ECH days. In spring 1998, TGM was detected at Lake Charles on 71% of ECH

days (all except March 4, March 6, March 13, March 14, and March 18), whereas TGM was detected on 

just 41% of ECH days at New Orleans (not detected on the same days as Lake Charles as well as on 

March 15, March 16, March 17, May 25, and May 26). The pattern was nearly identical in 1999. In 

spring 1999, TGM was detected at Lake Charles on 75% of ECH days (all except March 4, March 8, 

March 16, March 27, March 29, May 29, and May 30) but on just 48% of ECH days at New Orleans 

(absent on the same days as Lake Charles except on March 29, when TGM was detected at New Orleans 

but not at Lake Charles; also absent on March 5, April 29, May 17, May 18, May 20, May 21, May 22, 

and May 31). The bias toward detection at Lake Charles on ECH days presumably reflects the general 

steering influence of synoptic winds on the broad-scale trajectory of migration (Chapter 7). 

Table 8.2. 

 

Influence of synoptic weather on the occurrence of radar-observed trans-Gulf migration in spring 1998 and 1999 

 Spring 1998 Spring 1999 

 Lake Charles New Orleans Lake Charles New Orleans 

Weather Type TGM No TGM TGM No TGM TGM No TGM TGM No TGM 

BH 20 2 18 4 18 0 18 0 

ECH 12 5 7 10 21 7 13 14 

GH 10 9 8 11 12 6 9 9 

GF 8 8 12 4 7 11 7 10 

MCH 0 9 2 7 1 4 0 5 

ELOW 0 8 2 6 0 4 0 4 

TGM was detected by radar on roughly half of GH days. In spring 1998, TGM was detected at Lake 

Charles on 53% of GH days (absent on March 23, March 24, May 17, May 18, May 19, May 21, May 22, 

May 29, and May 30). At New Orleans, TGM was detected on 41% of GH days (absent on the same 

days as Lake Charles except May 21 and May 22, when TGM was detected at New Orleans but not at 
129 

Lake Charles; also absent on April 24, May 1, May 28, and May 31). The pattern was similar in 

1999. In spring 1999, TGM was detected at Lake Charles on 67% of GH days (absent on March 1, 

March 2, March 19, April 19, May 24, and May 28). At New Orleans, TGM was detected on 50% of GH

days (absent on the same days as Lake Charles except March 19, when TGM was detected at New 

Orleans but not at Lake Charles; also absent on April 20, May 23, May 25, and May 27). 

TGM was rarely detected by radar on MCH and ELOW days, and when it was it was only trace amounts. 

Radar detection on GF days was highly variable. At least in 1998, TGM was detected more frequently on 

GF days at New Orleans than at Lake Charles, presumably due to eastward displacement of the main 

migration stream by the fronts (e.g., Figure 7.7). 

The largest flights observed from platforms were often on GF days, but platform-observed migration 

peaks also occurred on BH, ECH, and GH days (Figure 8.3). Contrary to popular expectation, platformobserved migration was not strictly associated with precipitation. Indeed, in 1998 only one significant 

migration event was associated with precipitation. Association with precipitation was more prevalent in 

1999 and, especially, in 2000; however, the largest platform-observed migration events in those years 

occurred on precipitation-free days (Figure 8.3). 

Broad temporal patterns of platform-observed migration generally followed the radar patterns, but 

relatively more birds seemed to be detected from platforms in March and relatively fewer migrants 

seemed to be detected in May. These biases probably reflect the high early-season frequency of weather 

events unfavorable for trans-Gulf migrants and the generally benign weather toward the end of the season 

(Figure 5.2). 

A comparison of platform counts with radar estimates in relation to synoptic weather is provided in Table 

8.3 (1998) and Table 8.4 (1999). In general, BH and ECH days tended to yield large radar-observed 

events and relatively small platform-observed events. This association was not universal since, as 

previously mentioned, significant platform events did sometimes occur on such “fair-weather” days. On 

GF days, migration was usually absent or minor on radar, but often spectacular on platforms. 

Table 8.3. 

 

Comparison of daily platform counts and radar-observed migration 

traffic in relation to synoptic weather in spring 1998 

Date Synoptic 

Type 

Radar-Observed 

Migration Traffic 

Platform 

Counts 

1-Mar ELOW 0 na 

2-Mar ELOW 0 na 

3-Mar MCH 0 na 

4-Mar ECH 0 na 

5-Mar BH 39,919 na 

6-Mar ECH 0 na 

7-Mar BH 0 na 

8-Mar GF 0 na 

9-Mar ELOW 0 na 

10-Mar MCH 0 na 

11-Mar MCH 0 na 

12-Mar MCH 0 na 

13-Mar ECH 0 25 

14-Mar ECH 0 40 
130 

Table 8.3. 

 

Comparison of daily platform counts and radar-observed migration 

traffic in relation to synoptic weather in spring 1998 

Date Synoptic 

Type 

Radar-Observed 

Migration Traffic 

Platform 

Counts 

15-Mar ECH 167,252 78 

16-Mar ECH 238,151 14 

17-Mar ECH 34,340 256 

18-Mar ECH 0 218 

19-Mar GF 0 108 

20-Mar ELOW 0 9 

21-Mar ELOW 0 5 

22-Mar MCH 0 8 

23-Mar GH 0 6 

24-Mar GH 0 63 

25-Mar ECH 2,306,963 10 

26-Mar BH 2,150,481 8 

27-Mar BH 1,797,596 6 

28-Mar BH 623,348 57 

29-Mar BH 520,190 12 

30-Mar BH 2,956,739 14 

31-Mar GF 849,083 984 

1-Apr GF 1,005,184 81 

2-Apr BH 52,663 164 

3-Apr GF 2,453,045 237 

4-Apr GF 11,397 152 

5-Apr MCH 517 84 

6-Apr ECH 208,480 29 

7-Apr BH 1,063,162 183 

8-Apr BH 2,942,194 190 

9-Apr GF 3,939 35 

10-Apr ELOW 2,182 49 

11-Apr MCH 0 22 

12-Apr ECH 74,205 18 

13-Apr BH 8,732,527 49 

14-Apr BH 6,199,372 59 

15-Apr BH 4,485,806 47 

16-Apr BH 13,512,286 18 

17-Apr GF 5,020,577 281 

18-Apr GF 3,585,184 242 

19-Apr GF 5,221 104 

20-Apr MCH 1,953 89 

21-Apr MCH 0 78 

22-Apr ELOW 781 73 

23-Apr ELOW 0 36 

24-Apr GH 20,695 42 

25-Apr BH 19,423,956 189 

26-Apr BH 26,682,504 50 
131 

Table 8.3. 

 

Comparison of daily platform counts and radar-observed migration 

traffic in relation to synoptic weather in spring 1998 

Date Synoptic 

Type 

Radar-Observed 

Migration Traffic 

Platform 

Counts 

27-Apr BH 23,278,464 359 

28-Apr GF 4,179,621 1,025 

29-Apr GF 157,679 1,814 

30-Apr GF 7,201 589 

1-May GH 0 59 

2-May GH 4,118,728 68 

3-May BH 5,261,354 132 

4-May GF 3,055,877 494 

5-May ECH 27,566,460 57 

6-May BH 23,881,607 152 

7-May BH 12,840,200 213 

8-May BH 3,764,298 134 

9-May BH 12,628,122 100 

10-May GF 0 123 

11-May GF 14,350 73 

12-May ECH 2,306,692 19 

13-May ECH 27,891,872 139 

14-May ECH 21,574,463 145 

15-May GH 21,730,740 2 

16-May GH 4,133,416 na 

17-May GH 0 na 

18-May GH 0 na 

19-May GH 0 na 

20-May GH 3,126,921 na 

21-May GH 125,156 na 

22-May GH 240,092 na 

23-May GH 3,857,084 na 

24-May GH 1,946,344 na 

25-May ECH 444,565 na 

26-May ECH 324,916 na 

27-May ND 7,200 na 

28-May GH 13,554 na 

29-May GH 0 na 

30-May GH 0 na 

31-May GH 15,738 na 
132 

Table 8.4. 

 

Comparison of daily platform counts and radar-observed migration traffic 

in relation to synoptic weather in spring 1999 

 Synoptic 

type 

Radar-observed 

migration traffic 

Platform 

Counts 

1-Mar GH 0 na 

2-Mar GH 0 na 

3-Mar GF 0 na 

4-Mar ECH 0 na 

5-Mar ECH 171,425 na 

6-Mar BH 695,082 na 

7-Mar GF 10,167 na 

8-Mar ECH 0 na 

9-Mar GF 336,908 na 

10-Mar GF 0 na 

11-Mar GF 0 na 

12-Mar GF 102,003 na 

13-Mar GF 322,958 na 

14-Mar ELOW 0 na 

15-Mar ELOW 0 na 

16-Mar ECH 0 na 

17-Mar BH 320,542 na 

18-Mar BH 787,340 8 

19-Mar GH 8,560 225 

20-Mar GF 0 16 

21-Mar GF 0 73 

22-Mar MCH 0 55 

23-Mar GH 137,642 93 

24-Mar BH 869,919 132 

25-Mar GF 100,488 307 

26-Mar GF 0 63 

27-Mar ECH 0 258 

28-Mar ECH 2,234,816 954 

29-Mar ECH 957,955 266 

30-Mar GF 0 728 

31-Mar ND 0 65 

1-Apr BH 171,457 143 

2-Apr BH 7,351,551 7 

3-Apr BH 5,938,882 11 

4-Apr ECH 3,261,131 11 

5-Apr BH 2,439,647 3 

6-Apr GH 402,674 69 

7-Apr ECH 357,003 50 

8-Apr BH 2,326,992 8 

9-Apr BH 4,909,449 32 

10-Apr BH 5,846,485 97 

11-Apr GH 3,446,789 210 
133 

Table 8.4. 

 

Comparison of daily platform counts and radar-observed migration traffic 

in relation to synoptic weather in spring 1999 

 Synoptic 

type 

Radar-observed 

migration traffic 

Platform 

Counts 

12-Apr GH 537,339 257 

13-Apr ECH 64,459 29 

14-Apr ECH 991,489 61 

15-Apr GF 0 689 

16-Apr ELOW 0 578 

17-Apr ELOW 0 144 

18-Apr MCH 0 54 

19-Apr GH 0 40 

20-Apr GH 554,514 77 

21-Apr GH 6,735,496 109 

22-Apr BH 25,332,693 84 

23-Apr BH 11,667,590 29 

24-Apr GH 7,759,154 132 

25-Apr GH 3,724,672 307 

26-Apr BH 831,125 606 

27-Apr BH 1,744,102 2,900 

28-Apr GF 95,605 457 

29-Apr ECH 133,093 279 

30-Apr MCH 0 211 

1-May MCH 332,029 109 

2-May ECH 695,177 382 

3-May ECH 5,856,132 1,781 

4-May ECH 10,407,673 300 

5-May BH 7,825,020 41 

6-May GF 1,452,163 330 

7-May GF 81,571 239 

8-May GF 1,296,378 92 

9-May ECH 3,382,813 147 

10-May ECH 3,783,530 103 

11-May ECH 1,843,354 429 

12-May ECH 1,600,698 83 

13-May GF 553,457 1 

14-May BH 167,619 na 

15-May BH 901,284 na 

16-May ECH 978,990 na 

17-May ECH 407,363 na 

18-May ECH 23,823 na 

19-May MCH 0 na 

20-May ECH 37,834 na 

21-May ECH 395,288 na 

22-May ECH 276,390 na 

23-May GH 52,762 na 

24-May GH 0 na 
134 

Table 8.4. 

 

Comparison of daily platform counts and radar-observed migration traffic 

in relation to synoptic weather in spring 1999 

 Synoptic 

type 

Radar-observed 

migration traffic 

Platform 

Counts 

25-May GH 108,364 na 

26-May GH 233,555 na 

27-May GH 142,276 na 

28-May GH 0 na 

29-May ECH 0 na 

30-May ECH 0 na 

31-May ECH 15,695 na 
135 

8.3.3 Interspecific Variation in Migration Schedules 

Data on species-specific spring migration schedules are tabulated in Table 8.5 and summarized 

graphically in Figure 8.4. Waterfowl and herons peaked by early April. Shorebirds had widely varying 

migration schedules, with different species peaking as early as mid-March and as late as the end of May. 

Landbird migrants showed peaks throughout the season, but a majority of species peaked in the second 

half of April. 

Table 8.5. 

 

Species-specific timing of spring trans-Gulf migration in 2000 

Species are listed in order from earliest to latest migrants (based on mean date). Sample 

sizes indicate total numbers of individuals of each species recorded over the entire 

season, including both stopovers and flyby migrants. 95% confidence intervals are 

calculated only for sample sizes $ 5. 

Species First 

Date 

95% CI 

Lower 

Bound 

Mean 

Date 

95% CI 

Upper 

Bound 

Last 

Date 

N 

Greater White-fronted Goose --- --- 11-Mar --- --- 1 

Blue-headed Vireo --- --- 14-Mar --- --- 1 

Common Snipe 14-Mar --- 14-Mar --- 15-Mar 2 

Yellow-throated Vireo 13-Mar 14-Mar 14-Mar 15-Mar 15-Mar 7 

Ring-necked Duck --- --- 15-Mar --- --- 1 

White-faced Ibis --- --- 15-Mar --- --- 1 

Wilson's Plover --- --- 15-Mar --- --- 1 

Killdeer 11-Mar 13-Mar 15-Mar 17-Mar 19-Mar 11 

Green-winged Teal 16-Mar 16-Mar 16-Mar 2 

Northern Pintail 15-Mar 29-Feb 16-Mar 1-Apr 11-Apr 5 

Lesser Scaup 12-Mar 6-Mar 16-Mar 27-Mar 3-Apr 12 

Pied-billed Grebe 9-Mar 6-Mar 17-Mar 28-Mar 3-Apr 6 

Yellow-crowned NightHeron 11-Mar 12-Mar 17-Mar 22-Mar 29-Apr 61 

Louisiana Waterthrush 14-Mar 12-Mar 18-Mar 24-Mar 24-Apr 23 

Cinnamon Teal --- --- 19-Mar --- --- 1 

Eastern Meadowlark 11-Mar --- 19-Mar --- 28-Mar 4 

Red-winged Blackbird --- --- 20-Mar --- --- 1 

Black-crowned Night-Heron 15-Mar 1-Mar 20-Mar 7-Apr 1-May 10 

unid. Meadowlark 16-Mar 15-Mar 22-Mar 28-Mar 30-Mar 6 

White Ibis 12-Mar 14-Mar 22-Mar 30-Mar 21-Apr 15 

Redhead --- --- 23-Mar --- --- 1 

Burrowing Owl 10-Mar --- 23-Mar --- 5-Apr 2 

American Wigeon 15-Mar 29-Feb 23-Mar 14-Apr 16-Apr 6 

Myrtle Warbler 10-Mar 19-Mar 23-Mar 27-Mar 22-Apr 32 
136 

Table 8.5. 

 

Species-specific timing of spring trans-Gulf migration in 2000 

Species are listed in order from earliest to latest migrants (based on mean date). Sample 

sizes indicate total numbers of individuals of each species recorded over the entire 

season, including both stopovers and flyby migrants. 95% confidence intervals are 

calculated only for sample sizes $ 5. 

Species First 

Date 

95% CI 

Lower 

Bound 

Mean 

Date 

95% CI 

Upper 

Bound 

Last 

Date 

N 

Swallow-tailed Kite 14-Mar --- 24-Mar --- 13-Apr 4 

Tree Swallow 9-Mar 22-Mar 24-Mar 27-Mar 21-May 151 

Yellow-throated Warbler 10-Mar 18-Mar 24-Mar 31-Mar 22-Apr 21 

Northern Parula 15-Mar 21-Mar 25-Mar 28-Mar 22-May 69 

Purple Martin 8-Mar 22-Mar 25-Mar 28-Mar 19-May 214 

Black Rail --- --- 26-Mar --- --- 1 

Song Sparrow --- --- 26-Mar --- --- 1 

Violet-Green Swallow --- --- 26-Mar --- --- 1 

Ruby-crowned Kinglet 26-Mar --- 26-Mar --- 27-Mar 2 

American Robin 11-Mar --- 27-Mar --- 13-Apr 3 

American Bittern 15-Mar --- 27-Mar --- 7-Apr 3 

Black-and-white Warbler 9-Mar 24-Mar 28-Mar 31-Mar 25-May 174 

White-eyed Vireo 14-Mar 21-Mar 28-Mar 4-Apr 29-Apr 22 

Great Blue Heron 8-Mar 24-Mar 28-Mar 2-Apr 21-May 203 

Clapper Rail --- --- 29-Mar --- --- 1 

Marsh Wren --- --- 29-Mar --- --- 1 

Great Egret 7-Mar 25-Mar 29-Mar 1-Apr 26-May 320 

Little Blue Heron 7-Mar 25-Mar 29-Mar 3-Apr 22-May 317 

American Golden Plover 14-Mar 15-Mar 30-Mar 13-Apr 2-May 20 

Fulvous Whistling Duck 19-Mar 6-Feb 30-Mar 22-May 12-Apr 5 

Savannah Sparrow 11-Mar 27-Mar 30-Mar 3-Apr 9-May 63 

Western Meadowlark 29-Mar 31-Mar 2-Apr 2 

American Coot 14-Mar 25-Mar 1-Apr 8-Apr 1-May 16 

Eastern Phoebe --- --- 2-Apr --- --- 1 

Willet 10-Mar 13-Mar 2-Apr 21-Apr 24-Apr 9 

Belted Kingfisher 11-Mar 29-Mar 2-Apr 6-Apr 20-May 58 

Hooded Warbler 14-Mar 31-Mar 2-Apr 5-Apr 21-May 174 

Lazuli Bunting --- --- 3-Apr --- --- 1 

Virginia Rail --- --- 3-Apr --- --- 1 

Reddish Egret 7-Mar 27-Feb 3-Apr 9-May 2-May 5 

Short-eared Owl 29-Mar --- 4-Apr --- 10-Apr 2 

Northern Shoveler 15-Mar 15-Mar 4-Apr 23-Apr 22-Apr 13 
137 

Table 8.5. 

 

Species-specific timing of spring trans-Gulf migration in 2000 

Species are listed in order from earliest to latest migrants (based on mean date). Sample 

sizes indicate total numbers of individuals of each species recorded over the entire 

season, including both stopovers and flyby migrants. 95% confidence intervals are 

calculated only for sample sizes $ 5. 

Species First 

Date 

95% CI 

Lower 

Bound 

Mean 

Date 

95% CI 

Upper 

Bound 

Last 

Date 

N 

Slate-colored Junco --- --- 5-Apr --- --- 1 

Sora 11-Mar 31-Mar 5-Apr 10-Apr 2-May 51 

Least Bittern 16-Mar 28-Mar 6-Apr 14-Apr 3-May 11 

Blue-gray Gnatcatcher 26-Mar 21-Mar 6-Apr 22-Apr 5-May 6 

Yellow-bellied Sapsucker 26-Mar 24-Mar 6-Apr 19-Apr 25-Apr 6 

Tricolored Heron 10-Mar 31-Mar 6-Apr 13-Apr 20-May 87 

Worm-eating Warbler 15-Mar 29-Mar 7-Apr 15-Apr 17-May 17 

Pectoral Sandpiper 15-Mar 15-Mar 7-Apr 30-Apr 10-May 28 

Black-necked Stilt 19-Mar 7-Mar 7-Apr 8-May 28-Apr 11 

Snowy Egret 8-Mar 4-Apr 8-Apr 12-Apr 24-May 131 

Lesser Nighthawk 15-Mar 3-Apr 8-Apr 13-Apr 20-May 35 

Blue-winged Teal 8-Mar 1-Apr 8-Apr 15-Apr 2-May 226 

Prothonotary Warbler 15-Mar 4-Apr 9-Apr 14-Apr 24-Apr 26 

Upland Sandpiper 14-Mar 22-Mar 9-Apr 27-Apr 21-May 18 

Grasshopper Sparrow 16-Mar 5-Apr 9-Apr 13-Apr 19-May 53 

Cassin's Sparrow 30-Mar --- 10-Apr --- 21-Apr 2 

Northern Rough-winged 

Swallow 

15-Mar 3-Apr 10-Apr 16-Apr 7-May 20 

Yellow-breasted Chat 27-Mar 6-Apr 11-Apr 15-Apr 24-Apr 19 

Osprey 15-Mar 4-Apr 11-Apr 18-Apr 28-May 35 

Varied Bunting --- --- 12-Apr --- --- 1 

Ash-throated Flycatcher 3-Apr --- 12-Apr --- 21-Apr 2 

Lincoln's Sparrow 16-Mar 9-Apr 12-Apr 15-Apr 12-May 85 

Cliff Swallow 18-Mar 7-Apr 12-Apr 17-Apr 21-May 50 

Orchard Oriole 16-Mar 10-Apr 12-Apr 14-Apr 21-May 126 

Western Palm Warbler 18-Mar 28-Mar 12-Apr 27-Apr 12-May 9 

Common Moorhen 22-Mar 30-Mar 12-Apr 25-Apr 3-May 8 

Green Heron 7-Mar 10-Apr 12-Apr 15-Apr 29-May 301 

Northern Waterthrush 14-Mar 4-Apr 12-Apr 21-Apr 22-May 26 

Brewster's Warbler --- --- 14-Apr --- --- 1 

Carolina Wren --- --- 14-Apr --- --- 1 

Blue-winged Warbler 30-Mar 10-Apr 14-Apr 18-Apr 28-Apr 18 
138 

Table 8.5. 

 

Species-specific timing of spring trans-Gulf migration in 2000 

Species are listed in order from earliest to latest migrants (based on mean date). Sample 

sizes indicate total numbers of individuals of each species recorded over the entire 

season, including both stopovers and flyby migrants. 95% confidence intervals are 

calculated only for sample sizes $ 5. 

Species First 

Date 

95% CI 

Lower 

Bound 

Mean 

Date 

95% CI 

Upper 

Bound 

Last 

Date 

N 

Lark Sparrow 2-Apr 9-Apr 14-Apr 19-Apr 24-Apr 16 

American Kestrel 29-Mar 10-Apr 15-Apr 20-Apr 14-May 18 

Lesser Yellowlegs 14-Mar 5-Apr 16-Apr 27-Apr 5-May 39 

Black-bellied Plover 27-Mar --- 16-Apr --- 24-Apr 4 

Kentucky Warbler 16-Mar 13-Apr 16-Apr 18-Apr 6-May 65 

Scissor-tailed Flycatcher 29-Mar 10-Apr 16-Apr 22-Apr 21-May 29 

Cattle Egret 7-Mar 15-Apr 17-Apr 19-Apr 29-May 945 

Greater Yellowlegs 14-Mar 29-Mar 17-Apr 5-May 2-May 19 

Wood Thrush 19-Mar 10-Apr 17-Apr 24-Apr 29-Apr 14 

Aplomado Falcon --- --- 18-Apr --- --- 1 

Eurasian Starling --- --- 18-Apr --- --- 1 

White-throated Sparrow --- --- 18-Apr --- --- 1 

Northern Mockingbird 26-Mar 7-Apr 18-Apr 28-Apr 20-May 12 

Purple Gallinule 15-Mar 13-Apr 18-Apr 23-Apr 8-May 27 

Chipping Sparrow 29-Mar 10-Apr 18-Apr 26-Apr 24-Apr 13 

Sanderling 14-Mar --- 18-Apr --- 2-May 3 

Ruby-throated Hummingbird 15-Mar 16-Apr 18-Apr 21-Apr 20-May 116 

Chimney Swift 18-Mar 15-Apr 18-Apr 22-Apr 29-May 122 

Nashville Warbler 27-Mar 7-Apr 18-Apr 30-Apr 2-May 10 

Barn Swallow 8-Mar 17-Apr 19-Apr 21-Apr 29-May 538 

Brown-headed Cowbird 11-Mar 15-Apr 19-Apr 22-Apr 17-May 100 

Clay-colored Sparrow 29-Mar 14-Apr 19-Apr 23-Apr 26-Apr 26 

Eastern Kingbird 15-Mar 16-Apr 19-Apr 22-Apr 20-May 88 

Blue Grosbeak 9-Apr 14-Apr 19-Apr 25-Apr 3-May 11 

Swainson's Warbler 2-Apr 3-Apr 20-Apr 6-May 15-May 6 

Prairie Warbler 2-Apr 6-Apr 20-Apr 4-May 3-May 5 

Least Sandpiper 14-Mar 8-Apr 20-Apr 2-May 22-May 19 

Acadian Flycatcher 11-Apr 16-Apr 20-Apr 25-Apr 29-Apr 11 

Summer Tanager 27-Mar 16-Apr 20-Apr 25-Apr 6-May 23 

Audubon's Warbler 17-Apr --- 21-Apr --- 25-Apr 2 

House Wren 18-Apr --- 21-Apr --- 24-Apr 2 

unid. dowitcher 13-Mar --- 21-Apr --- 2-May 4 
139 

Table 8.5. 

 

Species-specific timing of spring trans-Gulf migration in 2000 

Species are listed in order from earliest to latest migrants (based on mean date). Sample 

sizes indicate total numbers of individuals of each species recorded over the entire 

season, including both stopovers and flyby migrants. 95% confidence intervals are 

calculated only for sample sizes $ 5. 

Species First 

Date 

95% CI 

Lower 

Bound 

Mean 

Date 

95% CI 

Upper 

Bound 

Last 

Date 

N 

Vesper Sparrow 3-Apr 15-Apr 21-Apr 27-Apr 24-Apr 8 

Painted Bunting 12-Apr 17-Apr 21-Apr 25-Apr 29-Apr 13 

Chuck-Will's-Widow 26-Mar 15-Apr 21-Apr 27-Apr 18-May 23 

White-winged Dove 17-Mar 18-Apr 21-Apr 24-Apr 21-May 113 

Mourning Dove 19-Mar 19-Apr 21-Apr 23-Apr 28-May 100 

Merlin 3-Apr 18-Apr 21-Apr 25-Apr 4-May 19 

Eastern Towhee --- --- 22-Apr --- --- 1 

Inca Dove --- --- 22-Apr --- --- 1 

Cerulean Warbler 14-Apr 18-Apr 22-Apr 25-Apr 2-May 11 

Brown Thrasher 14-Apr --- 22-Apr --- 27-Apr 4 

Indigo Bunting 26-Mar 20-Apr 22-Apr 23-Apr 21-May 173 

Scarlet Tanager 9-Apr 19-Apr 23-Apr 26-Apr 2-May 29 

Veery 14-Apr 15-Apr 23-Apr 1-May 2-May 5 

Solitary Sandpiper 2-Apr 15-Apr 23-Apr 30-Apr 1-May 9 

Warbling Vireo 18-Apr 23-Apr 30-Apr 4 

Common Yellowthroat 10-Mar 21-Apr 23-Apr 25-Apr 26-May 397 

Tennessee Warbler 7-Apr 22-Apr 23-Apr 25-Apr 21-May 111 

Brewer's Blackbird --- --- 24-Apr --- --- 1 

Loggerhead Shrike --- --- 24-Apr --- --- 1 

Marbled Godwit --- --- 24-Apr --- --- 1 

Northern Cardinal --- --- 24-Apr --- --- 1 

Orange-crowned Warbler --- --- 24-Apr --- --- 1 

Golden-winged Warbler 14-Apr 21-Apr 24-Apr 27-Apr 29-Apr 9 

Whimbrel 8-Apr 16-Mar 24-Apr 3-Jun 21-May 5 

Stilt Sandpiper 24-Apr --- 24-Apr --- 1-May 2 

Baltimore Oriole 4-Apr 24-Apr 25-Apr 26-Apr 11-May 120 

Gray Catbird 8-Apr 25-Apr 25-Apr 26-Apr 24-May 363 

Black-throated Green 

Warbler 

17-Mar 23-Apr 25-Apr 28-Apr 21-May 58 

Band-tailed Pigeon --- --- 26-Apr --- --- 1 

Common Grackle --- --- 26-Apr --- --- 1 

Yellow-headed Blackbird 21-Apr --- 26-Apr --- 7-May 4 
140 

Table 8.5. 

 

Species-specific timing of spring trans-Gulf migration in 2000 

Species are listed in order from earliest to latest migrants (based on mean date). Sample 

sizes indicate total numbers of individuals of each species recorded over the entire 

season, including both stopovers and flyby migrants. 95% confidence intervals are 

calculated only for sample sizes $ 5. 

Species First 

Date 

95% CI 

Lower 

Bound 

Mean 

Date 

95% CI 

Upper 

Bound 

Last 

Date 

N 

Yellow Warbler 14-Apr 24-Apr 26-Apr 27-Apr 21-May 60 

Bobolink 22-Apr 16-Apr 26-Apr 5-May 21-May 10 

Buff-breasted Sandpiper 24-Apr 15-Apr 27-Apr 8-May 2-May 5 

Cape May Warbler 21-Apr 20-Apr 27-Apr 3-May 3-May 5 

Rose-breasted Grosbeak 12-Apr 23-Apr 27-Apr 1-May 10-May 26 

Red-eyed Vireo 29-Mar 22-Apr 27-Apr 2-May 29-May 50 

Philadelphia Vireo 14-Apr 22-Apr 28-Apr 3-May 6-May 8 

Ovenbird 29-Mar 26-Apr 28-Apr 30-Apr 21-May 99 

Dickcissel 20-Apr 25-Apr 28-Apr 1-May 15-May 31 

Chestnut-sided Warbler 13-Apr 26-Apr 28-Apr 30-Apr 14-May 47 

Great Crested Flycatcher 3-Apr 9-Apr 28-Apr 17-May 21-May 6 

Yellow-billed Cuckoo 10-Apr 26-Apr 29-Apr 1-May 30-May 89 

Peregrine Falcon 11-Mar 26-Apr 29-Apr 1-May 28-May 108 

unid. Catharus thrush 13-Apr 24-Apr 29-Apr 5-May 14-May 44 

Swainson's Thrush 16-Apr 21-Apr 29-Apr 8-May 6-May 8 

Blackburnian Warbler 22-Apr 28-Apr 30-Apr 2-May 21-May 65 

Baird's Sandpiper --- --- 1-May --- --- 1 

Spotted Sandpiper 9-Apr 19-Apr 1-May 14-May 21-May 8 

Bay-breasted Warbler 22-Apr 1-May 1-May 2-May 21-May 77 

Magnolia Warbler 21-Apr 1-May 2-May 3-May 31-May 317 

unid. Empidonax flycatcher 22-Apr 21-Apr 2-May 13-May 23-May 8 

Rock Dove --- --- 3-May --- --- 1 

Black-whiskered Vireo --- --- 4-May --- --- 1 

American Pipit 8-Apr --- 4-May --- 30-May 2 

Western Kingbird 25-Apr --- 4-May --- 23-May 3 

American Redstart 30-Mar 1-May 4-May 7-May 24-May 112 

Black-billed Cuckoo 22-Apr 23-Apr 4-May 15-May 21-May 8 

Bank Swallow 28-Mar 1-May 5-May 9-May 22-May 58 

Least Flycatcher 24-Apr 22-Apr 6-May 19-May 22-May 5 

Blackpoll Warbler 19-Apr 3-May 6-May 9-May 31-May 57 

unid. Cuckoo 13-Apr 3-May 6-May 9-May 23-May 89 

Canada Warbler 24-Apr --- 7-May --- 20-May 2 
141 

Table 8.5. 

 

Species-specific timing of spring trans-Gulf migration in 2000 

Species are listed in order from earliest to latest migrants (based on mean date). Sample 

sizes indicate total numbers of individuals of each species recorded over the entire 

season, including both stopovers and flyby migrants. 95% confidence intervals are 

calculated only for sample sizes $ 5. 

Species First 

Date 

95% CI 

Lower 

Bound 

Mean 

Date 

95% CI 

Upper 

Bound 

Last 

Date 

N 

Ruddy Turnstone 1-May --- 7-May --- 14-May 3 

Sharp-shinned Hawk 8-May --- 9-May --- 11-May 2 

Hudsonian Godwit --- --- 10-May --- --- 1 

Eastern Wood-Pewee 22-Apr 7-May 10-May 14-May 30-May 45 

Common Nighthawk 3-Apr 8-May 11-May 15-May 27-May 114 

Wilson's Phalarope --- --- 14-May --- --- 1 

Mourning Warbler --- --- 18-May --- --- 1 

Brown-crested Flycatcher --- --- 19-May --- --- 1 

White-rumped Sandpiper 25-Apr 30-Apr 19-May 6-Jun 22-May 8 

Alder Flycatcher --- --- 21-May --- --- 1 

Couch's Kingbird --- --- 21-May --- --- 1 

Western Sandpiper --- --- 21-May --- --- 1 

Black-throated Blue Warbler 14-May --- 21-May --- 28-May 2 

Traill's Flycatcher 21-May --- 21-May --- 22-May 3 

Gray Kingbird --- --- 22-May --- --- 1 
142 

8

Neotropical migrants

Herons

Waterfowl

Shorebirds

14 20 26 1 7 13 19 25 1 7 13 19

March April May

Figure 8.4. Frequency distribution of species-specific peak arrival times in spring 2000. Each 

symbol indicates the mean date of occurrence for one of the species listed in Table 8.5. 

“Neotropical migrants” includes aerial insectivores and diurnal raptors. Species 

represented by single observations are not included. 
143 

8.3.4 Sexual Differences in Migration Timing 

In most sexually dichromatic species, males had earlier seasonal peaks than females (Table 8.6). Such 

differential migration has been documented widely and is generally believed to result evolutionarily from 

selective advantages accruing to early-arriving males that can secure the highest-quality territories. 

However, the mechanistic basis for differential migration (i.e., differences in onset versus rate of 

migration) remain poorly known (Woodrey and Chandler 1997). 

Table 8.6. 

 

Sexual differences in migration timing in spring 2000 

Mean date is shown for each sex followed by the range of dates in parentheses. The difference between male and 

female peaks is given in the last column (positive for early males and negative for early females). Species are 

listed in descending order of magnitude of male-female difference. 

Species % & %-& (days)

Yellow-throated Warbler March 27 (March 10-April 14) April 14 (na) 18 

Prothonotary Warbler April 6 (March 15-April 22) April 21 (na) 15 

Hooded Warbler March 30 (March 14-April 26) April 13 (March 16-May 21) 14 

Blackpoll Warbler May 2 (April 19-May 22) 14-May (April 28-May 31) 12 

American Redstart April 29 (March 30-May 21) May 11 (April 14-May 24) 12 

Ruby-throated Hummingbird April 12 (March 15-May 3) April 23 (March 21-May 20) 11 

Blue Grosbeak April 18 (April 9-25) April 26 (April 14-May 3) 8 

Black-and-white Warbler March 28 (March 9-May 18) April 5 (March 14-May 25) 8 

Belted Kingfisher March 29 (March 11-April 24) April 6 (March 15-May 20) 8 

Kentucky Warbler April 13 (March 16-May 2) April 20 (April 15-24) 7 

Blackburnian Warbler April 30 (April 22-May 21) May 7 (April 24-May 21) 7 

Black-throated Green Warbler April 24 (March 17-May 5) May 1 (April 22-May 21) 7 

Common Yellowthroat April 20 (March 10-May 26) April 27 (March 16-May 25) 7 

Summer Tanager April 18 (March 27-May 6) April 25 (April 24-May 2) 7 

Magnolia Warbler May 2 (April 21-May 22) May 8 (April 25-May 31) 6 

Rose-breasted Grosbeak April 26 (April 12-May 6) May 1 (April 24-May 10) 5 

Orchard Oriole April 10 (March 16-May 1) April 15 (March 29-May 21) 5 

Baltimore Oriole April 25 (April 4-May 6) April 29 (April 22-May 11) 4 

Indigo Bunting April 21 (March 26-May 2) April 25 (April 12-May 21) 4 

Yellow Warbler April 25 (April 14-May 3) April 29 (April 22-May 21) 4 

Purple Martin March 21 (March 8-May 6) March 25 (March 11-May 19) 4 

Chestnut-sided Warbler April 27 (April 13-May 6) April 30 (April 22-May 14) 3 

Golden-winged Warbler April 24 (April 14-29) April 27 (April 25-28) 3 

Scarlet Tanager April 22 (April 9-May 2) April 25 (April 9-May 2) 3 

Brown-headed Cowbird April 18 (March 11-May 13) April 20 (March 12-May 17) 2 

Tennessee Warbler April 23 (April 14-May 17) April 25 (April 13-May 13) 2 

Bay-breasted Warbler May 2 (April 22-May 13) May 3 (April 30-May 21) 1 

Cerulean Warbler April 22 (April 14-May 2) April 22 (April 15-25) 0 

Northern Parula March 26 (March 15-April 28) March 23 (March 15-May 3) -3 

Blue-winged Warbler April 14 (April 12-25) April 7 (March 30-April 12) -7 

Myrtle Warbler April 9 (April 8-9) March 24 (March 10-April 17) -16 
145 

 Chapter 9 

 

Ecology of Fall Trans-Gulf Migration 

Robert W. Russell 

9.1 INTRODUCTION

Cooke (1905) was the first to address meaningfully the possibility of fall trans-Gulf migration, postulating 

that the main route of long-distance neotropical migrants is from northwestern Florida across the Gulf. 

Since that time, there has been remarkably little progress in studying fall trans-Gulf migration, and 

knowledge on the subject has derived largely from occasional anecdotal reports from “ships of 

opportunity” (e.g., Paynter 1951, 1953; Siebenaler 1954) and a thesis that has remained unpublished 

(Buskirk 1968). 

Several problems inhibit the study of fall trans-Gulf migration. A fundamental difference between spring 

and fall migration over the northern Gulf is proximity of origin and its consequences for diel timing. 

During the spring, migrants departing northward from the southern shores of the Gulf on a given night do 

not reach our study area until the following morning, and are therefore actively migrating through our 

study area during daylight hours. In contrast, during the fall, migrants departing southward from the 

northern shore of the Gulf are aloft over the study platforms primarily at night. This difference in timing 

of activity with respect to ambient light levels has implications for detectability of migrants as well as for 

the possible ways that platforms may attract or influence migrants. 

In addition to the diel timing of migration, the abundance of migrating and dispersing insects during the 

fall is a significant problem hindering the quantitative study of fall trans-Gulf bird migration. Radar 

echoes over the Gulf in the spring can be interpreted unambiguously as birds (Chapters 7 and 8) for two 

main reasons. First, spring trans-Gulf migration by insects is rare and confined to just a handful of 

species (notably the green darner [Anax junius], spot-winged glider [Pantala hymenaea], and exposed 

bird-dropping moth [Acontia aprica]). Thus, the source population for any contribution to spring radar 

echoes by truly migratory insects on days favorable for northbound trans-Gulf migration is necessarily 

small. Second, classic spring radar patterns showing trans-Gulf arrivals (e.g., Figure 8.1) are confined to 

synoptic weather scenarios that are conducive to northbound trans-Gulf flight but completely prohibitive 

to passive advection southward over the Gulf. Although large advection events do sometimes occur in 

the spring (see Figure 13.4), they are strictly associated with frontal events and southward winds that are 

hostile to northbound migrants. On such days, both birds and drifted insects travel at low altitudes to 

minimize the impact of the unfavorable winds, and the low-altitude flight results in a near absence of any 

radar echoes of biological origin over the Gulf. 

In other words, while aerial plankton is indeed common over the Gulf during the spring, it occurs only 

under circumstances that result in the absence of significant radar echoes of any sort over the Gulf, and 

therefore, questions about the source of radar echoes are moot. 

Unfortunately, the above considerations do not apply similarly to the fall. During the fall, conditions 

favorable to both southward trans-Gulf bird migration and southward advection of aerial plankton 

coincide. In addition, very large numbers of dragonflies and noctuid and sphingid moths occur over the 

Gulf — in some cases, apparently intentionally — and these insects can have radar cross sections that 

approach those of migrant landbirds (Russell and Wilson 1997). Complicating the problem of evaluating 
146 

the relative contributions of birds and insects to radar echoes in the fall is the fact that migrating 

birds and insects exhibit remarkably similar patterns of diel and seasonal flight activity (see Chapter 

13). 

9.2 RADAR EVIDENCE OF MIGRATION ROUTES

Understanding the relative contributions of birds and insects to radar echoes is an essential prerequisite 

for being able to interpret fall radar data quantitatively. Since we know of no method for objectively 

decomposing NEXRAD reflectivity data into bird and insect contributions, we decided not to attempt to 

interpret fall radar data quantitatively. Nonetheless, it is useful to review the qualitative types of radar 

patterns that we observed during the fall as a starting point for future efforts. 

The most common pattern in the fall around the Gulf Coast is the “bull’s-eye” or “exploding flower” 

pattern, in which reflectivity was greatest near the radar site and declined rapidly with distance from the 

radar site (e.g., Figure 9.1). This pattern can be characterized by a negative exponential function of the 

form Z = a C e-b Cd

, where Z is reflectivity factor, d is distance from the radar site (or altitude above ground, 

which is proportional to distance), and b is a parameter that may be related to boundary-layer stability or 

other atmospheric conditions. This pattern differs dramatically from the “donut” pattern that 

characterizes major spring trans-Gulf bird migrations (Chapters 7, 8). This difference between spring and 

fall in the prevalence of the two types of radar signatures (i.e., donut vs. exploding flower) is a result of 

dramatic seasonal differences in the altitude of radar targets over the northern Gulf. In spring, radar 

profiles and direct observations from platforms and helicopters indicated that most migrants travel at 

altitudes from 300-1300 m on days with favorable flying conditions, and occasionally as high as 1900 m. 

In contrast, surveillance radar studies during September and October at Lake Charles showed that most 

nocturnal migrants were in the lowest altitudinal stratum (<300 m) with a rapid decline in density at 

higher altitudes (Able 1970). 

Rapid transformations between donuts and exploding flowers at sunrise and sunset provide strong 

evidence that the prevalence of one or the other type of radar signature is primarily due to quantitative 

differences in flight altitude, rather than some other qualitative factor such as species composition. 

Nearly instantaneous transformations from donuts to exploding flowers occur predictably along the 

northern Gulf Coast in spring shortly after sunset, when migrants rapidly descend in altitude. Conversely, 

rapid transformations from exploding flowers to donuts can be seen predictably at sunrise in fall 

NEXRAD images from Brownsville and Key West, associated with a rapid ascent by overwater migrants 

to daytime flight altitudes. 
147 

Figure 9.1. NEXRAD base reflectivity image from the Mobile site (MOB) showing a large movement 

of migrating birds and insects departing southward from the northeastern Gulf Coast 

shortly after nightfall on September 30, 1998. Synoptic weather for this flight was 

classified as ELOW. The radar scan was obtained in clear-air mode at an elevation of 

0.5E. Image is courtesy of WSI Corporation. 

The classic fall exploding flower begins shortly after sunset and reaches peak “bloom” within two hours 

(e.g., Figure 9.2). This temporal pattern corresponds closely with the known timing of departure by 

nocturnal bird migrants. Unfortunately (from the perspective of identifying the source of the radar 

echoes), the pattern also corresponds closely with the known timing of departure by nocturnal insect 

migrants. A number of previous studies along the northern Gulf Coast have shown that large numbers of 

insects — particularly noctuid moths — begin ascending into the airspace about a half hour after sunset 

(e.g., Beerwinkle et al. 1994). 

Fall exploding flowers occurred in the study area from August through November, but seemed to be most 

intense (highest reflectivity) from late September through mid-October. This period corresponds roughly 

to the seasonal peak of passerine bird migration along the northern Gulf Coast. However, complicating 

matters of radar image interpretation is the fact that many migratory insects have fall activity patterns that 

are remarkably similar to those of migrating birds (reviewed by Russell et al. 1998). For example, studies 

in the Brazos River Valley of coastal Texas found fall peaks in the migration of corn earworm moths 

(Heliothis zea) around October 5 in one year and September 25 in another year (Beerwinkle et al. 1995). 

Classic post-sunset exploding flowers (e.g., Figures 9.1, 9.2) were common during the fall along the 

northern Gulf Coast from Houston (HGX) to Tallahassee (TLH). At Corpus Christi (CRP), the pattern 
148 

was characteristically truncated at the coast, with a wave of offshore radar echoes sweeping 

southward or southwestward as the night progressed (Figure 9.3) 

At the Brownsville NEXRAD site (BRO), exploding flowers were rare and confined to November. 

Classic-looking post-sunset exploding flowers at Brownsville (e.g., Figure 9.4) were strictly associated 

with major insect dispersal events that brought large numbers of moths and butterflies to PN975. On 

other occasions, the principal radar signature was a “semi-donut” indicating an arrival of targets at high 

altitudes from the northeast during morning daylight hours (Figure 9.5). Similar to the spring, fallouts 

were occasionally evident along the lower Texas coast or on the coast of Tamaulipas (Figure 9.6). 

Migrants that had departed from the northeastern Gulf Coast the previous evening were often visible 

heading south offshore from the west coast of the Florida Peninsula in morning NEXRAD images from 

the Tampa Bay (TBW) site, and this migration stream could often be seen in Key West (BYX) imagery 

making landfall in Cuba in late morning or early afternoon (Figure 9.7). 

9.3 DIRECT OBSERVATIONS OF THE FLIGHT DIRECTIONS OF MIGRANTS

Direct studies of flight orientation were difficult during the fall since most migrants were aloft over the 

platforms during hours of darkness. 

Platform lighting and other circumstances at VK786 permitted a series of visual studies from the helipad. 

The results of these studies are summarized in Table 9.1. Mean vanishing bearing varied from 136°-197° 

(SE-SSW). At the observed western extreme, migrants flying directly toward 197E would arrive in the 

eastern Bay of Campeche, with landfall near the Tabasco/Campeche border. At the observed eastern 

extreme, migrants flying directly toward 136° would make landfall in Cuba just west of Havana. 

Assuming the birds in these studies maintained their flight directions with no lateral drift, mean landfall 

would have been along the eastern Bay of Campeche in two cases, on the Yucatan in four cases, on the 

northern shore of Honduras in one case, and in western Cuba in two cases. 
149 

A B

C D

Figure 9.2. Time series of NEXRAD base reflectivity images from the Houston site (HGX) showing a large 

movement of migrating birds and insects departing the Upper Texas Coast after nightfall on October 7, 

1998. Synoptic weather for this flight was classified as ELOW. A. Sunset. B. Sunset + 0.5 h. C. 

Sunset + 1.5 h. D. Sunset + 2.5 h. Radar scans were obtained in clear-air mode at an elevation of 

0.5E. Images are courtesy of WSI Corporation. 
150 

Figure 9.3. NEXRAD base reflectivity images from the Corpus Christi site (CRP) showing a large movement of presumed mig

traveling southwestward off the central Texas coast on the night of September 23-24, 1998. Synoptic weather for t

classified as MCH. Left: 02:32 Z (21:32 CDT). Right: 04:35 Z (23:35 CDT). Radar scans were obtained in precian elevation of 0.5E. Images are courtesy of WSI Corporation. 
151 

18:27 CST 18:51 CST

19:19 CST 22:10 CST

Figure 9.4. Time series of NEXRAD images showing the development of an “exploding flower” at 

Brownsville (BRO) after nightfall on November 8, 2000. Synoptic weather for this event was 

classified as GF/ELOW. Radar scans were obtained in precipitation mode at an elevation of 

0.5E. Images are courtesy of TAP Publishing Company. 
152 

Figure 9.5. NEXRAD images from the Brownsville site (BRO) showing a large movement of migrating birds arriving on the lo

the morning of September 30, 1998. Synoptic weather for this flight was classified as TLOW, though a high pressu

developed over the western Gulf overnight, resulting in ambient winds from the south in the radar coverage area at timages. Left: Reflectivity image showing the arriving migrants just offshore. Right: Corresponding velocity image of movement. Red indicates movement away from the radar site and green indicates movement toward the radar. Nintense echoes offshore were moving against the wind, generally toward the southwest. The sparse echoes over land northward with the wind and probably were attributable to dispersing moths. Radar scans were obtained in precipitaelevation of 0.5E. Images are courtesy of WSI Corporation. 
153 

Figure 9.6. NEXRAD base reflectivity image from the Brownsville site (BRO) showing a fallout 

of migrants along the Tamaulipas coast on the morning of November 9, 2000 (14:22 

Z). Synoptic weather for this event was classified as GF/ELOW. The radar scan was 

obtained in clear-air mode at an elevation of 0.5E. Image is courtesy of TAP 

Publishing Company. 
154 

Figure 9.7. NEXRAD base reflectivity image from the Key West site (BYX) showing a large flight 

of migrants approaching Cuba from the NNW on the morning of October 8, 2000 (15:29 

Z). Synoptic weather for this event was classified as MCH. The radar scan was 

obtained in precipitation mode at an elevation of 0.5E. Image is courtesy of TAP 

Publishing Company. 
155 

Table 9.1. 

Summary of case studies of nocturnal flight direction at VK786 in fall 1999 

Aug 20-21 Aug 21-22 Sep 5-6 Sep 10-11

(early) 

Sep 10-11 

(late) 

Sep 11-12 Sep 12-13 Sep 13-

Wind Direction: W SW WSW N NE E E NNE/NE Wind Speed (m/s): 4 2 7 4 7 7 7 10 Sky Condition: Clear P. cloudy P. cloudy Clear Clear Clear Clear Clear Synoptic Weather Type: GH GH GH MCH MCH ECH ECH MCH 

NNE 0 0 1 0 0 0 0 0 NE 0 0 0 0 0 0 0 0 ENE 0 0 0 0 0 0 0 0 E 0 0 0 0 0 0 0 0 ESE 1 0 4 0 0 0 0 0 SE 5 1 14 5 2 1 0 0 SSE 2 4 6 14 2 0 1 2 S 3 2 1 14 27 7 3 14 SSW 1 0 0 2 11 10 4 12 SW 0 0 0 2 2 4 2 1 WSW 0 0 0 0 0 0 0 0 W 0 0 0 0 0 0 0 0 WNW 0 0 0 0 0 0 0 0 NW 0 0 0 0 0 0 0 0 NNW 0 0 0 0 0 0 0 0 

Number of vanishing bearings N 0 0 0 0 0 0 0 0 

Mean Vector: 153° 161° 136° 169° 185° 197° 196° 189° N: 12 7 26 37 42 22 10 29 Length of Mean Vector (r): 0.90 0.97 0.91 0.93 0.95 0.94 0.94 0.97 Rayleigh P: <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 
156 

The regression equation relating mean vanishing bearing to the direction toward which the wind was 

blowing was VB = 135E + 0.23·wind (r2

 = 0.84, P = 0.0005). Setting VB = wind and solving the 

equation yields a wind direction for which migrants fly strictly downwind and do not compensate. This 

direction of no compensation is 175E, suggesting that the preferred flight direction of fall migrants over 

VK786 is slightly east of due south. Direct linear flight from VK786 toward 175E would take migrants to 

the eastern portion of the northern shore of the Yucatan Peninsula. 

Variation in mean nightly flight directions in relation to the lateral wind component is depicted in Figure 

9.8. The relationship was strong and clear, and fortuitous circumstances provided evidence that 

individual flight behavior was responsible for the pattern, as opposed to pseudodrift. On the night of 

September 10-11, there was a distinct shift in wind direction from N to NE after an adequate sample of 

vanishing bearings had already been obtained for analysis, and a second study was undertaken for 

comparison. Along with the shift in wind from N to NE, mean flight direction shifted from 169° to 185° 

(Table 9.1). This change between observation periods was highly significant (Watson-Williams test: F1,77

= 12.8, P = 0.0006). Because it is most unlikely that an entirely new assemblage of migrants materialized 

in the airspace over the course of several hours, it can be concluded that the flight behavior of individual 

migrants was being influenced over a short time period by the changing wind. 

Over the course of the study we made numerous efforts to study nocturnal flight orientation using the 

“moonwatching” method. This technique involves watching for migrants passing in front of the face of 

the moon, recording their entry and exit points, and using spherical geometry along with data on the 

moon’s path to compute the migrants’ trajectories (Nisbet 1959). Most of our efforts were foiled by 

clouds or lack of adequate samples of bird targets due to synoptic weather circumstances. One successful 

and informative case study was undertaken at MI622 on the night of September 12-13, 2000 (Figure 9.9). 

Synoptic weather for this flight was classified as ECH. This study yielded a sample of 39 tracks with a 

mean angle of 209E (95% CI = 198E-220E, r = 0.83, Rayleigh P < 0.0001). Virtually all tracks were west 

of due S. Although based on just a single night, results from this study were consistent with our general 

impression based on many anecdotal observations and other lines of evidence that nocturnal movements 

in the far western Gulf in fall are usually toward the SW or SSW. 

9.4 “OVERSHOOTING” OF THE GULF OF MEXICO BY NON-TRANS-GULF 

MIGRANTS

One of the interesting features of the fall migration offshore was the frequent occurrence of a variety of 

species that do not typically winter south of the northern Gulf Coast. These species were evidently 

mostly “overshoots” that inadvertently traveled past their intended destinations and found themselves 

unexpectedly over water at first light, or else circum-Gulf migrants that inadvertently drifted eastward 

over the Gulf during nocturnal flight. Accordingly, these species were often observed flying north or 

west during daylight hours, presumably trying to get back to land. 
157 

963 0 3 6 9

Westward Eastward

Lateral wind component (m/s)

205E

195E

185E

175E

165E

155E

145E

135E

125E

SSW

S

SSE

SE

Mean Vanishing Bearing

Y = 167 – 4.2CX

F1,7 = 408

r2 = 0.98

P << 0.0001

Figure 9.8. Relationship between lateral wind strength and the orientation of nocturnal 

migration at VK786 during fall 1999. Each point in the figure represents results 

from one night’s study (Table 9.1). 
158 

Figure 9.9. Frequency distribution of flight tracks from “moonwatching” at MI622 on the 

night of September 12-13, 2000. The radial line and arc indicate the circular 

mean and 95% confidence interval, respectively. 
159 

All species displaying behavior suggestive of overshooting were either medium-distance migrants 

(Table 6.6) or short-distance migrants (Table 6.7). Mean vanishing bearings for all medium- and 

short-distance migrants observed in flight during the fall are shown in Table 9.2. Among the species 

listed in Table 9.2, only three were also commonly observed during the spring migration (N ≥ 50 

individuals; Table 7.7), and therefore believed to be “intentional” trans-Gulf migrants. These three 

species (Lincoln’s Sparrow, Grasshopper Sparrow, Brown-headed Cowbird) were classified as mediumdistance migrants, and had vanishing bearings of ~SSW to S that were not unexpected for fall trans-Gulf 

migrants. 

Among the presumed overshoot species, three species had adequate samples of vanishing bearings for 

separate analyses (Figure 9.10). These species showed significant orientation toward the WSW (House 

Wren), W (Marsh Wren), and NW (Ruby-crowned Kinglet). Intraspecifically, all three species showed 

some indication of bimodality in flight direction, with one group headed NW to NE and another group 

headed primarily W to WSW. Among these three, the short-distance migrant (Ruby-crowned Kinglet) 

was more likely to be seen heading in a northerly (as opposed to westerly) direction than the two mediumdistance migrants (Marsh Wren and House Wren). 

A comparison of vanishing bearings of all medium- versus short-distance migrant species is shown in 

Figure 9.11. Short-distance migrant species had a mean vanishing bearing that was more to the north ( ̄x = 

303°, r = 0.79, Rayleigh P = 0.004) than medium-distance migrant species ( ̄x = 263°, r = 0.71, Rayleigh P 

< 0.0001). The difference between the two groups was statistically significant (Watson-Williams test: 

F1,27 = 4.29, P = 0.048). 

In summary, among the medium- and short-distance migrant species, only Lincoln’s Sparrow, 

Grasshopper Sparrow, and Brown-headed Cowbird showed indications of being intentional trans-Gulf 

migrants. All other species are presumed to have been over the Gulf inadvertently. Vanishing bearings 

of overshoots tended to cluster in the W-WSW and NW-NE sectors, with medium-distance migrants 

tending toward the former and short-distance migrants tending toward the latter. 

The distinction between the westbound and northbound migrants could result either from facultative 

individual decisions or from inter-individual differences. In the case of facultative individual decisions, 

an overshoot migrant may “decide” whether to attempt to head back to the northern Gulf Coast based 

upon its body condition and remaining fat stores in relation to its assessment of the energy requirements 

needed to complete the corrective flight. Migrants lacking adequate energy stores for such corrective 

action may simply travel downwind (usually toward the WSW or SW in the fall) to maximize the amount 

of time before they deplete their energy stores. Alternatively, westbound birds may be individuals with 

intended winter destinations along the western (as opposed to northern) Gulf Coast. The fact that 

medium-distance migrants (which have more southerly wintering distributions) were more likely to be 

westbound than were short-distance migrants is consistent with the hypothesis of inter-individual 

variation. However, some short-distance migrants were observed traveling in directions that, if 

maintained until landfall, would have taken them to points well south of their species’ normal wintering 

range (e.g., Figure 9.10), in support of the hypothesis of condition-dependence. 
160 

Table 9.2. 

 

Mean vanishing bearings of medium-distance migrants 

(MDM) and short-distance migrants (SDM) 

The value for each species represents the mean from fall 

1998 and fall 1999 combined. 

Species MDM/SDM 0 N 

Brown Thrasher SDM 348E 4 

Pine Siskin MDM 338E 2 

Long-eared Owl SDM 338E 1 

Eastern Phoebe MDM 326E 4 

Ruby-crowned Kinglet SDM 326E 14 

Lark Sparrow MDM 315E 1 

Brewer's Blackbird MDM 315E 1 

White-throated Sparrow SDM 315E 1 

Common Grackle SDM 315E 1 

American Pipit MDM 312E 3 

Northern Mockingbird MDM 304E 2 

Vesper Sparrow MDM 300E 3 

Marsh Wren MDM 280E 10 

Yellow-shafted Flicker MDM 275E 4 

White-crowned Sparrow MDM 270E 1 

Golden-crowned Kinglet SDM 270E 2 

Eastern Towhee SDM 270E 1 

Clay-colored Sparrow MDM 267E 3 

House Wren MDM 256E 13 

Western Meadowlark MDM 248E 4 

Hermit Thrush MDM 248E 3 

Swamp Sparrow MDM 225E 1 

Sedge Wren MDM 225E 1 

Loggerhead Shrike MDM 225E 1 

Song Sparrow SDM 225E 1 

Brown-headed Cowbird MDM 214E 2 

Grasshopper Sparrow MDM 202E 1 

Orange-crowned Warbler MDM 191E 2 

Lincoln's Sparrow MDM 180E 2 
161 

2 2

2

2

1 1

1

1

3 2 2

2

2

1 1

1

1

6 6

6

6

4 4

4

4

2 2

2

2

Ruby-crowned Kinglet

0 = 325E

r = 0.63

P = 0.003

Marsh Wren

0 = 280E

r = 0.57

P = 0.034

House Wren

0 = 256E

r = 0.67

P = 0.002

Figure 9.10. Distributions of vanishing bearings of Rubycrowned Kinglets, Marsh Wrens, and House 

Wrens. Data are from fall 1998 and fall 1999 

combined. 
162 

Medium-Distance

Migrants

Short-Distance

Migrants

2 2

2

2

1 1

1

1

4 4

4

4

3 3

3

3

2 2

2

2

1 1

1

1

Figure 9.11. Comparison of vanishing bearings of medium-distance migrants and shortdistance migrants. Each data point represents the mean for one species. 

Data are from fall 1998 and fall 1999 combined. 

9.5 EFFECTS OF SYNOPTIC WEATHER ON PATTERNS OF ABUNDANCE ON 

PLATFORMS

Patterns of abundance across the platforms were compared among weather types in fall 1999. Longdistance migrants were most abundant on the easternmost platform studied (VK786) on ECH, GH, and 

TLOW days, which prevailed during the early part of the migration in August and September (Figure 

9.12). A significant western component to the migration became evident during the latter half of the 

migration in October and November, when MCH and ELOW days prevailed, and migrants then showed 

a bimodal pattern of highest abundance in the far east and far west. Long-distance migrants were very 

scarce on BH days. 

Medium- and short-distance migrants, which tended to have migration peaks later in the season, exhibited 

the bimodal geographic pattern on MCH and ELOW days, with peak abundance on the far eastern and 

far western platforms (Figures 9.13, 9.14). Medium-distance migrants and short-distance migrants were 

generally scarce under other weather conditions. 

9.6 INTERSPECIFIC VARIATION IN MIGRATION ROUTES

There were dramatic differences among species in patterns of abundance across the Gulf (Table 9.3), 

implicating species-specific migration routes. 

The frequency distribution of mean species-specific longitudinal positions is shown in Figure 9.15. It 

should be noted that peak abundance of many long-distance migrants occurred at VK786 (Table 9.3), 

implying that the true center of abundance for these species lies in the eastern Gulf where we were unable 

to sample due to absence of platforms (Figure 2.1). Thus, the “mean species-specific longitudinal 

positions” indicated in Figure 9.15 are severely biased toward the west for many species, and should only 

be interpreted relative to other species. Medium-distance migrants had more western peaks of abundance 

than both long- and short-distance migrants, consistent with the idea that some of these species may 

intentionally cross the western Gulf of Mexico (Chapter 9.4). 
163 

103

63

23 27

23 27 32 35

21

72

ELOW

267

877

64 286

130

138 165 185

60

574

MCH

131

61

25 82

32 45 97 108

83

625

ECH

68

143

44 35

51 37 60 128

41

241

GH

54

18

31 26

18 32 84 68

19

428

TLOW

4

7

6 9

5 1 6 1

0

5

BH

Figure 9.12. Patterns of abundance of long-distance migrant landbirds (aerial insectivores and neotropical 

migrants) on platforms in fall 1999 in relation to synoptic weather type. Numbers shown for a 

given weather type indicate total numbers of individuals initiating stopovers on each platform 

across all days characterized by that weather type. 
164 

175

81

13 12

17 9 13 13

16

40

ELOW

199

82

8 25

31 13 23 20

5

60

MCH

36

9

5 5

6 1 7 6

1

5

ECH

18

9

3 4

6 1 5 8

4

9

GH

19

7

2 2

6 1 3 3

2

5

TLOW

Figure 9.13. Patterns of abundance of medium-distance migrants on platforms in fall 1999 in relation to 

synoptic weather type. Numbers shown for a given weather type indicate total numbers of 

individuals initiating stopovers on each platform across all days characterized by that weather 

type. 
165 

24

16

5 6

5 4 4 4

3

14

ELOW

38

21

3 11

12 6 10 12

5

19

MCH

7

3

1 4

3 1 1 3

3

5

ECH

1

2

0 1

1 0 2 0

0

5

GH

4

3

1 0

2 2 1 1

0

0

TLOW

Figure 9.14. Patterns of abundance of short-distance migrants on platforms in fall 1999 in relation to synoptic 

weather type. Numbers shown for a given weather type indicate total numbers of individuals 

initiating stopovers on each platform across all days characterized by that weather type. 
166 

Table 9.3. 

 

Comparative longitudinal distributions of fall trans-Gulf migrants 

Numbers shown indicate numbers of individuals undertaking stopovers on each platform during fall 1999. 

Species are listed in order from most westerly distribution at top to most easterly distribution at bottom. Species 

with <5 stopovers are not included. 

Species PN975 MI622 HI561 GB189 VE265 SM147 GC18 EW826 SP89 VK786 Total 

Western Meadowlark 23 31 0 0 0 0 0 0 0 0 54 

Clapper Rail 2 4 0 0 0 0 0 0 0 0 6 

Pyrrhuloxia 2 4 0 0 0 0 0 0 0 0 6 

Scissor-tailed Flycatcher 24 16 0 3 0 0 0 0 0 0 43 

Lincoln's Sparrow 53 138 4 2 4 0 0 2 3 4 210 

Common Ground-Dove 14 9 0 2 1 0 0 0 0 1 27 

Pied-billed Grebe 6 0 0 1 0 0 1 0 0 0 8 

Yellow-shafted Flicker 0 4 0 0 0 1 0 0 0 0 5 

Sora 30 0 1 2 0 0 0 0 0 5 38 

Grasshopper Sparrow 16 48 1 1 1 3 3 3 2 2 80 

Osprey 4 2 1 0 0 0 0 0 0 1 8 

Wilson's Warbler 12 12 3 1 4 1 2 1 0 0 36 

Burrowing Owl 2 2 0 1 0 0 1 0 0 0 6 

Yellow-headed Blackbird 0 4 0 1 0 1 0 0 0 0 6 

Barn Owl 2 2 0 0 0 0 0 0 0 1 5 

House Wren 37 99 5 13 13 4 12 8 2 11 204 

Traill's Flycatcher 0 22 2 0 0 1 2 0 2 2 31 

Chipping Sparrow 6 21 1 1 2 0 1 4 1 2 39 

Chuck-Will's-Widow 18 12 6 9 2 0 8 1 1 0 57 

Great Blue Heron 9 0 0 0 1 0 3 0 0 1 14 

Sedge Wren 1 10 0 1 1 2 0 1 0 1 17 

Nashville Warbler 23 10 0 2 1 1 2 2 0 7 48 

Indigo Bunting 205 15 5 14 0 4 9 6 1 63 322 

Loggerhead Shrike 2 2 0 1 1 0 0 0 1 0 7 

Mourning Warbler 6 9 2 3 0 0 2 0 0 3 25 

Ruby-throated Hummingbird 22 32 8 5 6 5 5 0 1 9 93 

Brown-headed Cowbird 6 8 0 1 5 0 1 1 1 1 24 

Black-throated Green 

Warbler 

216 6 8 42 1 17 8 14 7 51 370 

Lark Sparrow 4 7 0 1 0 0 0 4 0 1 17 

White-winged Dove 40 86 9 26 7 7 17 13 6 12 223 

Blue-headed Vireo 0 2 0 2 1 0 0 0 0 0 5 

Northern Mockingbird 9 11 2 1 4 0 2 0 1 4 34 

Eastern Meadowlark 3 8 2 2 4 1 0 0 0 2 22 

Yellow-bellied Flycatcher 2 2 1 0 0 0 1 0 0 1 7 

Ruby-crowned Kinglet 28 37 6 9 5 3 6 6 2 14 116 

Virginia Rail 1 4 1 0 0 0 0 0 0 2 8 

Cliff Swallow 17 1 3 12 0 0 10 1 0 2 46 

Savannah Sparrow 20 32 4 7 4 3 7 8 4 13 102 

Orange-crowned Warbler 13 15 3 4 6 3 2 5 3 6 60 

Dickcissel 13 21 6 7 2 2 4 4 6 8 73 

Common Yellowthroat 165 69 6 36 34 13 20 25 12 82 462 

Clay-colored Sparrow 4 15 6 4 8 1 4 2 1 6 51 

Canada Warbler 6 2 2 1 2 0 0 0 1 4 18 

Purple Gallinule 2 0 1 0 1 0 3 0 0 0 7 

Barn Swallow 39 8 7 37 4 2 10 6 2 21 136 

Least Flycatcher 9 12 7 4 12 0 12 0 2 6 64 

Vesper Sparrow 2 2 1 0 0 1 0 1 0 2 9 

Yellow-breasted Chat 18 65 7 11 5 1 14 6 9 37 173 

Belted Kingfisher 6 3 1 3 0 0 3 1 1 4 22 

White-throated Sparrow 1 12 0 0 1 2 0 3 0 6 25 
167 

Table 9.3. 

 

Comparative longitudinal distributions of fall trans-Gulf migrants 

Numbers shown indicate numbers of individuals undertaking stopovers on each platform during fall 1999. 

Species are listed in order from most westerly distribution at top to most easterly distribution at bottom. Species 

with <5 stopovers are not included. 

Species PN975 MI622 HI561 GB189 VE265 SM147 GC18 EW826 SP89 VK786 Total 

Bank Swallow 6 1 0 4 1 2 4 0 0 3 21 

White-crowned Sparrow 1 4 0 0 0 1 0 2 0 2 10 

Cattle Egret 41 14 19 21 13 2 35 45 4 6 200 

Warbling Vireo 1 1 0 1 1 0 1 0 0 1 6 

Peregrine Falcon 52 37 12 22 16 5 62 29 26 19 280 

Eastern Phoebe 3 13 1 3 3 2 3 3 2 8 41 

Field Sparrow 0 2 0 3 1 0 0 0 1 1 8 

Baltimore Oriole 6 8 1 0 0 1 4 4 1 7 32 

Acadian Flycatcher 2 3 5 2 2 2 2 0 0 5 23 

Mourning Dove 109 133 28 45 27 35 84 83 28 131 703 

Orchard Oriole 6 8 2 3 1 0 0 2 3 10 35 

Brown Thrasher 9 6 3 3 4 4 4 2 5 9 49 

Blue-gray Gnatcatcher 0 4 1 3 1 0 1 1 1 3 15 

Myrtle Warbler 16 22 11 9 16 15 14 13 7 25 148 

Green Heron 6 2 3 5 8 7 6 8 3 2 50 

Eastern Wood-Pewee 5 10 2 2 1 2 3 2 0 13 40 

Summer Tanager 0 3 2 3 3 1 2 2 1 2 19 

Rose-breasted Grosbeak 2 1 1 2 1 0 2 1 0 3 13 

Winter Wren 1 1 0 1 3 0 1 1 1 1 10 

Great Crested Flycatcher 4 7 0 3 9 0 10 2 2 7 44 

Prairie Warbler 3 2 1 3 2 2 4 3 1 4 25 

Golden-crowned Kinglet 0 1 1 1 2 1 0 0 0 2 8 

Purple Martin 4 0 4 6 0 1 2 0 0 8 25 

Eastern Kingbird 3 4 1 2 6 3 2 0 5 6 32 

Merlin 3 2 0 2 0 0 7 3 1 3 21 

Song Sparrow 2 6 0 1 3 0 3 2 0 8 15 

unid. Empidonax flycatcher 0 7 1 1 3 2 3 1 3 6 27 

Red-eyed Vireo 1 8 2 3 6 2 1 2 0 12 37 

Yellow-throated Warbler 6 0 2 3 0 3 2 2 1 8 27 

Blue-winged Teal 1 0 0 0 0 1 3 0 0 1 6 

Marsh Wren 9 28 7 11 12 5 16 13 4 51 156 

Gray Catbird 5 8 1 9 3 0 7 10 2 19 64 

Blue Grosbeak 1 4 0 3 0 0 3 0 0 8 19 

Pine Warbler 1 0 0 2 2 1 0 5 0 2 13 

Black-throated Blue Warbler 3 0 0 4 0 2 2 2 0 7 20 

American Kestrel 5 9 1 5 2 1 6 2 4 25 60 

Ovenbird 32 12 4 12 8 3 5 18 7 84 185 

Louisiana Waterthrush 0 0 0 1 0 2 0 1 0 1 5 

Yellow-billed Cuckoo 1 1 0 2 3 2 0 1 1 6 17 

Cape May Warbler 1 0 0 1 0 0 0 0 1 2 5 

Western Palm Warbler 3 2 4 5 8 7 8 8 3 19 67 

Blue-winged Warbler 1 0 0 1 1 0 2 0 0 3 8 

White-eyed Vireo 1 3 3 0 1 2 12 11 0 9 42 

Northern Parula 13 1 10 32 8 23 21 23 3 65 199 

Swainson's Thrush 0 3 1 1 2 1 3 7 1 7 26 

Northern Waterthrush 4 2 2 4 2 3 7 7 2 18 51 

Swainson's Warbler 1 0 0 1 0 0 1 2 0 3 8 

Swamp Sparrow 1 5 0 1 5 0 6 3 6 14 41 

Red-winged Blackbird 0 2 0 0 0 0 0 0 0 4 6 

Yellow Warbler 25 14 3 5 3 8 10 51 6 98 223 
168 

Table 9.3. 

 

Comparative longitudinal distributions of fall trans-Gulf migrants 

Numbers shown indicate numbers of individuals undertaking stopovers on each platform during fall 1999. 

Species are listed in order from most westerly distribution at top to most easterly distribution at bottom. Species 

with <5 stopovers are not included. 

Species PN975 MI622 HI561 GB189 VE265 SM147 GC18 EW826 SP89 VK786 Total 

Scarlet Tanager 1 2 0 1 1 0 2 1 2 8 18 

Black-and-white Warbler 11 6 3 6 6 5 3 12 5 55 112 

Kentucky Warbler 2 0 0 0 6 1 2 6 3 11 31 

Northern Rough-winged 

Swallow 

0 0 1 1 0 0 0 0 0 3 5 

American Robin 0 0 0 1 0 1 1 0 1 2 6 

Hooded Warbler 2 3 4 13 5 11 8 17 3 54 120 

Tennessee Warbler 4 7 2 13 1 20 8 15 1 77 148 

Worm-eating Warbler 1 0 1 1 3 2 2 1 1 13 25 

Yellow-throated Vireo 1 0 0 1 0 0 0 0 0 5 7 

Bobolink 0 0 0 0 0 0 1 2 1 1 5 

Eurasian Collared-Dove 0 0 0 0 0 1 1 2 2 2 8 

Prothonotary Warbler 1 0 0 3 9 8 44 47 12 72 196 

American Redstart 16 13 13 16 8 19 10 27 32 250 404 

Magnolia Warbler 7 3 9 29 24 43 25 58 28 356 582 

Bay-breasted Warbler 1 0 0 2 2 3 1 11 3 37 60 

Blackburnian Warbler 1 0 1 3 1 3 1 2 5 34 51 

Chestnut-sided Warbler 0 1 0 4 1 3 1 3 3 45 61 

Cerulean Warbler 0 0 0 1 0 0 0 1 1 13 16 

PN975 HI561 GB 189 GC 18

Long-distance migrants

Medium-distance migrants

Short-distance migrants

MI622 SM147 EW 826 SP89 VK 786

Figure 9.15. Frequency distribution of mean species-specific longitudinal positions in fall 1999. Each 

symbol indicates the mean position calculated from the entire sample of stopovers (i.e., all 

weather types) for one of the species listed in Table 9.3. Longitude is indicated by reference 

to the relative positions of selected study platforms. 
169 

9.7 AGE-RELATED VARIATION IN MIGRATION ROUTES

For a number of species of warblers we were able to determine the age of many individuals based on 

plumage, and to analyze geographic patterns of age ratio. Geographic variation in age ratio provides 

important insights concerning migration strategies because a high percentage of young (along with a 

comparatively low absolute density of individual migrants) characterizes the periphery of a species’ 

migration route (Ralph 1981). 

Adult males generally have the most distinctive plumage, and in many species it is difficult to determine 

the age of females or to distinguish adult females from young males. These problems mean that while 

virtually all adult males can be identified and counted, many adult females are excluded from samples of 

individuals of determined age; as a result, the proportion of adults in visually identified samples will be 

biased low. To address this problem, we estimated the proportion of adults in a given sample as (2·X)/N, 

where N is the sample size and X is the proportion of adult males in the sample. 

Geographic patterns of abundance and age ratio for 16 species of warblers are presented in Figures 9.16-

9.23. 

Bay-breasted Warblers were relatively more common toward the east and had a higher proportion of 

adults in the eastern region, consistent with an easterly trans-Gulf route (Figure 9.16). However, the 

proportion of adults even in the eastern portion of our study area was very low (8%), implying that the 

main route of this species is considerably to the east of our study area, and perhaps even along the Florida 

peninsula. Prairie Warblers showed a pattern similar to Bay-breasted Warblers, except that they were 

very scarce even in the eastern region (Figure 9.16). 

Chestnut-sided Warblers, Blackburnian Warblers, Black-and-white Warblers, Kentucky Warblers, and 

Prothonotary Warblers were more common toward the east, with peak abundance at VK786, and all 

showed an obvious gradient in age ratio across the Gulf, with the largest proportion of adults in the 

eastern region (Figures 9.17-9.20). These patterns are consistent with a primarily eastern trans-Gulf route 

that includes VK786 along its western periphery. 

Like the previous group, Northern Parulas, American Redstarts, and Yellow Warblers exhibited peaks in 

both abundance and proportion of adults at the eastern end of our study area (Figures 9.18-9.19, 9.21). 

However, these species were also moderately common in the central and western regions of the Gulf, with 

moderately high proportions of adults (≥ 18%) in the western region. These patterns are consistent with a 

primarily eastern trans-Gulf route that is nonetheless broader and less geographically concentrated than in 

the previous group. 

Mourning Warblers and Wilson’s Warblers had peak abundance in the western region and showed an 

obvious gradient in age ratio across the Gulf, with the largest proportion of adults in the western region 

(Figure 9.23). These patterns indicate a primarily western trans-Gulf route – or perhaps even a primarily 

circum-Gulf route – for these species, with PN975 and MI622 along the eastern periphery. 
170 

Prairie

Warbler

3

2 1 3

2 2 4

4

3

1

0% 9%

14%

Bay-breasted

Warbler

1

0 0 2

2 3 1

37

11

3

0% 0%

8%

Figure 9.16. Patterns of abundance and age ratios of Bay-breasted Warblers and Prairie 

Warblers across the Gulf during fall 1999. The numbers in circles indicate 

the total numbers of individuals of the species recorded on each platform 

over the course of the season. The numbers shown below indicate the 

percentages of adults in the samples of individuals identifiable to age in the 

western (PN975, MI622, HI561), central (GB189, VR265, SM147, GC18), 

and eastern (EW826, SP89, VK786) portions of the study region. 
171 

Chestnut-sided

Warbler

Blackburnian

Warbler

1

0 1 3

1 3 1

34

2

5

0% 12%

22%

0

1 0 4

1 3 1

45

3

3

0% 0%

46%

Figure 9.17. Patterns of abundance and age ratios of Chestnut-sided Warblers and 

Blackburnian Warblers across the Gulf during fall 1999. The numbers in 

circles indicate the total numbers of individuals of the species recorded on 

each platform over the course of the season. The numbers shown below 

indicate the percentages of adults in the samples of individuals identifiable to 

age in the western (PN975, MI622, HI561), central (GB189, VR265, 

SM147, GC18), and eastern (EW826, SP89, VK786) portions of the study 

region. 
172 

Black-and-White

Warbler

Northern

Parula

13

1 10 32

8 23 21

65

23

3

20% 10%

30%

10

6 3 6

6 5 3

55

12

5

0% 0%

32%

Figure 9.18. Patterns of abundance and age ratios of Black-and-white Warblers and 

Northern Parulas across the Gulf during fall 1999. The numbers in circles 

indicate the total numbers of individuals of the species recorded on each 

platform over the course of the season. The numbers shown below indicate 

the percentages of adults in the samples of individuals identifiable to age in 

the western (PN975, MI622, HI561), central (GB189, VR265, SM147, 

GC18), and eastern (EW826, SP89, VK786) portions of the study region. 
173 

American

Redstart

Kentucky

Warbler

2

0 0 0

6 1 2

11

6

3

0% 33%

50%

16

13 13 16

8 19 10

250

27

32

28% 24%

31%

Figure 9.19. Patterns of abundance and age ratios of American Redstarts and Kentucky 

Warblers across the Gulf during fall 1999. The numbers in circles indicate 

the total numbers of individuals of the species recorded on each platform over 

the course of the season. The numbers shown below indicate the percentages 

of adults in the samples of individuals identifiable to age in the western 

(PN975, MI622, HI561), central (GB189, VR265, SM147, GC18), and 

eastern (EW826, SP89, VK786) portions of the study region. 
174 

Prothonotary

Warbler

Hooded

Warbler

1

0 0 3

9 8 44

72

47

12

0% 21%

25%

2

3 4 13

5 11 8

54

17

3

17% 52%

32%

Figure 9.20. Patterns of abundance and age ratios of Prothonotary Warblers and Hooded 

Warblers across the Gulf during fall 1999. The numbers in circles indicate 

the total numbers of individuals of the species recorded on each platform over 

the course of the season. The numbers shown below indicate the percentages 

of adults in the samples of individuals identifiable to age in the western 

(PN975, MI622, HI561), central (GB189, VR265, SM147, GC18), and 

eastern (EW826, SP89, VK786) portions of the study region. 
175 

Yellow

Warbler

Magnolia

Warbler

7

3 9 29

24 43 25

356

58

28

29% 8%

26%

25

14 3 5

3 8 10

98

51

6

18% 0%

19%

Figure 9.21. Patterns of abundance and age ratios of Yellow Warblers and Magnolia 

Warblers across the Gulf during fall 1999. The numbers in circles indicate 

the total numbers of individuals of the species recorded on each platform 

over the course of the season. The numbers shown below indicate the 

percentages of adults in the samples of individuals identifiable to age in the 

western (PN975, MI622, HI561), central (GB189, VR265, SM147, GC18), 

and eastern (EW826, SP89, VK786) portions of the study region. 
176 

Common

Yellowthroat

Black-throated

Green 

Warbler

216

6 8 42

1 17 8

51

14

7

44% 20%

32%

165

69 6 36

34 13 20

82

25

12

18% 12%

19%

Figure 9.22. Patterns of abundance and age ratios of Common Yellowthroats and Blackthroated Green Warblers across the Gulf during fall 1999. The numbers in 

circles indicate the total numbers of individuals of the species recorded on 

each platform over the course of the season. The numbers shown below 

indicate the percentages of adults in the samples of individuals identifiable to 

age in the western (PN975, MI622, HI561), central (GB189, VR265, SM147, 

GC18), and eastern (EW826, SP89, VK786) portions of the study region. 
177 

Mourning

Warbler

Wilson’s

Warbler

12

12 3 1

4 1 2

0

1

0

47% 29%

0%

6

9 2 3

0 0 2

3

0

0

20% 0%

0%

Figure 9.23. Patterns of abundance and age ratios of Mourning Warblers and Wilson’s 

Warblers across the Gulf during fall 1999. The numbers in circles indicate the 

total numbers of individuals of the species recorded on each platform over the 

course of the season. The numbers shown below indicate the percentages of 

adults in the samples of individuals identifiable to age in the western (PN975, 

MI622, HI561), central (GB189, VR265, SM147, GC18), and eastern 

(EW826, SP89, VK786) portions of the study region. 

The remaining species showed more complex patterns. Hooded Warblers and Magnolia Warblers had 

peak abundance at VK786 but had the highest proportion of adults in the central or western region 

(Figures 9.20, 9.21). Common Yellowthroats and Black-throated Green Warblers had peak abundance in 

the far west at PN975 but a secondary peak in the far east at VK786, with high proportions of adults in 

both the western and eastern regions (Figure 9.22). 
178 

9.8 SEASONAL TIMING

As was the case in the spring (Chapter 8.3.1), the volume of trans-Gulf migration detected on platforms 

varied dramatically from day to day during the fall (Figure 9.24). 

In 1999, migration peaks seemed to be approximately Gaussian and symmetric around the peak of 

approximately October 1. The broad seasonal pattern was more difficult to evaluate in 1998 because of 

the series of irregular coverage gaps resulting from hurricane evacuations. 

Early in the fall, most migration peaks occurred on ECH, GH, and TLOW days (Figure 9.24). Starting 

around the last week in September, migration peaks occurred mostly on MCH and ELOW days. 

Contrary to popular expectation, peaks in platform-observed migration were not usually associated with 

precipitation (Figure 9.24). Lack of association with precipitation during the fall makes sense because of 

the proximity of our platforms to points of migratory takeoff: rain over our platforms usually meant that 

rain was also occurring along the northern Gulf Coast, precluding takeoff by southbound trans-Gulf 

migrants. 

9.9 INTERSPECIFIC VARIATION IN MIGRATION SCHEDULES

Data on species-specific fall migration schedules are tabulated in Table 9.4 and summarized graphically 

in Figure 9.25. Neotropical migrants peaked from late August through mid-October, with maximum 

diversity in early October. Medium-distance migrants peaked in late October, and short-distance 

migrants peaked in early November. Herons peaked from mid-September through mid-October. 

Waterfowl peaked in late October and early November, except the two teal species, which peaked in late 

September and early October. Shorebirds had widely varying migration schedules, with different species 

peaking from late August through the end of October, but with maximum diversity in early September. 
179 

0

August September October

0

500

1000

1500

2000

November

10 20 1 10 20 1 10 20 1 10

1

10

100

1000

10000

200

400

600

800

1000

2500

1

10

100

1000

10000

1200

1400

1600

1800

2000

Birds Observed Per Day

ECH

GH

MCH

TL

GH

GH

GH

EL

EL

EL

EL

MCH

ECH

MCH

MCH

TL

MCH

MCH

MCH

MCH MCH

MCH

EL

ECH GH TL TL ECH

TL

ECH

GH

1998

1999

Figure 9.24. Phenology of platform-observed fall migration in 1998 and 1999. The inset figures show the same 

data (x+1)-transformed on a logarithmic scale for perspective. Synoptic weather types are indicated 

above intraseasonal migration peaks, and inside a box on days with rain. Note that several 

considerable coverage gaps occurred in 1998 due to a series of hurricane evacuations (Table 2.3). 
180 

Table 9.4. 

 

Species-specific timing of fall trans-Gulf migration in 1999 

Species are listed in order from earliest to latest migrants (based on mean date). Sample sizes indicate total numbers 

of individuals of each species recorded over the entire season, including both stopovers and flyby migrants. 95% 

confidence intervals are calculated only for sample sizes ≥ 5. 

Species First 

Date 

95% CI 

Lower 

Bound 

Mean 

Date 

95% CI 

Upper 

Bound 

Last 

Date 

N 

Ringed Kingfisher --- --- 5-Aug --- --- 1 

Purple Martin 5-Aug 17-Aug 20-Aug 24-Aug 2-Oct 70 

Cerulean Warbler 15-Aug 12-Aug 21-Aug 30-Aug 20-Oct 18 

Prothonotary Warbler 5-Aug 18-Aug 21-Aug 23-Aug 10-Nov 209 

Least Sandpiper 16-Aug 26-Jul 21-Aug 16-Sep 9-Oct 6 

Yellow Warbler 4-Aug 20-Aug 23-Aug 26-Aug 20-Oct 189 

Louisiana Waterthrush 17-Aug 14-Aug 28-Aug 11-Sep 10-Sep 5 

Kentucky Warbler 14-Aug 24-Aug 29-Aug 3-Sep 5-Oct 35 

Orchard Oriole 5-Aug 24-Aug 29-Aug 3-Sep 16-Oct 44 

Hooded Warbler 5-Aug 24-Aug 30-Aug 5-Sep 20-Oct 130 

Worm-eating Warbler 15-Aug 19-Aug 30-Aug 10-Sep 19-Oct 29 

American Golden Plover --- --- 1-Sep --- --- 1 

Canada Warbler 22-Aug 29-Aug 1-Sep 4-Sep 8-Sep 18 

MacGillivray's Warbler 25-Aug --- 2-Sep --- 7-Sep 3 

Tropical/Couch's Kingbird 19-Aug --- 2-Sep --- 17-Sep 2 

Solitary Sandpiper 16-Aug 27-Aug 3-Sep 9-Sep 30-Sep 18 

Pectoral Sandpiper 9-Aug --- 4-Sep --- 25-Sep 3 

Cliff Swallow 8-Aug 1-Sep 4-Sep 8-Sep 31-Oct 67 

Willet 3-Sep --- 5-Sep --- 7-Sep 2 

Spotted Sandpiper 18-Aug 25-Aug 6-Sep 17-Sep 21-Sep 8 

Bank Swallow 7-Aug 3-Sep 6-Sep 10-Sep 10-Oct 54 

Prairie Warbler 6-Aug 29-Aug 7-Sep 15-Sep 3-Nov 28 

Eastern Kingbird 13-Aug 4-Sep 7-Sep 10-Sep 24-Sep 51 

Acadian Flycatcher 5-Aug 1-Sep 8-Sep 15-Sep 12-Oct 24 

Baird's Sandpiper --- --- 9-Sep --- --- 1 

Black-chinned 

Hummingbird 

--- --- 9-Sep --- --- 1 

Bell's Vireo 30-Aug --- 9-Sep --- 19-Sep 2 

Yellow-bellied Flycatcher 9-Aug 20-Aug 9-Sep 28-Sep 16-Oct 7 

Least Flycatcher 5-Aug 6-Sep 9-Sep 13-Sep 5-Oct 59 

Great Crested Flycatcher 15-Aug 7-Sep 10-Sep 13-Sep 9-Oct 55 

Mourning Warbler 19-Aug 4-Sep 10-Sep 16-Sep 18-Oct 26 

unid. Empidonax flycatcher 5-Aug 5-Sep 11-Sep 17-Sep 20-Oct 44 

Loggerhead Kingbird --- --- 12-Sep --- --- 1 

Western Sandpiper --- --- 12-Sep --- --- 1 

Yellow-breasted Chat 16-Aug 10-Sep 12-Sep 14-Sep 21-Oct 182 

Western Kingbird 11-Sep --- 12-Sep --- 14-Sep 2 

Upland Sandpiper 14-Aug 3-Sep 13-Sep 22-Sep 18-Oct 33 

Eurasian Collared-Dove 16-Aug 29-Aug 13-Sep 27-Sep 9-Oct 8 
181 

Table 9.4. 

 

Species-specific timing of fall trans-Gulf migration in 1999 

Species are listed in order from earliest to latest migrants (based on mean date). Sample sizes indicate total numbers 

of individuals of each species recorded over the entire season, including both stopovers and flyby migrants. 95% 

confidence intervals are calculated only for sample sizes ≥ 5. 

Species First 

Date 

95% CI 

Lower 

Bound 

Mean 

Date 

95% CI 

Upper 

Bound 

Last 

Date 

N 

Tricolored Heron 18-Aug 29-Aug 13-Sep 29-Sep 17-Oct 12 

Rock Dove --- --- 14-Sep --- --- 1 

Baltimore Oriole 27-Aug 11-Sep 14-Sep 18-Sep 7-Oct 33 

Green Heron 14-Aug 10-Sep 15-Sep 21-Sep 4-Nov 98 

Chuck-Will's-Widow 16-Aug 11-Sep 16-Sep 20-Sep 3-Nov 65 

Red-winged Blackbird 15-Aug 9-Aug 16-Sep 23-Oct 15-Nov 9 

Ruby-throated 

Hummingbird 

14-Aug 14-Sep 16-Sep 17-Sep 1-Nov 239 

Lesser Yellowlegs 9-Aug --- 16-Sep --- 31-Oct 4 

Traill's Flycatcher 4-Aug 10-Sep 16-Sep 23-Sep 11-Oct 29 

Little Blue Heron 9-Aug 9-Sep 17-Sep 24-Sep 20-Oct 70 

Northern Waterthrush 15-Aug 12-Sep 17-Sep 22-Sep 20-Oct 54 

Swainson's Warbler 16-Aug 28-Aug 17-Sep 7-Oct 18-Oct 9 

Barn Swallow 6-Aug 15-Sep 18-Sep 22-Sep 10-Nov 403 

Rufous Hummingbird --- --- 19-Sep --- --- 1 

Yellow-headed Blackbird 10-Sep 10-Sep 19-Sep 27-Sep 30-Sep 6 

Red-eyed Vireo 15-Aug 12-Sep 19-Sep 26-Sep 24-Oct 43 

unid. Dowitcher 28-Aug --- 19-Sep --- 12-Nov 4 

Groove-billed Ani 8-Sep --- 20-Sep --- 30-Sep 3 

White Ibis 20-Aug 8-Aug 20-Sep 1-Nov 1-Nov 18 

Loggerhead Shrike 14-Aug 26-Aug 22-Sep 19-Oct 1-Nov 7 

Western Wood-Pewee 23-Sep --- 23-Sep --- 24-Sep 2 

Blue-winged Warbler 7-Sep 11-Sep 23-Sep 5-Oct 12-Oct 8 

Belted Kingfisher 6-Aug 14-Sep 23-Sep 3-Oct 19-Oct 30 

Northern Parula 14-Aug 20-Sep 25-Sep 29-Sep 24-Oct 188 

Blue-winged Teal 25-Aug 15-Sep 25-Sep 5-Oct 11-Nov 106 

Northern Rough-winged 

Swallow 

14-Aug 6-Sep 25-Sep 14-Oct 9-Nov 14 

Cattle Egret 9-Aug 19-Sep 25-Sep 2-Oct 17-Nov 366 

Least Bittern 5-Aug 15-Sep 25-Sep 6-Oct 18-Oct 21 

Ruddy Turnstone 4-Sep 17-Aug 26-Sep 5-Nov 6-Nov 6 

Snowy Egret 15-Aug 9-Sep 26-Sep 13-Oct 20-Oct 43 

Lark Sparrow 28-Aug 19-Sep 26-Sep 3-Oct 18-Oct 18 

Black-and-white Warbler 5-Aug 22-Sep 26-Sep 30-Sep 22-Oct 112 

Gray-cheeked Thrush 22-Aug --- 27-Sep --- 16-Oct 3 

Inca Dove 24-Sep --- 27-Sep --- 4-Oct 3 

unid Plegadis ibis 29-Aug 6-Feb 27-Sep 17-May 1-Nov 5 

Common Nighthawk 1-Sep 21-Sep 27-Sep 4-Oct 18-Oct 19 

Yellow-billed Cuckoo 5-Aug 20-Sep 28-Sep 5-Oct 21-Oct 33 
182 

Table 9.4. 

 

Species-specific timing of fall trans-Gulf migration in 1999 

Species are listed in order from earliest to latest migrants (based on mean date). Sample sizes indicate total numbers 

of individuals of each species recorded over the entire season, including both stopovers and flyby migrants. 95% 

confidence intervals are calculated only for sample sizes ≥ 5. 

Species First 

Date 

95% CI 

Lower 

Bound 

Mean 

Date 

95% CI 

Upper 

Bound 

Last 

Date 

N 

Purple Gallinule 15-Aug 21-Sep 28-Sep 5-Oct 17-Oct 27 

Blue-gray Gnatcatcher 10-Sep 19-Sep 28-Sep 8-Oct 6-Nov 14 

Cave Swallow 29-Sep --- 29-Sep --- 29-Sep 2 

Wilson's Warbler 30-Aug 23-Sep 29-Sep 5-Oct 8-Nov 37 

Blackburnian Warbler 31-Aug 23-Sep 29-Sep 5-Oct 20-Oct 47 

Merlin 14-Sep 26-Sep 29-Sep 1-Oct 2-Nov 56 

Eurasian Starling --- --- 30-Sep --- --- 1 

Painted Bunting 21-Sep --- 1-Oct --- 18-Oct 4 

Sanderling 30-Aug 12-Sep 1-Oct 20-Oct 19-Oct 8 

Vermilion Flycatcher 22-Sep --- 1-Oct --- 11-Oct 2 

Green-winged Teal 9-Sep 3-May 1-Oct 1-Mar 8-Nov 5 

Burrowing Owl --- --- 2-Oct --- --- 1 

Yellow-throated Vireo 26-Aug 20-Sep 2-Oct 13-Oct 19-Oct 9 

Whip-Poor-Will 15-Sep --- 2-Oct --- 18-Oct 4 

Osprey 23-Aug 24-Sep 2-Oct 10-Oct 2-Nov 26 

Ovenbird 13-Aug 29-Sep 2-Oct 5-Oct 3-Nov 181 

Yellow-throated Warbler 16-Aug 24-Sep 2-Oct 10-Oct 2-Nov 27 

Red-breasted Nuthatch 14-Sep --- 2-Oct --- 21-Oct 2 

Yellow-crowned NightHeron 11-Aug 8-Sep 2-Oct 26-Oct 20-Oct 23 

Great Egret 6-Aug 23-Sep 3-Oct 13-Oct 7-Nov 67 

Black-throated Blue Warbler 12-Sep 29-Sep 4-Oct 8-Oct 20-Oct 22 

Semipalmated Plover 19-Aug --- 4-Oct --- 8-Oct 2 

Philadelphia Vireo 22-Sep --- 4-Oct --- 15-Oct 3 

Eastern Wood-Pewee 25-Aug 29-Sep 4-Oct 9-Oct 27-Oct 42 

Bobolink 7-Sep 10-Sep 4-Oct 28-Oct 27-Oct 5 

Golden-winged Warbler --- --- 5-Oct --- --- 1 

Chestnut-sided Warbler 31-Aug 1-Oct 5-Oct 9-Oct 21-Oct 47 

Peregrine Falcon 3-Sep 4-Oct 5-Oct 6-Oct 12-Nov 372 

American Redstart 5-Aug 3-Oct 5-Oct 7-Oct 20-Oct 277 

Clay-colored Sparrow 29-Aug 1-Oct 5-Oct 10-Oct 1-Nov 51 

Long-billed Dowitcher 25-Aug --- 6-Oct --- 17-Oct 2 

Summer Tanager 7-Aug 28-Sep 6-Oct 14-Oct 4-Nov 21 

Black-crowned Night-Heron 15-Aug 14-Sep 6-Oct 28-Oct 19-Oct 10 

Cooper's Hawk --- --- 7-Oct --- --- 1 

Dickcissel 14-Aug 1-Oct 7-Oct 12-Oct 9-Nov 96 

Scarlet Tanager 21-Sep 2-Oct 7-Oct 11-Oct 20-Oct 20 

Black Swift --- --- 8-Oct --- --- 1 

Brown Thrasher 17-Sep 5-Oct 8-Oct 11-Oct 3-Nov 50 
183 

Table 9.4. 

 

Species-specific timing of fall trans-Gulf migration in 1999 

Species are listed in order from earliest to latest migrants (based on mean date). Sample sizes indicate total numbers 

of individuals of each species recorded over the entire season, including both stopovers and flyby migrants. 95% 

confidence intervals are calculated only for sample sizes ≥ 5. 

Species First 

Date 

95% CI 

Lower 

Bound 

Mean 

Date 

95% CI 

Upper 

Bound 

Last 

Date 

N 

Red-headed Woodpecker 15-Sep --- 8-Oct --- 24-Oct 4 

Swainson's Thrush 17-Aug 2-Oct 8-Oct 14-Oct 20-Oct 33 

unid. Catharus thrush 1-Oct 1-Oct 8-Oct 16-Oct 20-Oct 22 

Cuban Pewee --- --- 9-Oct --- --- 1 

Nashville Warbler 29-Aug 3-Oct 9-Oct 15-Oct 5-Nov 42 

Blue Grosbeak 9-Sep 3-Oct 9-Oct 15-Oct 11-Nov 24 

Red-necked Phalarope --- --- 10-Oct --- --- 1 

House Wren 16-Sep 8-Oct 10-Oct 12-Oct 3-Nov 197 

Warbling Vireo 24-Sep 28-Sep 10-Oct 21-Oct 20-Oct 6 

Flammulated Owl 10-Oct --- 10-Oct --- 11-Oct 2 

Cassin's Sparrow 21-Sep --- 10-Oct --- 31-Oct 3 

Black-billed Cuckoo 22-Sep 25-Sep 10-Oct 26-Oct 21-Oct 5 

Townsend's Warbler --- --- 11-Oct --- --- 1 

Veery 7-Oct 5-Oct 11-Oct 16-Oct 19-Oct 5 

Cape May Warbler 25-Sep 21-Sep 11-Oct 30-Oct 3-Nov 5 

Wood Thrush 24-Sep 4-Oct 11-Oct 18-Oct 23-Oct 9 

Clapper Rail 23-Sep 30-Sep 11-Oct 22-Oct 18-Oct 6 

Great Blue Heron 26-Aug 7-Oct 11-Oct 15-Oct 16-Nov 143 

Magnolia Warbler 11-Sep 11-Oct 11-Oct 12-Oct 3-Nov 439 

Common Grackle 18-Sep --- 12-Oct --- 3-Nov 4 

Common Yellowthroat 3-Sep 10-Oct 12-Oct 13-Oct 16-Nov 366 

Chimney Swift 8-Oct 8-Oct 12-Oct 17-Oct 19-Oct 13 

White-eyed Vireo 23-Sep 11-Oct 13-Oct 14-Oct 19-Oct 57 

Bay-breasted Warbler 17-Sep 11-Oct 13-Oct 14-Oct 20-Oct 62 

Tennessee Warbler 6-Sep 11-Oct 13-Oct 14-Oct 4-Nov 135 

American Kestrel 15-Sep 9-Oct 13-Oct 16-Oct 15-Nov 78 

Greater Yellowlegs 11-Sep --- 13-Oct --- 20-Oct 2 

Western Palm Warbler 23-Sep 11-Oct 13-Oct 15-Oct 3-Nov 68 

Sharp-shinned Hawk --- --- 14-Oct --- --- 1 

Audubon's Warbler 10-Oct --- 14-Oct --- 21-Oct 3 

Downy Woodpecker 11-Oct --- 14-Oct --- 18-Oct 2 

Sora 1-Sep 10-Oct 14-Oct 18-Oct 10-Nov 63 

Marsh Wren 15-Sep 12-Oct 14-Oct 16-Oct 3-Nov 161 

Pied-billed Grebe 12-Sep 11-Oct 14-Oct 18-Oct 1-Nov 30 

Rose-breasted Grosbeak 30-Aug 10-Oct 14-Oct 19-Oct 21-Oct 28 

Green-tailed Towhee 24-Sep --- 15-Oct --- 31-Oct 3 

Virginia Rail 30-Sep 4-Oct 16-Oct 28-Oct 16-Nov 8 

Northern Mockingbird 14-Aug 8-Oct 16-Oct 25-Oct 8-Nov 35 

Mourning Dove 22-Aug 15-Oct 17-Oct 18-Oct 16-Nov 603 
184 

Table 9.4. 

 

Species-specific timing of fall trans-Gulf migration in 1999 

Species are listed in order from earliest to latest migrants (based on mean date). Sample sizes indicate total numbers 

of individuals of each species recorded over the entire season, including both stopovers and flyby migrants. 95% 

confidence intervals are calculated only for sample sizes ≥ 5. 

Species First 

Date 

95% CI 

Lower 

Bound 

Mean 

Date 

95% CI 

Upper 

Bound 

Last 

Date 

N 

Lesser Nighthawk 7-Sep 24-Sep 17-Oct 8-Nov 11-Nov 6 

Gray Catbird 8-Sep 14-Oct 17-Oct 19-Oct 16-Nov 75 

Yellow-shafted Flicker 30-Sep 7-Oct 17-Oct 26-Oct 5-Nov 9 

Scissor-tailed Flycatcher 21-Sep 15-Oct 17-Oct 19-Oct 1-Nov 66 

Common Ground-Dove 30-Sep 14-Oct 17-Oct 21-Oct 13-Nov 29 

Blackpoll Warbler 12-Oct --- 17-Oct --- 27-Oct 4 

Blue-headed Vireo 23-Sep 29-Sep 17-Oct 5-Nov 2-Nov 5 

Black-throated Green 

Warbler 

2-Sep 16-Oct 17-Oct 19-Oct 3-Nov 181 

Indigo Bunting 26-Sep 16-Oct 17-Oct 19-Oct 12-Nov 152 

Black-bellied Whistling 

Duck 

--- --- 18-Oct --- --- 1 

Black-necked Stilt --- --- 18-Oct --- --- 1 

Broad-winged Hawk --- --- 18-Oct --- --- 1 

Couch's Kingbird --- --- 18-Oct --- --- 1 

Short-eared Owl --- --- 18-Oct --- --- 1 

American Bittern 9-Oct --- 18-Oct --- 27-Oct 4 

Ruby-crowned Kinglet 10-Sep 15-Oct 18-Oct 21-Oct 17-Nov 128 

American Coot 18-Oct 17-Oct 18-Oct 19-Oct 22-Oct 11 

Reddish Egret --- --- 19-Oct --- --- 1 

Yellow Rail --- --- 19-Oct --- --- 1 

Long-billed Thrasher 1-Oct --- 19-Oct --- 14-Nov 3 

Common Snipe 23-Sep 8-Oct 19-Oct 31-Oct 13-Nov 11 

Burrowing Owl 18-Oct 17-Oct 20-Oct 22-Oct 23-Oct 6 

White-breasted Nuthatch --- --- 21-Oct --- --- 1 

Yellow-bellied Sapsucker 18-Oct --- 21-Oct --- 24-Oct 2 

King Rail 11-Oct --- 21-Oct --- 1-Nov 2 

American Wigeon 29-Sep 12-Sep 21-Oct 29-Nov 15-Nov 7 

Brown-headed Cowbird 13-Aug 12-Oct 21-Oct 30-Oct 16-Nov 31 

Northern Harrier 4-Oct 10-Oct 21-Oct 2-Nov 12-Nov 7 

Great-tailed Grackle --- --- 22-Oct --- --- 1 

Spotted Towhee --- --- 22-Oct --- --- 

Bronzed Cowbird 11-Oct --- 22-Oct --- 2-Nov 2 

Chipping Sparrow 30-Aug 16-Oct 22-Oct 28-Oct 16-Nov 38 

Sage Thrasher --- --- 23-Oct --- --- 1 

Lincoln's Sparrow 20-Sep 21-Oct 23-Oct 25-Oct 17-Nov 135 

Boat-tailed Grackle --- --- 24-Oct --- --- 1 

Sedge Wren 10-Oct 20-Oct 24-Oct 27-Oct 3-Nov 17 

White-winged Dove 14-Sep 22-Oct 24-Oct 25-Oct 16-Nov 197 
185 

Table 9.4. 

 

Species-specific timing of fall trans-Gulf migration in 1999 

Species are listed in order from earliest to latest migrants (based on mean date). Sample sizes indicate total numbers 

of individuals of each species recorded over the entire season, including both stopovers and flyby migrants. 95% 

confidence intervals are calculated only for sample sizes ≥ 5. 

Species First 

Date 

95% CI 

Lower 

Bound 

Mean 

Date 

95% CI 

Upper 

Bound 

Last 

Date 

N 

Barn Owl 18-Oct 14-Oct 24-Oct 3-Nov 3-Nov 5 

Eastern Meadowlark 10-Oct 21-Oct 25-Oct 28-Oct 14-Nov 21 

Northern Shoveler 7-Sep 1-Oct 25-Oct 19-Nov 1-Nov 7 

Eastern Phoebe 1-Oct 23-Oct 25-Oct 28-Oct 16-Nov 41 

Surf Scoter --- --- 26-Oct --- --- 1 

Hermit Thrush 17-Oct 20-Oct 26-Oct 31-Oct 3-Nov 7 

Winter Wren 10-Oct 21-Oct 26-Oct 31-Oct 3-Nov 11 

Orange-crowned Warbler 5-Oct 23-Oct 26-Oct 29-Oct 16-Nov 57 

Common Moorhen 17-Oct --- 27-Oct --- 1-Nov 3 

Slate-colored Junco 15-Oct --- 27-Oct --- 7-Nov 4 

Western Meadowlark 17-Oct 24-Oct 27-Oct 30-Oct 14-Nov 30 

Killdeer 17-Oct 16-Oct 28-Oct 8-Nov 15-Nov 8 

Golden-crowned Kinglet 21-Oct 23-Oct 28-Oct 2-Nov 3-Nov 8 

Swamp Sparrow 5-Oct 25-Oct 28-Oct 1-Nov 16-Nov 41 

Grasshopper Sparrow 30-Sep 26-Oct 28-Oct 31-Oct 16-Nov 77 

Northern Pintail 23-Oct 19-Oct 29-Oct 7-Nov 16-Nov 15 

Pyrrhuloxia 22-Oct 25-Oct 29-Oct 1-Nov 2-Nov 10 

Brewer's Blackbird 23-Oct --- 29-Oct --- 2-Nov 4 

unid. Meadowlark 7-Oct 27-Oct 30-Oct 1-Nov 4-Nov 37 

Vesper Sparrow 14-Oct 25-Oct 30-Oct 4-Nov 12-Nov 12 

White-crowned Sparrow 20-Oct 26-Oct 30-Oct 4-Nov 12-Nov 11 

Savannah Sparrow 16-Sep 28-Oct 30-Oct 1-Nov 17-Nov 109 

American Avocet --- --- 31-Oct --- --- 1 

Say's Phoebe --- --- 31-Oct --- --- 1 

Sprague's Pipit --- --- 31-Oct --- --- 1 

Mallard 23-Oct --- 31-Oct --- 16-Nov 2 

American Goldfinch --- --- 1-Nov --- --- 1 

Pine Siskin --- --- 1-Nov --- --- 1 

Pink-sided Junco --- --- 1-Nov --- --- 1 

Gray-headed Junco 1-Nov --- 1-Nov 1-Nov 2 

Greater White-fronted 

Goose 

1-Nov 30-Oct 1-Nov 2-Nov 2-Nov 9 

Gadwall 31-Oct 31-Oct 1-Nov 2-Nov 2-Nov 6 

Eastern Towhee 23-Oct --- 1-Nov --- 11-Nov 2 

Ring-necked Duck --- --- 2-Nov --- --- 1 

Ross's Goose --- --- 2-Nov --- --- 1 

Redhead 2-Nov --- 2-Nov --- 2-Nov 2 

Myrtle Warbler 12-Oct 31-Oct 2-Nov 3-Nov 17-Nov 157 

Lesser Scaup 23-Oct 24-Oct 2-Nov 11-Nov 15-Nov 19 
186 

Table 9.4. 

 

Species-specific timing of fall trans-Gulf migration in 1999 

Species are listed in order from earliest to latest migrants (based on mean date). Sample sizes indicate total numbers 

of individuals of each species recorded over the entire season, including both stopovers and flyby migrants. 95% 

confidence intervals are calculated only for sample sizes ≥ 5. 

Species First 

Date 

95% CI 

Lower 

Bound 

Mean 

Date 

95% CI 

Upper 

Bound 

Last 

Date 

N 

White-throated Sparrow 11-Oct 30-Oct 2-Nov 6-Nov 16-Nov 28 

Field Sparrow 24-Oct 27-Oct 2-Nov 9-Nov 16-Nov 8 

Henslow's Sparrow --- --- 3-Nov --- --- 1 

Short-eared/Long-eared Owl 3-Nov --- 3-Nov --- 3-Nov 2 

Long-eared Owl 24-Oct --- 3-Nov --- 16-Nov 4 

Song Sparrow 21-Oct 30-Oct 3-Nov 7-Nov 17-Nov 25 

American Pipit 21-Oct 25-Oct 3-Nov 12-Nov 16-Nov 8 

Pine Warbler 20-Oct 28-Oct 3-Nov 10-Nov 17-Nov 13 

LeConte's Sparrow 2-Nov 4-Nov 8-Nov 4 

Snow Goose 22-Oct 27-Oct 5-Nov 14-Nov 16-Nov 19 

Tree Swallow 2-Nov 2-Nov 6-Nov 9-Nov 15-Nov 12 

Chestnut-collared Longspur 1-Nov --- 6-Nov --- 12-Nov 2 

Hooded Merganser --- --- 8-Nov --- --- 1 

Nelson's Sharp-tailed 

Sparrow 

2-Nov --- 9-Nov --- 16-Nov 2 

American Robin 3-Nov 8-Nov 13-Nov 18-Nov 17-Nov 7 

Ash-throated Flycatcher --- --- 15-Nov --- --- 1 

Lesser Goldfinch --- --- 15-Nov --- --- 1 

Red-breasted Merganser --- --- 17-Nov --- --- 1 
187 

21 1 11 21 1 11 21 1 11

August September October November

Long-distance migrants

Medium-distance migrants

Short-distance migrants

Herons

Waterfowl

Figure 9.25. Frequency distribution of species-specific peak fall arrival times in 1999. Each 

symbol indicates the mean date of occurrence for one of the species listed in Table 

9.4. “Long-distance migrants” includes aerial insectivores and diurnal raptors. 

Species represented by single observations are not included. 
189 

Chapter 10 

 

Fall Trans-Gulf Migration and Platform Use by Peregrine Falcons 

Robert W. Russell 

10.1 INTRODUCTION

Peregrine Falcons (Falco peregrinus) proved to be among the most obvious and interesting beneficiaries 

of the platform archipelago. Populations of this apex predator had declined precipitously and approached 

extinction in parts of its range by the mid-1960’s as a result of exposure to the organochlorine pesticide 

DDT and its breakdown product DDE, which accumulated in fatty tissues and induced reproductive 

failure by interfering with normal calcium deposition during eggshell formation (reviewed by Kiff 1988). 

The American and Arctic subspecies (F. p. anatum and F. p. tundrius) were listed as Endangered in 1970 

under the Endangered Species Conservation Act of 1969 (which preceded the Endangered Species Act of 

1973), and DDT was banned in the United States starting in 1973. After reaching a nadir of as few as 324 

nesting pairs by 1975 (Fyfe et al. 1976), the continental population subsequently rebounded by over two 

orders of magnitude to perhaps 59,000 individuals in the mid-1980s (Cade et al. 1988). The Arctic 

subspecies was officially downgraded from Endangered to Threatened in 1984, and was delisted 

altogether in 1994; removal of the American subspecies from the endangered species list followed in 

1999 (U.S. Fish and Wildlife Service 1999). 

This chapter documents the migration ecology of Peregrine Falcons and their use of Gulf platforms 

during the fall 1999 field season, when we achieved our most extensive geographical and seasonal 

coverage. All patterns were similar or identical in other years. 

10.2 SEASONAL TIMING

The seasonal timing of Peregrine trans-Gulf migration was highly compressed into the period from late 

September to mid-October (Figure 10.1). 60% of all recorded individuals were initially detected in the 2-

week period from September 29-October 12. 

10.3 POPULATION STRUCTURE

Among 284 individuals that could be identified to age and sex, 61% were male (%:&= 1.6:1) and 60% 

were juveniles (HY:AHY = 1.5:1). There were significant seasonal and geographical differences among 

the age-sex classes in patterns of occurrence. A two-way ANOVA on Julian day of arrival (Figure 10.1) 

indicated that adults arrived earlier than juveniles (F1,280 = 15.3, P < 0.001) and that females arrived 

earlier than males (F1,280 = 4.62, P = 0.032), with no interaction between age and sex (F1,280 = 0.14, P = 

0.71). However, the absolute differences in seasonal timing were minor: there was only a week’s 

difference between peak arrival of the earliest (AHY & = October 2) and latest (HY % = October 9) agesex classes. Interestingly, there was a significant longitudinal gradient in sex ratio, with a female bias 

toward the east and a male bias toward the west (Figure 10.2). Overall sex ratio did not differ between 

the age classes in the population we sampled (G = 2.1, df = 1, P = 0.15). 
190 

Sept 1-5 Sept 6-10 Sept 11-15 Sept 16-20 Sept 21-25 Sept 26-30 Oct 1-5 Oct 6-10 Oct 11-15 Oct 16-20 Oct 21-25 Oct 26-30 Oct 31-Nov 

Number of Peregrines 102030405060

7080

90

100

0

Figure 10.1. Frequency distribution of the dates of initial detection of all 372 Peregrine Falcons recorded during the fall 1999 sea

both platform stopovers and flybys. 
191 

r2 = 0.59

P = 0.009

Longitude (EW)

% Female

90

80

70

60

50

40

30

20

10

98 96 94 92 90 88 86

Figure 10.2. Longitudinal trend in sex ratios of Peregrine Falcons in fall 1999. 

10.4 POPULATION SIZE

We documented 273 Peregrines using our 10 study platforms in fall 1999. There was substantial 

variation among platforms (range = 5-63 birds per platform; Figure 10.3), with falcons tending to favor 

larger platforms with taller towers. 

Based on our direct observations and interviews with platform workers, fishermen, and supply boat 

personnel from all parts of the Gulf, it is certain that virtually every “major” platform complex in the Gulf 

(i.e., excluding complexes consisting of caissons with fewer than six completions and three pieces of 

production equipment) hosts Peregrines during peak migration periods. Our platforms constituted a 

representative sample of the major platform space (i.e., we selected a range of platform types and sizes for 

the study). According to the latest MMS database (Minerals Management Service 2004), there were 2434 

major platform complexes in the northern Gulf of Mexico as of October 1, 1999. A simple extrapolation 

of our results to the total platform space yields a population estimate of more than 66,000 Peregrines 

using platforms during the entire fall migration. 

While most individuals remained on or in the immediate vicinity of one platform complex during their 

stopovers, we sometimes noted birds patrolling nearby platforms. Thus, our straightforward estimate of 

27.3 Peregrines per platform per season is probably too liberal for extrapolation because of replication 

among platforms, though we have no direct data bearing on the degree of multiple platform use. 
192 

36

46

12 22

16 5 63 29

26

18

Peregrine Falcon

Fall 1999

Figure 10.3. Numbers of Peregrines undertaking stopovers on each of the 10 study 

platforms in fall 1999. 

An extreme lower bound for a population estimate can be obtained by assessing the number of unique 

individuals that could be accounted for during a short period. During the peak 3-day period in fall 1999 

(September 29-October 1), we recorded 45 unique individuals using our 10 platforms. Extrapolation 

yields an instantaneous point estimate of about 11,000 Peregrines on platforms throughout the Gulf at that 

time. This figure is very conservative because it only considers birds in the Gulf at the peak of the 

migration (cf. Figure 10.1), and also because Peregrines also frequently used minor caissons that were 

excluded from the calculations. 

Our observations and extrapolations thus suggest that the number of Peregrines using Gulf platforms 

during fall 1999 fell somewhere in the range from 11,000-66,000. We were unable to find any very 

recent estimates of continental Peregrine populations. However, based on an analysis of migration 

statistics from banding and retrapping, Cade et al. (1988) estimated that in the mid-1980s, the migratory 

population consisted of 26,000 breeding adults, 13,000 nonbreeders, and 20,000 young of the year — i.e., 

a total population size of 59,000. This estimated population size is larger than the number of migrants 

that could potentially use platforms because it includes a component from western North America that 

probably does not routinely cross the Gulf of Mexico, and it does not account for an estimated 15-20% 

mortality of juveniles before reaching trapping points south of Canada (Cade et al. 1988). 

It is also noteworthy that juveniles comprised 60% of the population of Peregrines on Gulf platforms, 

compared to a maximum of ~34% in the hypothetical total migratory population (Cade et al. 1988). This 

discrepancy indicates that proportionally more adults may use routes that avoid trans-Gulf crossings, or 

that adults are less likely than juveniles to use platforms. The former explanation is supported by 

radiotelemetry work suggesting that Peregrines traveling offshore along the Atlantic seaboard in the fall 

are predominantly adult males (Cochran 1985). 

Despite the considerable uncertainty involved in both our estimates and the estimates of Cade et al. 

(1988), it is clear that a substantial portion of the North American population of Peregrine Falcons — and 

perhaps a majority of the juveniles — now uses Gulf platforms during fall migration. 
193 

10.5 PLATFORM USE

Peregrines typically arrived on platforms in the afternoon and stayed up to several days, hunting primarily 

at night when most migrant landbirds were aloft. Peregrines took avian prey of a wide variety (at least 69 

species; Table 10.1) and of all sizes, but relied almost entirely on migrants rather than local marine birds 

(which comprised only 1% of all prey items) and clearly favored larger species. In addition to the avian 

prey listed in Table 10.1, Peregrines also frequently captured and consumed migrating green darners 

(Anax junius). 

Of the 389 avian prey items in our total sample, 92 were known to have been taken by individual 

Peregrines identifiable to sex. Based on this sample, avian prey items taken by male Peregrines had a 

median body mass of 48 g compared to a median body mass of 112 g for prey taken by females (Figure 

10.4); this difference was statistically significant (Mann-Whitney U = 1374, P = 0.006). Also, all 

Peregrines observed preying on migrant dragonflies were males. 

Table 10.1. 

 

Avian prey species taken by Peregrine Falcons using offshore 

platforms during fall migration 

Total sample consisted of 389 prey items from 1998-2000. 

 Proportion of 

Prey Species N Prey Items 

Sora 41 0.105 

unid. passerine 34 0.087 

Gray Catbird 23 0.059 

Lincoln's Sparrow 23 0.059 

Yellow-billed Cuckoo 23 0.059 

Mourning Dove 22 0.057 

White-winged Dove 17 0.044 

Scissor-tailed Flycatcher 15 0.039 

Purple Gallinule 12 0.031 

Upland Sandpiper 11 0.028 

Common Snipe 9 0.023 

Common Yellowthroat 9 0.023 

unid. warbler 9 0.023 

Pied-billed Grebe 8 0.021 

Wood Thrush 8 0.021 

House Wren 7 0.018 

Magnolia Warbler 6 0.015 

unid. sparrow 6 0.015 

Blue-winged Teal 5 0.013 

Belted Kingfisher 4 0.010 

Rose-breasted Grosbeak 4 0.010 

Yellow-breasted Chat 4 0.010 

unid. shorebird 4 0.010 

Chuck-Will's-Widow 3 0.008 

Black-billed Cuckoo 3 0.008 

unid. dove 3 0.008 

Indigo Bunting 3 0.008 

Nashville Warbler 3 0.008 
194 

Table 10.1. 

 

Avian prey species taken by Peregrine Falcons using offshore 

platforms during fall migration 

Total sample consisted of 389 prey items from 1998-2000. 

 Proportion of 

Ovenbird 3 0.008 

Yellow-bellied Sapsucker 3 0.008 

American Kestrel 3 0.008 

Baird's Sandpiper 2 0.005 

Brown Thrasher 2 0.005 

Cattle Egret 2 0.005 

Eastern Kingbird 2 0.005 

Eastern Wood-Pewee 2 0.005 

Greater Yellowlegs 2 0.005 

Long-billed Dowitcher 2 0.005 

Least Bittern 2 0.005 

Northern Mockingbird 2 0.005 

Ruby-crowned Kinglet 2 0.005 

Summer Tanager 2 0.005 

unid. meadowlark 2 0.005 

Western Meadowlark 2 0.005 

White-eyed Vireo 2 0.005 

Wilson's Warbler 2 0.005 

American Redstart 1 0.003 

Black Tern 1 0.003 

Black-throated Blue Warbler 1 0.003 

Clay-colored Sparrow 1 0.003 

Chimney Swift 1 0.003 

Common Ground-Dove 1 0.003 

Flammulated Owl 1 0.003 

Grasshopper Sparrow 1 0.003 

Hooded Warbler 1 0.003 

Inca Dove 1 0.003 

Least Sandpiper 1 0.003 

Lesser Yellowlegs 1 0.003 

Myrtle Warbler 1 0.003 

Northern Waterthrush 1 0.003 

Northern Rough-winged Swallow 1 0.003 

Prairie Warbler 1 0.003 

Red-shafted Flicker 1 0.003 

Sanderling 1 0.003 

Sandwich Tern 1 0.003 

Short-billed Dowitcher 1 0.003 

unid. storm-petrel 1 0.003 

Stilt Sandpiper 1 0.003 

Swainson's Thrush 1 0.003 

Tree Swallow 1 0.003 

Sooty/Bridled Tern 1 0.003 
195 

Table 10.1. 

 

Avian prey species taken by Peregrine Falcons using offshore 

platforms during fall migration 

Total sample consisted of 389 prey items from 1998-2000. 

 Proportion of 

unid. flycatcher 1 0.003 

Veery 1 0.003 

Western Tanager 1 0.003 

Yellow-shafted Flicker 1 0.003 

Least Grebe 1 0.003 

Yellow-throated Vireo 1 0.003 

0

5

10

15

0-39 40-

119

0

5

10

15

20

25

Prey Body Mass (g)

Number of Observations

%

&

120-

249

250+

Figure 10.4. Comparison of body masses of prey taken during the fall by male and female 

Peregrine Falcons. Data from three seasons (1998-2000) are combined. 

10.6 FLIGHT STRATEGIES

Among 44 individuals observed in high-altitude, direct migratory flight, vanishing bearings were 

significantly oriented toward the southwest (Figure 10.5;  ̄x = 218E, 95% CI = 203E-232E, r = 0.70, 
196 

Rayleigh P << 0.001), suggesting that many or most of the birds we saw were destined for the 

western shore of the Gulf of Mexico (as opposed to Cuba or the Yucatan Peninsula). Most 

Peregrines arrived on platforms or were observed flying by platforms after early morning. This pattern 

differed dramatically from the daily pattern of arrival of most trans-Gulf migrant passerines (Figure 10.6), 

indicating that Peregrines usually departed the northern Gulf coast during hours of daylight. 

N

Figure 10.5. Vanishing bearings of Peregrine Falcons in fall 1999. The long arrow 

indicates the mean angle of the sample, and the arc shows the 95% 

confidence interval around the mean. 
197 

SS

+11⁄2h

0

20

40

60

80

100

120 Prothonotary Warbler

Sunrise

(SR)

SR

+1h

SR

+3h

4:00

PM

Sunset

(SS)

0

20

40

60

80

100 Peregrine

Falcon

Arrival Time

Number of Observations

Figure 10.6. Comparison of arrival times of Peregrines and a representative trans-Gulf migrant 

passerine in fall 1999. 

10.7 POSSIBLE SELECTIVE INFLUENCE OF PLATFORMS

This study showed that large numbers of Peregrine Falcons — including perhaps the majority of North 

American juveniles — now undertake stopovers on Gulf platforms for resting and hunting during fall 

migration. Peak migration occurred during a well-defined period from late September through midOctober. Because of their high success preying on nocturnal migrant landbirds, it seems that Peregrines 

are now “anticipating” ephemeral periods of high resource availability on platforms. It is possible that 

this adaptive behavior may be a precursor to more dramatic and perhaps evolutionary changes, and that 

Peregrines are being influenced by the installation of an artificial archipelago of oil and gas production 

platforms in the Gulf in a fashion similar to how Eleonora’s Falcons (Falco eleonorae) have evolved a 

strategy of breeding on Mediterranean islands during the fall, when abundant trans-Mediterranean 

migrant landbird prey are available for provisioning young. Wondering why there is no ecological 

counterpart of Eleonora’s Falcon in the Gulf of Mexico region, Walter (1979) concluded that a similar 

niche does not exist because of the lack of islands. With the explosion of the North American Peregrine 

population in the last three decades and the concurrent completion of the world’s largest artificial 

archipelago in the Gulf of Mexico, Walter’s conclusion may need to be revised in coming decades. 
199 

Chapter 11 

 

Synoptic Weather and Vagrancy 

Robert W. Russell 

One of the interesting results of this study was the discovery of a large number of “vagrants” offshore. 

Vagrants are individuals that occur well outside of their species’ typical geographic range. 

Over the course of the study, 29 species were recorded as spring vagrants (Table 11.1). Among the 231 

total individuals involved, all were independent occurrences with the exception of three flocks of 

Bobolinks involving 2, 7, and 29 individuals. In general, spring Caribbean vagrants tended to be more 

common in the eastern part of the study area, whereas western vagrants tended to be more common 

toward the west. In spring 1998 (Figure 11.1), 74% of Caribbean vagrants occurred from the South 

Marsh Island platform eastward, whereas 86% of western vagrants were recorded west of South Marsh 

Island (G = 25.5, df = 1, P < 0.0001). The pattern was similar in spring 1999 (Figure 11.2), except a 

higher percentage of western vagrants occurred in the eastern part of the study area (44%, compared to 

71% of Caribbean vagrants), and the difference was not significant (G = 2.1, df = 1, P = 0.15). In spring 

2000 (Figure 11.3), 55% of Caribbean vagrants occurred from the South Marsh Island platform eastward, 

whereas 92% of western vagrants were recorded west of South Marsh Island (G = 23.8, df = 1, P < 

0.0001). Most spring vagrants were males (Table 11.1). 

Geographic patterns of vagrancy were similar in the fall, when 42 species were recorded as vagrants 

(Table 11.2). As in spring, Caribbean vagrants tended to be more common in the eastern part of the study 

area, whereas western vagrants tended to be more common toward the west. In fall 1998 (Figure 11.4), 

76% of Caribbean vagrants occurred from the South Marsh Island platform eastward, whereas 83% of 

western vagrants were recorded west of South Marsh Island (G = 7.5, df = 1, P = 0.006). The pattern was 

similar in fall 1999 (Figure 11.5): 61% of Caribbean vagrants occurred from the South Marsh Island 

platform eastward, whereas 89% of western vagrants were recorded west of South Marsh Island (G = 

47.0, df = 1, P << 0.0001). 

Adequate samples of vanishing bearings were available for a comparison of the orientation of western 

vagrants and Caribbean vagrants in spring and fall (Figure 11.6). During the fall, vanishing bearings of 

both western and Caribbean vagrants were random. In the spring, both western and Caribbean vagrants 

were significantly oriented toward the NNW. There was no significant difference between vanishing 

bearings of western and Caribbean vagrants in the spring (Watson-Williams test: F1,30 = 0.16, P = 0.69). 
200 

Table 11.1. 

 

Spring vagrants recorded in 1998-2000 

Species are listed in descending order of abundance. Numbers of each sex are shown for species routinely 

identifiable to sex in the field. Confidence intervals are shown for species with N > 5. 

Species N % &

Mean 

Date 95% CI (range) 

Bobolink 69 27 13 Apr 27 Apr 24-30 (Apr 18-May 21) 

Lesser Nighthawk 45 34 4 Apr 8 Apr 4-13 (Mar 15-May 20) 

Western Palm Warbler 30 ----- ----- Apr 20 Apr 14-26 (Mar 18-May 12) 

Western Kingbird 13 ----- ----- May 4 Apr 28-May 10 (Apr 20-May 23) 

Prairie Warbler 13 9 2 Apr 16 Apr 9-23 (Apr 2-May 5) 

Cape May Warbler 10 10 0 Apr 28 Apr 25-May 1 (Apr 21-May 7) 

Yellow-headed Blackbird 9 3 6 Apr 24 Apr 13-May 4 (Mar 31-May 11) 

Black-throated Blue Warbler 6 5 1 May 10 Apr 28-May 21 (Apr 29-May 28) 

Ash-throated Flycatcher 4 ----- ----- Apr 14 ----- (Apr 3-21) 

Gray Kingbird 3 ----- ----- May 8 ----- (Apr 30-May 22) 

Brown-crested Flycatcher 3 ----- ----- May 12 ----- (May 9-19) 

Burrowing Owl 2 ----- ----- Mar 23 ----- (Mar 10-Apr 5) 

Tropical Kingbird 2 ----- ----- May 10 ----- (May 6-14) 

Couch's Kingbird 2 ----- ----- May 13 ----- (May 6-21) 

Tropical/Couch's Kingbird 2 ----- ----- Apr 30 ----- (Apr 21-May 9) 

Black-whiskered Vireo 2 ----- ----- May 4 ----- (May 4-5) 

Western Tanager 2 1 1 Apr 18 ----- (na) 

Audubon's Warbler 2 1 1 Apr 21 ----- (Apr 17-25) 

Cassin's Sparrow 2 ----- ----- Apr 10 ----- (Mar 30-Apr 21) 

Band-tailed Pigeon 1 ----- ----- Apr 26 

Cave Swallow 1 ----- ----- May 11 

Violet-green Swallow 1 1 0 Mar 26 

Townsend's Warbler 1 1 0 Apr 8 

Hermit Warbler 1 1 0 Apr 15 

Black-throated Gray Warbler 1 1 0 May 6 

Connecticut Warbler 1 1 0 Apr 30 

Bronzed Cowbird 1 1 0 Mar 31 

Lazuli Bunting 1 1 0 Apr 3 

Varied Bunting 1 1 0 Apr 12 
201 

Caribbean

vagrants

Western

vagrants

8

2 8

3

18

16

8 2

0

2

Figure 11.1. Comparative distribution of occurrences of Caribbean vagrants and 

western vagrants in spring 1998. 
202 

2

6 1

8

11

2

3 0

0

4

Caribbean

vagrants

Western

vagrants

Figure 11.2. Comparative distribution of occurrences of Caribbean vagrants and western 

vagrants in spring 1999. 
203 

1

4 1 6

3 1 7 6

4

13

24 6 6

5 0 4 0

1

Caribbean

vagrants

Western

vagrants

Figure 11.3. Comparative distribution of occurrences of Caribbean vagrants and western 

vagrants in spring 2000. 
204 

Table 11.2. 

 

Fall vagrants recorded in 1998 and 1999 

Species are listed in descending order of abundance. Confidence intervals are shown for species 

with N > 5. 

Species N Mean 

Date 95% CI (range) 

Prairie Warbler 41 Sept 4 Aug 28-Sept 11 (Aug 6-Nov 3) 

Black-throated Blue Warbler 31 Oct 1 Sept 26-Oct 6 (Sept 11-Oct 24) 

Lesser Nighthawk 10 Oct 24 Oct 10-Nov 7 (Sept 7-Nov 15) 

Pyrrhuloxia 10 Oct 29 Oct 25-Nov 1 (Oct 22-Nov 2) 

Bobolink 7 Sept 28 Sept 11-Oct 16 (Sept 7-Oct 27) 

Yellow-headed Blackbird 7 Sept 21 Sept 12-Oct 1 (Sept 10-Oct 8) 

Burrowing Owl 6 Oct 20 Oct 17-22 (Oct 18-23) 

Cape May Warbler 6 Oct 5 Sept 14-Oct 26 (Sept 7-Nov 3) 

Cave Swallow 5 Sept 28 ----- (Sept 23-Oct 3) 

Blackpoll Warbler 4 Oct 17 ----- (Oct 12-27) 

Groove-billed Ani 3 Sept 20 ----- (Sept 8-30) 

Western Kingbird 3 Sept 25 ----- (Sept 11-Oct 21) 

Gray Kingbird 3 Sept 22 ----- (Aug 27-Oct 16) 

Long-billed Thrasher 3 Oct 19 ----- (Oct 1-Nov 14) 

Audubon's Warbler 3 Oct 14 ----- (Oct 10-21) 

MacGillivray's Warbler 3 Sept 2 ----- (Aug 25-Sept 7) 

Green-tailed Towhee 3 Oct 15 ----- (Sept 24-Oct 31) 

Cassin's Sparrow 3 Oct 10 ----- (Sept 21-Oct 31) 

Flammulated Owl 2 Oct 10 ----- (Oct 10-11) 

Vermilion Flycatcher 2 Oct 1 ----- (Sept 22-Oct 11) 

Tropical/Couch's Kingbird 2 Sept 2 ----- (Aug 19-Sept 17) 

Western Wood-Pewee 2 Sept 23 ----- (Sept 23-24) 

Bell's Vireo 2 Sept 9 ----- (Aug 30-Sept 19) 

Bronzed Cowbird 2 Oct 22 ----- (Oct 11-Nov 2) 

Bullock's Oriole 2 Sept 28 ----- (Sept 24-Oct 2) 

Gray-headed Junco 2 Nov 1 ----- (na) 

Chestnut-collared Longspur 2 Nov 6 ----- (Nov 1-12) 

Ringed Kingfisher 1 Aug 5 

Rufous Hummingbird 1 Sept 19 

Black-chinned Hummingbird 1 Sept 9 

Black Swift 1 Oct 8 

Couch's Kingbird 1 Oct 18 

Loggerhead Kingbird 1 Sept 12 
205 

Table 11.2. 

 

Fall vagrants recorded in 1998 and 1999 

Species are listed in descending order of abundance. Confidence intervals are shown for species 

with N > 5. 

Species N Mean 

Date 95% CI (range) 

Ash-throated Flycatcher 1 Nov 15 

Say's Phoebe 1 Oct 31 

Cuban Pewee 1 Oct 9 

Sage Thrasher 1 Oct 23 

Sprague's Pipit 1 Oct 31 

Townsend's Warbler 1 Oct 11 

Western Tanager 1 Nov 10 

Spotted Towhee 1 Oct 22 

Lesser Goldfinch 1 Nov 15 

Pink-sided Junco 1 Nov 1 
206 

4

1 1

0

0

3

4 6

6

10

Caribbean

vagrants

Western

vagrants

Figure 11.4. Comparative distribution of occurrences of Caribbean vagrants and western 

vagrants in fall 1998. 
207 

2

12

1 10

2 4 9 12

4

14

31

31

3 8

3 1 4 1

1

2

Caribbean

vagrants

Western

vagrants

Figure 11.5. Comparative distribution of occurrences of Caribbean vagrants and western vagrants in 

fall 1999. 
208 

Fall

Spring

Western

Vagrants

Caribbean

Vagrants

r = 0.14

P = 0.78

r = 0.30

P = 0.37

0 = 348E

r = 0.64

P = 0.002

0 = 340E

r = 0.67

P = 0.0001

Figure 11.6. Comparison of vanishing bearings of Western vagrants and Caribbean vagrants in spring and fall. 
PART 3 

 

SUPPLEMENTAL STUDIES 
211 

Chapter 12 

 

“Winter” and “Summer” Migrations 

Robert W. Russell 

12.1 INTRODUCTION

Our first year’s data suggested that migratory movements probably continue well outside of the nominal 

seasonal limits of March-May in the spring and August-November in the fall. Although these 

“extraseasonal” flights are probably quantitatively small, they are potentially very informative because 

they involve individuals operating in the tail ends of their species’ frequency distributions for seasonal 

timing. In some cases, these individuals may be of unusually high or low genetic quality (e.g., very early 

spring migrants and very late spring migrants, respectively), and information on their occurrence and 

behavior may shed light on the environmental factors limiting trans-Gulf migration. Alternatively, 

extraseasonal migrants may possess aberrant spatiotemporal programs for migration, which might also be 

evident in unusual patterns of orientation or platform use. In addition, because of the strong genetic basis 

for migration timing (Berthold 1990, 1991; Berthold and Helbig 1992; Møller 2001; Pulido et al. 2001), 

changes in the occurrence of extraseasonal migration could be reflective of evolutionary changes in 

response to altered selective regimes associated with important environmental shifts such as global 

warming. A baseline understanding of the occurrence and ecology of extraseasonal migrants is therefore 

a prerequisite for developing any long-term strategies for monitoring trans-Gulf migration. 

To determine whether offshore migration continues during “winter” and “summer” months, we staffed 

one platform (GC18) on the following dates: November 18-December 16, 1999; January 6-February 29, 

2000; and June 1-August 1, 2000. Field methods during these periods were identical to those used during 

regular field seasons (Chapter 3). In the winter, we used the fall survey schedule (Table 3.2) during 

November-December, and the spring schedule (Table 3.1) during January-February. In the summer, we 

used the spring survey schedule (Table 3.1) through July 7 and the fall schedule (Table 3.2) thereafter. 

These extended seasonal observations provided us with a nearly complete year-round perspective from a 

single platform, with coverage gaps (due to staffing limitations) only on December 17-January 5, March 

1-6, and August 2-3. 

12.2 RESULTS AND DISCUSSION

Complete listings of all migrants detected at GC18 during winter 1999-2000 and summer 2000 are 

presented in Tables 12.1 and 12.2. 

Table 12.1 indicates that southbound trans-Gulf migration of neotropical migrant landbirds continued at 

least until November 25, and perhaps until December 6. In addition, overshooting by shorter-distance 

migrants continued at least until December 7. 
212 

Table 12.1. 

 

“Winter” observations at GC18: November 18-December 16, 1999, and January 6-February 29, 2000 

Synoptic weather type (Chapter 5) is listed in parentheses following each date of initial observation. Number of 

individuals, age and sex (when known), species, and behavior are listed after date and weather type. Inclusive 

times of occurrence and supplementary behavioral notes are given in parentheses following primary data. Times 

indicated by an asterisk are exact arrival or departure times. The condition of all individuals was “vigorous” unless 

otherwise noted. Abbreviation: unid = unidentified. 

Date Sightings 

Nov 20 (ND) 1 HY % American Robin on platform (06:15-Dec 1 20:00; foraged on rice and crackers 

provided by workers, drank) 

Nov 24 (GF/MCH) 1 Great Egret flew past to SE (09:54*; in flock with 4 Great Blue Herons) 

Nov 24 (GF/MCH ) 4 Great Blue Herons flew past to SE (09:54*; in flock with 1 Great Egret) 

Nov 25 (GF/MCH) 1 Tree Swallow flew past to S (11:35*) 

Nov 25 (GF/MCH) 2 Tree Swallows flew past to S (11:39*) 

Nov 25 (GF/MCH) 2 Tree Swallows flew past to S (11:40*) 

Nov 25 (GF/MCH) 1 Tree Swallow flew past to SW (11:50*) 

Nov 25 (GF/MCH ) 4 Tree Swallows flew past to S (11:57*; single individuals) 

Nov 26 (ELOW) 1 White-winged Dove approached platform (08:10*) 

Nov 26 (ELOW) 1 American Pipit approached platform (08:15*) 

Nov 27 (GH) 1 Mourning Dove flew past to N (08:15*) 

Nov 28 (GH) 1 HY % Peregrine Falcon on platform (17:18*-17:40; probably roosted overnight) 

Nov 29 (MCH) 1 HY Myrtle Warbler on platform (08:25) 

Nov 30 (MCH) 1 unid dove flew past (08:19*; landed on distant freighter) 

Dec 1 (ECH) 1 White-winged Dove on platform (09:50-Dec 2 07:45) 

Dec 6 (GF/ELOW) 1 unid passerine flew past to S (08:14*) 

Dec 7 (MCH) 1 HY Snow Goose swimming around platform (09:02) 

Dec 7 (MCH) 1 Myrtle Warbler on platform (departed to W 10:08*) 

Dec 11 (GF/MCH) 3 AHY Great Blue Herons flew past to ENE (08:00*) 

Jan 6 (ECH) 1 Snowy Egret approached platform from E (10:25*; departed to NW) 

Jan 27 (MCH) 1 Sora on platform (PC3-PC5; tired) 

Jan 28 (GF) 1 Common Snipe on platform (arrived at 15:57*) 

Jan 29 (GF) 1 ASY % Purple Martin approached platform (06:53*; soaring, drifted to SE until lost 

from sight) 

Jan 31 (GF) 1 & Mallard approached platform from SW (07:01*; departed to SSW) 

Jan 31 (GF) 1 ASY % Purple Martin approached platform from SSW (07:47*; cruised in lee of 

platform until 08:00) 

Feb 1 (GF) 1 ASY % Purple Martin flew past to N (16:29*) 

Feb 1 (GF) 1 ASY % Purple Martin approached platform (16:36*; cruised in lee of platform until 

16:48) 

Feb 5 (MCH) 2 ASY % Purple Martins flew past (17:45*) 

Feb 9 (GH) 1 Sora on platform (15:00) 

Feb 19 (GF) 4 Great Egrets flew past to N (10:09*) 

Feb 19 (GF) 1 % Northern Parula on platform (12:37-12:45) 

Feb 19 (GF) 1 Cattle Egret flew past to NNW (15:59*) 

Feb 19 (GF) 3 Purple Martins on platform (2 ASY %, 1 &/SY %; 18:10-19:20) 

Feb 28 (MCH) 1 White-winged Dove approached platform (09:45*) 
213 

Table 12.2. 

 

“Summer” observations at GC18: June 1-August 1, 2000 

Synoptic weather type (Chapter 5) is listed in parentheses following each date of initial observation. Number of 

individuals, age and sex (when known), species, and behavior are listed after date and weather type. Inclusive 

times of occurrence and supplementary behavioral notes are given in parentheses following primary data. Times 

indicated by an asterisk are exact arrival or departure times. The condition of all individuals was “vigorous” 

unless otherwise noted. Abbreviation: unid = unidentified. 

Date Sightings 

Jun 1 (ECH) 1 Cattle Egret flew past to N (FC1*) 

Jun 1 (ECH) 5 Cattle Egrets flew past to N (PC2*) 

Jun 1 (ECH) 1 Barn Swallow on platform (18:30) 

Jun 2 (ECH) 1 & Common Yellowthroat on platform (PC4; foraged) 

Jun 3 (GH) 1 Cattle Egret approached platform (FC1*) 

Jun 5 (BH) 1 American Redstart approached platform (FC1*) 

Jun 5 (BH) 1 % Blackpoll Warbler on platform (PC4) 

Jun 6 (GF) 1 Couch’s Kingbird on platform (PC1-PC5; foraged successfully) 

Jun 6 (GF) 1 Osprey flew past platform to N (12:00*) 

Jun 6 (GF) 1 % Blackpoll Warbler on platform (13:10-PC3) 

Jun 6 (GF) 1 & Blackpoll Warbler on platform (PC3) 

Jun 6 (GF) 1 unid &/HY martin on platform (18:00-20:00; foraged) 

Jun 7 (GF) 1 Barn Swallow on platform (FC1) 

Jun 7 (GF) 6 Cattle Egrets approached platform from S (FC1*) 

Jun 8 (ECH) 1 Northern Waterthrush (PC0-06:30*; tired) 

Jun 9 (ECH) 1 Cattle Egret approached platform (PC1*) 

Jun 15 (BH) 1 Cattle Egret approached platform (12:58*; departed to ENE) 

Jun 18 (BH) 1 Cattle Egret approached platform from N (10:44*; departed to ESE) 

Jun 22 (GH) 1 ASY % Indigo Bunting on platform (PC5) 

Jun 25 (GH) 1 Cattle Egret on platform (PC4) 

Jun 26 (GH) 1 Cattle egret approached platform (11:04*; departed to E) 

Jun 27 (GH) 1 ASY & Myrtle Warbler on platform (14:48*- Jul 1 PC5; arrived from SE) 

Jul 4 (ECH) 1 AHY White-winged Dove on platform (FC2-PC5) 

Jul 7 (GH) 1 AHY Cattle Egret approached platform from W (09:08*) 

Jul 10 (GH) 1 Snowy Egret approached platform from W (12:08*; departed to E) 

Jul 11 (GH) 1 &/HY Prothonotary Warbler on platform (PC1) 

Jul 11 (GH) 1 Willet flew past to WNW (08:23*) 

Jul 13 (GH) 1 AHY Yellow-throated Warbler on platform (PC1-FC1) 

Jul 13 (GH) 1 HY Barn Swallow on platform (PC6-Jul 14 00:20; tired) 

Jul 15 (GH) 1 unid white egret flew past to SE (07:56*) 

Jul 22 (GH) 1 Louisiana Waterthrush on platform (PC1) 

Jul 22 (GH) 1 unid warbler approached platform (07:24*; departed to S) 
214 

Table 12.2. 

 

“Summer” observations at GC18: June 1-August 1, 2000 

Synoptic weather type (Chapter 5) is listed in parentheses following each date of initial observation. Number of 

individuals, age and sex (when known), species, and behavior are listed after date and weather type. Inclusive 

times of occurrence and supplementary behavioral notes are given in parentheses following primary data. Times 

indicated by an asterisk are exact arrival or departure times. The condition of all individuals was “vigorous” 

unless otherwise noted. Abbreviation: unid = unidentified. 

Date Sightings 

Jul 23 (GH) 1 &/HY Yellow Warbler on platform (PC1) 

Jul 23 (GH) 1 Barn Swallow approached platform (07:56*; departed to S) 

Jul 23 (GH) 1 &/HY Purple Martin cruised platform (14:11*; foraged) 

Jul 24 (GF/MCH) 2 &/HY Purple Martins cruised platform (14:45*-14:50; foraged) 

Jul 24 (GF/MCH) 1 &/HY Purple Martin cruised platform (20:13*; foraged) 

Jul 25 (GF/MCH) 1 &/HY Prothonotary Warbler on platform (PC1) 

Jul 26 (ND) 1 AHY % Prothonotary Warbler on platform (PC3-09:08; tired) 

Jul 26 (ND) 1 &/HY Black-and-white Warbler on platform (PC4; foraged) 

Jul 27 (GH) 1 HY Cliff Swallow on platform (17:23*-Jul 28 FC1; foraged) 

Jul 28 (ECH) 1 unid warbler approached platform (07:12*; departed to W) 

Jul 28 (ECH) 1 HY Barn Swallow cruised platform (07:17*-FC1; foraged) 

Jul 28 (ECH) 1 Cattle Egret approached platform (09:54*; departed to NE) 

Jul 29 (GH) 1 &/HY Yellow Warbler on platform (PC1) 

Jul 29 (GH) 1 &/HY Prothonotary Warbler on platform (PC1-06:27*; departed to S) 

Jul 29 (GH) 1 &/HY American Redstart on platform (PC1-PC2) 

Jul 29 (GH) 1 HY Barn Swallow cruised platform (11:30*; foraged) 

Jul 30 (GH) 1 &/HY Black-and-white Warbler on platform (PC1) 

Aug 1 (GH) 1 HY Snowy Egret flew past to SW (09:59*) 

Heron movements oriented to the ENE and NW on December 11 and January 6 are difficult to 

characterize, and could represent overshooting, nonmigratory wandering, or extremely early northbound 

“spring” migration. 

Weather circumstances strongly suggested that the Sora and Common Snipe on January 27-28 were 

southbound “fall” migrants, yet these dates are so extreme that some behavioral phenomenon other than 

“normal” migration must be involved. Recent work has demonstrated that many aspects of migratory 

behavior are governed by endogenous “programs” that are under direct genetic control (reviewed by 

Berthold 1990, 1991). During the normal fall migration period, migration is generally obligate and 

determined by these endogenous programs. However, other evidence from both field and laboratory 

studies (reviewed by Terrill 1990a,b) indicates that migration can be extended facultatively well beyond 

the normal fall migratory period in response to exogenous factors such as changes in food availability in 

wintering areas. Haila et al. (1986) recognized such delayed fall migration and defined it as “an attempt 

to stay in the north through the winter combined with the ability to start migration as late as mid-winter if 

conditions become unbearable.” The most detailed studies of this phenomenon have shown that Yellow-
215 

rumped Warblers (Dendroica coronata) extend their fall migration into the winter months in 

response to varying food availability in wintering areas, which in turn depends on weather 

conditions (Terrill and Ohmart 1984). Extensive facultative nocturnal movements during midwinter have 

also been documented unambiguously (via tower kills) for American Tree Sparrows in the Midwest 

(Niles et al. 1969) and Myrtle Warblers along Gulf Coast (Terrill and Crawford 1988). We suggest that 

the January Sora and Common Snipe represent additional instances of facultatively extended fall 

migration. Interestingly, long-term data from the Tall Timbers Research Station on the Florida Panhandle 

(Crawford 1981) indicate that nocturnal tower kills of Soras and Common Snipes occur regularly from 

late December through January, supporting our contention. 

Unambiguous northbound “spring” migration began on January 29 with the arrival of the first Purple 

Martin, and small numbers of martins continued through February. A Mallard that approached the 

platform on January 31 could have been either southbound or northbound. A Sora recorded on February 

9 was probably a northbound migrant (on the basis of weather circumstances). Unambiguous northbound 

“spring” migration by species other than Purple Martins began on February 19 with the arrival of a 

Northern Parula and two species of herons. 

Table 12.2 indicates that northbound “spring” trans-Gulf migration by neotropical migrants continued at 

least to June 8. Unambiguous southbound “fall” trans-Gulf migration by neotropical migrants began on 

July 11 with the arrival of a Prothonotary Warbler. Observations made during the interim period between 

unambiguous northbound and southbound migration (June 9-July 10) were difficult to characterize, and 

were dominated by a largely eastward movement by Cattle Egrets. The Indigo Bunting and Myrtle 

Warbler on the platform on June 22 and 27 were so far removed from their species’ normal migration 

schedules that they probably involved injury, disease, or mutation of the spatiotemporal program. The 

only other species recorded during the midsummer period were a White-winged Dove and a Snowy Egret 

(which, curiously, was flying eastward like the Cattle Egrets). 

Several observations suggested that southbound “fall” migration began even earlier than indicated in 

Table 12.2 (i.e., technically during the spring field season). On April 23, 2000, during PC3, a female 

Purple Martin was observed flying due S past PN975. The synoptic weather on April 23 was classified as 

ECH, and local winds at PN975 were from the SSE at Beaufort force 2. On May 9, 2000, at 19:21, a 

juvenile Purple Martin was found on GB189. The synoptic weather on May 9 was classified as BH, and 

local winds at GB189 were from the SE at Beaufort force 4. On May 20, 2000, at 10:34, 30 unidentified 

small shorebirds (probably Least Sandpipers) were observed flying due S past HI561 at flight altitude 0. 

The synoptic weather on May 20 was classified as BH, and local winds at HI561 were from the SW at 

Beaufort force 2. 
217 

Chapter 13 

 

Insect Dispersal Over the Gulf of Mexico 

Robert W. Russell and Sidney A. Gauthreaux, Jr. 

13.1 INTRODUCTION 

 (by Robert W. Russell) 

The first two field seasons revealed an unexpected abundance and diversity of terrestrial insects offshore. 

During both spring and fall, a blanket of terrestrial arthropods — the “aerial plankton” — was often 

transported passively over the Gulf by offshore winds. The aerial plankton appeared to represent a 

significant food resource for birds that stopped to rest on platforms. Many of the migrant birds on the 

platforms were able to forage successfully, and in fact, energy intake rates of birds on platforms 

sometimes seemed to be higher than generally observed in “natural” habitats onshore (see Chapter 

15.2.5). Because of the possible importance of insects as a food resource for fuel-depleted migrant birds 

on platforms, we expanded the focus of the field program starting in spring 1999 to include insect 

monitoring using both visual surveys and quantitative sampling methods. 

13.2 METHODS 

 (by Robert W. Russell) 

13.2.1 Visual Surveys 

During each Platform Census, we visually assessed the abundance of insects that could potentially serve 

as prey for foraging birds. At the conclusion of each Platform Census, we made two estimates of insect 

numbers encountered on and about the platform. First, we estimated the abundance of “aerial plankton,” 

which was operationally defined as all small flying insects excluding butterflies, dragonflies, and moths 

(i.e., flies, lacewings, beetles, planthoppers, aphids, true bugs, damselflies, etc.). Second, we estimated 

the number of what we called “bird-food” moths (i.e., all moths excluding black witches [Ascalapha 

odorata] and sphinx moths [Family Sphingidae]). The aerial plankton and bird-food moth categories 

were distinguished because our initial observations suggested that small moths were overwhelmingly the 

most important resource for foraging birds. “Macro-insects” (dragonflies, butterflies, sphingid moths, 

Black Witches, and sphinx moths) were excluded because they are generally too large and robust to be 

taken alive by birds; however, these species were interesting in their own right because many of them 

appeared to be migratory, and we identified and counted them individually for a separate study. 

Abundance estimates reflected the approximate number of individuals detectable by the observer along 

the standard census route using a logarithmic abundance scale (Table 13.1). 
218 

Table 13.1. 

 

Logarithmic abundance scale for visual surveys of insects. 

Abundance Code Observed Abundance Range 

0 0 

1 1 

2 2-10 

3 11-100 

4 101-1000 

5 1001-10,000 

6 >10,000 

13.2.2 Ultraviolet Light Trapping 

During fall 1999 and spring 2000, we sampled insect abundance and biodiversity quantitatively using 

Universal Light Traps (#2851L, BioQuip Products, Inc.) equipped with 22-watt Circline black light tubes 

with full 360-degree visibility. To the extent possible, permanent sites for the traps were chosen in poorly 

lit, well sheltered areas that were easily accessible and away from areas of heavy use by platform 

workers. However, availability of a devoted power outlet was usually the primary determinant of trap 

location. Traps remained in the same locations throughout the study, except at PN975 where heavy 

construction required a location change beginning on April 27, 2000. 

The traps were deployed on all nights except when severe weather rendered the security of the trap 

doubtful or when power outlets were unavailable or production or drilling operations otherwise rendered 

trap deployment infeasible (Table 13.2). Incomplete samples resulting from nighttime power outages or 

wind blowovers were discarded. Light traps were turned on a half hour after sunset, and samples were 

retrieved from the traps the following morning at sunrise or as soon as possible thereafter. A paper towel 

or coffee filter was often used to filter the contents when large numbers of very small species were 

present. Trap contents (including any filters, when used) were transferred into Ziploc bags and then 

stored in a freezer for later enumeration and identification in the laboratory. 

Table 13.2. 

 

Dates of light trap deployment on the platforms 

Dates in parentheses are those when samples were not obtained for one of the reasons described in 

the text. 

Platform Fall 1999 Spring 2000 

VK786 Aug 19-Nov 17 (Sept 2) Platform not available 

SP89 Aug 6-Nov 16 Mar 8-May 30 (Mar 19, May 8-9) 

GC18 Aug 15-Nov 17 Mar 8-May 31 (Mar 15,16,19) 

VR265 Aug 6-Nov 17 (Aug 22, Sept 

17,24,26-29, Oct 6, Nov 13) 

Mar 9-May 28 (Mar 19,30-31, Apr 4-7) 

HI561 Platform not available Mar 12-May 30 (Mar 16,19, Apr 20, May 4,18) 

MI622 Aug 10-Nov 16 (Oct 14,19) Mar 9-May 29 (Mar 11,14,15,19,20, Apr 4) 

PN975 Aug 7-Nov 15 (Sept 16-24, Oct 

18-20) 

Mar 9-May 31 (Mar 15,18, Apr 1-6, May 17) 
219 

In addition, a light trap was deployed at GC18 from November 18-December 16, 1999, 

January 6-February 29, 2000, and June 1-August 1, 2000, as part of the seasonally extended 

program there (Chapter 12). 

13.2.3 Taxonomic Identification and Sorting of Insect Specimens 

The frozen samples were thawed, enumerated, measured, and identified to the lowest taxon possible 

under magnification in the laboratory. Body length was measured from the frons to the tip of the 

abdomen. Antennae, terminal abdominal appendages, and wings extending beyond these points were not 

included in length measurements. Identifications were made by comparison to specimens in the 

collections of the Louisiana State Arthropod Museum, and by using a variety of published guides and 

keys (e.g., Oliver and Chapin 1981). 

13.2.4 Computation of Insect Biomass 

Insect body mass was estimated from measured body length using the taxon-specific regression equations 

in Sage (1982) and Sample et al. (1993). Equations from Sample et al. (1993) were used for most taxa, 

since they were generally based on larger samples and yielded higher explained variances than the 

equations in Sage (1982). The only exception was for the order Orthoptera, which was not included in the 

study by Sample et al. (1993). The predictive equations for dry mass are summarized in Table 13.3. Dry 

mass was converted to wet mass assuming an average water content of 2.33 ml H2O/g dry mass (Bell 

1990). 

Table 13.3. 

 

Allometric equations used for estimating insect body mass from body length 

Abbreviations: DM = dry mass (mg); L = body length (mm). 

Taxon Predictive Equation Explained 

Variance 

Insecta: DM = 0.02657CL2.494 (R2 = 0.88) 

Orthoptera: ln(WM) = 1.043 + 0.2792CL — 0.002873CL2

 (R2

 = 0.89) 

Homoptera: DM = 0.05943CL2.225 (R2 = 0.81) 

 Cicadellidae: DM = 0.02387CL2.561 (R2 = 0.90) 

Hemiptera: DM = 0.008362CL3.075 (R2 = 0.93) 

 Pentatomidae: DM = 0.01504CL3.053 (R2 = 0.95) 

Neuroptera: DM = 0.01130CL2.570 (R2 = 0.97) 

Coleoptera: DM = 0.03889CL2.492 (R2 = 0.86) 

 Carabidae: DM = 0.02414CL2.755 (R2 = 0.96) 

 Chrysomelidae: DM = 0.08830CL2.171 (R2 = 0.86) 

 Coccinelidae: DM = 0.007248CL3.867 (R2 = 0.96) 

Lepidoptera: DM = 0.006500CL3.122 (R2 = 0.93) 

 Microlepidoptera: DM = 0.007350CL2.918 (R2 = 0.93) 

 Arctiidae: DM = 0.02340CL2.658 (R2 = 0.96) 

 Noctuidae: DM = 0.03554CL2.499 (R2 = 0.86) 

Diptera: DM = 0.04142CL2.213 (R2 = 0.85) 

Hymenoptera: DM = 0.01379CL2.696 (R2 = 0.89) 

 Ichneumonidae: DM = 0.01578CL2.464 (R2 = 0.90) 
220 

13.3 MAGNITUDE AND TAXONOMIC COMPOSITION OF THE AERIAL PLANKTON 

(by Robert W. Russell) 

A complete listing of the insects sampled by the light traps is provided in Table 13.4. 

Table 13.4. 

 

Taxonomic composition of the aerial plankton sampled by light traps 

Numbers shown are totals from all platforms across all dates during spring and fall (Table 13.2). 

Order Family Species Fall 

1999 

Spring 

2000 

Coleoptera 

 Anthicidae 10 0 

 Carabidae 

 Anisodactylus sp. 1 0 

 Ledia sp. 1 0 

 no common name (Pterostichus monedulus) 1 0 

 unidentified 56 0 

 Chrysomelidae 

 twelve-spotted cucumber beetle 

 (Diabrotica undecimpunctata) 1,440 4 

 Diabrotica punctella 31 0 

 unidentified 4 0 

 Cicindelidae 

 three-lined tiger beetle (Cicindela trifasciata) 13 0 

 Coccinellidae 16 32 

 Dytiscidae 

 Luccophilus sp. 5 0 

 Dytiscidae 12 0 

 Gyrinidae 3 0 

 Heterocerida 

 Heterocerus sp. 5 0 

 Hydrophilidae 

 Berosus sp. 1 0 

 unidentified 18 0 

 Nitidulidae 218 0 

 Noteridae 1 0 

 Oedemeridae 2 0 

 Scirtidae 3 0 

 Scolytidae 4 0 

 Staphylinida 167 9 

 unidentified 92 318 

 

Diptera 6,067 1,537 

Hemiptera 

 Alydidae 
221 

Table 13.4. 

 

Taxonomic composition of the aerial plankton sampled by light traps 

Numbers shown are totals from all platforms across all dates during spring and fall (Table 13.2). 

Order Family Species Fall 

1999 

Spring 

2000 

 Hyalymenus sp. 8 0 

 Coreidae 

 western leaf-footed bug (Leptoglossus 

clypealis) 

433 6 

 eastern leaf-footed bug (Leptoglossus 

phyllopus) 

34 0 

 citron bug (Leptoglossus gonagra) 2 0 

 Leptoglossus zonatus 13 0 

 Leptoglossus oppositus 3 0 

 Leptoglossus sp. 3 0 

 Corixidae 4 0 

 Lygaeidae 

 unidentified big-eyed bug (Geocoris sp.) 58 0 

 Pachygrontha sp. 1 0 

 unidentified 245 0 

 Miridae 

 Polymerus sp. 30 0 

 unidentified 111 0 

 Pentatomidae 

 predacious stink bug (Apateticus cynicus) 0 2 

 green juniper stink bug (Banasa euchlora) 84 15 

 unidentified stink bug (Banasa sp.) 7 0 

 Chlorochroa senilis 4 0 

 Euschistus crassus 2 0 

 brown-colored stink bug (Euschistus 

quadrator) 

1 0 

 unidentified stink bug (Euschistus sp.) 3 1 

 Mecidea major 81 0 

 southern green stink bug (Nezara viridula) 19 0 

 rice stink bug (Oebalus pugnax) 267 1 

 predatory stink bug (Alcaeorrhynchus 

grandis) 

17 0 

 spined soldier bug (Podisus maculiventris) 36 29 

 Thyanta custator 4 1 

 Thyanta sp. 53 0 

 Trichopepla sp 1 0 

 unidentified 4 0 

 Rhopalidae 

 red-shouldered bug (Jadera haematoloma) 1 0 

 Liorhyssus hyalinus 39 0 

 scentless plant bug (Niesthrea louisianica) 29 0 

 unidentified 10 0 

 Saldidae 12 0 
222 

Table 13.4. 

 

Taxonomic composition of the aerial plankton sampled by light traps 

Numbers shown are totals from all platforms across all dates during spring and fall (Table 13.2). 

Order Family Species Fall 

1999 

Spring 

2000 

 Scutelleridae 25 0 

 unidentified 1,405 1,801 

 

Homoptera 

 Aphidae 867 289 

 Cicadellidae 342 0 

 Cixiidae 519 0 

 Delphacidae 122 0 

 Psyllidae 95 254 

 unidentified 10 0 

 

Hymenoptera 

 Ichneumonidae 5 0 

 unidentified 53 241 

Lepidoptera 

 Papilionidae 

 pipevine swallowtail (Battus philenor) 0 1 

 Pieridae 

 orange sulphur (Colias eurytheme) 0 3 

 southern dogface (Colias cesonia) 0 4 

 little yellow (Eurema lisa) 6 0 

 lyside sulphur (Kricogonia lyside) 0 4 

 Lycaenidae 

 gray hairstreak (Strymon melinus) 6 6 

 Nymphalidae 

 gulf fritillary (Agraulis vanillae) 4 1 

 monarch (Danaus plexippus) 5 0 

 variegated fritillary (Euptoieta claudia) 0 7 

 common buckeye (Junonia coenia) 8 1 

 American snout (Libytheana carinenta) 4 5 

 phaon crescent (Phyciodes phaon) 0 1 

 American lady (Vanessa virginiensis) 0 1 

 red admiral (Vanessa atalanta) 3 0 

 painted lady (Vanessa cardui) 2 0 

 Hesperiidae 

 Brazilian skipper (Calpodes ethlius) 4 1 

 ocola skipper (Panoquina ocola) 1 0 

 long-tailed skipper (Urbanus proteus) 1 0 

 

 Oecophoridae 2 0 

 Yponomeutidae 
223 

Table 13.4. 

 

Taxonomic composition of the aerial plankton sampled by light traps 

Numbers shown are totals from all platforms across all dates during spring and fall (Table 13.2). 

Order Family Species Fall 

1999 

Spring 

2000 

 ailanthus webworm moth (Atteva punctella) 39 0 

 Sesiidae 

 oak stump borer moth (Paranthrene 

asilipennis) 

0 1 

 Pyralidae 

 no common name (Samea ecclesialis) 7 0 

 Sphingidae 

 titan sphinx (Aellopos titan) 0 1 

 pink-spotted hawkmoth (Agrius cingulatus) 1 0 

 mournful sphinx (Enyo lugubris) 10 0 

 banded sphinx (Eumorpha fasciata) 2 0 

 pandorus sphinx (Eumorpha pandorus) 1 0 

 white-lined sphinx (Hyles lineata) 2 1 

 tersa sphinx (Xylophanes tersa) 1 0 

 

 Arctiidae 

 salt marsh moth (Estigmene acrea) 0 1 

 joyful holomelina (Holomelina laeta) 1 0 

 Isabella tiger moth (Pyrrharctia isabella) 0 2 

 Noctuidae 

 four-spotted bird-dropping moth (Acontia 

tetragona) 

0 3 

 exposed bird-dropping moth (Acontia aprica) 0 19 

 unidentified dagger moth (Acronicta sp.) 1 0 

 black cutworm moth (Agrotis ipsilon) 2 1 

 subterranean dart (Agrotis subterranea) 24 1 

 rascal dart (Agrotis malefida) 0 2 

 velvetbean caterpillar moth (Anticarsia 

gemmatalis) 

12 0 

 no common name (Bulia deducta) 0 2 

 forage looper moth (Caenurgina erechtea) 7 0 

 Florida fern moth (Callopistria floridensis) 1 0 

 no common name (Condica mobilis) 1 0 

 no common name (Condica sutor) 1 0 

 chalcedony midget (Elaphria chalcedonia) 15 0 

 corn earworm moth (Heliothis zea) 173 22 

 tobacco budworm moth (Heliothis virescens) 1 0 

 tobacco budworm moth/subflexus straw 

 (Heliothis virescens/subflexus) 1 0 

 unidentified wainscot (Leucania sp.) 1 0 

 no common name (Melipotis acontioides) 0 1 

 indomitable melipotis (Melipotis indomita) 0 5 
224 

Table 13.4. 

 

Taxonomic composition of the aerial plankton sampled by light traps 

Numbers shown are totals from all platforms across all dates during spring and fall (Table 13.2). 

Order Family Species Fall 

1999 

Spring 

2000 

 small mocis (Mocis latipes) 17 0 

 large paectes (Paectes abrostoloides) 1 0 

 armyworm moth (Pseudaletia unipuncta) 11 3 

 soybean looper moth (Pseudoplusia 

includens) 

46 0 

 ragweed flower moth (Schinia rivulosa) 1 0 

 dolichos armyworm moth (Spodoptera 

dolichos) 

10 1 

 yellow-striped armyworm (Spodoptera 

ornithogalli) 

4 2 

 fall armyworm moth (Spodoptera frugiperda) 100 3 

 beet armyworm moth (Spodoptera exigua) 5 0 

 no common name (Spodoptera latifascia) 3 0 

 Spodoptera sp. 8 0 

 bicolored sallow (Sunira bicolorago) 1 0 

 small bird-dropping moth (Tarachidia 

erastrioides) 

0 1 

 cabbage looper moth (Trichoplusia ni) 7 0 

 lettered zanclognatha (Zanclognatha 

lituralis) 

0 1 

 unidentified 143 7 

 

 microlepidoptera 236 1 

 unidentified moths 189 246 

Neuroptera 

 Chrysopidae (green 

lacewing) 

 27 0 

 Hemerobiidae (brown 

lacewing) 

 1 0 

 unidentified 11 1 

 

Odonata 

 Coenagrionidae 

 Rambur's forktail (Ischnura ramburii) 4 1 

 unidentified damselfly 9 1 

 Aeshnidae 

 ringed darner (Anax amazili) 0 1 

 green darner (Anax junius) 203 2 

 Libellulidae 

 great pondhawk (Erythemis vesiculosa) 7 0 

 band-winged dragonlet (Erythrodiplax 

umbrata) 

4 0 
225 

Table 13.4. 

 

Taxonomic composition of the aerial plankton sampled by light traps 

Numbers shown are totals from all platforms across all dates during spring and fall (Table 13.2). 

Order Family Species Fall 

1999 

Spring 

2000 

 blue dasher (Pachydiplax longipennis) 19 4 

 wandering glider (Pantala flavescens) 131 1 

 spot-winged glider (Pantala hymenaea) 10 36 

 variegated meadowhawk (Sympetrum 

corruptum) 

19 1 

 black-mantled glider (Tramea lacerata) 15 1 

 violet-masked glider (Tramea carolina) 1 0 

 red-mantled glider (Tramea onusta) 18 0 

 hyacinth glider (Miathyria marcella) 1 0 

 unidentified dragonfly 1 0 

 

Orthoptera 

 Gryllidae 

 southeastern field cricket (Gryllus rubens) 35 0 

 Tettigoniidae 

 broad-tipped conehead (Neoconocephalus 

triops) 

4 0 

 unidentified 12 0 

 

Insecta 

 unidentified 55 40 

TOTAL 1,4952 4,991 

Altogether, we captured 14,952 insects with an estimated biomass of 1261 g in the fall compared to 4991 

insects with an estimated biomass of 116 g in the spring. Although light traps were deployed on six 

platforms in both seasons, VK786 was used only in fall and HI561 was used only in spring. A 

comparison between spring and fall based on the five platforms with replicated deployments is shown in 

Figure 13.1. The abundance of insects sampled on a given platform was significantly greater in the fall 

than in the spring (paired t = 4.93, P = 0.008; log-transformed data). Similarly, the biomass of insects 

sampled on a given platform was significantly larger in the fall than in the spring (paired t = 4.14, P = 

0.014; log-transformed data). 

Some of the variation among platforms in insect abundance and biomass was clearly related to distance 

from source populations (see Figure 2.1, Table 2.2). In the spring, both abundance and biomass were 

highest at MI622 (the platform closest to land) and lowest at GC18 (the platform farthest from land). In 

the fall, abundance and biomass were similarly highest at MI622, although abundance was lowest at 

PN975 and biomass was lowest at VR265. 

The composition of the insect assemblages sampled by light traps also differed between seasons (Figure 

13.2). Hemipterans were numerically dominant in the spring, whereas Dipterans were dominant in the 

fall. Dragonflies were the largest component of the captured insect fauna by mass in both spring and fall. 
226 

10

100

1000

10000

100000 0.1

1.0

10.0

100.0

1000.0

PN975

Fall Spring

Total Numbers

10

Total Biomass (g)

MI622 VR265 GC18 SP89

100

1000

0.1

10,000

1

10

100

1000

Figure 13.1. Comparison of total abundance and biomass of insects sampled by light 

traps on offshore platforms during spring and fall.
227 

Odonata

Lepidoptera

Hemiptera

Homoptera

Coleoptera

Diptera

Other

Fall

Spring

0

10

20

30

40

50

60

70

80

0

5

10

15

20

25

30

35

40

45

% of Total Numbers % of Total Biomass

Figure 13.2. Comparison of taxonomic composition of insect assemblages sampled by light traps 

on offshore platforms during spring and fall. “Other” includes Orthoptera, 

Hymenoptera, Neuroptera, and unidentified Insecta. 
228 

13.4 SEASONAL PATTERNS AND INFLUENCE OF WEATHER

 (by Robert W. Russell) 

During the fall, insect abundance and biomass peaked from late August through late October, with one 

very large pulse following in mid-November (Figure 13.3). During the spring, insect movements 

occurred from late March through late May (Figure 13.4). Spring occurrences were extremely episodic, 

however: 56% of the total spring biomass was collected during just two events (April 25 and May 13), 

and 60% of the entire season’s numbers was collected on April 25 alone. 

Large fall influxes of insects were primarily associated with the MCH and ELOW synoptic types, but 

smaller events also occurred on ECH, GH, and TLOW days. Large spring events were strictly 

associated with GF days. 

August

10 20 1 10 20 1 10 20 1

September October

0

400

600

800

1000

1200

0

20

40

60

80

100

Number of Insects Trapped

200

November

Biomass of Insects Trapped (g)

10

1

10

100

1000

10000

1

10

100100

10

1

1000

100

10

1

Figure 13.3. Intraseasonal variation in abundance and biomass of insects sampled by light traps on offshore 

platforms in fall 1999. Data from all platforms (N = 6) are combined for each day. The inset 

figures show the same data (x+1)-transformed on a logarithmic scale for perspective. 
229 

March

10 20 1 10 20 1 10 20 30

April May

0

500

1000

1500

2000

2500

3000

0

5

10

15

20

25

30

1

10

1

10

100

1000

10000

Biomass of Insects Trapped (g) Number of Insects Trapped

40

35

3500

1

10

40

1

10

100

1000

Figure 13.4. Intraseasonal variation in abundance and biomass of insects sampled by light traps on offshore 

platforms in spring 2000. Data from all platforms (N = 6) are combined for each day. The inset 

figures show the same data (x+1)-transformed on a logarithmic scale for perspective. 

13.5 INSECTS AS A SOURCE OF FALL RADAR ECHOES

(by Robert W. Russell and Sidney A. Gauthreaux, Jr.) 

Since we know of no method for decomposing NEXRAD reflectivity data into bird and insect 

contributions, we decided not to attempt to interpret the fall radar data quantitatively in this study. Many 

recent advocates of NEXRAD for studying overland bird migration have decided otherwise, assuming 

implicitly that nocturnal radar echoes over land can be interpreted directly in terms of bird migration. As 

a result of this and the straightforward availability of high-quality NEXRAD data, avocational “studies” 

of overland bird migration based on NEXRAD imagery have proliferated on the world wide web. 

However, the underlying assumption that birds are the primary source of the echoes has not yet been 

tested critically or discussed thoroughly. 
230 

Our offshore insect sampling provided an opportunity to examine the correlation between insect 

dispersal and nocturnal radar signatures, since SP89 was within the range of the New Orleans (LIX) 

NEXRAD (see Figures 2.1, 4.1). Figure 13.5 shows that there was a statistically significant correlation 

between light-trap loads from SP89 and peak nightly reflectivity factors measured at the LIX site across 

57 nights in fall 1999. 

1 10 100

Radar Cross Section of Insect Trap Contents (cm2)

Peak Z

e from LIX NEXRAD (mm6 

m-3)

10

100

1

r

s = 0.60

P < 0.001

Figure 13.5. Comparison of light-trap loads from SP89 with NEXRAD radar echoes from the LIX site in 

fall 1999. Each data point represents one night (N = 57). Both data sets were (x+1)-

transformed before plotting to allow the use of logarithmic scales. 

The observed correlation between insect trap loads and radar reflectivity factors is consistent with the 

hypothesis that insects are potentially an important — and possibly even the principal — source of 

nocturnal radar signatures. Of course, a simple correlation between radar reflectivity factors and a 

measure of insect abundance is no more convincing of a causal relationship than is an analogous 

correlation between radar reflectivity and bird abundance. Both birds and insects necessarily contribute 

to radar reflectivity factors, and the important questions concern the relative contributions from each 

under different circumstances, and whether ecological processes involving the two broad classes of biotic 

radar targets can be distinguished correctly. 

Unfortunately, the light traps used in this study are attractant traps that lure insects using a stimulus that 

operates with different efficiencies on different insect taxa. Thus, there is no way to use the resulting 

capture data to estimate absolute aerial densities even crudely. A rigorous determination of the 

quantitative contribution of insects to reflectivity signatures would require the collection of true 
231 

volumetric data on insect densities over the Gulf using non-attractant traps such as the JohnsonTaylor suction trap (Johnson and Taylor 1955; Taylor 1962), which continues to be manufactured by 

Burkard Manufacturing Company <http://www.burkard.co.uk/jttrap.htm>. 

13.6 DISCUSSION 

 (by Robert W. Russell) 

This study documented a large diversity of insects on offshore platforms in both spring and fall. Some of 

the common species recorded in this study are nondiapausing species that can only overwinter in areas far 

enough south to permit them to survive the coldest winter temperatures. These nondiapausing species are 

obligate migrants that must recolonize the northern parts of their range each year via northward flights of 

hundreds of kilometers in spring and early summer, and that evacuate the northern areas via southward 

flights of similar magnitude in late summer and fall (e.g., fall armyworm: Pair et al. 1987, 1991). While 

other species are physiologically capable of overwintering as diapausing pupae in most of their range, at 

least portions of the populations of these species also engage in seasonal long-distance migrations (e.g., 

corn earworm: Hartstack et al. 1982, 1986; Hendrix et al. 1987; Lingren et al. 1993, 1994; Westbrook et 

al. 1995a,b, 1997). Many of the migratory moth species recorded on our platforms may have originated 

from points well beyond the northern Gulf Coast, as the ability of insects to travel to the Gulf Coast 

region from long distances has been documented unequivocally. For example, a marked black cutworm 

moth (Agrotis ipsilon) released in Ankeny, Iowa, in mid-September was recaptured in Brownsville, 

Texas, in late September (Showers et al. 1993). 

Several previous studies have documented the occurrence of insects on oil platforms in the Gulf of 

Mexico (Sparks et al. 1975, 1986; Baust et al. 1981; Keaster et al. 1996). Sparks et al. (1975) deployed 

light traps on four platforms in blocks 32, 125, 198, and 292 of the Eugene Island area off the Louisiana 

coast (located 32, 74, 106, and 160 km offshore, respectively) for six weeks in fall 1973, and found that 

corn earworms occurred at all four locations but that their abundance decreased monotonically with 

distance from shore. In addition to corn earworms, 176 species of insects from 9 orders and 69 families 

were recorded (Sparks et al. 1986). As part of a study of black cutworm migration, Keaster et al. (1996) 

deployed pheromone traps on Gulf platforms for unspecified durations in September 1988 and in January 

1989-1991; they captured about 1700 insects from nine orders, but the study methods precluded complete 

identifications and detailed analysis. 

The abundance and diversity of insects over the Gulf of Mexico has a number of important ecological 

implications. 

First, while the majority of species are undoubtedly inadvertent visitors to the open Gulf, arriving as a 

result of passive transport in a blanket of “aerial plankton,” several species of dragonflies and moths 

exhibited behaviors and seasonal patterns indicating that they are “intentional” trans-Gulf migrants. 

Seasonal trans-Gulf migration in insects has important implications for understanding large-scale 

biogeographic patterns as well as life-history strategies of the species involved. These implications will 

be explored more fully in future publications. 

Second, the passively transported fauna of smaller insect species provides an important food resource that 

is used extensively by energy-depleted migrant birds on platforms. The foraging behavior and success of 

migrant birds is described in detail in Chapter 15. 

Third, it is possible that allochthonous input of carbon via mortality of drifting insects could be 

ecologically significant in food webs of the more depauperate waters of the Gulf. Undoubtedly, the vast 

majority of smaller insects advected over the Gulf must rapidly deplete their fuel stores, fall to the sea 
232 

surface, and become part of the marine neuston (Zaitsev 1970). Based on neuston tows in the Gulf 

of Mexico, Wolf et al. (1986) reported typical densities of up to 290 insects per hectare of ocean 

surface, with exceptional densities of up to 1350/ha in a convergence zone. While some of these 

individuals may survive and eventually be washed ashore alive, mortality is surely high (e.g., Howden 

1977). The role of terrestrial insects as a component of the marine neuston and pleuston, and their 

possible involvement in marine ecosystem dynamics, have only rarely been considered (Heydemann 

1967; Bowden and Johnson 1976; Cheng and Birch 1977, 1978; Wolf et al. 1986; Peck 1994b). Cheng 

and Birch (1977) estimated the organic input of terrestrial insects into marine systems to be 2-17 kg km-2

year-1, which is only about 0.01% of the primary productivity of relatively unproductive oceans. 

However, the importance of advective fluxes of insects in marine food chains might be much higher 

locally in the Gulf of Mexico, because primary productivity in the Gulf is relatively low and insect 

populations appear to be extremely high compared to other areas. To our knowledge, the possibility that 

terrestrial insects may figure significantly in Gulf food webs and carbon cycling has yet to be evaluated 

critically. 

Fourth, we often noted blue runners (Caranx crysos) as well as the otherwise herbivorous Bermuda chubs 

(Kyphosus sectatrix) consuming insects that fell into the water. It is possible that attraction of drifting 

insects to platforms creates local concentrations of entomological pleuston that in turn play a role in the 

attraction of the large schools of fish to the surface waters around platforms. 

Finally, the abundance of insects over the Gulf in the fall along with the documented correlation between 

insect trap loads and radar reflectivity factors casts doubt on simplistic attempts to interpret fall NEXRAD 

images strictly in terms of bird migration. Indeed, we believe that the problems inherent in distinguishing 

between birds and insects as the source of NEXRAD signatures have been underappreciated severely in 

virtually all circumstances other than spring trans-Gulf migration (Chapter 9.1). 

As discussed in Chapter 9.2, the pattern of diel timing of ascent into the atmosphere by migrating birds is 

identical to that for dispersing insects, with large numbers of both taxa typically ascending en masse into 

the airspace about a half hour after sunset. Complicating the problem of radar target identification is the 

fact that migratory insects also have seasonal activity patterns that are remarkably similar to those of 

migrating birds. In a comprehensive study of invertebrate populations of a deciduous forest, the total 

insect population showed seasonal peaks around May 1 and September 1 (Kendeigh 1979). In a study 

directly relevant to ours, Beerwinkle et al. (1994, 1995) used pheromone traps along with 3-cm scanning 

radar to study the phenology and nocturnal flight activity of a variety of agriculturally important noctuid 

moth pests in the Brazos River Valley of coastal Texas. They found that northward migration-type 

movements were regular in the spring and southward movements were regular in the fall. The most 

abundant species in their study was the corn earworm (Heliothis zea), which also turned out to be 

abundant offshore during the fall in our study (Table 13.4). Pheromone trapping indicated seasonal peaks 

in corn earworm migration at around April 20 and October 5 in one year, and April 25 and September 25 

in another year. These dates are nearly identical to the seasonal peaks of passerine bird migration along 

the northern Gulf Coast (Figures 8.3, 9.25). Even the durations of migration are similar: the bulk of the 

spring pulse of corn earworms occurred March 30-May 30 in one year and March 5-May 20 in the second 

year of trapping. Similarly, a fall pulse was evident from August 20-October 25 in one year and August 

25-October 25 in the second. The only detectable difference between birds and corn earworms is that 

activity of the latter continues variably through the summer, with periodic peaks from June 10-August 10. 

Ornithologists have routinely underestimated the magnitude of the biomass of migrating and dispersing 

insects in the atmosphere. Entomologists have documented discrete dispersal events involving ten billion 

moths and hundreds of tons of airborne moth biomass (Dickison et al. 1986). Beerwinkle et al. (1994) 

documented large numbers of insects moving southward in northerly winds immediately behind 

advancing cold fronts from early September through late October in coastal Texas, and measured a peak 
233 

concentration of >1200 noctuid moths/106

 m3 in an altitudinal band between 100 and 600 m on the 

night of October 16-17. Such an ambient density would have resulted in a NEXRAD-measured 

dBZe value over 24 (see Chapter 4.2, Table 13.3), which is comparable to the heaviest spring trans-Gulf 

bird migrations recorded in this study (e.g., Figure 8.1). The potential for peak insect movements to rival 

peak bird movements in terms of biomass and reflectivity suggests that proponents of using surveillance 

radar to monitor bird migrations both along the Gulf Coast and elsewhere should exercise extreme 

caution. 

In some cases, migrating birds may have velocities that are manifested clearly in radar-measured wind 

fields that differ dramatically from sonde measurements (e.g., Wilczak et al. 1995). However, obvious 

contamination of wind profiles by migrating birds does not mean that reflectivity contributions from 

insects can be ignored. In addition, nocturnal moth migrants are not always good tracers of the wind. 

Indeed, during the course of our nocturnal studies at VK786 (Table 9.1) we often observed apparent 

noctuids traveling at angles up to 90° with respect to the wind, and on several occasions we have 

observed pink-spotted hawkmoths (Agrius cingulata) traveling upwind under conditions that were 

challenging to small warblers. The presence of anomalous velocities in radar-observed wind fields is not 

alone an adequate criterion for interpreting nocturnal radar signatures solely in terms of bird migration. 
235 

Chapter 14 

 

Evaluation of a Method for Monitoring Audible 

Fall Bird Migration Traffic Over Platforms 

Andrew Farnsworth and Robert W. Russell 

14.1 INTRODUCTION

A major limitation to fall field work on platforms is that the vast majority of trans-Gulf migrants are aloft 

over the platforms during the night, and at the beginning of their journeys when they are probably much 

less likely to use the platforms. Moreover, because nocturnal fall migration over the northern Gulf 

proceeds at much lower altitudes than does the largely diurnal migration during the spring, large fall 

trans-Gulf flights are often partially or completely missed by radar. An additional problem is that, even 

when fall migrants are flying high enough to be detected by radar, vast numbers of migrating sphingid 

and noctuid moths and other insects often dominate the radar return at night, precluding unequivocal 

interpretations of bird migration based on radar data alone. Thus, except on rare nights when moon 

conditions permit the quantification of migration via “moonwatching” (Lowery and Newman 1963, 

1966), the ability to obtain independent information on fall migration traffic aloft for comparison with 

observations of migrants on platforms is compromised. 

Although we could not effectively study the flight behavior of nocturnal fall migrants using visual means, 

we frequently heard large numbers calling as they passed overhead. Birds are thought to vocalize at night 

during migration as a means of maintaining flock structure and facilitating orientation. We attempted to 

exploit this behavior and undertook a pilot study in fall 1999 to evaluate an acoustic recording method for 

monitoring audible bird migration traffic over the platforms. 

14.2 BACKGROUND ON STUDIES OF NOCTURNAL VOCALIZATIONS BY MIGRANTS

Nocturnal vocalizations have offered insight into the specific composition of nocturnal flights of 

migrating birds since the turn of the century (Libby 1899). As early as the 1950s, researchers were 

actively investigating nocturnal migration by examining bird vocalizations with electronic recording 

technology (Ball 1952; Graber and Cochran 1959, 1960; Hamilton 1962; Dorka 1966; Graber 1968; 

Dierschke 1989). Since 1985, Evans and colleagues (Evans 1994; Evans and Mellinger 1999; Evans and 

Rosenberg 2000) have determined the identity of many nocturnal vocalizations of migrant landbirds by 

comparing spectrographic analyses of nocturnal flight calls with known diurnal flight calls and correlating 

seasonal timing and geographic range patterns of these nocturnal recordings with known timing and 

migration routes of migrants. Groups known to give night-flight calls include cuckoos (Cuculidae), 

thrushes (Turdinae, especially the species in the genus Catharus), warblers (Parulinae), and sparrows 

(Emberizinae); many non-passerines including herons (Ardeidae), waterfowl (Anatidae), and rails 

(Rallidae) also give night-flight calls. As a result of the recent advances, it is now theoretically possible 

— simply by aiming a microphone at the sky — to record and analyze the vocalizations of passing 

migrants, yielding data on species composition, migration timing and routing, and the volume of 

nocturnal migration traffic. 

14.3 FIELD METHODS

Following the design for a pressure zone microphone (PZM) suggested and successfully implemented in 

recent onshore studies (Evans 1994; Evans and Mellinger 1999; Evans and Rosenberg 2000), Farnsworth 
236 

constructed two PZMs and housed them in flower pots to improve durability and shield the 

microphone from excessive wind noise. The configuration of a PZM looks and acts much like a 

tympanic membrane: a small microphone centered on a reflective dish which is sealed against moisture 

with a thin plastic membrane, and which acts in such a way as to amplify the bird vocalization. Housing a 

PZM in a flower pot also secures its position and maintains its skyward orientation. To protect the plastic 

membrane surface of the PZM from debris, the pot was covered with lightweight, perforated cloth. 

The range of detection of the flower pot PZM varies with conditions, but it can generally record calls in 

the 6-9 kHz range (most warblers and sparrows) within a cone of detection 300 m high and 250 m wide, 

as well as in the 2-5 kHz range (most thrushes, grosbeaks, and tanagers) within a cone of detection 600 m 

high and 1000 m wide (Evans and Rosenberg 2000). The detection heights are well within what are 

considered the densest layers of nocturnal migration over land. 

One of the PZM units was assigned to EW826, but failed shortly after deployment. The other unit was 

deployed to VK786 on September 9 and successfully operated on 41 nights through November 2. (A 

mechanical failure precluded operations in early October while replacement parts were gathered and 

shipped offshore.) 

On operational nights, calls were recorded to 8-hour VHS tapes on a Sony SLV-675 hi-fidelity 

videocassette recorder. Most recordings began at 21:00 CDT (range = 20:44-21:18 CDT) and ended 

close to 05:00 CDT. A portable Optimus tape deck was used to amplify the signal of the incoming audio 

stream. At the end of each night, tapes were labeled with the date and time of recording and stored for 

later analysis in the laboratory. 

14.4 LABORATORY ANALYSIS

Farnsworth conducted all laboratory analysis at Clemson University. The VHS tapes were played back 

into the line level input of a Dell XPS-600 computer with a SoundBlaster 4.61 

16-bit sound card to analyze the acoustic record for each night. During playback, software developed by 

Old Bird, Inc. <www.oldbird.org> was used to detect call notes automatically, following the methods of 

Larkin et al. (2002). This software examined two basic criteria to detect a call note: aberrations in the 

slope of incoming smoothed log-magnitude signal energy and concentrations in the Fourier spectrum in a 

given frequency band. If a signal met these criteria, it was logged with a time and date stamp as a digital 

.wav file. Once these .wav files were logged, they were examined using a visual spectrographic analysis 

software package called GlassOFire developed by Old Bird, Inc. This program permitted the viewing of 

large numbers of digital .wav files as spectrographs, and facilitated the classification of call notes and the 

rejection of spurious, non-avian detections. The identity of avian detections was also confirmed by 

reviewing all classified calls through headphones plugged into the computer’s sound card. 

These techniques were used to analyze a total of 333 hours of recordings made on VK786 over 41 nights 

during fall 1999. To check the program’s efficiency, each 8-h tape was manually audited four times each 

hour to confirm the detection success of the algorithms employed by the Old Bird, Inc. software. The 

software’s detection rate was approximately 98% of the manual auditory detection rate, and the software 

correctly detected a small percentage of calls that were missed manually (4%). 

Unfavorable recording conditions occurred on 11 nights and necessitated the rejection of all data collected 

by the PZM during those time periods. Such data are likely not salvageable by any means of corrective 

recording techniques, representing one of the drawbacks of this methodology in a noisy environment. 

Environmental noise and platform activities that were not as extreme also significantly affected the 

recordings, effectively reducing the efficiency of the PZM for any vocalizations in the 2-5 kHz range to 

zero. As a result, the data presented here mostly represent 6-9 kHz detections of warblers and sparrows 
237 

and a few lower-frequency vocalizations (e.g., bunting types). Some of these data were also 

contaminated by environmental noise, making detection possible but identification impossible. 

14.5 RESULTS

A summary of number of calls detected on a nightly and hourly basis is provided in Table 14.1. 

Altogether, 2762 calls were detected during 30 nights of recording, although some of the calls could not 

be analyzed properly due to contamination by non-avian signals. The last column in Table 14.1 shows 

the total number of recognizable calls corrected for contamination and recording problems. Table 14.2 

presents the identities of known species and their total calls as well as unidentified species and their total 

calls. Three species (Black-and-white Warbler, Common Yellowthroat, and American Redstart) could be 

identified specifically, representing approximately 9% (N = 156) of the total identifiable calls (N = 1664). 

Four call types attributable to species groups defined by Evans and Rosenberg (2000) and representing 

approximately 15-20 possible species were also identified, but not to the specific level. Call notes in the 

four species groups made up approximately 54% (873) of the total identifiable calls. Bunting types 

representing two species in the genus Passerina and a single species in the genus Guiraca represented 

approximately 3% (42) of the total identifiable calls. Unidentified calls made up the remaining 

percentage of approximately 34% (562) of the total identifiable calls. 

Table 14.1. 

 

Summary of hourly and nightly numbers of calls recorded over VK786 during fall 1999 

 Time (CDT) Total # 

 Hours 21:00- 22:00- 23:00- 00:00- 01:00- 02:00- 03:00- 04:00- 05:00- Total Corrected for 

Date Recorded 22:00 23:00 00:00 01:00 02:00 03:00 04:00 05:00 06:00 # Calls Contamination

9-Sep 8.0 0 0 3 23 12 20 39 21 0 118 76 

10-Sep 8.0 2 257 196 67 33 58 123 245 36 1,017 659 

12-Sep 7.4 1 2 0 0 2 8 0 10 0 23 18 

13-Sep 8.0 0 0 0 2 30 7 18 23 8 88 62 

14-Sep 8.0 0 45 25 35 10 14 45 33 5 212 143 

15-Sep 8.0 10 10 25 9 32 26 3 82 1 198 99 

16-Sep 7.0 0 0 0 1 0 0 0 0 0 1 1 

17-Sep 8.0 0 1 18 12 5 19 14 47 6 122 76 

18-Sep 8.0 1 0 8 4 40 3 16 40 13 125 94 

20-Sep 8.0 0 0 0 0 0 0 3 0 1 4 1 

21-Sep 8.0 14 8 0 0 0 0 4 0 0 26 2 

22-Sep 8.0 0 14 10 13 8 17 15 51 9 137 29 

23-Sep 8.0 0 0 0 0 0 1 1 14 0 16 12 

24-Sep 8.0 0 4 0 0 0 0 0 0 0 4 2 

25-Sep 7.0 0 0 0 0 0 0 0 0 0 0 0 

26-Sep 8.0 0 0 0 0 0 0 0 0 0 0 0 

27-Sep 8.0 0 0 0 1 0 0 0 0 0 1 0 

28-Sep 8.0 0 1 5 1 0 0 0 0 0 7 1 

29-Sep 8.0 2 3 7 37 34 13 41 122 46 305 168 
238 

Table 14.1. 

 

Summary of hourly and nightly numbers of calls recorded over VK786 during fall 1999 

 Time (CDT) Total # 

19-Oct 7.2 0 0 0 0 0 0 0 0 0 0 0 

20-Oct 8.0 0 3 3 6 5 4 17 8 2 48 29 

21-Oct 8.0 1 0 5 0 0 0 2 2 2 12 4 

22-Oct 8.0 0 0 0 0 1 0 1 0 0 2 2 

23-Oct 8.0 3 0 0 3 2 0 1 0 0 9 3 

24-Oct 8.1 1 1 1 1 1 2 1 0 1 9 9 

25-Oct 8.0 0 0 0 0 6 29 41 6 0 82 60 

26-Oct 8.0 5 0 0 0 0 1 1 1 7 15 5 

27-Oct 8.0 6 2 5 1 2 1 2 63 2 84 54 

1-Nov 8.0 4 65 5 5 0 2 0 3 2 86 52 

2-Nov 8.0 0 0 0 1 5 2 1 2 0 11 3 

Totals 236.7 50 416 316 222 228 227 389 773 141 2,762 1,664 
239 

Table 14.2. 

 

Species composition of calling migrants recorded over VK786 during fall 1999 

Groups follow the Evans and Rosenberg (2000) classification scheme (Table 14.3). “Bunting type” refers to a 

group containing two species of the genus Passerina and a single species in the genus Guiraca. Abbreviations: 

BAWW = Black-and-white Warbler; COYE = Common Yellowthroat; AMRE = American Redstart; UNID = 

unidentified. 

 Taxon 

Date BAWW COYE AMRE Bunting 

Type 

Group 

2 

Group 

3 

Group 

4 

Group 

5 UNID 

Total 

9-Sep 2 2 6 3 3 13 2 35 10 76

10-Sep 3 1 43 4 13 163 38 147 247 659

12-Sep 3 2 0 0 0 0 0 7 6 18

13-Sep 2 4 2 1 4 9 1 31 8 62

14-Sep 0 0 8 3 5 31 7 23 66 143

15-Sep 0 1 15 2 1 17 4 43 17 99

16-Sep 0 0 0 0 0 0 1 0 0 1

17-Sep 2 2 5 9 0 13 4 22 19 76

18-Sep 1 3 14 7 3 4 0 49 13 94

20-Sep 0 1 0 0 0 0 0 0 0 1

21-Sep 0 0 1 0 0 0 0 1 0 2

22-Sep 0 2 0 0 0 0 2 5 20 29

23-Sep 0 0 0 0 0 0 0 0 0 12

24-Sep 0 2 0 0 0 0 0 0 0 2

25-Sep 0 0 0 0 0 0 0 0 0 0

26-Sep 0 0 0 0 0 0 0 0 0 0

27-Sep 0 0 0 0 0 0 0 0 0 0

28-Sep 0 0 0 0 0 0 0 0 1 1

29-Sep 2 6 8 3 0 74 7 54 14 168

19-Oct 0 0 0 0 0 0 0 0 0 0

20-Oct 1 0 1 4 0 3 0 9 11 29

21-Oct 0 0 0 0 0 0 0 0 4 4

22-Oct 0 0 0 0 0 2 0 0 0 2

23-Oct 0 3 0 0 0 0 0 0 0 3

24-Oct 0 2 0 0 0 1 0 1 5 9

25-Oct 0 2 0 3 0 4 1 5 45 60

26-Oct 0 0 0 0 0 1 1 0 3 5

27-Oct 0 0 1 3 0 3 3 1 43 54

1-Nov 0 3 0 0 0 9 2 11 27 52

2-Nov 0 0 0 0 0 0 0 0 3 3

30 nights 16 36 104 42 29 347 73 444 562 1,664
240 

Table 14.3. 

 

Some nocturnal call note group classifications 

Each grouping represents a “complex” of species with similar call notes. From Table 

2 in Evans and Rosenberg (2000). 

Group 2

Blue-winged Warbler 

Golden-winged Warbler 

Louisiana Waterthrush 

A buzzy “kzeen” or “kzeep” note 

Group 3

Tennessee Warbler 

Nashville Warbler 

Orange-crowned Warbler 

Black-throated Green Warbler 

Vesper Sparrow 

Yellow-rumped Warbler 

White-crowned Sparrow 

A sibilant, rising “tseet” or “tsee” note 

Group 4

Pine Warbler 

Northern Parula 

Field Sparrow 

Yellow-throated Warbler 

Prairie Warbler 

A descending “tsew” note 

Group 5

Cerulean Warbler 

Blackburnian Warbler 

Magnolia Warbler 

Worm-eating Warbler 

Blackpoll Warbler 

Yellow Warbler 

Bay-breasted Warbler 

Connecticut Warbler 

A buzzy “zeet” note 

Figure 14.1 shows the temporal pattern of calls recorded per hour. The peak number of nocturnal calls 

during one hour (733 calls) occurred approximately 8 hours after the beginning of recording, during the 

period from 04:00-05:00 CDT. A secondary peak (416 calls) occurred approximately 2 hours after the 

beginning of recording, during the period from 23:00-00:00 CDT. 

We examined the relationship between the frequency of calls recorded during a given night and the 

abundance of landbird migrants on the platform the following day. Prior to analysis, stopover counts 

were ln(x+1)-transformed to normalize the highly skewed distribution. Overall, there was not a 
241 

significant correlation between nocturnal calling and subsequent stopovers (rp = 0.21, N = 30 nights, 

P = 0.26). However, we noticed that there seemed to be an influence of weather on the relationship 

between calling and stopovers. Two weather types dominated the overall sample and provided 

subsamples of sufficient size for analysis (ECH and MCH; N = 11, 12 nights respectively). We 

conducted an analysis of covariance (ANCOVA) with synoptic weather type (ECH versus MCH) as the 

independent categorical variable and nightly calling frequency as the covariate. This analysis indicated 

the following: 1) the number of stopovers was significantly positively related to calling frequency (F1,19 = 

4.91, P = 0.039); 2) the slope of the calling-stopover relationship did not differ between weather types 

(F1,19 = 1.12, P = 0.30); and 3) a given nocturnal call frequency was associated with significantly more 

stopovers when the weather type was MCH (F1,19 = 13.1, P = 0.002). 

0

100

200

300

400

500

600

700

800

900

123456789

Hours After 9 PM CDT

Calls Per Hour

Figure 14.1. Total calls per hour recorded at VK786 during fall 1999. 

The apparent weather influence on the magnitude of the relationship between calling and stopovers could 

result from an influence of weather on likelihood of calling, an influence of weather on likelihood of 

initiating a stopover, or perhaps both. Fortuitously, we conducted an ancillary study of nocturnal flight 

behavior that provides some evidence concerning the former possibility. On several days in September 

1999, an observer on VK786 conducted visual and auditory censuses of migrants while lying prone on the 

platform helipad through the night. On the night of September 17-18 (ECH), 22 small migrant passerine 

targets were detected visually and 5 of those were heard calling. On the night of September 21-22 

(MCH), 67 small migrant passerines were detected visually and 7 of those were heard calling. The 
242 

proportion of migrants calling was thus more than twice as high on the ECH night (23%) than on 

the MCH night (10%), though the difference was not quite statistically significant due to the 

relatively small sample on the ECH night (G = 1.95, P = 0.16). If this was indeed a true difference, it 

suggests that nocturnal migrants may call less frequently under favorable flight conditions. 

14.6 DISCUSSION

The data gathered during this study offered a unique opportunity to examine the behavior of nocturnal 

calling over water. All previous studies have focused on recording birds in migration over land. 

It is interesting to note that two marked peaks occurred in the total number of calls per hour over the 

course of the night (Figure 14.1). If the nearest stopover habitat is approximately 145 km to the north 

(Table 2.2), birds departing the coast a half hour after sunset and traveling south at 12 m·s-1 would reach 

the platform shortly before midnight. Thus, it seems likely that the peak in calling occurring at 23:00-

24:00 CDT represented the initial wave of migrants leaving the coast of the United States. The major 

peak occurring at 04:00-05:00 CDT likely represents what occurs with some consistency over land at 

approximately the same time — a communication of the message to flock and/or to land. It is possible 

that increased calling over VK786 late in the night resulted from migrants seeking to reform flocks for the 

remaining diurnal flight across the Gulf of Mexico, or perhaps attempting to find suitable landing sites on 

the platform. 

The finding of a positive relationship between calling frequency and subsequent stopovers, after 

accounting for effects of weather, suggests that it might be possible to use automated acoustic methods to 

generate quantitative data on nocturnal migrant traffic in the airspace above platforms. Other recent work 

has shown that calling does reflect bird density in a weak positive relationship, but the temporal and 

spatial bounds of the relationship’s occurrence and utility are still not well understood (Farnsworth 2001). 

The presence of a platform may itself represent a bias that positively affects calling rate in migrants, if 

migrants are calling as a means of communicating to maintain flock organization (Hamilton 1962) or to 

convey information about location (Farnsworth 2001), including potential landing situations on the 

platform. 

The need for quiet recording locations when sampling nocturnal migration with acoustic sensors has been 

mentioned frequently (e.g., Graber and Cochran 1959) and would be critical in the planning for any future 

acoustic work in offshore situations. The adverse acoustic environment usually found offshore (e.g., high 

winds, platform noise) means that the technique is not likely to be used very frequently. However, in this 

study, the microphone did not exhibit any signs of corrosion on the element or in its energy supply, 

making the implementation of this type of sensor in future studies a realistic possibility in terms of the 

performance of the equipment. 

Several other difficulties exist in using this technique to monitor nocturnal migration, e.g., the identity of 

many migrant species is still poorly understood, the rate at which different species vocalize is not 

constant, and some species may not vocalize at all. Given these caveats, a conservative approach is best 

when interpreting acoustic data. However, any knowledge of the magnitude and specific composition of 

offshore flights during periods when visual observation is unlikely or impossible is potentially very 

valuable. A recently published audio guide to nocturnal vocalizations (Evans and O’Brien 2002) and 

increasing popular interest in flight call notes will shed more light on issues of species identity in the very 

near future, making further studies of this kind a realistic option at least for qualifying nocturnal 

migration both onshore and offshore. 
PART 4 

 

THE IMPACT OF PLATFORMS 

ON MIGRANT BIRDS 
245 

Chapter 15 

 

Stopover Ecology of Migrants on Platforms 

Robert W. Russell 

15.1 PHYSIOLOGICAL CONDITION OF ARRIVING MIGRANTS

The majority of migrants arriving on platforms appeared to be in fine physical condition. However, initial 

body condition varied between seasons and among species types and was related to weather. 

15.1.1 Spring 

In the spring, small neotropical migrants were least frequently in poor condition upon arrival on BH, 

ECH, and GH days, and most frequently in poor condition on GF and especially MCH days (Table 15.1; 

G = 23.7, df = 8, P = 0.0026). 

Large neotropical migrants exhibited a pattern similar to that for small species, except that a larger overall 

proportion of birds arrived in poor condition (Table 15.2; excluding the 2 AE birds: G = 21.8, df = 4, P = 

0.0002). 

Table 15.1. 

 

Initial body condition of newly arrived small neotropical 

migrants in relation to synoptic weather type during spring 2000 

Proportions are shown. 

Weather Initial Body Condition 

Type AE AT AV N 

BH 0.015 0.088 0.897 522 

ECH 0.014 0.096 0.890 291 

GH 0.034 0.085 0.881 118 

GF 0.012 0.152 0.836 1,182 

MCH 0.018 0.193 0.789 57 

All types 0.014 0.127 0.859 

N 31 275 1,864 2,170 
246 

Table 15.2. 

 

Initial body condition of newly arrived large neotropical migrants 

in relation to synoptic weather type during spring 2000 

Proportions are shown. 

Weather Initial Body Condition 

Type AE AT AV N 

BH 0.009 0.150 0.841 107 

ECH 0.000 0.204 0.796 93 

GH 0.000 0.195 0.805 82 

GF 0.003 0.304 0.694 382 

MCH 0.000 0.395 0.605 86 

All types 0.003 0.268 0.729 

N 2 201 547 750 

15.1.2 Fall 

In the fall, small neotropical migrants were least frequently in poor condition on ECH, ELOW, and GH

days, and most frequently in poor condition on BH, MCH, and especially TLOW days (Table 15.3; 

excluding the 13 AE birds: G = 541.6, df = 5, P < 0.0001). 

In contrast to the spring, when large neotropical migrants were more likely than small species to arrive in 

poor condition (Tables 15.1, 15.2), large migrants were more likely than small species to arrive in good 

condition during the fall (Tables 15.3, 15.4). 

Table 15.3. 

 

Initial body condition of newly arrived small neotropical migrants 

in relation to synoptic weather type during fall 1999 

Proportions are shown. 

Weather Initial Body Condition 

type AE AT AV N 

ELOW 0.000 0.072 0.928 208 

MCH 0.002 0.284 0.714 2,033 

ECH 0.005 0.052 0.943 982 

GH 0.004 0.069 0.928 567 

BH 0.050 0.200 0.750 20 

TLOW 0.002 0.445 0.553 604 

All types 0.003 0.217 0.780 

N 13 956 3,445 4,414 
247 

Table 15.4. 

 

Initial body condition of newly arrived large neotropical migrants 

in relation to synoptic weather type during fall 1999 

Proportions are shown. 

Weather Initial Body Condition 

Type AE AT AV N 

ELOW 0.000 0.059 0.941 202 

MCH 0.007 0.094 0.899 684 

ECH 0.004 0.075 0.920 226 

GH 0.023 0.090 0.887 177 

BH 0.000 0.053 0.947 19 

TLOW 0.014 0.192 0.795 146 

All types 0.008 0.095 0.897 

N 12 138 1,304 1,454 

In addition, while arrival condition of large migrants was also related to weather type in fall (Table 15.4; 

excluding the 12 AE birds: G = 18.0, df = 5, P = 0.0029), large species exhibited a rather different pattern 

in fall than did small species. Specifically, the likelihood of arriving in poor condition on MCH days was 

not dramatically different than on days with other seemingly favorable weather types (Table 15.4). 

Oddly, large fall migrants were least likely to arrive in poor condition on BH days, though the sample size 

was very small. As was the case for small species, large neotropical migrants in fall were most likely to 

arrive in poor condition on TLOW days. 

15.2 FORAGING BEHAVIOR AND SUCCESS

Analyses of foraging behavior were restricted to neotropical migrants, medium- and short-distance 

migrants, and vagrants. Aerial insectivores often appeared to be foraging on aerial plankton in the lees of 

the platforms, but because it was impossible to observe their minute prey from a distance, we were unable 

to characterize their behavior with certainty. Ruby-throated Hummingbirds were often observed 

approaching red objects or materials, apparently in search of flowers and nectar. Because of the difficulty 

in ascertaining the motivation for this behavior — as well as the universal absence of real food resources 

for hummingbirds on platforms — they were also excluded from foraging analyses. 

15.2.1 Spring 

Overall, 11% of the 7,261 migrant landbirds that undertook stopovers on the platforms over the three 

springs of study were observed foraging. Likelihood of foraging varied significantly among the three 

springs studied (1998: 187 of 3,021 [6%]; 1999: 147 of 791 [19%]; 2000: 453 of 3,449 [13%]; G = 

137.8, df = 2, P < 0.0001). 

Likelihood of foraging was also related to body condition of the migrants upon arrival, though the form of 

the relationship varied among years. In spring 1998, 168 of 2,881 landbirds that were vigorous upon 

arrival were subsequently observed foraging (6%), whereas 19 of 101 tired and exhausted migrants were 

observed foraging (19%). This difference was statistically significant (G = 19.2, P < 0.0001). In spring 

1999, 118 of 552 vigorous migrants were observed foraging (21%), whereas 29 of 232 exhausted 

migrants were observed foraging (13%). This difference was statistically significant (G = 9.0, P = 0.003). 
248 

In spring 2000, 350 of 2,780 vigorous migrants were observed foraging (13%), whereas 103 of 577 

exhausted migrants were observed foraging (18%). This difference was statistically significant (G = 

10.6, P = 0.001). Thus, exhausted migrants were significantly more likely to forage than were vigorous 

migrants in two years (1998, 2000), whereas vigorous migrants were more likely to forage in the other 

year (1999). 

Overall, 38% of the 787 landbird migrants that were observed foraging over the three springs of study 

were successful. Likelihood of success varied significantly among the three springs studied (1998: 57 of 

187 [30%]; 1999: 40 of 147 [27%]; 2000: 201 of 453 [44%]; G = 19.9, df = 2, P < 0.0001), and was also 

variably related to body condition of the migrants upon arrival. Likelihood of success by foraging birds 

was unrelated to initial body condition in two of the three years of study (1998: 30% of 168 vigorous 

migrants successful, versus 32% of 19 exhausted migrants; G = 0.01, P = 0.91; 2000: 44% of 350 

vigorous migrants successful, versus 46% of 103 exhausted migrants; G = 0.09, P = 0.77). In the other 

year (1999), significantly more vigorous migrants foraged successfully (32% of 118, versus 7% of 29 

exhausted migrants; G = 9.2, P = 0.002). 

Drinking by migrants on the platforms was very rare, despite the constant availability of fresh water on 

most platforms. Overall, 0.5% of the 7,261 landbird migrants that undertook stopovers on the platforms 

over the three springs of study were observed drinking. Likelihood of drinking varied significantly 

among the three springs studied (1998: 21 of 3,021 [0.7%]; 1999: 11 of 791 [1.4%]; 2000: 7 of 3449 

[0.2%]; G = 18.2, df = 2, P = 0.0001), and was also variably related to body condition of the migrants 

upon arrival. In two of the three years of study, significantly more exhausted migrants were observed 

drinking than were vigorous migrants (1998: 0.6% of 2,881 vigorous migrants observed drinking, versus 

4.1% of 101 exhausted migrants; G = 7.9, P = 0.005; 2000: 0.07% of 2,780 vigorous migrants, versus 

0.9% of 577 exhausted migrants; G = 10.0, P = 0.002). In the other year (1999), there was no relationship 

between body condition and likelihood of drinking (1.4% of 552 vigorous migrants observed drinking, 

versus 1.3% of 232 exhausted migrants; G = 0.03, P = 0.86). 

15.2.2 Interspecific Variation 

A comparison of the foraging status and success of the different species groups during the fall is provided 

in Table 15.5. Long-distance migrants foraged less frequently than did medium-distance migrants, which 

foraged less frequently than did short-distance migrants and vagrants. These differences are 

understandable in terms of the migratory goals of the different groups and their proximities to points of 

original departure. The long-distance migrants presumably departed from nearby points along the 

northern Gulf Coast with adequate energy stores to complete a trans-Gulf crossing. In contrast, most of 

the medium- and especially short-distance migrants ended up over water inadvertently (Chapter 9.4) and 

had probably not deposited adequate energy reserves for a long water crossing. Vagrants were randomly 

oriented during the fall (Figure 11.6) and many of them had probably expended most of their energy 

reserves traveling long distances over water from their points of departure. The likelihood of foraging 

success was similar among all groups except vagrants, which were the most successful — perhaps 

because they were the most fat-depleted and had the greatest “motivation.” 

A summary of species-specific foraging status and success is provided in Table 15.6. Among warblers, 

the following species were observed foraging more than 50% of the time: Pine Warbler, Prairie Warbler, 

Canada Warbler, Western Palm Warbler, Orange-crowned Warbler, Myrtle Warbler, Mourning Warbler, 

and Wilson’s Warbler. These species are all overshoots (Pine Warbler, Orange-crowned Warbler, and 

probably some of the Myrtle Warblers), vagrants (Western Palm Warbler, Prairie Warbler), or primarily 

circum-Gulf migrants that do not routinely cross the widest part of the Gulf (Canada Warbler, Mourning 

Warbler, Wilson’s Warbler). Warbler species observed foraging infrequently (<50%) were all typical 
249 

trans-Gulf migrants. Among non-warblers, the list of frequent foragers was dominated by small 

flycatchers and ecologically similar species (gnatcatchers, kinglets). 

Among the species shown in Table 15.6, there was a significant correlation between likelihood of 

foraging and probability of success (rp = 0.35, N = 48, P = 0.014), though the amount of variance 

explained was rather low (12.5%). 

Table 15.5. 

 

Comparative foraging status of the different species groups in fall 

1999 

Sample size indicates total number of individuals studied in all 

synoptic weather types. “Proportion successful” is the proportion 

of the foraging birds that were successful. The “VAG” group 

includes both Caribbean and western vagrants. 

Species 

Group N 

Proportion 

Foraging 

Proportion 

Successful 

NEOS 3,217 0.19 0.45 

NEOL 1,175 0.12 0.42 

MDM 907 0.33 0.44 

SDM 276 0.47 0.46 

VAG 194 0.46 0.62 

Total 5,769 0.22 0.46 

Table 15.6. 

 

Species-specific foraging status in fall 1999 

Sample size shown is total number of individuals stopping over on 

platforms. The list excludes species with N < 10 stopovers. 

“Proportion successful” is the proportion of the foraging birds that 

were successful. Data on proportion successful are shown only for 

species with N ≥ 8 individuals observed foraging. Species are listed 

in descending order of foraging frequency. 

Species N Proportion 

Foraging 

Proportion 

Successful 

Pine Warbler 13 0.846 0.455 

Least Flycatcher 64 0.688 0.705 

Acadian Flycatcher 23 0.652 0.733 

Blue-gray Gnatcatcher 14 0.643 0.778 

Prairie Warbler 25 0.640 0.500 

Canada Warbler 18 0.611 0.364 

Western Palm Warbler 68 0.603 0.683 

Orange-crowned Warbler 60 0.600 0.500 

Winter Wren 10 0.600 na 

Eastern Phoebe 41 0.585 0.750 

Ruby-crowned Kinglet 115 0.574 0.545 

Myrtle Warbler 148 0.568 0.464 

Mourning Warbler 25 0.560 0.357 
250 

Table 15.6. 

 

Species-specific foraging status in fall 1999 

Sample size shown is total number of individuals stopping over on 

platforms. The list excludes species with N < 10 stopovers. 

“Proportion successful” is the proportion of the foraging birds that 

were successful. Data on proportion successful are shown only for 

species with N ≥ 8 individuals observed foraging. Species are listed 

in descending order of foraging frequency. 

Species N Proportion 

Foraging 

Proportion 

Successful 

Northern Mockingbird 34 0.559 0.737 

“Traill’s” Flycatcher 31 0.548 0.647 

Chipping Sparrow 39 0.538 0.429 

Wilson’s Warbler 36 0.528 0.474 

Song Sparrow 25 0.520 0.308 

Clay-colored Sparrow 51 0.471 0.417 

Eastern Kingbird 32 0.469 0.200 

Great Crested Flycatcher 44 0.432 0.632 

White-crowned Sparrow 10 0.400 na 

Savannah Sparrow 102 0.392 0.375 

Lark Sparrow 18 0.389 na 

Sedge Wren 17 0.353 na 

Marsh Wren 156 0.353 0.309 

Black-throated Blue Warbler 20 0.350 na 

Swamp Sparrow 41 0.341 0.071 

Gray Catbird 64 0.328 0.476 

Brown Thrasher 49 0.327 0.688 

White-throated Sparrow 25 0.320 0.000 

House Wren 204 0.319 0.431 

Common Yellowthroat 459 0.318 0.219 

Eastern Wood-Pewee 40 0.300 0.667 

Nashville Warbler 48 0.292 0.429 

Yellow Warbler 222 0.284 0.540 

Baltimore Oriole 31 0.258 0.125 

Scissor-tailed Flycatcher 43 0.233 0.800 

Northern Waterthrush 50 0.220 0.545 

Black-and-white Warbler 112 0.205 0.348 

Yellow-breasted Chat 172 0.186 0.313 

Western Meadowlark 54 0.185 0.600 

Swainson’s Thrush 26 0.154 na 

Brown-headed Cowbird 24 0.125 na 

Yellow-billed Cuckoo 16 0.125 na 

Worm-eating Warbler 25 0.120 na 

American Redstart 404 0.111 0.378 

Yellow-throated Warbler 27 0.111 na 

Scarlet Tanager 18 0.111 na 

Summer Tanager 18 0.111 na 

Dickcissel 73 0.110 0.250 

Kentucky Warbler 31 0.097 na 
251 

Table 15.6. 

 

Species-specific foraging status in fall 1999 

Sample size shown is total number of individuals stopping over on 

platforms. The list excludes species with N < 10 stopovers. 

“Proportion successful” is the proportion of the foraging birds that 

were successful. Data on proportion successful are shown only for 

species with N ≥ 8 individuals observed foraging. Species are listed 

in descending order of foraging frequency. 

Species N Proportion 

Foraging 

Proportion 

Successful 

Orchard Oriole 35 0.086 na 

Tennessee Warbler 148 0.074 0.455 

White-eyed Vireo 42 0.071 na 

Lincoln’s Sparrow 210 0.067 0.500 

Hooded Warbler 120 0.067 0.250 

Chestnut-sided Warbler 61 0.066 na 

Blackburnian Warbler 51 0.059 na 

Ovenbird 185 0.054 0.400 

Red-eyed Vireo 37 0.054 na 

Blue Grosbeak 19 0.053 na 

Bay-breasted Warbler 60 0.050 na 

Eastern Meadowlark 22 0.045 na 

Magnolia Warbler 581 0.043 0.440 

Prothonotary Warbler 196 0.041 0.375 

White-winged Dove 223 0.040 0.444 

Grasshopper Sparrow 80 0.038 na 

Common Ground-Dove 27 0.037 na 

Northern Parula 199 0.025 na 

Black-throated Green Warbler 370 0.024 0.556 

Mourning Dove 703 0.011 0.000 

Indigo Bunting 322 0.009 na 

Cerulean Warbler 16 0.000 na 

Rose-breasted Grosbeak 13 0.000 na 

15.2.3 Weather Influences 

The relationship between foraging status and weather upon arrival is summarized in Table 15.7. Large 

neotropical migrants, medium- and short-distance migrants, and vagrants were all most likely to forage 

when arriving on GH days. One possible explanation for this pattern is that migrants arriving on GH

days attempt to compensate for the much weaker winds (and reduced potential for wind assistance) that 

generally prevail in association with a high pressure system over the Gulf. Small neotropical migrants 

also foraged more frequently when arriving on GH days than on most days with more favorable winds, 

except that they showed a striking tendency to forage most frequently on ELOW days. One possible 

explanation for this finding is that some of the smaller species arriving offshore on ELOW days did so 

inadvertently as a result of drift by the strong NW winds that generally accompany the ELOW type, and 

consequently had insufficient energy stores to complete the crossing or correct for the displacement. 

 
252 

The relationship between foraging success and weather upon arrival is summarized in Table 15.8. 

Foraging success was generally rather high, and few consistent patterns emerged. All groups were 

at least as successful when arriving on MCH days as on ECH days, presumably reflecting the greater 

insect biomass advected offshore on MCH days. 

Table 15.7. 

 

Relationship of foraging status to weather in fall 1999 

Data shown are proportions of all individuals stopping over that were ever observed foraging in 

relation to weather type upon arrival, classified by species group. Only common weather types are 

shown; meaningful samples were not available for BH and ND types. Sample size is given in 

parentheses and indicates total number of stopovers (i.e., both foraging and not). Caribbean and 

western vagrants are combined into one group due to limited sample sizes. 

Species Synoptic Weather Type 

Group ECH ELOW GH MCH TLOW 

NEOS 0.18(698) 0.45 (197) 0.31 (1,498) 0.16 (1,337) 0.08 (462) 

NEOL 0.11(210) 0.07 (132) 0.27 (171) 0.08 (509) 0.09 (130) 

MDM 0.36 (73) 0.31 (290) 0.45 (67) 0.34 (419) 0.17 (46) 

SDM 0.42 (31) 0.39 (83) 0.58 (12) 0.54 (132) 0.43 (14) 

VAG 0.30 (33) 0.48 (31) 0.68 (28) 0.42 (72) 0.52 (29) 

Table 15.8. 

 

Relationship of foraging success to weather in fall 1999 

Data shown are proportions of all individuals observed foraging that were ever successful in relation to 

weather type upon arrival, classified by species group. Only common weather types are shown; 

meaningful samples were not available for BH and ND types. Sample size is given in parentheses and 

indicates total number of individuals observed foraging. Caribbean and western vagrants are 

combined into one group due to limited sample sizes. 

Species Synoptic Weather Type 

Group ECH ELOW GH MCH TLOW 

NEOS 0.45 (125) 0.44 (88) 0.49 (152) 0.45 (214) 0.40 (38) 

NEOL 0.33 (24) 0.56 (9) 0.36 (47) 0.56 (41) 0.25 (12) 

MDM 0.19 (26) 0.49 (90) 0.37 (30) 0.48 (141) 0.38 (8) 

SDM 0.46 (13) 0.28 (32) 0.29 (7) 0.54 (71) 0.83 (6) 

VAG 0.50 (10) 0.67 (15) 0.53 (19) 0.77 (30) 0.47 (15) 

15.2.4 Comparison Between Platform Foraging and “Natural” Foraging 

Over the course of the study it became apparent that the foraging behavior of many species in the artificial 

environment of the platforms closely resembled the foraging behavior of these species in their natural 

terrestrial environments, despite the profound structural and ecological differences between the artificial 

and natural habitats. In fall 2000 we conducted a focused study on PN975, MI622, and MP252 to 

quantify in greater detail the foraging behavior of selected species on platforms. The specific objectives 

of this study were to test the hypothesis that species-specific behavioral stereotypes are preserved in a 

completely artificial environment, and to estimate energy intake rates of actively foraging birds in a 

completely novel environment for comparison to natural stopover habitats. 
253 

Species selected for detailed observational work were chosen because of the availability of 

comparative data on the foraging behavior of these species during stopovers in natural habitats in 

chenieres along the northern Gulf coast (Chen 1996). We defined searching movements following Chen 

(1996). “Hop” means any movement made only by legs. “Flutter” means movements made mainly by 

legs with the support of wings. “Fly” means movements made by the beating of wings. An “attack” is 

any attempt to capture a prey item, and a successful attack is a capture (i.e., a prey item is captured and 

consumed). At the beginning of an observation period for an individual migrant, we recorded the start 

time and the bird’s platform microhabitat and began taking observations. All searching movements, 

attacks, and captures were recorded until the bird moved to a new platform microhabitat or disappeared 

from view, and the elapsed time was then recorded. 

Results from the focused foraging studies are summarized in Table 15.9 and Table 15.10. Among the 9 

species for which data were available from both platforms and chenieres, search rate was positively 

correlated between the two habitats (rs = 0.63, P < 0.05) but was significantly higher in chenieres (paired t 

= 3.91, df = 8, P = 0.0045). Attack rate was positively correlated between the two habitats, though not 

significantly (rs = 0.45, P > 0.05), but again was significantly higher in chenieres (paired t = 6.70, df = 8, 

P = 0.0002). Thus, relative search rates and attack rates among species tended to be preserved in the 

artificial platform environment, although all species searched more deliberately and attacked prey less 

frequently on platforms than in chenieres. 

Table 15.9. 

 

Comparison of foraging behavior of fall migrants on platforms to foraging behavior of the same species in coastal 

chenieres 

All cheniere data are from Chen (1996). G and P refer to results from log-likelihood ratio tests (df = 2) of the 

hypothesis that the relative frequencies of different types of foraging movements were independent of study site. 

 Foraging Movements 

Species Location 

Total 

Observation 

Time (min)

Hop Flutter Fly N G P 

Search 

Rate 

(moves/

min) 

Eastern Wood-Pewee platforms 61.37 1.000 0.000 0.000 8 --- --- 0.13

 chenieres na na na 0.30

Empidonax spp. platforms 90.48 0.033 0.016 0.951 61 8.83 0.012 0.67

Acadian Flycatcher chenieres 0.174 0.000 0.826 69 1.70

Pine Warbler platforms 51.98 0.238 0.000 0.762 42 --- --- 0.81

 chenieres na na na na na

Hooded Warbler platforms 58.65 0.811 0.027 0.162 111 3.10 0.22 1.89

 chenieres 0.737 0.038 0.226 1,166 18.20

Magnolia Warbler platforms 35.12 0.580 0.086 0.333 81 52.3 <0.0001 2.31

 chenieres 0.883 0.048 0.069 4,785 30.90

Myrtle Warbler platforms 38.43 0.800 0.060 0.140 150 0.19 0.91 3.90

 chenieres 0.813 0.059 0.128 4,643 20.50

American Redstart platforms 78.67 0.749 0.047 0.204 358 15.9 0.0004 4.55

 chenieres 0.841 0.032 0.127 1,527 26.60

Ruby-crowned Kinglet platforms 25.58 0.781 0.050 0.169 302 5.54 0.063 11.80

 chenieres 0.825 0.055 0.120 2,882 24.70

Common Yellowthroat platforms 47.22 0.924 0.014 0.063 733 17.2 0.0002 15.52

 chenieres 0.883 0.049 0.068 837 21.20

Yellow Warbler platforms 39.28 0.948 0.025 0.028 686 40.9 <0.0001 17.46

 chenieres 0.848 0.065 0.087 790 25.90

House Wren platforms 14.92 0.981 0.010 0.009 1,022 --- --- 68.51

 chenieres na na na na na
254 

Table 15.10. 

 

Comparison of foraging rates of fall migrants on platforms to foraging rates of the same species in coastal chenieres

All cheniere data are from Chen (1996). 

 This Study 

 Chen (1996) Proportion Successful Total 

 Attack Rate Attack Rate of Attacks Attack Rate Observation 

Species (prey items h-1) (prey items h-1) Successful (prey items h-1) Time (min) 

Common Yellowthroat 240.0 55.9 0.23 12.71 47.22 

Myrtle Warbler 300.0 35.9 0.35 12.49 38.43 

American Redstart 216.0 33.6 0.55 18.31 78.67 

Yellow Warbler 324.0 30.5 0.35 10.69 39.28 

Pine Warbler na 25.4 0.77 19.62 51.98 

Eastern Wood-Pewee 84.0 21.5 0.23 4.89 61.37 

Magnolia Warbler 270.0 18.8 0.36 6.83 35.12 

Ruby-crowned Kinglet 258.0 18.8 0.38 7.04 25.58 

Hooded Warbler 144.0 12.3 0.42 5.12 58.65 

House Wren na 12.1 0.33 4.02 14.92 

Empidonax spp. 84.0 11.9 0.44 5.30 90.48 

15.2.5 Energetic Consequences of Foraging on Platforms 

Unfortunately, our offshore foraging studies were conducted near the end of a prolonged and intense 

drought that affected most of the U.S. Gulf Coast. Insect populations offshore during our focused 

foraging studies in fall 2000 seemed conspicuously lower than in prior seasons, which we believe may 

have been the result of diminished reproductive rates in source populations on the Gulf Coast caused by 

the persistent hot and dry weather. Thus, we feel that the foraging rates summarized in Table 15.10 are 

atypically low for migrants on platforms. 

Figure 15.1 shows the results from an opportunistic case study of a Magnolia Warbler and an American 

Redstart that actively foraged on EW826 during spring 1999, when insect numbers were high. Both birds 

foraged in the same area on the lee sides of the living quarters all day, though the foraging styles of the 

two species differed. The American Redstart foraged above the grating, from rails, ladders, and other 

exposed perches, making sallies to catch flying moths and, especially, moths off the walls. The Magnolia 

Warbler foraged more steadily largely underneath the grating of the living quarters walkways, picking off 

moths from a variety of surfaces, and was never seen sallying. The success rate of both individuals 

seemed high, but appeared to decline through the day as moth availability declined. The resource 

depression reflected in the birds’ foraging performance may have resulted either directly from the 

reduction in actual numbers owing to consumption by the birds, or indirectly as the moths selected safer 

roosting locations over the course of the day (Charnov et al. 1976). 
255 

30

40

50

60

7 AM

1.25

1.5

1.75

1

2

1.25

1.5

1.75

1

2

30

40

50

60

8 9 10 11 noon 1 PM 2 3 4 5

American Redstart

Magnolia Warbler

Foraging Rate (moths/min)

Foraging Success (%)

Figure 15.1. Case studies of the foraging performance of two warblers on EW826 on May 7, 1999. 

Synoptic weather for May 7 was classified as GF. Foraging success is defined as the 

percentage of attacks that were successful. Foraging rate is defined as the number of prey 

items consumed per minute. Total duration of observations was 43.6 min. 

Foraging rates on platforms are not directly comparable to “natural” foraging because warblers rely 

primarily on lepidopteran larvae (caterpillars) in terrestrial habitats during both migration and breeding 

seasons in North America, and adult moths are relatively infrequent prey items (Robinson and Holmes 

1982; Graber and Graber 1983). However, for comparison we can convert the data in Figure 15.1 to units 

of energy. The moths being consumed by the warblers on May 7 had an average body length of 10 mm, 

yielding a dry mass estimate of 6 mg per moth (Table 13.3). Adult Lepidopterans have an average energy 

content of 21.25 kJ/g dry mass (Bell 1990), which yields an estimated energy content of 0.13 kJ per moth 

consumed by the warblers on May 7. Assuming a 9-h foraging day, the average foraging rate of the 

Magnolia Warbler translates into estimated daily intake rates of about 17 g of wet biomass and 111 kJ for 

the Magnolia Warbler and 14 g wet biomass and 89 kJ for the American Redstart. 

Spring migrant warblers studied in the field in an area with large populations of lepidopteran larvae and 

that behaved as if not limited by food supply achieved consumption rates of 13-17 g wet mass/day or 63-

80 kJ/day (Graber and Graber 1983). 

Nice and Nice (1950) fed a captive migrant Black-and-white Warbler ad libitum for two days. Using the 

equations in Table 13.3, it can be estimated from the information provided that the warbler consumed 5 g 

of wet biomass with an energy content of 36 kJ on day 1, and 12 g of wet biomass with an energy content 

of 82 kJ on day 2. 
256 

The maximum possible rates of energy intake and mass gain can be estimated from theory. 

Estimated maximum rate of metabolizable energy intake (kJ/d) is given by 1713@M0.72, where M is 

given in kg (Kirkwood 1983). Based on a fat-depleted body mass of 6.4 g for small warblers 

(McClintock et al. 1978), maximum rate of metabolizable energy intake is calculated to be 45 kJ/d. 

Allometric analysis suggests that maximum fat deposition rate for passerines should scale as FDRmax = 

2.22 H M-0.27, where FDR is given in daily mass gain as a percentage of lean body mass, and M is lean 

body mass in kg (Lindström 1991). Again assuming a lean body mass of 6.4 g, maximum fat deposition 

rate is estimated by this method to be about 0.6 g/d. Lipids from the adipose tissue of migrant birds have 

a mean energy density of 38.1 kJ/g (Johnston 1970). Assuming efficiency of fat deposition to be about 

90% (Kirkwood 1985; Kersten and Piersma 1987), the theoretical maximum fat deposition rate of 0.6 g/d 

would require an energy intake rate of 25 kJ/d. 

In summary, platform-foraging warblers are capable of achieving estimated daily intake rates of 14-17 g 

wet biomass or 89-111 kJ. These figures compare to observed maximum daily intake rates of 12-17 g wet 

biomass or 80-82 kJ in small warblers in natural habitats and in the laboratory, and theoretical maximum 

rates of energy intake of 25-45 kJ/d. Clearly, platforms can at least sometimes provide a foraging 

environment that is at least as profitable as natural habitats, and that permits migrants to refuel as fast as is 

physiologically possible. 

15.3 MICROHABITAT SELECTION

It was clear that migrants did not use different parts of the platform indiscriminately. Unfortunately, it is 

not possible to test the hypothesis that migrants selected particular habitats, since we were unable to 

quantify objectively the relative availability of the different habitat types. Thus, our analyses focus on 

demonstrating species-specific patterns of habitat selection. The existence of species-specific patterns in 

the use of different platform habitats confirms indirectly that at least some species select particular habitat 

types on platforms, because if all species used all habitats randomly, then there should be no detectable 

differences in habitat use among species. 

15.3.1 Spring 

Patterns of microhabitat use for longer-staying spring migrants are summarized in Table 15.11. Spring 

habitat use differed significantly among the ecological groups (G = 629, df = 25, P < 0.0001). Aerial 

insectivores used high cover and exposed perches. Hawks almost exclusively used exposed perches. 

Herons primarily used exposed perches and grating. Medium-distance migrants used a wide range of 

habitats, but appeared to prefer grating. Neotropical migrants used a wide range of habitats, but there was 

an interesting difference in habitat use between small and large species. Small species primarily selected 

high cover, whereas large species primarily selected exposed perches. Small neotropical migrants were 

the only birds that frequently became trapped. 
257 

Table 15.11. 

 

Microhabitat use by migrants on platforms during spring 2000 

 Usual Habitat 

Group EP GG HC LC OD TR N 

AER 31 4 123 1 2 1 162 

HAWK 23 3 0 0 0 0 26 

HER 117 157 4 11 9 0 298 

MDM 10 25 10 10 5 0 60 

NEOL 66 27 22 32 7 3 157 

NEOS 84 66 175 31 30 40 426 

Total 331 282 334 85 53 44 1,129 

15.3.2 Fall 

Patterns of microhabitat use for longer-staying fall migrants are summarized in Table 15.12. Fall habitat 

use differed significantly among the ecological groups (G = 1,764, df = 30, P < 0.0001). Aerial 

insectivores and hawks mostly used exposed perches. Herons used a wide variety of habitats. Among 

neotropical migrants, there was an interesting difference in habitat use between small and large species. 

Small species usually selected high cover, whereas large species usually selected exposed perches. Shortand medium-distance migrants preferred grating. Small neotropical migrants were the only birds that 

frequently became trapped. 

Table 15.12. 

 

Microhabitat use by migrants on platforms during fall 1999

 Usual Habitat 

Group EP GG HC LC OD TR Total 

AER 41 2 7 0 0 2 52 

HAWK 141 0 4 0 0 1 146 

HER 85 26 6 23 19 0 159 

MDM 45 165 4 74 74 1 363 

NEOL 314 127 62 68 40 15 626 

NEOS 210 162 766 151 115 280 1,684 

SDM 23 39 18 22 29 1 132 

Total 859 521 867 338 277 300 3,162 

15.3.3 Interspecific Variation 

Interspecific variation in microhabitat use by migrants during spring and fall is summarized in Tables 

15.13 and 15.14, respectively. 

Most species were consistent in their patterns of microhabitat use between spring and fall. A list of 

species that differed between seasons in microhabitat use is presented in Table 15.15. The most notable 

feature of this list is the number of neotropical migrants that used EP habitat in spring and switched to HC 

in fall. 
258 

Table 15.13. 

 

Interspecific variation in microhabitat use by migrants in spring 2000 

Data shown for each species are the proportions of all individuals using each microhabitat. The 

microhabitat most frequently used by each species is boldfaced. Species are grouped according to 

most frequently used microhabitat, and listed within each group in descending order of frequency 

of use of the preferred microhabitat. 

Most Frequent Habitat 

Species OD GG EP LC HC TR N 

grating (GG): 

Chipping Sparrow 0.00 0.85 0.10 0.00 0.05 0.00 20 

Clay-colored Sparrow 0.09 0.66 0.19 0.03 0.00 0.03 32 

Lincoln's Sparrow 0.01 0.53 0.26 0.16 0.04 0.00 85 

Sora 0.07 0.51 0.12 0.17 0.10 0.02 41 

all shorebirds 0.39 0.43 0.09 0.00 0.09 0.00 23 

Purple Gallinule 0.05 0.42 0.11 0.16 0.26 0.00 19 

Common Yellowthroat 0.03 0.33 0.15 0.22 0.24 0.03 389 

 

exposed perch (EP): 

Scissor-tailed Flycatcher 0.00 0.00 1.00 0.00 0.00 0.00 12 

Great Blue Heron 0.11 0.00 0.89 0.00 0.00 0.00 19 

Peregrine Falcon 0.03 0.03 0.88 0.00 0.05 0.00 60 

Mourning dove 0.08 0.08 0.78 0.03 0.03 0.00 65 

Belted Kingfisher 0.00 0.07 0.77 0.00 0.16 0.00 44 

American Kestrel 0.08 0.17 0.75 0.00 0.00 0.00 12 

Green Heron 0.02 0.24 0.71 0.03 0.02 0.00 200 

Chuck-Will's-Widow 0.00 0.00 0.70 0.05 0.20 0.05 20 

Lark Sparrow 0.06 0.19 0.69 0.00 0.06 0.00 16 

White-winged Dove 0.07 0.09 0.66 0.03 0.14 0.01 100 

Eastern Wood-Pewee 0.00 0.04 0.64 0.00 0.25 0.07 28 

Yellow-breasted Chat 0.00 0.07 0.64 0.07 0.14 0.07 14 

Yellow-crowned Night-Heron 0.00 0.21 0.64 0.00 0.14 0.00 14 

White-eyed Vireo 0.00 0.11 0.63 0.05 0.21 0.00 19 

Bank Swallow 0.08 0.17 0.58 0.00 0.17 0.00 12 

Cattle Egret 0.05 0.34 0.58 0.01 0.02 0.00 587 

Great Egret 0.06 0.39 0.56 0.00 0.00 0.00 18 

Yellow-billed Cuckoo 0.02 0.16 0.55 0.16 0.10 0.00 49 

Orchard Oriole 0.04 0.11 0.55 0.13 0.18 0.00 95 

Northern Rough-winged Swallow 0.08 0.23 0.54 0.00 0.15 0.00 13 

Blackpoll Warbler 0.12 0.09 0.53 0.14 0.12 0.00 43 

Tree Swallow 0.00 0.05 0.53 0.00 0.41 0.00 94 

Purple Martin 0.00 0.01 0.53 0.02 0.43 0.00 277 

Myrtle Warbler 0.04 0.26 0.52 0.00 0.17 0.00 23 

 

exposed perch (EP): 

Brown-headed Cowbird 0.07 0.18 0.51 0.06 0.19 0.00 89 

Cliff Swallow 0.00 0.06 0.50 0.00 0.44 0.00 18 

Grasshopper Sparrow 0.00 0.42 0.48 0.06 0.02 0.02 52 

Snowy Egret 0.06 0.47 0.47 0.00 0.00 0.00 17 
259 

Table 15.13. 

 

Interspecific variation in microhabitat use by migrants in spring 2000 

Data shown for each species are the proportions of all individuals using each microhabitat. The 

microhabitat most frequently used by each species is boldfaced. Species are grouped according to 

most frequently used microhabitat, and listed within each group in descending order of frequency 

of use of the preferred microhabitat. 

Most Frequent Habitat 

Species OD GG EP LC HC TR N 

Baltimore Oriole 0.10 0.02 0.46 0.00 0.41 0.00 41 

Savannah Sparrow 0.03 0.41 0.46 0.07 0.02 0.02 59 

Barn Swallow 0.02 0.05 0.44 0.02 0.42 0.05 257 

Prothonotary Warbler 0.21 0.14 0.43 0.07 0.14 0.00 14 

Ruby-throated Hummingbird 0.04 0.09 0.43 0.04 0.26 0.15 54 

Summer Tanager 0.17 0.00 0.42 0.17 0.25 0.00 12 

Yellow Warbler 0.05 0.25 0.40 0.13 0.15 0.03 40 

American Redstart 0.11 0.13 0.39 0.10 0.24 0.04 84 

Gray Catbird 0.20 0.14 0.38 0.14 0.12 0.03 236 

Dickcissel 0.07 0.21 0.36 0.07 0.29 0.00 14 

Indigo Bunting 0.09 0.13 0.35 0.07 0.29 0.07 86 

Hooded Warbler 0.06 0.16 0.33 0.15 0.29 0.01 141 

 

high cover (HC): 

Bay-breasted Warbler 0.01 0.01 0.03 0.02 0.78 0.16 125 

Yellow-throated Vireo 0.00 0.07 0.27 0.00 0.67 0.00 15 

Worm-eating Warbler 0.00 0.00 0.29 0.12 0.59 0.00 17 

Magnolia Warbler 0.05 0.14 0.06 0.03 0.58 0.13 315 

Blue-winged Warbler 0.00 0.00 0.33 0.11 0.56 0.00 18 

Blackburnian Warbler 0.04 0.16 0.00 0.09 0.56 0.16 45 

Yellow-throated Warbler 0.05 0.10 0.29 0.05 0.52 0.00 21 

Chestnut-sided Warbler 0.03 0.13 0.16 0.09 0.47 0.13 32 

Black-and White Warbler 0.03 0.28 0.13 0.07 0.46 0.04 150 

Black-throated Green Warbler 0.09 0.20 0.13 0.07 0.44 0.07 45 

Northern Waterthrush 0.00 0.33 0.22 0.00 0.44 0.00 27 

Tennessee Warbler 0.10 0.20 0.17 0.05 0.40 0.08 104 

Louisiana Waterthrush 0.00 0.19 0.32 0.03 0.39 0.06 31 

Scarlet Tanager 0.08 0.15 0.23 0.15 0.38 0.00 13 

Kentucky Warbler 0.04 0.17 0.19 0.19 0.38 0.02 47 

Northern Parula 0.05 0.12 0.33 0.12 0.37 0.02 60 

Ovenbird 0.11 0.21 0.12 0.12 0.33 0.11 91 

 

Trapped (TR): 

Red-eyed Vireo 0.03 0.00 0.24 0.06 0.30 0.36 33 
260 

Table 15.14. 

 

Interspecific variation in microhabitat use by migrants in fall 1999 

Data shown for each species are the proportions of all individuals using each 

microhabitat. The microhabitat most frequently used by each species is boldfaced. 

Species are grouped according to most frequently used microhabitat, and listed within 

each group in descending order of frequency of use of the preferred microhabitat. 

Most Frequent Habitat 

Species OD GG EP LC HC TR N 

open deck (OD): 

Savannah Sparrow 0.37 0.32 0.22 0.05 0.03 0.01 93 

Western Palm Warbler 0.29 0.15 0.26 0.12 0.15 0.03 66 

all shorebirds 0.72 0.08 0.20 0.00 0.00 0.00 25 

 

grating (GG): 

Sora 0.06 0.72 0.00 0.08 0.11 0.03 36 

Western Meadowlark 0.04 0.69 0.27 0.00 0.00 0.00 26 

Lincoln's Sparrow 0.06 0.67 0.14 0.10 0.03 0.00 137 

White-throated Sparrow 0.09 0.64 0.00 0.14 0.05 0.09 22 

Swamp Sparrow 0.17 0.63 0.12 0.07 0.00 0.00 41 

Marsh Wren 0.19 0.62 0.03 0.17 0.00 0.00 151 

Sedge Wren 0.06 0.59 0.18 0.18 0.00 0.00 17 

Grasshopper Sparrow 0.12 0.52 0.18 0.04 0.07 0.07 73 

Song Sparrow 0.33 0.50 0.13 0.04 0.00 0.00 24 

Clay-colored Sparrow 0.24 0.48 0.20 0.04 0.04 0.00 46 

Eastern Meadowlark 0.21 0.47 0.32 0.00 0.00 0.00 19 

Common Ground-Dove 0.18 0.45 0.23 0.09 0.05 0.00 22 

Nashville Warbler 0.12 0.45 0.21 0.10 0.10 0.02 42 

Common Yellowthroat 0.07 0.44 0.07 0.12 0.24 0.05 432 

Chipping Sparrow 0.22 0.41 0.22 0.11 0.05 0.00 37 

Wilson's Warbler 0.07 0.34 0.31 0.17 0.10 0.00 29 

House Wren 0.12 0.34 0.21 0.28 0.05 0.01 127 

Canada Warbler 0.00 0.33 0.22 0.11 0.28 0.06 18 

Yellow-breasted Chat 0.05 0.32 0.23 0.23 0.16 0.01 137 

 

exposed perch (EP): 

Peregrine Falcon 0.00 0.00 0.98 0.00 0.02 0.00 225 

Traill's Flycatcher 0.00 0.03 0.97 0.00 0.00 0.00 29 

Least Flycatcher 0.00 0.05 0.91 0.02 0.02 0.00 57 

American Kestrel 0.00 0.00 0.91 0.00 0.07 0.02 55 

Merlin 0.11 0.00 0.89 0.00 0.00 0.00 19 

Eastern Phoebe 0.00 0.06 0.85 0.03 0.03 0.03 34 

Belted Kingfisher 0.00 0.00 0.85 0.05 0.10 0.00 20 

Eastern Kingbird 0.03 0.03 0.83 0.00 0.07 0.03 30 

 

exposed perch (EP): 

Great Crested Flycatcher 0.00 0.00 0.80 0.08 0.13 0.00 40 

Acadian Flycatcher 0.05 0.00 0.79 0.00 0.16 0.00 19 

Orchard Oriole 0.00 0.10 0.72 0.00 0.14 0.03 29 
261 

Table 15.14. 

 

Interspecific variation in microhabitat use by migrants in fall 1999 

Data shown for each species are the proportions of all individuals using each 

microhabitat. The microhabitat most frequently used by each species is boldfaced. 

Species are grouped according to most frequently used microhabitat, and listed within 

each group in descending order of frequency of use of the preferred microhabitat. 

Most Frequent Habitat 

Species OD GG EP LC HC TR N 

Barn Swallow 0.02 0.02 0.72 0.00 0.20 0.04 123 

Scissor-tailed Flycatcher 0.03 0.06 0.67 0.00 0.25 0.00 36 

Chuck-Will's-Widow 0.04 0.15 0.65 0.13 0.04 0.00 48 

unid. Empidonax flycatcher 0.04 0.08 0.63 0.00 0.13 0.13 24 

Bank Swallow 0.00 0.00 0.63 0.06 0.31 0.00 16 

Cattle Egret 0.16 0.10 0.61 0.10 0.02 0.00 186 

Purple Martin 0.07 0.00 0.60 0.13 0.20 0.00 15 

Cliff Swallow 0.00 0.00 0.60 0.00 0.40 0.00 45 

Mourning Dove 0.13 0.16 0.59 0.05 0.04 0.01 601 

Baltimore Oriole 0.00 0.17 0.59 0.03 0.21 0.00 29 

Scarlet Tanager 0.00 0.00 0.56 0.06 0.33 0.06 18 

Summer Tanager 0.00 0.00 0.56 0.06 0.33 0.06 18 

White-winged Dove 0.14 0.12 0.55 0.04 0.15 0.01 179 

Ruby-throated Hummingbird 0.01 0.01 0.54 0.07 0.27 0.09 74 

Pine Warbler 0.15 0.15 0.54 0.08 0.08 0.00 13 

Eastern Wood-Pewee 0.00 0.00 0.51 0.03 0.35 0.11 37 

Green Heron 0.11 0.16 0.51 0.04 0.18 0.00 45 

Northern Mockingbird 0.26 0.06 0.48 0.16 0.03 0.00 31 

Brown-headed Cowbird 0.14 0.33 0.43 0.10 0.00 0.00 21 

Ruby-crowned Kinglet 0.08 0.26 0.43 0.07 0.16 0.01 89 

Myrtle Warbler 0.17 0.14 0.41 0.14 0.11 0.02 140 

Yellow-billed Cuckoo 0.08 0.08 0.38 0.15 0.31 0.00 13 

Prairie Warbler 0.29 0.17 0.33 0.04 0.17 0.00 24 

Lark Sparrow 0.25 0.17 0.33 0.17 0.08 0.00 12 

Dickcissel 0.07 0.23 0.32 0.13 0.20 0.04 69 

Orange-crowned Warbler 0.21 0.15 0.28 0.21 0.13 0.02 53 

 

low cover (LC): 

Mourning Warbler 0.17 0.21 0.17 0.38 0.08 0.00 24 

Black-throated Blue Warbler 0.05 0.11 0.11 0.37 0.32 0.05 19 

Gray Catbird 0.23 0.07 0.17 0.33 0.18 0.02 60 

Brown Thrasher 0.17 0.17 0.23 0.32 0.11 0.00 47 

 

high cover (HC): 

Cerulean Warbler 0.00 0.00 0.00 0.00 0.92 0.08 13 

Blackburnian Warbler 0.00 0.07 0.02 0.00 0.83 0.09 46 

Yellow-throated Warbler 0.04 0.08 0.08 0.00 0.73 0.08 26 

Worm-eating Warbler 0.00 0.09 0.09 0.00 0.73 0.09 22 

American Redstart 0.02 0.03 0.06 0.05 0.72 0.11 372 

Bay-breasted Warbler 0.00 0.10 0.00 0.03 0.71 0.15 59 
262 

Table 15.14. 

 

Interspecific variation in microhabitat use by migrants in fall 1999 

Data shown for each species are the proportions of all individuals using each 

microhabitat. The microhabitat most frequently used by each species is boldfaced. 

Species are grouped according to most frequently used microhabitat, and listed within 

each group in descending order of frequency of use of the preferred microhabitat. 

Most Frequent Habitat 

Species OD GG EP LC HC TR N 

Magnolia Warbler 0.01 0.01 0.03 0.04 0.68 0.23 516 

Chestnut-sided Warbler 0.02 0.06 0.04 0.06 0.68 0.14 50 

Northern Parula 0.03 0.04 0.02 0.01 0.67 0.24 189 

Black-throated Green Warbler 0.01 0.21 0.02 0.01 0.67 0.07 360 

Red-eyed Vireo 0.00 0.06 0.12 0.03 0.67 0.12 33 

Indigo Bunting 0.01 0.27 0.04 0.05 0.62 0.03 311 

Black-and-white Warbler 0.02 0.07 0.11 0.08 0.61 0.10 106 

Tennessee Warbler 0.03 0.10 0.06 0.03 0.58 0.21 147 

Ovenbird 0.08 0.16 0.07 0.05 0.56 0.08 179 

Hooded Warbler 0.05 0.02 0.09 0.11 0.56 0.18 104 

Kentucky Warbler 0.00 0.04 0.00 0.19 0.56 0.22 27 

Prothonotary Warbler 0.02 0.01 0.14 0.09 0.54 0.20 166 

White-eyed Vireo 0.10 0.05 0.12 0.07 0.50 0.17 42 

Blue Grosbeak 0.00 0.26 0.26 0.11 0.37 0.00 19 

Swainson's Thrush 0.16 0.20 0.12 0.12 0.32 0.08 25 

Northern Waterthrush 0.16 0.22 0.16 0.07 0.24 0.16 45 

 

Trapped (TR): 

Yellow Warbler 0.07 0.13 0.24 0.12 0.17 0.27 145 

Table 15.15. 

 

Species that differed between spring and fall in preferred microhabitat 

Only species with N ≥ 12 in both seasons are included. G and P refer to results from 

log-likelihood ratio tests of the hypothesis that patterns of microhabitat use were 

independent of season. All tests were based on 5 degrees of freedom, except in the case 

of shorebirds (df = 3), which were never recorded as LC or TR. 

 Most Frequent Habitat 

Species Spring Fall G P 

all shorebirds GG OD 12.9 0.0049 

Yellow-breasted Chat EP GG 14.5 0.013 

White-eyed Vireo EP HC 23.0 0.0003 

Grasshopper Sparrow EP GG 22.9 0.0003 

Savannah Sparrow EP OD 29.7 <0.0001 

Prothonotary Warbler EP HC 28.3 <0.0001 

Yellow Warbler EP TR 18.1 0.003 

American Redstart EP HC 104.1 <0.0001 

Gray Catbird EP LC 20.6 0.001 

Indigo Bunting EP HC 87.9 <0.0001 
263 

Table 15.15. 

 

Species that differed between spring and fall in preferred microhabitat 

Only species with N ≥ 12 in both seasons are included. G and P refer to results from 

log-likelihood ratio tests of the hypothesis that patterns of microhabitat use were 

independent of season. All tests were based on 5 degrees of freedom, except in the case 

of shorebirds (df = 3), which were never recorded as LC or TR. 

 Most Frequent Habitat 

Hooded Warbler EP HC 69.4 <0.0001 

Scarlet Tanager HC EP 9.1 ns 

Red-eyed Vireo TR HC 14.7 0.012 

15.3.4 Movement Among Microhabitats 

We had initially hypothesized that a migrant’s initial habitat would be a function of where it happened to 

land on the platform, which itself would be largely random, and that preferred habitats would be 

discovered and utilized after a period of exploration of and movement around the platform. Under this 

hypothesis, the population of migrants would be expected to settle to an equilibrium distribution among 

habitats that would be independent of initial habitat. However, among the 1679 fall migrants that showed 

persistent habitat utilization, 1415 (84%) utilized the same habitat as when detected initially, and initial 

habitat was a strong predictor of subsequent habitat use (Table 15.16; G = 3191, df = 25, P < 0.0001). 

Table 15.16. 

 

Movement among platform microhabitats by small neotropical migrants in fall 1999 

Initial Usual Habitat 

Habitat EP GG HC LC OD TR N 

EP 176 12 13 10 16 0 227 

GG 14 135 1 12 11 1 174 

HC 9 5 736 10 1 89 850 

LC 2 4 5 101 6 0 118 

OD 8 4 0 14 78 1 105 

TR 0 0 11 4 1 189 205 

Total 209 160 766 151 113 280 1,679 

15.3.5 Relationships Between Habitat Selection and Foraging Behavior 

Fall habitat use was strongly related to foraging status (Table 15.17; G = 762, df = 5, P < 0.0001). 

Migrants that did not forage avoided open deck habitat and mostly used high cover. Migrants that 

foraged avoided high cover. Migrants that were trapped almost never foraged. 

Foraging success of small neotropical migrants in fall was significantly related to habitat use (Table 

15.18; G = 28.4, df = 5, P < 0.0001). The percentage of foragers that were successful was much higher in 

exposed perch habitat (67%) than in the other habitats (35-45%). 

Foraging success of medium-distance migrants was also significantly related to habitat use in fall (Table 

15.19; G = 13.4, df = 3, P = 0.0039). As was the case for small neotropical migrants, the percentage of 
264 

foraging medium-distance migrants that were successful was much higher in exposed perch habitat 

(76%) than in the other habitats (39-52%). 

Table 15.17. 

Microhabitat use by small neotropical migrants in fall 1999 in relation 

to foraging status 

 Usual Habitat 

Any Foraging? EP GG HC LC OD TR N 

No 75 78 732 72 27 278 1,262 

Yes 135 84 34 79 88 2 422 

Total 210 162 766 151 115 280 1,684 

Table 15.18. 

 

Foraging success in relation to microhabitat use by small 

neotropical migrants in fall 1999 

The table summarizes data only for those individuals observed 

foraging. 

 Usual Habitat 

Success? EP GG HC LC OD TR N 

No 45 50 22 48 48 2 215 

Yes 90 34 12 31 40 0 207 

Total 135 84 34 79 88 2 422 

Table 15.19. 

 

Foraging success in relation to 

microhabitat use by medium-distance 

migrants in fall 1999 

The table summarizes data only for those 

individuals observed foraging. 

 Usual Habitat 

Success? EP GG LC OD N 

No 7 43 27 27 104 

Yes 22 28 17 29 96 

Total 29 71 44 56 200 
265 

Chapter 16 

 

Nocturnal Circulation Events 

Robert W. Russell 

Early on during the study we discovered that migrants would sometimes arrive at certain platforms 

shortly after nightfall and proceed to circle those platforms for variable periods ranging from minutes to 

hours. We were relatively ill-prepared to study these “nocturnal circulations” systematically for two 

reasons: 1) birds in flight could not be detected visually and identified reliably beyond a distance of a few 

dozen meters; and 2) the platform observers were already committed to a strenuous daily program of field 

work, and required the nighttime hours to satisfy basic sleep requirements. Nevertheless, anecdotal 

observations were made at night as often as possible. Because of the potentially important implications of 

the nocturnal circulation phenomenon (particularly with respect to possible impacts of platforms), we 

present a summary of our observations in this chapter, along with tentative interpretations. 

Table 16.1 provides a complete enumeration of all circulation events observed in spring 2000, when we 

put forth our best effort to document this phenomenon. During that season, PN975 recorded 40 events 

involving 1 to 800+ individuals; GB189 recorded 22 events involving 1 to 1,260 individuals; MI622 

recorded five events involving 20 to 700 individuals; HI561 recorded four events involving 2 to 410 

individuals; and SM147 recorded two events, one involving a single individual and the other involving 

50-100 birds. Circulations were never detected at the other platforms (SP89, EW826, GC18, and VR265; 

note that VK786 was not available for study in spring 2000). 

Circulations clearly occurred because nocturnal migrants were attracted to platform lights. Circulations 

usually began shortly after sunset and the onset of darkness, undoubtedly involving birds that were in the 

general vicinity of the platform. On some occasions there were no birds in circulation when the observer 

retired for the night, but they would be present during the observer’s first round prior to sunrise. Once 

started, circulations usually peaked within an hour or two in terms of numbers of participants, then 

continued through the night with a gradual diminution of numbers. They invariably ended abruptly at the 

first indication of ambient light over the horizon, when the birds apparently regained their orientation and 

then departed. 

Circulations were highly variable in size and composition. The numbers of birds involved varied from a 

single individual to many hundreds of migrants. Although a wide variety of species was recorded in 

circulations, herons, shorebirds, swallows, and warblers were the dominant components. Certain species 

seemed especially prone to becoming involved in circulations, including Pied-billed Grebes, Soras, 

Upland Sandpipers, American Golden-Plovers, Bay-breasted Warblers, and Magnolia Warblers. 

Once an individual or flock of migrants began circulating, it typically continued to do so rather uniformly 

for an indefinite period of time. Passerines typically stayed close to the platform and circulated at nearly 

constant rates, while flocks of herons and shorebirds often deviated considerably in their circular flight 

paths. At VK786 in fall 1999, we timed a distinctive flock of 8 small shorebirds as it circled around the 

platform during a 39-min period. The average duration of each lap was about 21⁄2 min (0 = 155 s " 21 se, 

N = 15 laps) but lap duration ranged from 68 to 307 s. Assuming a flight speed of 10 m/s, this range in 

lap duration implies that the shorebirds traveled at radii that varied from about 100 to 500 m from the 

center of the platform. 
266 

Perhaps the most striking feature of the circulation events was their directional organization. With 

just a few exceptions, the vast majority of birds always traveled counterclockwise (with respect to 

the sea when viewed from above). 

Weather clearly played a role in the development of circulations. Clear nights rarely attracted numbers, 

whereas overcast nights (especially with rain) regularly attracted large numbers. Birds seemed more 

likely to circulate when haze or extensive cloud cover prevailed and on otherwise moonless nights. Wind 

direction did not appear to play any role at all in the development of circulations. Major circulation 

events often occurred on BH and ECH days with strong winds from favorable directions. 

The specific mechanisms involved in the onset and maintenance or continuity of a circulation event 

remain a matter of speculation. Most of the observers felt that birds are attracted to the lights, and once 

they get inside the cone of light surrounding the platform they are either reluctant to leave or have a 

difficult time getting out, seemingly becoming trapped by the surrounding “wall of darkness.” The 

prevalence of circulations on overcast nights suggests that initial attraction to the platform lights occurs 

when birds become confused or disoriented by the lack of navigational reference points like the stars and 

moon. Birds seemed more likely to continue circulating for prolonged periods when haze or fog 

prevailed, perhaps because the “outside world” beyond the cone of platform lighting appeared even 

darker under such circumstances, making the birds more reluctant to venture out beyond the wall of 

darkness. These admittedly anthropomorphic characterizations are consistent with the more formal ideas 

of Herbert (1970), who proposed that birds become spatially disoriented by bright light sources because 

of the loss of true visual cues to the horizontal. 

As described above, flocks of herons and shorebirds often varied dramatically from lap to lap in the radius 

of their circular flight path. We felt that this behavior represented an intentional effort to probe the wall 

of darkness surrounding the cone of platform lighting looking for an escape point. Sometimes smaller 

birds would suddenly disappear from a circulation after circling uniformly for an hour or more. Usually 

these birds could then be found roosting on the platform. In some cases, disappearances resulted from 

collisions with the platform or with other circulating birds (Chapter 17). Unexplained disappearances 

seemed to be most common on windy days, when warblers and other small birds often seemed able to 

escape the circulation and evidently continue their migration. We suspected that this pattern was an 

essentially random effect of the migrants being blown by wind and unintentionally reaching a point where 

they can see the outside world and are no longer trapped within the cone of light. 

Reasons for the differences among platforms in frequency of circulation events were not clear. The three 

platforms where circulations were frequent (PN975, GB189, VK786) were extremely isolated (Figure 

2.2), suggesting that the differences could simply arise as a sampling artifact because the more isolated 

platforms have a larger “hinterland” (Cairns 1989) and can draw from a larger source population of 

airborne migrants. Casting doubt on this explanation, however, is the fact that circulation events were 

never recorded at the other two extremely isolated platforms (GC18, EW826; Figure 2.2). 

We also noted that the three platforms that routinely recorded significant nocturnal circulations were 

painted extensively in yellow (PN975) or orange (GB189 and VK786), whereas most of the platforms 

without frequent circulations were not conspicuously yellow or orange. The exceptions were HI561 and 

SM147; both were extensively yellow yet recorded few circulations. It is not clear why platform color 

might influence the development of circulations. From any distance greater than 200 m, platforms 

usually appeared pinkish yellow or peachy yellow at night due to the sodium vapor lights on most 

platforms. 

 

One of the field observers who worked on GB189, VK786, and HI561 noted that the prevalence of 

circulations seemed to be correlated with the amount of “projected light” on the platforms. Specifically, 
267 

HI 561 has very little projected light; looking at the sea at night only gives an observer a small field 

of view. In contrast, GB189 and VK786 have extensive light systems with considerably more night 

visibility. 

The impact of circulations on trans-Gulf migrants is difficult to evaluate. Any delay in migration induced 

by a circulation clearly must have some impact on the migrants involved, because they expend energy 

during the delay that might otherwise be needed to complete the crossing successfully, particularly if 

adverse weather conditions are encountered later on. Whether or not the impact of a circulation is 

significant probably depends on the initial energetic condition of the participants. Migrants that deposited 

sufficient fat stores prior to departure to maintain them through the entire journey and then some are 

likely unaffected. However, for migrants that departed with just enough fat to get them to their 

destination under normal circumstances, prolonged participation in a circulation could be lethal. On a 

few occasions we observed individually recognizable migrants circulating all night long (i.e., 8+ hours), 

but on many occasions we observed a particular species making only one or a few passes during an 

extended event. Migrants that dropped out of circulations had a tendency to become trapped in the well 

bays or other well-lit interior areas of the platforms; these individuals often appeared to us to be highly 

stressed. Because of the anecdotal nature of our circulation observations, we are reluctant even to 

speculate about the average duration of participation in circulations or the typical energetic consequences 

of participating in these events. 

On the other hand, circulations clearly put birds at risk for collision with the platform or with each other 

(Chapter 17). 
268 

Table 16.1. 

Summary of all nocturnal circulation events detected during spring 2000 

Date shown is that for the beginning of the night. Flight direction is given with respect to the sea surface as a reference. Events arfrom east to west. Platform Date 

Synoptic Weather 

Local Weather 

Flight Direction 

Maximum # Involved Species Composition Notes 

SP89 none EW826 none GC18 none SM147 Apr 28 GH W B2 

clear vis 4 

CCW 1 Indigo Bunting Post-suns

hazy. 

SM147 May 2 BH SE B4 

p. cloudy vis 4 

? 50-100 Bay-breasted Warblers 

Swainson’s Thrushes Spotted Sandpipers 1 Veery 1 Gray Catbird 1 Blackpoll Warbler 

Post-sunshaze. 

VR265 none 
269 

Table 16.1. 

Summary of all nocturnal circulation events detected during spring 2000 

Date shown is that for the beginning of the night. Flight direction is given with respect to the sea surface as a reference. Events arfrom east to west. Platform Date 

Synoptic Weather 

Local Weather 

Flight Direction 

Maximum # Involved Species Composition Notes 

GB189 Mar 14 ECH NW B7 

heavy rain vis 1 

CCW 23 5 unid shorebirds 

4 Great Blue Herons 3 Black-crowned Night-Herons 2 Little Blue Herons 2 White Ibises 2 Barn Swallows 1 Great Egret 1 Wilson’s Plover 1 Pectoral Sandpiper 1 Osprey 1 Ruby-throated Hummingbird 

Circulation Mar 1thunderst

GB189 Mar 15 ECH S B6 

overcast vis 2 

CCW 5 unid swallows Post-suns

stars visi

GB189 Mar 31 ECH SSE B6 

overcast vis 4 

CCW 6 Barn Swallows Pre-sunri
270 

Table 16.1. 

Summary of all nocturnal circulation events detected during spring 2000 

Date shown is that for the beginning of the night. Flight direction is given with respect to the sea surface as a reference. Events arfrom east to west. Platform Date 

Synoptic Weather 

Local Weather 

Flight Direction 

Maximum # Involved Species Composition Notes 

GB189 Apr 1 BH SW B4 

overcast vis 3 

CCW + CW 121 64 Barn Swallows 

15 Purple Martins 15 unid dowitchers 10 Tricolored Herons  5 Tree Swallows  3 Ospreys  2 Green Herons  2 Cliff Swallows  1 Little Blue Heron  1 Great Egret  1 Cattle Egret  1 Lesser Nighthawk  1 Orchard Oriole 

Pre-sunri

GB189 Apr 2 BH SSW B5 

p. cloudy vis 3 

CCW 12 8 Barn Swallows 

 3 Tricolored Heron  1 Tree Swallow 

Pre-sunri

GB189 Apr 7 GF NNW B3 

clear vis 4 

CW 1 unid swallow Pre-sunri

GB189 Apr 11 BH SE B4 

clear vis 4 

CCW 1 Least Bittern Pre-sunri
271 

Table 16.1. 

Summary of all nocturnal circulation events detected during spring 2000 

Date shown is that for the beginning of the night. Flight direction is given with respect to the sea surface as a reference. Events arfrom east to west. Platform Date 

Synoptic Weather 

Local Weather 

Flight Direction 

Maximum # Involved Species Composition Notes 

GB189 Apr 13 GF NNE B5 

overcast vis 4 NNW B3 overcast vis 4 

CCW CW 

 14  137 

 6 Cattle Egrets  2 unid warblers  1 Northern Waterthrush  1 Common Yellowthroat  1 Chestnut-sided Warbler  1 Orchard Oriole  1 Baltimore Oriole  1 unid passerine 112 Cattle Egrets  10 Little Blue Herons  5 Great Egrets  4 Great Blue Herons  3 Snowy Egrets  3 Yellow-crowned 

Night-Herons 

 1 Green Heron 

Post-sunsvisible in sky. At acover broto dissipaPre-sunri

GB189 Apr 24 GF N B3 

clear vis 4 

CCW 1 ? Post-suns

visible in sky. 

GB189 Apr 27 GH WSW B2 

clear vis 4 

CCW 2 Tree Swallows Pre-sunri
272 

Table 16.1. 

Summary of all nocturnal circulation events detected during spring 2000 

Date shown is that for the beginning of the night. Flight direction is given with respect to the sea surface as a reference. Events arfrom east to west. Platform Date 

Synoptic Weather 

Local Weather 

Flight Direction 

Maximum # Involved Species Composition Notes 

GB189 Apr 29 GF ESE B4 

clear vis 4 

CCW 4 3 Magnolia Warblers 

1 Chestnut-sided Warbler 

Post-sunsvisible in sky. 

GB189 Apr 30 ECH SE B5 

clear vis 3 

CCW 3 Bay-breasted Warblers Post-suns

visible in sky. 

GB189 May 1 ECH SE B6 

clear vis 4 

CCW 6 Bay-breasted Warblers Post-suns

No stars 
273 

Table 16.1. 

Summary of all nocturnal circulation events detected during spring 2000 

Date shown is that for the beginning of the night. Flight direction is given with respect to the sea surface as a reference. Events arfrom east to west. Platform Date 

Synoptic Weather 

Local Weather 

Flight Direction 

Maximum # Involved Species Composition Notes 

GB189 May 2 BH E B6 

heavy rain vis 2 

CCW ~1260 100 Bay-breasted Warblers 

 30 Catharus thrushes 

 30 Lesser Yellowlegs  30 unid dowitchers  30 unid peeps  18 Rose-breasted Grosbeaks  16 unid cuckoos  10 American Golden Plovers  10 Greater Yellowlegs  8 Buff-breasted Sandpipers  8 White-rumped Sandpipers  6 Cattle Egrets  6 Barn Swallows  5 Gray Catbirds  3 Sanderlings  3 Scarlet Tanagers  2 Ospreys  many others 

Post-sunsbegan at very rapivisible 20declined change. 

GB189 May 4 ECH SE B2 

p. cloudy vis 4 

? 11 Bank Swallows, other species Post-suns

visible in 

GB189 May 10 BH SSE B5.5 

overcast vis 4 

CCW 6 2 Chimney Swifts 

1 Osprey 1 Yellow Warbler 1 Magnolia Warbler 1 unid & tanager 

Post-sunsvisible in sky. 
274 

Table 16.1. 

Summary of all nocturnal circulation events detected during spring 2000 

Date shown is that for the beginning of the night. Flight direction is given with respect to the sea surface as a reference. Events arfrom east to west. Platform Date 

Synoptic Weather 

Local Weather 

Flight Direction 

Maximum # Involved Species Composition Notes 

GB189 May 12 BH S B6 

overcast vis 4 

CCW 5 2 unid warblers 

1 Black-and-white Warbler 1 Magnolia Warbler 1 Chimney Swift 

Post-sunsvisible in sky. 

GB189 May 13 GF NNE B4 

overcast vis 4 

CCW 4 ? Post-suns

visible in sky. 

GB189 May 16 GH SE B5 

p. cloudy vis 4 

CCW 1 Least Sandpiper Post-suns

visible in sky. 

GB189 May 17 BH S B6 

p. cloudy vis 4 

CCW 1 ? Post-suns

visible in sky. 
275 

Table 16.1. 

Summary of all nocturnal circulation events detected during spring 2000 

Date shown is that for the beginning of the night. Flight direction is given with respect to the sea surface as a reference. Events arfrom east to west. Platform Date 

Synoptic Weather 

Local Weather 

Flight Direction 

Maximum # Involved Species Composition Notes 

GB189 May 20 BH E B3 

p. cloudy vis 4 NE B3 light rain vis 4 

CCW CCW 

 2  22 

1 unid passerine 1 unid warbler 6 unid warblers 5 unid passerines 4 Cattle Egrets 3 Common Nighthawks 2 Snowy Egrets 1 Spotted Sandpiper 1 Magnolia Warbler 

Post-sunsvisible in sky. Pre-sunriNo stars 

GB189 May 21 BH E B1 

clear vis 4 

CCW 10 4 unid passerines 

3 unid warblers 3 Common Nighthawks 

Post-sunsvisible in 
276 

Table 16.1. 

Summary of all nocturnal circulation events detected during spring 2000 

Date shown is that for the beginning of the night. Flight direction is given with respect to the sea surface as a reference. Events arfrom east to west. Platform Date 

Synoptic Weather 

Local Weather 

Flight Direction 

Maximum # Involved Species Composition Notes 

HI561 Mar 14 ECH E B8 

heavy rain vis 2 

CCW 410 100 unid warblers 

100 Little Blue Herons  50 Great Blue Herons  30 Great Egrets  25 Yellow-crowned 

Night-Herons 

 20 Snowy Egrets  15 Cattle Egrets  15 Black-crowned 

Night-Herons 

 15 Purple Martins  12 unid shorebirds  10 Tricolored Herons  10 Green Herons  6 Soras  2 Black-and-white Warblers 

Circulati00:30 on 

HI561 Apr 7 GF N B2 

clear vis 4 

CW + CCW 2 unid swallows Pre-sunri
277 

Table 16.1. 

Summary of all nocturnal circulation events detected during spring 2000 

Date shown is that for the beginning of the night. Flight direction is given with respect to the sea surface as a reference. Events arfrom east to west. Platform Date 

Synoptic Weather 

Local Weather 

Flight Direction 

Maximum # Involved Species Composition Notes 

HI561 May 2 BH SE B7 

overcast vis 3 

CCW 24 9 unid shorebirds 

4 unid warblers 2 Little Blue Herons 2 Cattle Egrets 2 Blackburnian Warblers 2 Bay-breasted Warblers 1 Reddish Egret 1 Black-and-white Warbler 1 Ovenbird 

Post-suns

HI561 May 10 BH S B5 

clear vis 4 

CCW 2 1 Bank Swallow 

1 Northern Rough-winged 

Swallow 

Pre-sunri

MI622 Apr 2 GF E B4 

overcast vis 3 

? 77 35 Cattle Egrets 

17 Upland Sandpipers 15 unid passerines  3 unid shorebirds  2 Green Herons  1 Great Blue Heron  1 Least Bittern  1 Sora  1 Solitary Sandpiper  1 Lesser Nighthawk 

Post-suns
278 

Table 16.1. 

Summary of all nocturnal circulation events detected during spring 2000 

Date shown is that for the beginning of the night. Flight direction is given with respect to the sea surface as a reference. Events arfrom east to west. Platform Date 

Synoptic Weather 

Local Weather 

Flight Direction 

Maximum # Involved Species Composition Notes 

MI622 Apr 11 BH SE B3 

overcast vis 4 

? 43 27 Cattle Egrets 

12 unid passerines  2 unid white egrets  1 Snowy Egret  1 Little Blue Heron 

Pre-sunriThundersapproachto N duri

MI622 May 1 ECH ESE B4 

clear vis 4 

? ~20 unid warblers, Indigo Buntings Post-suns

MI622 May 2 BH NE B3 

overcast vis 4 

CW ~700 almost all Magnolia Warblers 

~10 Common Yellowthroats 1 Kentucky Warbler 1 Blackburnian Warbler 

Post-sunsAll Magnappeared stars visi

MI622 May 5 ECH SE B5 

clear vis 4 

? 54 50 Magnolia Warblers 

 4 Common Yellowthroats 

Post-sunsAll Magnappeared 

PN975 Mar 8 BH SSE B3 

overcast vis 3 

CCW 52 45 Tree Swallows 

 5 Barn Swallows  2 unid warblers 

Pre-sunriVery haz

PN975 Mar 13 ECH SE B3 

overcast vis 3 

CCW 8 6 Tree Swallows 

 2 unid warblers 

Pre-sunriof light r
279 

Table 16.1. 

Summary of all nocturnal circulation events detected during spring 2000 

Date shown is that for the beginning of the night. Flight direction is given with respect to the sea surface as a reference. Events arfrom east to west. Platform Date 

Synoptic Weather 

Local Weather 

Flight Direction 

Maximum # Involved Species Composition Notes 

PN975 Mar 14 ECH NE B7 

heavy rain vis 2 S B3.5 overcast vis 4 

CCW ? 

 41  12 

10 unid warblers  6 American Golden-Plovers  4 Louisiana Waterthrushes  5 Killdeers  3 Lesser Yellowlegs  2 Green Herons  1 Yellow-crowned Night-Heron 1 Little Blue Heron  1 Sora  1 Upland Sandpiper  1 Willet  1 Greater Yellowlegs  1 Sanderling  1 Least Sandpiper  1 Purple Martin  1 Barn Swallow  1 Yellow-throated Warbler  7 Pectoral Sandpipers  2 Tree Swallows  1 Great Blue Heron  1 Black-and-white Warbler  1 Hooded Warbler 

Post-sunsoccurred with torrelightningPre-sunri

PN975 Mar 16 GF NNE B5 

overcast vis 4 

CCW 8 6 unid warblers 

 1 Black-throated Green Warbler 1 Cattle Egret 

Pre-sunri
280 

Table 16.1. 

Summary of all nocturnal circulation events detected during spring 2000 

Date shown is that for the beginning of the night. Flight direction is given with respect to the sea surface as a reference. Events arfrom east to west. Platform Date 

Synoptic Weather 

Local Weather 

Flight Direction 

Maximum # Involved Species Composition Notes 

PN975 Mar 17 GF N B5 

overcast vis 4 

CCW 11 5 Black-and-white Warblers 

 2 Hooded Warblers  1 Purple Martin  1 Willet  1 Yellow-throated Warbler  1 Northern Parula 

Post-suns

PN975 Mar 18 GF NNW B6 

overcast vis 4 

? ~41 ~40 unid warblers 

 1 Great Egret 

Pre-sunri

PN975 Mar 20 GF 

ECH 

SE B4 clear vis 4 SE B4 p. cloudy vis 4 

CCW ? 

 49  1 

 40 Tree Swallows  5 Barn Swallows  2 Black-and-White Warblers  1 Purple Martin  1 Willet  1 Yellow-throated Warbler 

Post-sunsPre-sunri

PN975 Mar 23 ECH E B3 

overcast vis 4 

CW 70 Tree Swallows Post-suns

PN975 Mar 25 ECH SE B3.5 

clear vis 4 

CW 2 Tree Swallows Post-suns
281 

Table 16.1. 

Summary of all nocturnal circulation events detected during spring 2000 

Date shown is that for the beginning of the night. Flight direction is given with respect to the sea surface as a reference. Events arfrom east to west. Platform Date 

Synoptic Weather 

Local Weather 

Flight Direction 

Maximum # Involved Species Composition Notes 

PN975 Mar 26 GH SSW B2 

p. cloudy vis 3 

CW 1 Tree Swallow Pre-sunri

PN975 Mar 27 GF E B2 

overcast vis 3 SE B1 overcast vis 3 

CCW CCW 

 345  289 

160 Tree Swallows  40 unid warblers  35 Cattle Egrets  20 unid passerines  20 Upland Sandpipers  13 American Golden Plovers  10 Greater Yellowlegs  10 Willets  9 Least Sandpipers  7 Pectoral Sandpipers  5 Black-bellied Plovers  5 Great Blue Herons  3 unid peep  2 Snowy Egrets  2 Tricolored Herons  1 Yellow-crowned NightHeron  1 Summer Tanager  1 Black-and-white Warbler  1 Hooded Warbler 280 Tree Swallows  6 unid warblers  2 Barn Swallows  1 Bank Swallow 

Post-sunswere cleacover durwas someabout 21clouded o15 minutmasse. Aclear agasuddenly event thuminutes awith a brcover. Pre-sunrithe Barn CW. 
282 

Table 16.1. 

Summary of all nocturnal circulation events detected during spring 2000 

Date shown is that for the beginning of the night. Flight direction is given with respect to the sea surface as a reference. Events arfrom east to west. Platform Date 

Synoptic Weather 

Local Weather 

Flight Direction 

Maximum # Involved Species Composition Notes 

PN975 Mar 28 BH SE B3.5 

p. cloudy vis 4 S B3 overcast vis 3 

? ? 

 15  32 

 12 Tree Swallows  1 Barn Swallows  1 Least Sandpiper  1 unid warbler  30 Tree Swallows  2 Barn Swallows 

Post-sunsPre-sunri

PN975 Mar 30 GF E B2 

overcast vis 4 

CCW 73 23 Cattle Egrets 

15 unid warblers 12 Chimney Swifts  5 Little Blue Herons  2 Lesser Yellowlegs  2 Lesser Nighthawks  2 Tree Swallows  1 Tricolored Heron  1 Snowy Egret  1 Green Heron  1 American Golden-Plover  1 Pectoral Sandpiper  1 Barn Swallow  1 Common Yellowthroat  1 American Redstart  1 Black-and-white Warbler  1 Hooded Warbler  1 Blue-winged Warbler  1 Worm-eating Warbler 

 Post-sun
283 

Table 16.1. 

Summary of all nocturnal circulation events detected during spring 2000 

Date shown is that for the beginning of the night. Flight direction is given with respect to the sea surface as a reference. Events arfrom east to west. Platform Date 

Synoptic Weather 

Local Weather 

Flight Direction 

Maximum # Involved Species Composition Notes 

PN975 Apr 10 BH SE B3 

overcast vis 3 

? 10 8 American Golden-Plovers 

 2 unid passerines 

Pre-sunri

PN975 Apr 11 BH E B2 

p. cloudy vis 4 

? 4 3 unid warblers 

 1 Purple Martin 

Pre-sunri

PN975 Apr 12 GF N B4 

overcast vis 4 

? 170 40 Common Yellowthroats 

40 unid warblers 20 unid passerines 13 Indigo Buntings  9 Blue-winged Warblers  7 Hooded Warblers  6 Northern Parulas  4 Red-eyed Vireos  4 Kentucky Warblers  4 Tennessee Warblers  4 Black-and-white Warblers  3 Worm-eating Warblers  3 Ovenbirds  3 American Redstarts  2 Least Sandpipers  2 Black-throated Green Warblers  1 Merlin  1 Wood Thrush  1 Gray Catbird  1 Scarlet Tanager 

Post-sunsbegan rig~80 birds 120 birds 22:00. Bbegan to distinct rdown to 
284 

Table 16.1. 

Summary of all nocturnal circulation events detected during spring 2000 

Date shown is that for the beginning of the night. Flight direction is given with respect to the sea surface as a reference. Events arfrom east to west. Platform Date 

Synoptic Weather 

Local Weather 

Flight Direction 

Maximum # Involved Species Composition Notes 

PN975 Apr 12 

(cont.) 

GF N B4 

overcast vis 4 N B4 p. cloudy vis 4 

? ? 

170 

 8 

 1 Orchard Oriole  1 Rose-breasted Grosbeak  8 Cattle Egrets Pre-sunri

PN975 Apr 13 GF SE B1 

clear vis 4 

? 6 3 unid warblers 

 2 Cattle Egrets  1 Yellow-billed Cuckoo 

Pre-sunri

PN975 Apr 21 GF SE B2 

clear vis 4 

? 2 1 Gray Catbird 

 1 unid warbler 

Post-suns

PN975 Apr 25 GF E B3 

clear vis 4 

? 9 4 unid warblers 

 3 unid passerines  1 Red-eyed Vireo  1 Chestnut-sided Warbler 

Post-suns
285 

Table 16.1. 

Summary of all nocturnal circulation events detected during spring 2000 

Date shown is that for the beginning of the night. Flight direction is given with respect to the sea surface as a reference. Events arfrom east to west. Platform Date 

Synoptic Weather 

Local Weather 

Flight Direction 

Maximum # Involved Species Composition Notes 

PN975 Apr 26 MCH E B3.5 

clear vis 4 

? 36 14 unid warblers 

 8 Cattle Egrets  3 Red-eyed Vireos  3 unid sparrows  2 Indigo Buntings  2 Common Yellowthroats  1 Little Blue Heron  1 Bank Swallow  1 Gray Catbird  1 Ovenbird 

Post-suns

PN975 Apr 27 GH SE B3.5 

p. cloudy vis 4 SE B1 p. cloudy vis 4 

? ? 

 7  26 

 2 unid warblers  1 Barn Swallow  1 Red-eyed Vireo  1 Blackpoll Warbler  1 Black-and-white Warbler  1 Common Yellowthroat 25 unid peeps  1 unid passerine 

Post-sunsPre-sunri
286 

Table 16.1. 

Summary of all nocturnal circulation events detected during spring 2000 

Date shown is that for the beginning of the night. Flight direction is given with respect to the sea surface as a reference. Events arfrom east to west. Platform Date 

Synoptic Weather 

Local Weather 

Flight Direction 

Maximum # Involved Species Composition Notes 

PN975 Apr 28 GH ESE B3 

clear vis 4 SSE B2 overcast vis 4 

CCW CW + CCW 

 69  41 

46 Magnolia Warblers  6 unid warblers  5 Common Yellowthroats  2 Baltimore Orioles  1 Chimney Swift  1 Ruby-throated Hummingbird  1 Gray Catbird  1 Yellow Warbler  1 Blackburnian Warbler  1 Chestnut-sided Warbler  1 Blue-winged Warbler  1 Northern Parula  1 American Redstart  1 Kentucky Warbler 24 Cattle Egrets  6 Dickcissels  3 Little Blue Herons  2 Solitary Sandpipers  1 Great Blue Heron  1 Yellow-crowned Night-Heron 1 Upland Sandpiper  1 Lesser Yellowlegs  1 Lesser Nighthawk  1 Baltimore Oriole 

Post-sunsbird in ciat 20:20. very quicdecline blanded on departed. remained Pre-sunriherons wothers CC
287 

Table 16.1. 

Summary of all nocturnal circulation events detected during spring 2000 

Date shown is that for the beginning of the night. Flight direction is given with respect to the sea surface as a reference. Events ar

from east to west. 

Platform Date 

Synoptic Weather 

Local Weather 

Flight Direction 

Maximum # Involved Species Composition Notes 

PN975 Apr 29 GF ESE B4 

overcast vis 4 

CCW 800+ 283 Magnolia Warblers 

 67 Common Yellowthroats  31 Ovenbirds  17 Blackburnian Warblers  15 Tennessee Warblers  12 Little Blue Herons  12 Indigo Buntings  8 Least Sandpipers  4 Yellow Warblers  3 Cattle Egrets  3 Philadelphia Vireos  3 American Redstarts  3 Chestnut-sided Warblers  3 Painted Buntings  2 Gray Catbirds  2 Baltimore Orioles  2 Rose-breasted Grosbeaks  2 Black-throated Green 

Warblers 

 2 Black-and-white Warblers  2 Kentucky Warblers  2 Northern Waterthrushes  1 Peregrine Falcon  1 Merlin  1 Purple Martin  1 Barn Swallow  1 Chimney Swift  1 Eastern Kingbird  1 Warbling Vireo  1 Golden-winged Warbler 

Post-sunsafter sunto appear by 20:50 migrants birds consome timof 800+ anumbers platform. 30 (when bed), therin flight. 
288 

Table 16.1. 

Summary of all nocturnal circulation events detected during spring 2000 

Date shown is that for the beginning of the night. Flight direction is given with respect to the sea surface as a reference. Events ar

from east to west. 

Platform Date 

Synoptic Weather 

Local Weather 

Flight Direction 

Maximum # Involved Species Composition Notes 

PN975 Apr 29 

(cont.) 

GF ESE B4 

overcast vis 4 

CCW 800+ 1 Summer Tanager 

 1 Dickcissel  1 Lincoln’s Sparrow 

PN975 Apr 29 GF SSE B5 

overcast vis 4 

CCW 25 10 Magnolia Warblers 

 4 Common Yellowthroats  3 Blackburnian Warblers  3 Dickcissels  2 Ovenbirds  1 Cattle Egret  1 Barn Swallow  1 Black-and-white Warbler 

Pre-sunri

PN975 Apr 30 ECH SE B5 

clear vis 4 

CCW 18 6 unid warblers 

 2 Buff-breasted Sandpipers  2 Chimney Swifts  2 Blackburnian Warblers  1 Least Sandpiper  1 Barn Swallow  1 Yellow Warbler  1 Magnolia Warbler  1 Ovenbird  1 Tennessee Warbler 

Post-suns
289 

Table 16.1. 

Summary of all nocturnal circulation events detected during spring 2000 

Date shown is that for the beginning of the night. Flight direction is given with respect to the sea surface as a reference. Events ar

from east to west. 

Platform Date 

Synoptic Weather 

Local Weather 

Flight Direction 

Maximum # Involved Species Composition Notes 

PN975 May 1 ECH E 3.5 

overcast vis 4 

CCW 81 20 unid warblers 

13 Magnolia Warblers  5 Least Sandpipers  5 Barn Swallows  5 Blackburnian Warblers  3 Great Blue Herons  2 Lesser Yellowlegs  2 Baird’s Sandpipers  2 Chimney Swifts  2 Dickcissels  2 Blackpoll Warblers  2 Blackburnian Warblers  2 Ovenbirds  1 Cattle Egret  1 Black-crowned Night-Heron  1 American Coot  1 Sora  1 Upland Sandpiper  1 Ruddy Turnstone  1 Stilt Sandpiper  1 Solitary Sandpiper  1 Merlin  1 Yellow-billed Cuckoo  1 Eastern Kingbird  1 Baltimore Oriole  1 Swainson’s Thrush  1 Common Yellowthroat  1 Chestnut-sided Warbler  1 American Redstart 

Post-suns
290 

Table 16.1. 

Summary of all nocturnal circulation events detected during spring 2000 

Date shown is that for the beginning of the night. Flight direction is given with respect to the sea surface as a reference. Events ar

from east to west. 

Platform Date 

Synoptic Weather 

Local Weather 

Flight Direction 

Maximum # Involved Species Composition Notes 

PN975 May 1 

(cont.) 

ECH E 2 

overcast vis 4 

CCW 147 40 Dickcissels 

 30 Bank Swallows  30 unid passerines  20 Barn Swallows  9 Cattle Egrets  4 Baltimore Oriole  3 Green Herons  3 Brown-headed Cowbirds  1 Little Blue Heron  1 Tricolored Heron  1 Spotted Sandpiper  1 Scissor-tailed Flycatcher  1 Gray Catbird  1 Philadelphia Vireo  1 Bay-breasted Warbler  1 Black-and-white Warbler 

Pre-sunri
291 

Table 16.1. 

Summary of all nocturnal circulation events detected during spring 2000 

Date shown is that for the beginning of the night. Flight direction is given with respect to the sea surface as a reference. Events ar

from east to west. 

Platform Date 

Synoptic Weather 

Local Weather 

Flight Direction 

Maximum # Involved Species Composition Notes 

PN975 May 2 BH SSE B4.5 

overcast vis 4 

CCW 229 60 unid warblers 

56 Magnolia Warblers 20 Common Yellowthroats 19 Cattle Egrets 13 Blackburnian Warblers  9 Least Sandpipers  8 Lesser Yellowlegs  5 American Redstarts  5 Gray Catbirds  3 Baltimore Orioles  3 Dickcissels  2 Great Blue Herons  2 Little Blue Herons  2 Eastern Kingbirds  2 Eastern Wood-Pewees  2 Black-and-White Warblers  2 Black-throated Green 

Warblers 

Post-sunsmonitorepeak of thobserver birds flyiat the top The next Eastern WRuby-thrwere fouwith theiburnt off
292 

Table 16.1. 

Summary of all nocturnal circulation events detected during spring 2000 

Date shown is that for the beginning of the night. Flight direction is given with respect to the sea surface as a reference. Events ar

from east to west. 

Platform Date 

Synoptic Weather 

Local Weather 

Flight Direction 

Maximum # Involved Species Composition Notes 

PN975 May 2 

(cont.) 

BH SSE B4.5 

overcast vis 4 

CCW 229 2 Chestnut-sided Warblers 

 2 Tennessee Warblers  2 Yellow Warblers  1 Snowy Egret  1 Green Heron  1 Bank Swallow  1 Blue Grosbeak  1 Indigo Bunting  1 Lesser Nighthawk  1 Nashville Warbler  1 Northern Waterthrush  1 unid passerine  1 White-winged Dove 

PN975 May 4 ECH SE B5 

p. cloudy vis 4 

? 1 Cattle Egret Pre-sunri
293 

Table 16.1. 

Summary of all nocturnal circulation events detected during spring 2000 

Date shown is that for the beginning of the night. Flight direction is given with respect to the sea surface as a reference. Events ar

from east to west. 

Platform Date 

Synoptic Weather 

Local Weather 

Flight Direction 

Maximum # Involved Species Composition Notes 

PN975 May 5 ECH SE B5 

p. cloudy vis 4 SE B5 p. cloudy vis 4 

? ? 

 118  4 

 61 unid warblers  24 Magnolia Warblers  10 Common Yellowthroats  6 Chestnut-sided Warblers  3 Barn Swallows  3 Blackpoll Warblers  3 Black-throated Green 

Warblers 

 2 Bay-breasted Warblers  2 Blackburnian Warblers  2 Ovenbirds  1 Lesser Yellowlegs  1 Chimney Swift  2 Magnolia Warblers  1 Brown-headed Cowbird  1 unid warbler 

Post-sunsmajority were ASY 

Pre-sunri
294 

Table 16.1. 

Summary of all nocturnal circulation events detected during spring 2000 

Date shown is that for the beginning of the night. Flight direction is given with respect to the sea surface as a reference. Events ar

from east to west. 

Platform Date 

Synoptic Weather 

Local Weather 

Flight Direction 

Maximum # Involved Species Composition Notes 

PN975 May 6 BH SE B4.5 

p. cloudy vis 4 

? 206 133 Magnolia Warblers 

 22 unid warblers  14 Common Yellowthroats  7 Swainson’s Thrushes  7 Ovenbirds  6 Baltimore Orioles  5 Barn Swallows  3 American Redstarts  2 Cattle Egrets  1 Rose-breasted Grosbeak  1 Peregrine Falcon  1 Least Sandpiper  1 Philadelphia Vireo  1 Blackburnian Warbler  1 Kentucky Warbler  1 Dickcissel 

Post-suns

PN975 May 7 BH SSE B4.5 

clear vis 4 SE B5 clear vis 4 

? ? 

 1  4 

 1 unid Empidonax flycatcher 

 2 Magnolia Warblers  1 Barn Swallow  1 unid warbler 

Post-sunsPre-sunri

PN975 May 8 BH SE B5 

clear vis 4 

? 1 unid passerine Post-suns
295 

Table 16.1. 

Summary of all nocturnal circulation events detected during spring 2000 

Date shown is that for the beginning of the night. Flight direction is given with respect to the sea surface as a reference. Events ar

from east to west. 

Platform Date 

Synoptic Weather 

Local Weather 

Flight Direction 

Maximum # Involved Species Composition Notes 

PN975 May 9 BH SE B4 

clear vis 3 

? 94 42 Pectoral Sandpipers 

12 Cattle Egrets  9 unid warblers  8 Magnolia Warblers  7 Bank Swallows  6 Tricolored Herons  4 Barn Swallows  1 Peregrine Falcon  1 Spotted Sandpiper  1 Chimney Swift  1 Red-eyed Vireo  1 American Redstart  1 unid small flycatcher 

Pre-sunri

PN975 May 10 BH SE B5 

clear vis 4 

? 70 45 Magnolia Warblers 

 7 Ovenbirds  6 Common Yellowthroats  4 Barn Swallows  4 Chestnut-sided Warblers  2 Bank Swallows  1 Rose-breasted Grosbeak  1 American Redstart 

Post-suns

PN975 May 10 

(cont.) 

BH S B5 

clear vis 3 

? 53 34 Bank Swallows 

11 Barn Swallows  6 Magnolia Warblers  2 unid warblers 

Pre-sunri
296 

Table 16.1. 

Summary of all nocturnal circulation events detected during spring 2000 

Date shown is that for the beginning of the night. Flight direction is given with respect to the sea surface as a reference. Events ar

from east to west. 

Platform Date 

Synoptic Weather 

Local Weather 

Flight Direction 

Maximum # Involved Species Composition Notes 

PN975 May 11 BH SSE B5.5 

overcast vis 3 

? 8 5 Barn Swallows 

 2 Bank Swallows  1 Magnolia Warbler 

Pre-sunri

PN975 May 12 BH SE B4 

clear vis 4 

? 8 7 Cattle Egrets 

 1 Peregrine Falcon 

Post-suns

PN975 May 13 GF 

MCH 

NE B4 p. cloudy vis 4 E B4 p. cloudy vis 4 

? ? 

 4  7 

 2 unid passerines  1 Barn Swallow  1 Magnolia Warbler  2 Cattle Egrets  2 Barn Swallows  2 unid passerines  1 unid Catharus thrush 

Post-sunsPre-sunri

PN975 May 14 MCH E B3.5 

p. cloudy vis 4 

? 17 17 unid passerines Pre-sunri

of the birsized and 
297 

Table 16.1. 

Summary of all nocturnal circulation events detected during spring 2000 

Date shown is that for the beginning of the night. Flight direction is given with respect to the sea surface as a reference. Events ar

from east to west. 

Platform Date 

Synoptic Weather 

Local Weather 

Flight Direction 

Maximum # Involved Species Composition Notes 

PN975 May 19 BH SE B3 

overcast vis 3 

? 101 74 unid passerines 

13 unid flycatchers  4 Eastern Wood-Pewees  3 Common Yellowthroats  2 Magnolia Warblers  2 Ovenbirds  1 Lesser Nighthawk  1 Bank Swallow  1 unid swallow 

Circulatiminutes iwith fronthe horiz

PN975 May 20 BH ESE B4 

p. cloudy vis 4 NE B3 clear vis 4 

? ? 

 32  1 

 8 unid warblers  5 unid passerines  5 Magnolia Warblers  5 Ovenbirds  3 Barn Swallows  2 Common Yellowthroats  1 Blackburnian Warbler  1 Yellow-billed Cuckoo  1 unid flycatcher  1 Eastern Kingbird  1 Blackburnian Warbler 

Post-sunsunid wararound thdrawn to (present ovalve leaconstructwarblers by the flaPre-sunri

PN975 May 21 BH ENE B4 

clear vis 4 

? 15 6 unid passerines 

 5 unid warblers  3 American Redstarts  1 Magnolia Warbler 

Post-sunr
299 

Chapter 17 

 

Sources of Mortality 

Robert W. Russell 

17.1 SPRING

Among the total of 787 cases of migrant mortality we documented on or near platforms during the three 

springs of study, a specific cause of death could be ascribed to 758 (96%). Starvation was the most 

common cause of death (46% of deaths in which a cause was assignable), followed by collision (34%) 

and predation (14%). Summaries of sources of spring mortality by platform are given in Tables 17.1-17.3 

for 1998, 1999, and 2000, respectively. 

Table 17.1. 

 

Sources of mortality in spring 1998 

Cause of Mortality GB189 VR265 SM66 PL10 EW826 TOTAL 

Starvation 54 22 16 17 21 130 

Predation 

 Raptors 12 11 2 4 3 32 

 Gulls/other 1 1 0 0 0 2 

Collision 112 11 1 1 1 126 

Other 1 0 0 1 0 2 

Unknown 0 2 1 0 0 3 

TOTAL 180 47 20 23 25 295 

Table 17.2. 

 

Sources of mortality in spring 1999 

Cause of Mortality GB189 VR265 SM66 GC18 EW826 TOTAL 

Starvation 13 2 10 29 21 75 

Predation 

 Raptors 1 1 0 13 3 18 

 Gulls/other 0 0 0 0 0 0 

Collision 7 1 0 4 1 13 

Other 0 0 0 0 1 1 

Unknown 1 4 0 0 0 5 

TOTAL 22 8 10 46 26 112 
300 

Table 17.3. 

Sources of mortality in spring 2000 

Cause of Mortality PN975 MI622 HI561 GB189 VR265 SM66 GC18 EW826 SP8

Starvation 7 9 17 30 18 15 17 18 Predation  Raptors 7 40 0 2 2 0 3 1  Gulls/other 0 23 0 0 0 0 0 2 Collision 87 11 5 17 0 0 1 1 Other 6 0 1 4 0 0 0 2 Unknown 12 3 1 3 1 0 0 1 TOTAL 119 86 24 56 21 15 21 25 1
301 

Differences among years primarily reflected large-scale differences in migration route and the 

occurrence of a few extreme events. 

In spring 1998, the large number of collision deaths at GB189 was almost entirely attributable to an 

unusual weather situation in which a cold front became stationary over the platform from April 27-30. 

During that period, conditions were favorable for departure from Mexico, and massive circulations 

developed around the platform each night as incoming migrants encountered the foul weather and GB189 

almost simultaneously. Of the 112 collision deaths at GB189 in spring 1998, 111 (>99%) occurred 

during the April 27-30 event. 

After accounting for the unusual event at GB189 in spring 1998, patterns of mortality in 1999 differed 

from 1998 in two important ways. First, starvation deaths were nearly 50% less frequent in 1999. The 

relative scarcity of starvation deaths in 1999 apparently reflected the more benign synoptic weather 

patterns in April (i.e., around the time of peak migration) in 1999 than in 1998 (Figure 5.2). Second, 

there was a trend of more deaths toward the east in 1999, whereas the trend was in the opposite direction 

in 1998. This difference coincided with several other lines of evidence (e.g., see Figures 7.11, 7.12) 

suggesting that the large-scale trajectory of migration was shifted toward the east in 1999 compared to 

1998. 

Patterns of mortality in spring 2000 were more similar to 1998. In 2000, the number of collision deaths 

was dominated by a massive circulation event that developed at PN975 on the night of April 29-30, which 

accounted for 49 of that platform’s 87 collisions for the season (56%). 

17.2 FALL

Among the total of 780 cases of migrant mortality we documented on or near platforms during the two 

falls of study, a specific cause of death could be ascribed to 653 (84%). In contrast to spring, when 

starvation was the most common cause of death (even when the two extreme collision events at GB189 

and PN975 are included in the sample), collision was the most common cause of death in the fall, 

accounting for 48% of deaths in which a cause was assignable. Predation was relatively more common in 

the fall (36% of deaths, compared to 14% in spring). However, starvation was uncommon in the fall, with 

only 76 documented cases in the two years. Summaries of sources of fall mortality by platform are given 

in Tables 17.4 and 17.5 for 1998 and 1999, respectively. 

Table 17.4. 

 

Sources of mortality in fall 1998 

Cause of Mortality GB189 VR265 SM66 PL10 EW826 TOTAL 

Starvation 4 5 4 10 13 36 

Predation 

 Raptors 11 5 1 4 2 23 

 Gulls/other 0 0 0 0 0 0 

Collision 3 21 13 9 8 54 

Other 1 4 0 0 1 6 

Unknown 4 3 0 0 19 26 

TOTAL 23 38 18 23 43 145 
302 

Table 17.5. 

Sources of mortality in fall 1999 

Cause of Mortality PN975 MI622 HI561 GB189 VR265 SM66 GC18 EW826 SP89 

Starvation 2 2 3 1 1 6 4 3 5 Predation  Raptors 95 22 0 7 7 4 14 23 11  Gulls/other 0 0 0 0 0 0 0 0 0 Collision 48 16 1 8 9 5 47 3 2 Other 0 2 1 1 5 1 3 0 4 Unknown 23 15 1 5 17 1 15 19 1 TOTAL 168 57 6 22 39 17 83 48 23 
303 

17.3 “OTHER” SOURCES OF MORTALITY

Incidences of mortality attributable to specific causes other than starvation, predation, and collision were 

rare (Table 17.6). We were surprised by the ability of migrants to avoid what initially seemed to us to be 

an abundance of potential dangers on the platforms. In fact, most of the birds that died from oiling and 

poisoning were individuals that were already moribund from starvation. 

Table 17.6. 

 

“Other” sources of mortality during the study 

Cause of 

Mortality 

Spring 

1998 

Spring 

1999 

Spring 

2000 

Fall 

1998 

Fall 

1999 TOTAL 

Drowning 1 0 3 2 1 7 

Oiling 1 0 5 2 1 9 

Burning 0 0 3 0 0 3 

Fans 0 0 2 0 6 8 

Entrapment 0 0 0 1 1 2 

Poisoning 0 0 0 1 7 8 

Workers 0 0 0 0 3 3 

TOTAL 2 0 13 6 19 40 

17.4 DISCUSSION

Deaths of migrants during this study were attributable to a variety of causes, but most resulted from 

starvation, predation, and trauma resulting from collision with platforms. Surprisingly, mortality 

resulting from consumption of toxic chemicals or oiling appeared to be very rare. 

With the exception of the extraordinary circulation event at GB189 in spring 1998, it is clear that 

collisions are generally rare in spring. The reason for this is straightforward: migrants are usually not 

aloft over platforms at night during the spring. A closer look at the collision deaths in Tables 17.1-17.3 

(excluding the April 1998 event at GB189) revealed that most occurred very early in the morning on BH 

days with strong winds from the south, when the vanguard of northbound migrants actually reached our 

platforms before the onset of daylight. 

In contrast to spring, starvation was relatively rare and collision was the most common cause of death in 

the fall. Starvation was rare in part because the platforms are much closer to points of departure in the 

fall (i.e., migrants arriving on platforms have had less time to deplete their fat stores), and in part because 

food availability was greater in the fall (Figure 13.1). Collision mortality is more significant in the fall 

because most migrants in that season are aloft over the northern Gulf during hours of darkness. 

Mortality of migrants at tall towers and other land-based structures has been reviewed extensively (Weir 

1976; Avery et al. 1978; Hebert and Reese 1995; Ogden 1996; Trapp 1998), and the mechanisms 

involved in platform collisions appear to be similar. 

In some cases, migrants simply do not see a part of the platform until it is too late. At VK786 in fall 

1999, we made direct observations from the tip of the flare boom at night which indicated that landbird 

migrants often avoided colliding with the boom only via last-minute evasive movements. It is not clear 

why these migrants apparently did not see the structure almost until it was too late, since we were able to 
304 

see them long before they took evasive action. It is possible that nocturnal landbird migrants in 

flight over the Gulf pay attention primarily to the sky and/or the sea surface; presumably this would 

reflect the fact that, through most of their evolutionary history (i.e., until the advent of offshore 

platforms), no selective advantage would have accrued to individuals on the lookout for obstacles in the 

Gulf. 

Another factor that may be involved in attracting migrants and inducing collisions is noise. At VK786, 

we recovered 5 migrants that died in collisions before September 25 (the approximate peak of the fall 

migration) and 114 migrants that died in collisions after. Prior to September 25, VK786 was extremely 

quiet due to the lack of virtually any activity on the platform. After September 25, a large population of 

workers arrived, construction activities commenced, and the platform was considerably noisier. Thake 

(1981) hypothesized that birds maintain loose associations by detecting Doppler shifts in calls effected by 

relative velocity changes in neighboring migrants. The dramatic increase in frequency of collisions at 

VK786 following the increase in ambient noise levels is consistent with the idea that disruption of the 

aural environment may inhibit orientation performance and increase the risk of collision. 

It is noteworthy that virtually no birds were burned in gas flares in this study. The only platforms that 

routinely flared gas in this study were VR265 and SP89. At VR265, the primary observer during fall 

1999 commented as follows: “We always had a rather small flame. On one or two mornings when there 

was heavy flyover traffic, I could see up to 5 birds circling, or at least flying in and out of, the light 

around the flare. I didn’t think the flame affected birds at any other times.” At SP89, the primary 

observer during fall 1999 commented as follows: “I never observed a circulation event as described by 

others. During the fallout on October 12, the birds repeatedly approached the active flare, but did not 

circle the platform as I imagine a circulation event to be.” The absence of a significant impact of offshore 

gas flares is consistent with observations at terrestrial oil fields around the Gulf Coast indicating that, 

while many migrants may sometimes be attracted to gas flares, only a small proportion of those attracted 

are killed (Davies 1940). Our finding that mortality associated with gas flares in the Gulf is rare is also 

consistent with the conclusion by Bourne (1979) that — despite sensational reports in the popular science 

literature (e.g., Sage 1979) — verifiable reports of birds burned in gas flares in the North Sea are scarce 

(Bourne 1979). Most documented incidents of mass mortality in the North Sea were natural, resulting 

from lost migrants arriving in the fog and dying of starvation rather than incineration (Bourne 1979). 

Based on the seasons with heaviest observed collision mortality (spring 1998, fall 1999), an average Gulf 

platform may cause 50 deaths by collision per year, suggesting that the platform archipelago may cause 

roughly 200,000 deaths per year. This number may be biased low because some birds that collide with 

platforms undoubtedly fall into the sea and avoid detection. Nevertheless, the numbers still pale 

compared to human-related sources of bird mortality on land. At least 100 million — and perhaps as 

many as a billion — birds die each year from striking plate glass windows, including about 25% (225) of 

917 bird species in the U.S. and Canada and many long-distance migrants (Klem 1989,1990; Dunn 1993). 

Hundreds of millions more are killed each year by communication towers, power transmission lines, cars, 

pesticides, and domestic and feral cats. Overall, bird mortality currently caused by offshore platforms is 

negligible within the larger scheme of anthropogenic impacts on migratory birds. 

An important caveat to this conclusion is that future development of the eastern Gulf of Mexico may 

result in a disproportionately large increase in collision mortality in neotropical migrants. This possibility 

is raised by the numerous lines of evidence suggesting that the main fall migration route of neotropical 

migrants is across the eastern Gulf of Mexico (Chapter 9), along with the high observed mortality at 

VK786, at the eastern end of our study area and on the western fringe of the putative fall route (Table 

17.5). In addition, future platforms in the eastern Gulf are likely to be geographically isolated due to the 

economics of deepwater production, and geographical isolation seems to be a factor associated with the 

development of nocturnal circulations (Chapter 16), which put birds at higher risk of collision. 
305 

Chapter 18 

 

Conclusions and Recommendations 

Robert W. Russell 

18.1 CONCLUSIONS

18.1.1 Ecology of Trans-Gulf Bird Migration 

This was the first quantitative study of trans-Gulf migration based on direct evidence and systematic data 

collection. Previously, most information on trans-Gulf migration was based on remote radar 

observations, anecdotal reports from ships (e.g., Brooks 1922; Packard 1947; Bullis and Lincoln 1952; 

Bullis 1954), or casual observations made by platform workers or platform-based marine biologists 

(Ortego 1977; Pulich and Dellinger 1980; Childs 1998). 

Prior to this study, the conventional wisdom had been that spring trans-Gulf migration involves a roughly 

straight-line, shortest-distance flight from the Yucatan Peninsula to the upper Gulf Coast. Our results 

support parts of this scenario but also indicate that the situation is considerably more complex. 

Backtracking from radar images and arrival times on platforms indicates that most spring migrants initiate 

their flights from the Yucatan Peninsula and/or the northern coast of the Isthmus of Tehuantepec. Radar 

and direct observational evidence indicates that most trans-Gulf migration takes place over the western 

Gulf and suggests that the route of migrants is curvilinear and divergent, veering from a probable mean 

heading of northwest at points of origin, to north off the south Texas coast, to northeast off the Upper 

Texas Coast and Louisiana. Large flights are usually associated with Eastern Continental High (ECH) or 

Bermuda High (BH) synoptic weather patterns, in which winds similarly veer clockwise around the 

western Gulf. We therefore suggest that the route of trans-Gulf migrants is influenced by the availability 

of tailwinds, with migrants attempting to minimize the time or energy expenditure required for crossing. 

This hypothesis is strengthened by the finding that centers of offshore abundance as well as areas of 

eventual landfall varied in concert with synoptic weather. On ECH days when winds typically had a 

stronger westward component over the southern Gulf and often maintained a westward component over 

the northern Gulf, migrants were most abundant on platforms in the far western Gulf and landfall was 

usually along the Texas coast. In contrast, on BH days, when winds had a weaker westerly component 

over the southern Gulf and usually an eastward component over the northern Gulf, peak offshore 

abundance shifted eastward and landfall was more likely to take place farther east along the northern Gulf 

Coast, occasionally as far as the Florida Panhandle. All available evidence indicates that the main 

migration stream is at least partially “steered” by synoptic-scale winds. 

In addition to being subject to geographic displacement via steering by synoptic winds, the migration 

stream itself showed evidence of having a complex geographic structure. In at least several species of 

warblers, females apparently take a more direct route across the Gulf, and males tend to take a more 

westerly route. Species with different goals prefer to depart under different synoptic weather types. 

When winds across the northwestern Gulf were unfavorable for trans-Gulf migrants, the usual pattern of 

clockwise travel was absent. Strong, fast-moving cold fronts sometimes induced a large-scale 

displacement of the migration stream eastward as far as Florida. Under less adverse conditions, most of 

the migrants that did undertake the crossing apparently opted for a more direct route toward the Texas 
306 

coast, where they could be seen in radar images arriving from the SSE instead of continuing on 

toward the NNE or NE. 

The diel timing of spring trans-Gulf migration followed a predictable pattern that was evident both in 

radar imagery and from direct visual observations on the platforms. Spring migration over the northern 

Gulf began between early morning and early afternoon, peaked 3-4 h after first detection, and continued 

until 7-12 h after first detection. Patterns of diel timing varied geographically and were related to 

weather, again consistent with a strong synoptic steering influence on migration routes across the Gulf. 

The bulk of spring trans-Gulf migration detected by radar occurred between March 25 and May 24, but 

very large flights (>25 million migrants) occurred only in the 3-week period from April 22 to May 13. 

Waterfowl and herons peaked by early April. Shorebirds had widely varying migration schedules, with 

different species peaking as early as mid-March and as late as the end of May. Landbird migrants showed 

peaks throughout the season, but a majority of species peaked in the second half of April. Theoretical 

analyses of radar data yielded estimated total seasonal estimates of 316 million trans-Gulf migrants in 

spring 1998 and 147 million trans-Gulf migrants in spring 1999. In both years, about two thirds of all 

migrants made landfall west of South Marsh Island. 

Radar-observed spring migration was characterized by a series of pulses and tended to be “all-ornothing”, i.e., either significant trans-Gulf migration was evident on radar or else it was essentially 

entirely absent. Dramatic hiatuses in radar-observed migration were always associated with strong cold 

fronts that penetrated deep into Mexico and set up persistent northerly winds over most of the Gulf. 

Conversely, radar-observed migration peaks were almost strictly associated with ECH and BH days. 

Analyses of radar data indicated significant differences in both diel and seasonal timing between 1998 

and 1999. The diel timing of migration over the northern Gulf averaged 1-3 h later in 1999 than in 1998, 

even after accounting for synoptic weather type. In 1999, migration peaks seemed to be approximately 

Gaussian and symmetric around the peak date of April 22. In contrast, radar-observed migration peaked 

much later in 1998 and the overall pattern was highly skewed to the left. Reasons for these interannual 

differences are not clear. 

Fall trans-Gulf migration was more difficult to study because the extensive presence of aerial insects 

precluded quantitative interpretation of radar imagery. In addition, one of the two field seasons was 

partly compromised by prolonged absences from the platforms due to obligatory evacuations in response 

to developing tropical weather systems. Nevertheless, we argue that the heaviest trans-Gulf migration 

traffic in fall originates from the stretch of the northern Gulf Coast running eastward from Alabama. 

Although we were constrained from sampling much of this area by the absence of platforms in the eastern 

Gulf during the study period, our contention is supported by observed longitudinal trends in abundance 

and age ratios. Importantly, we witnessed many massive nocturnal migration events at VK786 in fall 

1999 which were effectively unquantifiable due to their magnitude and chaotic character. These massive 

flights occurred fairly consistently from mid-August through mid-September, despite the scarcity of 

particularly favorable weather conditions during that period. 

Southbound “fall” migrants were observed as early as May 20 and as late as January, but the vast majority 

of the migration occurred from mid-August to early November. There seemed to be several phases in the 

fall migration. During the early fall, migration appeared to be obligate and was not strongly influenced 

by weather. Later in the fall, major trans-Gulf movements were generally associated with cold fronts and 

northerly winds. Most of the species peaking in early fall are long-distance migrants, whereas species 

peaking later in the fall are dominated by shorter-distance migrants. Thus, our findings are consistent 

with the idea that the decision to initiate a migratory flight is influenced more strongly by weather in 
307 

short-distance migrants, and more strongly by an endogenous time program in longer-distance 

migrants (Zehnder and Karlsson 2001). 

The most prominent players in the early fall movements seemed to be Yellow Warblers and the common 

warblers breeding in moist southeastern woodlands (Prothonotary, Hooded, Kentucky), along with 

smaller numbers of Cerulean Warblers and Worm-eating Warblers. Shorebirds were also an important 

component of this early fall eastern-Gulf migration. The early fall flights frequently occurred on nights 

with light southerly or easterly winds, and were almost completely invisible on NEXRAD because most 

migrants flew at low altitudes. Direct observations at VK786 indicated that the direction of flight was 

most often due south but varied from south-southwest to south-southeast. As with spring, variation in the 

direction of travel was clearly influenced by wind. 

We also detected considerable fall migration over the far western Gulf, where flight direction usually had 

a westerly component. The western-Gulf route was used by a high proportion of juveniles, and appeared 

to represent a risk-averse migration strategy favoring a shorter, less risky overwater flight leg at the 

expense of a more circuitous overall migration route. We suspect that many of the adults traveling over 

the western Gulf were individuals that reached the northeastern Gulf Coast with inadequate fat stores for a 

direct trans-Gulf and worked their way westward along the coast, perhaps stopping over along the way. 

This hypothesis is consistent with findings from experimental work. Orientation cage experiments during 

September and October at Fort Morgan, Alabama, showed that lean birds oriented to the west-northwest 

(i.e., parallel to coast), whereas fat birds oriented to the south-southwest, in a direction consistent with 

intentional trans-Gulf crossing (Sandberg and Moore 1996). Free-flight release tests yielded similar 

results: fat birds disappeared to the south, whereas lean birds moved inland (albeit to the northeast). 

One of the interesting features of the fall migration offshore was the frequent occurrence of a variety of 

species that do not typically winter south of the northern Gulf Coast. These species were evidently 

mostly “overshoots” that inadvertently traveled past their intended destinations and found themselves 

unexpectedly over water at first light, or else circum-Gulf migrants that inadvertently drifted eastward 

over the Gulf during nocturnal flight. Accordingly, these species were often observed flying north or 

west during daylight hours, presumably trying to get back to land. This behavior was reminiscent of the 

daytime reorientation of fall migrants toward the Atlantic Coast following nocturnal wind drift, which has 

been well documented (Stone 1937; Baird and Nisbet 1960; Murray 1976; Able 1977; Richardson 1978) 

and seems to involve mostly immatures (Able 1977). 

All species displaying behavior suggestive of overshooting were either medium-distance migrants or 

short-distance migrants. Among the medium- and short-distance migrant species, only Lincoln’s 

Sparrow, Grasshopper Sparrow, and Brown-headed Cowbird showed indications of being intentional 

trans-Gulf migrants. All other species are presumed to have been over the Gulf inadvertently. Vanishing 

bearings of overshoots tended to cluster in the W-WSW and NW-NE sectors, with medium-distance 

migrants tending toward the former and short-distance migrants tending toward the latter. The distinction 

between the westbound and northbound migrants could result either from facultative individual decisions 

or from inter-individual differences. In the case of facultative individual decisions, an overshoot migrant 

may “decide” whether to attempt to head back to the northern Gulf Coast based upon its body condition 

and remaining fat stores in relation to its assessment of the energy requirements needed to complete the 

corrective flight. Migrants lacking adequate energy stores for such corrective action may simply travel 

downwind (usually toward the WSW or SW in the fall) to maximize the amount of time before they 

deplete their energy stores. Alternatively, westbound birds may be individuals with intended winter 

destinations along the western (as opposed to northern) Gulf Coast. The fact that medium-distance 

migrants (which have more southerly wintering distributions) were more likely to be westbound than 

were short-distance migrants is consistent with the hypothesis of inter-individual variation. However, 

some short-distance migrants were observed traveling in directions that, if maintained until landfall, 
308 

would have taken them to points well south of their species’ normal wintering range, in support of 

the hypothesis of condition-dependence. 

The year-round observations at GC18 indicated that northbound (“spring”) trans-Gulf migration spans 

late January to early June, and southbound (“fall”) trans-Gulf migration and overshooting spans early July 

to early December. Surprisingly, we found that northbound and southbound migrations overlapped 

temporally at the extremes: The latest southbound migrant recorded during the study (Common Snipe) 

occurred on 28 January, and the earliest northbound migrant (Purple Martin) occurred the following day. 

Southbound migration of Purple Martins began in late April and southbound shorebird migration began in 

late May, well before the latest northbound migrant was recorded on 8 June (Northern Waterthrush). 

Overwater movements during the brief interim periods between spring and fall migrations (midDecember to mid-January, mid-June to early July) seemed to be dominated by herons traveling along an 

east-west axis. The nature of these movements is completely unknown, but deserves further study. 

Death of migrants by starvation was fairly common in the spring. Although fat scores were not recorded 

systematically, most of the spring dead birds categorized as “starved” lacked any trace of furcular or 

abdominal fat and had fat scores of zero on the Helms and Drury (1960) scale. Similar to migrants 

landing on ships during overwater flights, which often have essentially no fat reserves (Johnston 1968), 

starving migrants on platforms usually had conspicuously protruding keels and appeared to have begun to 

catabolize nonfat dry body components. In contrast, fall collision mortalities typically had obvious 

furcular fat pads that were either convex or level with the pectoralis muscle, and would have scored 4 or 5 

on the Helms and Drury (1960) scale. These observations are consistent with the idea that maximum fat 

levels are reached at or near points where long nonstop flights begin, i.e., along the Gulf Coast in fall 

(Caldwell et al. 1963). 

Whether migrants are limited by water or energy has long been a contentious question (Carmi et al. 1992; 

Biebach and Klaassen 1994). Some workers have suggested that water might limit flight range because 

evaporative water loss exceeds metabolic water production (Yapp 1956). Our observations have a direct 

bearing on this question. Fresh water was generally always available on all platforms. Sources of water 

included morning dew, condensation from air conditioners that formed shallow pools on decks or the roof 

of living quarters, condensation on pipes, cooling water running down the outside of compressors that 

sometimes formed shallow puddles on production decks, and pools of rain water on the tops of storage 

barrels. Despite the widespread availability of fresh water, we very rarely observed migrants drinking. 

Our observation that water consumption by trans-Gulf migrants is very rare, together with the observation 

that desperate migrants that have depleted their fat reserves and begun to catabolize nonfat dry body 

components showed no signs of dehydration (Johnston 1968), indicate that water is not a limiting factor 

to trans-Gulf migrants. 

18.1.2 The Impact of Platforms on Migrant Birds 

Platforms have three primary proximate impacts on migrant birds: 1) they provide habitat for resting and 

refueling; 2) they induce nocturnal circulations; and 3) they result in some mortality through collisions. 

The availability of suitable habitats en route where migrants can safely replenish energy reserves is 

critical to successful migration, especially in the vicinity of major ecological barriers such as the Gulf of 

Mexico (reviewed by Moore and Simons 1992). Differences in stopover durations and rates of mass gain 

by migrants among different stopover sites are thought to be related to the intrinsic suitability of the 

different habitats available (Moore and Simons 1992). The suitability of a habitat is difficult to quantify, 

but presumably reflects food supply (Martin 1980,1985; Hutto 1985; Russell et al. 1994), competition 

(Moore and Yong 1991; Carpenter et al. 1993a,b), safety from predators (Metcalfe and Furness 1984; 

Lindström 1989,1990; Moore et al. 1990), shelter from severe weather (Gass and Lertzman 1980), and the 
309 

physical structure of the habitat (Holmes and Robinson 1981; Robinson and Holmes 1982, 1984). 

Physiognomy is an important determinant of habitat suitability because habitat structure influences 

how birds are able to move through the habitat, how they are able to detect and capture prey, and which 

prey they are able to detect. 

Platforms appeared to be suitable stopover habitats for most species, and most of the migrants that 

stopped over on platforms probably benefited from their stay, particularly in spring. Many of these 

migrants were able to feed successfully, and some appeared to achieve rates of mass gain that exceeded 

what is typical in terrestrial habitats. Platforms thus exert some selective influence on trans-Gulf 

migrants, since a positive association between fat levels at migratory departure and subsequent survival 

(estimated via return rates) has been demonstrated (Pfister et al. 1998). Even the individuals that do not 

feed probably benefit physiologically from the availability of the platforms. Yapp (1956) suggested that 

migrants may be affected by sources of fatigue other than total depletion of fat stores, such as excessive 

accumulation of lactic acid, failure of the nerve-muscle junction, or upset of central nervous coordination. 

These types of fatigue may be eliminated by simple rest. Many of the migrants that rested quietly on the 

platforms for hours to days were probably recovering from such sources of fatigue. 

Our findings suggest that migrants use platforms in highly nonrandom ways. Migrants selected specific 

microhabitats on platforms (i.e., used alternative microhabitats nonrandomly), much in the same way that 

they select specific habitats during terrestrial stopovers (e.g., Moore et al. 1990; Moore and Simons 

1992). Preferred platform microhabitats were species-specific and generally consistent between spring 

and fall. 

One of the most interesting, albeit speculative, impacts of platforms involves their possible role in the 

natural selection of migration strategies. Experimental studies of captive populations (Berthold 1991; 

Berthold and Helbig 1992) as well as analyses of banding recoveries from populations of recently 

introduced species (Able and Belthoff 1998) indicate that migratory behavior, including preferred 

orientation, can evolve very rapidly. Observations from this study suggest that the archipelago of 

offshore oil platforms may be facilitating the evolution of novel trans-Gulf migration strategies in several 

species. Cattle Egrets colonized eastern North America only in the last half-century (reviewed by Telfair 

1983), but have already become one of the most common species on platforms. The fact that Cattle 

Egrets were frequently observed at unexpected times of the year and flying in seasonally unexpected 

directions (e.g., Chapter 12) suggests the existence of a significant pool of genetic variation and the 

potential for rapid further selection. A rapid evolution of trans-Gulf migration may be implicated in what 

appears to be a population explosion and major range expansion of White-winged Doves into the 

southeastern United States. The Eurasian Collared-Dove has recently colonized North America, and 

began showing up on platforms in fall 1999. Platforms may facilitate the evolution of trans-Gulf 

migration strategies in certain species by providing “steppingstones” that allow incipient migrants to cross 

the Gulf successfully via a series of shorter flights. However, critical evaluation of hypotheses 

concerning selection and life-history evolution will require longer-term data than are currently available. 

Another intriguing possibility concerning selective influences of platforms concerns the life-history 

ecology of Peregrine Falcons. Although the spectacular recovery of Peregrine Falcons has generally been 

attributed to the ban on DDT along with extensive reintroduction efforts (reviewed by Enderson et al. 

1995; Cade et al. 2000), it seems reasonable to consider whether the growth of the platform archipelago in 

the Gulf may be involved. Indeed, since the first platform was installed in 1942, the period 1975-1985 

saw the most dramatic increase in the size of the platform archipelago of any 10-year period, with a net 

addition of 1,395 platforms (i.e., new platforms installed minus those removed; Minerals Management 

Service 2004). This period was coincident with the recovery period of exponential population growth of 

Peregrine Falcons. The majority of juveniles in the North American population of this species (which 

was listed as Federally endangered until recently) now uses oil platforms in the northern Gulf during the 
310 

fall for resting and hunting. Peregrines often appear on platforms immediately before weather 

events that induce large landbird movements, and now appear to be “anticipating” ephemeral 

periods of high resource availability. This adaptive behavior may be a precursor to evolutionary changes 

in the life history of the species, since an ecologically similar species — Eleonora’s Falcon — has 

evolved a strategy of breeding on islands in the Mediterranean during the fall, when abundant transMediterranean migrant landbird prey are available for provisioning young. In other words, we 

hypothesize that Peregrine Falcons may be in the early stage of a dramatic life-history evolution through 

natural selection occurring via the Gulf platform archipelago, and we predict that Peregrine Falcons will 

establish a breeding population on platforms in the near future. 

The existence of the platform archipelago also has potential selective implications for overshooting 

migrants (Chapter 9.4). During the fall, many shorter-distance migrants that spend the winter along the 

Gulf Coast (such as wrens and sparrows) inadvertently overshoot the coastline during nocturnal migratory 

flights and end up over Gulf waters. These overshoot migrants, which are evolutionarily ill-equipped to 

deal with the rigors of overwater migration, were among the heaviest users of platforms during the fall, 

and the availability of platform rest stops probably enabled many individuals to return to land 

successfully. This alleviation of selection pressure may in turn be implicated in large-scale southward 

shifts in the wintering distributions of some of these species. Long-term Christmas Bird Count (CBC) 

data may be useful for documenting trends in winter distributions of overshooting migrants in the Gulf 

region for tests of the “selection reduction hypothesis.” 

According to the latest MMS database (Minerals Management Service 2004), the number of platforms in 

the northern Gulf of Mexico peaked at 4,064 in 2001 and has since been in decline as a result of removals 

outpacing new installations. Future evaluations of the possible influence of platforms on the evolution of 

migration strategies will have to take into account the changing size, structure, and spatial extent of the 

platform archipelago. 

18.2 RECOMMENDATIONS

18.2.1 Use of Platforms as Ecological Observatories 

Gulf platforms provide a unique opportunity for monitoring migratory bird populations. Unlike most 

terrestrial habitats, platforms concentrate migrants in a small area that can be surveyed comprehensively 

in a highly standardized and repeatable fashion. With the exception of the extremely important effects of 

weather (which can be controlled for and factored out using multivariate statistical methods), platform 

counts are free from a variety of important biases that affect censusing schemes in natural terrestrial 

environments (e.g., temporal trends in vegetation development) and can provide useful samples of 

ambient migration traffic. Unlike migrants observed in terrestrial environments, migrants detected on 

platforms are effectively “caught in the act” of migrating. Platform monitoring may yield powerful time 

series for detecting trends in populations of common migratory species and for answering questions about 

the interrelationships among various environmental factors, migration, and population dynamics. 

We suggest that attention be paid to the possibility of developing and maintaining a network of 

decommissioned platforms as permanent “observatories” for long-term ecological research. Dokken 

(1993) has previously advocated the idea of converting a platform to a permanent offshore research 

station, and suggested that the annual cost for operating such a station might be $300,000 to $400,000, 

including staffing and research expenses but excluding transportation (which presumably could be 

supplied by the cooperating petroleum companies). Although such costs would be prohibitive for 

ornithological research alone, designation as general ecological observatories would provide novel 

opportunities in multiple research areas and would permit cost-sharing among a wide variety of agencies 

and organizations. 
311 

During this study we also monitored the occurrence and behavior of fishes and seabirds in the 

vicinity of our study platforms. This opportunistic work suggested a complex series of ecological 

interactions centered on platforms. Platforms appear to attract surface-dwelling macrozooplankton and 

“baitfish,” either by behavioral attraction or by mechanical entrainment in convergent flows induced by 

the platform substructures. In addition, terrestrial insects advected offshore are attracted to platforms and 

become part of the neuston around platforms. This diverse neustonic fauna in turn appears to attract 

predatory fishes such as blue runners and tunas. Obligate surface-feeding seabird species are in turn 

attracted to the vicinity of the schools of predatory fish, where they take buoyant scraps from the fish 

feeding frenzies. These complex trophic relationships merit further attention, because concentration of 

marine bird populations in the vicinity of platforms may be a conservation concern in the event of oil 

spills. Our seabird observations also suggested that Gulf populations of several species (e.g., Black Tern, 

Cory’s Shearwater) are concentrated in the vicinity of platforms, and consequently highly vulnerable to 

anthropogenic impacts. Understanding the mechanisms underlying the attraction of some species to 

platforms, and the lack of attraction or even repulsion in other species, is an important first step toward 

the formulation of seabird management and conservation plans, and could probably best be accomplished 

via a devoted platform observatory where, for example, experiments could be conducted with 

deployments of fish oil slicks to determine the area from which seabirds potentially attracted to platforms 

are drawn (e.g., Haney et al. 1992). 

During this study we discovered an unexpected abundance of terrestrial insects offshore and expanded our 

focus to include insect monitoring using both visual censuses and quantitative sampling methods. In 

addition to the passively transported fauna of smaller insect species (the “aerial plankton”), we discovered 

that several species of dragonflies and moths are “intentional” trans-Gulf migrants. Trans-Gulf migration 

in insects has important implications for understanding large-scale biogeographic patterns as well as lifehistory strategies of the species involved. On many occasions we detected insects on floating debris. 

Rafting on floating debris and long-distance transport on the sea surface as pleuston may be significant 

modes of dispersal and long-distance colonization (Peck 1994b); however, the relative biogeographical 

significance of transport in pleuston versus direct aerial transport (Peck 1994a) remains poorly known. A 

more speculative but potentially important implication of the aerial plankton is the possibility that 

allochthonous input of carbon via mortality of drifting insects could be ecologically significant in food 

webs of the more depauperate waters of the Gulf (Chapter 13). A better understanding of the insect 

biodiversity over and on the Gulf thus has implications for a wide variety of environmental issues, 

including the foraging success of migrant birds on platforms, regional pest management strategies (since 

many of the common offshore species are known to be important agricultural pests on the Gulf coastal 

plain), the relative importance of different dispersal modes in insect biogeography, and perhaps even 

carbon fluxes in marine ecosystems of the Gulf. Monitoring of insect biodiversity over the Gulf could 

best be accomplished on devoted platform observatories, where non-attractant traps such as the JohnsonTaylor suction trap (Johnson and Taylor 1955; Taylor 1962) could be deployed permanently to collect 

long-term time series of true volumetric data on aerial insect densities. 

Many other important applications can easily be envisioned for permanent offshore ecological 

observatories. A blood sampling program would permit blood chemistry studies to monitor levels of 

organochlorines and other pesticides and environmental pollutants that may be accumulated by migrants 

on wintering grounds and transported to the U.S. from other countries (Elliott and Shutt 1993; Mora 

1997). Automated weather instrumentation could be housed for long-term meteorological studies. Fulltime staff members could regularly monitor hydrographic structure and current dynamics with standard 

oceanographic sampling equipment, and the observatories could be used as bases for launching periodic 

plankton and fish sampling programs. As totally artificial large-scale environments, platforms-turnedobservatories could be made available to academic researchers as habitat laboratories for experimental 

studies of bird foraging behavior (Chapter 15). 
312 

18.2.2 Management of the Platform Archipelago 

This study suggests that, during the spring, the vast majority of trans-Gulf migrants are not influenced by 

platforms and simply pass high overhead en route to the northern Gulf Coast or points beyond. Those 

migrants that do drop out of the migration stream and interact with platforms probably realize a slightly 

positive net benefit from the existence of the platform archipelago. For these individuals, platforms 

provide a place to rest, replenish energy reserves, and find refuge from severe weather. Most of the 

mortality that occurs on platforms in the spring is the result of physiological damage that occurs prior to 

arrival on platforms, as a result of complete exhaustion of energy reserves and the onset of catabolism of 

essential tissues, and would result even in the absence of the platforms. 

In light of this, there is little need for specific management recommendations with respect to spring transGulf migration. Keeping platforms as clean as possible, and in particular, keeping decks and pans free 

from residual oil, is probably the most effective way that petroleum companies can maintain a safe 

environment for their spring migrant visitors, and cleanliness is presumably already fully addressed 

within existing environmental regulations. 

Our findings suggest that the heaviest trans-Gulf migration in fall emanates from the stretch of the 

northern Gulf Coast running eastward from Alabama, and thus that the majority of fall migrants may 

travel over waters in which there are currently virtually no platforms. Historically, only a very limited 

amount of exploration and appraisal activity took place in the eastern Gulf, and most of the discoveries 

were noncommercial (Gohrbandt 2001). Subsequently, the waters of the MMS Eastern Planning area off 

Alabama and Florida were formally closed to exploration and production activities because of 

congressional actions and presidential moratoria in response to opposition by Florida. However, the 

verification in 1995 of 2.6 tcf of potentially recoverable natural gas reserves from a 1987 gas find in the 

Destin Dome area—together with breakthroughs in seismic data analysis and deepwater production 

technology—have rekindled interest in the eastern Gulf. With the new political climate that is 

emphasizing increased domestic oil and gas production, the status of the eastern Gulf as one of the 

nation’s last petroleum frontiers is apparently overriding the historical objections by Florida, and Lease 

Sale 181 (Wetuski 2001) seems destined to be the start of a longer-term reopening of the eastern Gulf to 

the petroleum industry. The industrial development of the eastern Gulf will inevitably become an 

important component of the ecological context for fall trans-Gulf migration. 

Nocturnal collisions of fall migrants were the most significant adverse impact of platforms documented in 

this study. Although the overall numbers of migrants killed were relatively small, the highest mortality 

occurred at Viosca Knoll 786, which is on the western boundary of the hypothesized principal fall 

migration corridor (and on the eastern boundary of the MMS Central Planning Area). The high fall 

mortality at Viosca Knoll 786 suggests that the development of the eastern Gulf will not be benign to 

migrating birds. Although subsea production systems are likely to figure prominently in the eastern Gulf, 

permanent structures above the surface of the water are inevitable, and they will certainly cause some 

mortality in low-altitude night-migrating landbirds during the fall migration season (July-November). In 

addition, the economics of deep-sea platform-based production dictate that the eastern Gulf will probably 

be populated by a relatively small number of major platforms that are geographically isolated. Since 

isolation is suspected to be implicated in the likelihood of nocturnal circulation events (Chapter 16), 

circulations and their attendant problems (collisions and probably exhaustion) will also be associated with 

development of the eastern Gulf. 

We therefore suggest that the Minerals Management Service should consider implementing an ongoing 

platform monitoring program in the eastern Gulf of Mexico as that area is developed by the petroleum 

industry. The observer program operated by NOAA’s National Marine Fisheries Service (NMFS) may 

serve as a useful model for developing such a platform monitoring program. Since 1972, NMFS has been 
313 

deploying trained fishery observers to collect catch and bycatch data from commercial fishing 

vessels in a variety of different fisheries (National Marine Fisheries Service 1998, 2003). Nearly 20 

different fisheries are monitored by observer programs annually along all U.S. coasts. The trained 

observers collect data for a range of conservation and management issues as well as to ensure regulatory 

compliance. 

The impact of nocturnal circulation events (Chapter 16) on both spring and fall trans-Gulf migrants 

remains poorly known, and this phenomenon should be examined in a focused observational study using 

night-vision optics and thermal imaging equipment. The goals of such a study should be to quantify in 

greater detail the dimensions of the circulation phenomenon, to try to determine why some platforms 

often induce circulations and others never do, to assess the rate of turnover during major circulation 

events, and to model the energetic impact on migrants of participation in circulations with respect to 

synoptic weather, geography, and taxon. 

If fall collision mortality in the eastern Gulf proves to be significant or if results from the study of 

circulations suggest that the adverse impacts of this phenomenon should be addressed, experiments 

should be undertaken to evaluate the role of different color schemes and lighting regimes in the attraction 

of migrants to platforms. In some cases, simple changes in light signatures have seemingly resulted in 

dramatic reductions in avian attraction and mortality at tall lighted structures on land during migration 

(e.g., Jones and Francis 2003). One interesting idea that was arrived at independently by several of the 

field observers was that the continuity of lighting around a platform may dictate whether circulations 

occur or not; specifically, it is suspected that lighting completely around a platform (i.e., with no dark 

areas) is necessary for circulations to develop and be maintained. If this is the case, then a simple system 

incorporating baffles or more directional lighting might completely eliminate circulations. 

During this study we were surprised to find that very few people involved in the offshore petroleum 

industry are aware of the existence of trans-Gulf bird migrations. Virtually all platform workers 

attributed the presence of birds on platforms to navigational errors, storm events that blew the birds 

offshore, and a variety of other inventive explanations. Once informed, a number of platform workers 

became interested in learning more about the migrations and wanted to find out how they might be able to 

assist in our studies or to assist the migrant birds themselves. We suggest that production of a colorful 

informational brochure about trans-Gulf migration for distribution to offshore workers and other people 

involved in the industry would be a useful way to promote a wider awareness of the ecological 

importance of the Gulf, which in turn may be an incentive to platform workers to help maintain a safe 

environment for avian visitors to platforms. 

18.2.3 International Outreach 

The Bay of Campeche has a well-developed oil and gas exploration and production infrastructure. 

However, to our knowledge, no information is available concerning the possible influence of Mexican 

platforms on trans-Gulf migrants. Available evidence suggests that a large proportion of spring transGulf migrants probably travels over the Bay of Campeche (Chapter 7). These migrants are aloft over 

Mexican platforms during hours of darkness near the beginning of their journey, so the circumstances are 

very similar to those encountered by fall trans-Gulf migrants initiating southbound flights across the 

eastern Gulf of Mexico. Thus, significant mortality events associated with circulations and/or direct 

collisions probably occur at least occasionally on Mexican platforms. Our understanding is that Mexican 

platforms routinely burn gas, and persistent haze resulting from the smoke would likely exacerbate any 

unfavorable nocturnal interactions. Efforts during this study to communicate with Pemex (the Mexican 

national petroleum company) to exchange information and perhaps launch an exploratory study on 

Mexican platforms were unsuccessful, but should be continued in the future. 
314 

The Cuban sector of the southeastern Gulf of Mexico also has significant hydrocarbon potential, and 

Cuba recently decided to offer deepwater acreage in the Cuban Exclusive Economic Zone to foreign 

companies for exploration and development on a production-sharing basis (Pérez and Blickwede 2000). 

This Cuban deepwater acreage extends to the maritime boundaries with Mexico and the U.S., and its 

development could affect both spring and fall bird migrations (e.g., Figures 7.10, 9.7). The northernmost 

of the 59 Cuban blocks available is just 100 km south of the acreage offered in OCS Lease Sale 181, 

suggesting that platforms could eventually stretch in a continuous band along the pathway traveled by 

many fall trans-Gulf migrants. 

Trans-Gulf migrants are by definition international travelers, so conservation strategies targeting these 

species must consider the international implications of their migrations. Biologists interested in the 

ecology and conservation of trans-Gulf migrants should initiate outreach efforts to involve international 

colleagues in the development of a network for information exchange concerning events in all geographic 

sectors of the Gulf. 
315 

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