United States

Department

of Agriculture

Forest Service

Rocky Mountain

Research Station

General Technical

Report RMRS-GTR-60

September 2000

Status, Ecology, and

Conservation of the

Southwestern Willow

Flycatcher
Telephone (970) 498-1392

FAX (970) 498-1396

E-mail rschneider@fs.fed.us

Web site http://www.fs.fed.us/rm

Mailing Address Publications Distribution

Rocky Mountain Research Station

240 W. Prospect Road

Fort Collins, CO 80526-2098

Abstract ____________________________________

Finch, Deborah M.; Stoleson, Scott H., eds. 2000. Status, ecology, and conservation of the

Southwestern Willow Flycatcher. Gen. Tech. Rep. RMRS-GTR-60. Ogden, UT: U.S. Department

of Agriculture, Forest Service, Rocky Mountain Research Station. 131 p.

This publication was prepared in response to a need expressed by southwestern agencies and

organizations for a comprehensive assessment of the population status, history, biology, ecology,

habitats, threats, and conservation of the southwestern willow flycatcher (Empidonax traillii extimus).

The southwestern willow flycatcher was federally listed as an Endangered subspecies in 1995. A team

of flycatcher experts from multiple agencies and organizations identified components of the publication, wrote chapters, and cooperatively assembled management recommendations and research

needs. We hope this publication will be useful in conserving populations and habitats of the

southwestern willow flycatcher.

Key words: southwestern willow flycatcher, endangered species, riparian, Southwest, exotic woody

plants, rivers, recovery, habitat restoration, Neotropical migratory bird, brown-headed

cowbird

Top left photo: Adult southwestern willow flycatcher, White Mountains, Arizona.

Photo by Suzanne Langridge

Top right photo: Southwestern willow flycatcher adult, nest, and nestlings, Kern River

Preserve. Photo by Sean Rowe

Bottom photo: Southwestern willow flycatcher adult, nest, and nestlings, along

irrigation ditch, Gila National Forest. Photo by Jean-Luc Cartron

Acknowledgments____________________________

Financial assistance for preparing this report was provided by the Southwest Region and Rocky

Mountain Research Station’s Rio Grande Ecosystem Program of the U.S. Forest Service. The

editors and authors thank The Ornithological Council, and particularly Ellen Paul, for arranging a

formal blind review of this publication, and they thank the peer reviewers themselves for their

generous willingness to comment on the entire manuscript. We also extend our appreciation to John

Rotenberry for insightful comments on the manuscript and to Nancy Chadwick for her assistance

during the publication process, and to John Gustafson, Sheldon Plentovich, Susan Sferra, and

Sandy Williams for their participation in the discussion process.
Status, Ecology, and Conservation of

the Southwestern Willow Flycatcher

Editors:

Deborah M. Finch and Scott H. Stoleson

USDA Forest Service

Rocky Mountain Research Station

2205 Columbia SE

Albuquerque, NM 87106
Contents____________________________

Page

Chapter 1: Introduction of the Conservation Assessment Concept........................... 1

Deborah M. Finch

Chapter 2: Population Status on Breeding Grounds ..................................................... 3

Robert M. Marshall

Chapter 3: Threats .......................................................................................................... 13

Robert M. Marshall

Scott H. Stoleson

Chapter 4: The Dynamic Environmental History of Southwest Willow

Flycatcher Habitat: A Survey of Changing Riparian Conditions

Through Time ............................................................................................... 25

Richard D. Periman

Jeffrey F. Kelly

Chapter 5: A Survey of Current Breeding Habitats ..................................................... 43

Mark K. Sogge

Robert M. Marshall

Chapter 6: Breeding Season Ecology ........................................................................... 57

Mark K. Sogge

Chapter 7: Migration and Winter Ecology..................................................................... 71

Deborah M. Finch

Jeffrey F. Kelly

Jean-Luc E. Cartron

Chapter 8: Demographic Characteristics and Population Modeling.......................... 83

Scott H. Stoleson

Mary J. Whitfield

Mark K. Sogge

Chapter 9: The Ecology of Brown-Headed Cowbirds and Their Effects on

Southwestern Willow Flycatchers .............................................................. 95

J. C. Uyehara

Mary J. Whitfield

Lloyd Goldwasser

Chapter 10: Management Recommendations ............................................................ 107

Deborah M. Finch

Janie Agyagos

Tracy McCarthey

Robert M. Marshall

Scott H. Stoleson

Mary J. Whitfield

Chapter 11: Research Needs ....................................................................................... 119

Scott H. Stoleson

Janie Agyagos

Deborah M. Finch

Tracy McCarthey

Jamie Uyehara

Mary J. Whitfield

Appendix: Author Biosketches and Addresses ......................................................... 129
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 1

Deborah M. Finch

Chapter 1:

Introduction of the Conservation

Assessment Concept

The goal of this document is to describe the current

status, ecology, habitat, and threats of the southwestern willow flycatcher (Empidonax traillii extimus); to

offer guidance for managing and protecting this Neotropical migrant and its habitats; and to identify gaps

in our knowledge of the bird and its requirements.

Goals, processes, and target species for U.S. Forest

Service (USFS) conservation assessments of southwestern organisms were first outlined at a meeting

held by the USFS Southwestern Region on May 11-12,

1994. At that time, the Forest Service had identified

the southwestern willow flycatcher in a general category called “riparian birds” whose conservation assessments would be drafted in 1999. Yet, by July 23,

1993, the U.S. Fish and Wildlife Service had proposed

to list E. t. extimus based on findings of a petition

submitted in 1992 (U.S. Fish and Wildlife Service

1993). When the southwestern willow flycatcher was

federally listed as endangered in March 1995 (U.S.

Fish and Wildlife Service 1995), the need for developing management guidance became a higher priority,

and the date for completing the assessment was accelerated. In 1997, the USFS Southwestern Region asked

the Rocky Mountain Research Station’s Albuquerque

office to prepare an assessment of the flycatcher that

would provide guidance for conserving its populations

on national forests.

Prior to and since its listing, federal, state, and

municipal agencies have been working together and

with private landowners and conservation organizations to survey riparian habitats in California, Arizona,

New Mexico, western Texas, southern Utah, southern

Nevada, and southern Colorado with the hope of

finding and protecting additional flycatchers and their

habitats. Because populations of the flycatcher reside

on lands under mixed ownerships, I concluded that the

most effective conservation strategy for this subspecies of the willow flycatcher (WIFL) would have to be

developed by multiple stakeholders. With this in mind,

I assembled a team comprised of representatives of several southwestern state and federal agencies, including

U.S. Fish and Wildlife Service, U.S. Forest Service,

U.S. Geological Survey, Bureau of Reclamation, Arizona

Game and Fish, New Mexico Department of Game and

Fish, and California Department of Fish and Game to

prepare the conservation assessment. To ensure sufficient technical expertise, additional representatives of

two nongovernmental organizations, Kern River Research Center and The Nature Conservancy, were also

invited to participate in the development of specific

technical review chapters. Biographical sketches and

institutional affiliations of authors who contributed

chapters to the assessment are given in the Appendix.

Our first meeting of the conservation assessment

team was held in Albuquerque on May 6, 1997. At that

time, we outlined the chapter topics, content, authors,

and schedule needed for completing the conservation

assessment. We agreed that most chapters of the
2 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

assessment would be prepared by one or more experts

on the chapter topic, but that the Management Recommendations and Research Needs chapters would be

developed through a group consensus process. The

team met on multiple occasions through the remainder of 1997 and part of 1998 to discuss management

recommendations and research needs, and to review

progress. On March 2, 1998, the team met to evaluate

the final product and to initiate the review process.

External reviews of the conservation assessment

were solicited from a wide variety of stakeholders, and

their input was considered in the final document. In

addition, arrangements were made with The Ornithological Council to conduct a formal “blind review” of

the document. The Council requested reviews of the

entire document from three referees and received two

thorough but favorable reviews in return which were

forwarded to the Editors, Scott Stoleson and myself.

We distributed the Council reviews to senior authors

of each chapter with requests for revisions, and authors revised their individual papers accordingly.

This document is therefore defined as peer-reviewed

and should be cited as such.

In the context of WIFL habitat requirements and

consistent with the goals of Forest Service Conservation Assessments, our report emphasizes the Chief’s

national priorities for protecting watersheds and riparian ecosystems, and restoring rangeland and forest

health. This document is also consistent with the

“Company’s Coming” program that the USFS Southwestern Region implemented in 1997. One of three

major components of Company’s Coming focuses on

sustainability of riparian ecosystems; a second component stresses forest and rangeland health. This report

also emphasizes interagency collaboration in conserving flycatchers and their habitats, a strategy

that dovetails well with the new Southwestern Interagency Initiative referred to as the Southwest Strategy

(http://www.swstrategy.org) called for by the Secretaries of Agriculture and Interior in 1997. This initiative stresses the need for southwestern agencies and

associated partners to work cooperatively together to

develop strategies for managing natural resources.

The Scientific Information Working Group has highlighted the southwestern willow flycatcher as a flagship

species for initiating interagency research and conservation under the auspices of the Southwest Strategy.

In 1998, the U.S. Fish and Wildlife Service (FWS)

initiated the process for developing a comprehensive

recovery plan that involves input by numerous

technical experts and other stakeholders (http://

ifw2es.fws.gov/swwf). The conservation assessment

presented herein was prepared as an interim document to help guide WIFL habitat management on

southwestern national forests and other lands prior to

the release of the recovery plan. Several members of

the original Conservation Assessment Team (i.e.,

Deborah Finch, Rob Marshall, Susam Sferra, Mark

Sogge, Sartor Williams III, and Mary Whitfield) were

selected by FWS to be on the Recovery Team. Members

of the Conservation Team circulated chapters of the

Assessment to the Technical Subgroup of the Recovery

Team. Information compiled and synthesized in this

Assessment report served as a stepping stone and

useful reference for drafting the technical portion of

the recovery plan. Individual chapters of the Assessment report are cited liberally throughout the recovery plan. The management chapter provided in our

Assessment report is viewed as interim guidance and

should be promptly replaced by the Recovery Team’s

stepdown outline once the Recovery Plan is formally

released.

For Internet information about the southwestern

WIFL, the U.S. Geological Survey’s Colorado Plateau

Field Station hosts an excellent web site at http://

www.usgs.nau.edu/swwf. For Internet information

about the USFS Rocky Mountain Research Station’s

program of research on the flycatcher, refer to the

Albuquerque Forestry Sciences Laboratory’s web page:

http://www.fs.fed.us/rm/albuq. For status and updates of the recovery plan, check the FWS Southwest

Region’s web site: http://ifw2es.fws.gov/swwf.

Acknowledgments ________________

I would like to acknowledge John Peterson, Leader

of the Forest Service’s 1997 Seven-Species Team, for

his helpful role in getting the ball rolling and in setting

Seven-Species funds aside for this effort; Janie Agyagos,

Cara Staab, and Cecilia Dargan for their earlier teamwork that helped to inspire this effort; and Jim Lloyd

for his support. Thanks are also extended to Ellen Paul

for arranging manuscript reviews through The Ornithological Council, and to Chuck Hunter and an anonymous reviewer for their reviews of the complete document. Research and Management chapters benefited

from oral discussions with John Gustafson, David Leal,

Susan Sferra, and Mark Sogge during team meetings.

Financial support for this publication was provided by

the U.S. Forest Service’s Southwestern Region and by

the Rocky Mountain Research Station. In addition,

team members contributed travel funds and organizational time in preparing this document.

Literature Cited ___________________

U.S. Fish and Wildlife Service. 1993. Notice of 12-month petition

finding/proposal to list Empidonax traillii extimus as an endangered species, and to designate critical habitat. Federal Register

58: 39495-39522.

U.S. Fish and Wildlife Service. 1995. Endangered and threatened wildlife and plants: Final rule determining endangered

status of the Southwestern Willow Flycatcher. Federal Register: 10694-10715.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 3

Robert M. Marshall

Chapter 2:

Population Status on Breeding Grounds

In his review of the overall population status of the

southwestern willow flycatcher, Unitt (1987) concluded

that “Even if a few hundred pairs persist in New

Mexico, the total population of the subspecies is well

under 1000 pairs; I suspect 500 is more likely.” Since

Unitt’s review, a substantial amount of information

has been collected rangewide on the distribution and

abundance of the southwestern willow flycatcher. The

1992 petition to list the flycatcher under the Endangered Species Act and the subsequent proposal by the

U.S. Fish and Wildlife Service in 1993 to list the

subspecies as endangered with critical habitat encouraged survey efforts rangewide. This chapter synthesizes these new distribution and population data to

provide a current baseline from which to evaluate the

conservation needs of this subspecies.

Unitt (1987) reviewed historical museum specimens

and contemporary survey data for E.t. extimus to

estimate subspecies’ population status and geographic

boundaries. Unitt identified critical gaps in museum

specimen records which confounded the delineation of

distinct sub-specific boundaries in the Central Valley

of California, southern Nevada, south-central Utah,

southwestern Colorado, and northern New Mexico.

These gaps have resulted in confusion over management needs, particularly among state and federal

agencies responsible for the conservation of this subspecies. To reduce confusion, some agencies have drawn

up provisional boundaries which expand the known

distribution of this subspecies, particularly in central

Utah and Colorado. Data are presented in this chapter

that include those areas for which the taxonomic

status of the willow flycatcher remains unclear.

Methods _________________________

To document the current rangewide distribution

and population status of the southwestern willow

flycatcher, data were compiled from a variety of

sources, including federal and state agency reports,

reports prepared by private contractors available to

the public, and from individuals working at specific

study sites. Data were summarized for the years 1993

through 1996, the latter being the most recent year for

which complete rangewide data are available. Data

sources include the following: Arizona (Sogge and

Tibbitts 1992, Sogge et al. 1993, Muiznieks et al. 1994,

Sogge and Tibbitts 1994, Sferra et al. 1995, Sogge

1995a, Sogge et al. 1995, Peterson and Sogge 1996,

Spencer et al. 1996, Sferra et al. 1997, Sogge et al.

1997b, McKernan 1997 in litt.); California (Camp

Pendleton 1994, Whitfield 1994, Griffith and Griffith

1995, Holmgren and Collins 1995, San Diego Natural

History Museum 1995, Whitfield and Strong 1995,

Pike et al. 1995, Griffith and Griffith 1996 in litt.,

Haas 1996 in litt., Kus 1996, Whitfield and Enos

1996); Colorado (T. Ireland 1994 in litt., Stransky

1995); New Mexico (Maynard 1995, Cooper 1996,
4 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

1997, Skaggs 1996); Nevada (C. Tomlinson 1995 in

litt.); Utah (McDonald et al. 1995, Sogge 1995b).

Since 1993 more than 800 historical and new locations have been surveyed rangewide to document the

status of the southwestern willow flycatcher (some

sites in southern California have been surveyed since

the late 1980s). Statewide survey and monitoring

efforts were conducted in Arizona and New Mexico,

and smaller-scale efforts in Colorado, Nevada, and

Utah. A small number of sites in California have

been surveyed and/or intensively monitored by several dedicated individuals, but the state as a whole

lacks a coordinated survey and monitoring program.

Formal survey efforts have not been attempted in

southwestern Texas, largely due to difficulties in gaining access to private lands. Survey efforts in some

states were conducted under the auspices of state

Partners In Flight programs (e.g., Arizona), which

served as the coordinating body for survey training

sessions and data reporting. Tibbitts et al. (1994)

developed the first survey protocol, which served as

the basis for training efforts held annually throughout

the range of E.t. extimus. Sogge et al. (1997a) published an updated version of the survey protocol.

Both Tibbitts et al. (1994) and Sogge et al. (1997a)

rely on the use of tape-playback to elicit responses

from territorial birds. The revised protocol prepared

by Sogge et al. (1997a) requires a minimum of three

visits stratified among three survey periods. In addition to summarizing flycatcher biology and habitat

use, the protocol provides guidance on interpreting

field responses and a set of standardized reporting

forms for data collection.

The basic unit of analysis for this dataset is the

“site.” In most cases, site refers to a manageable

survey unit of one to several hectares that can be

reported on standardized data forms provided in

Tibbitts et al. (1994) and Sogge et al. (1997a). However, in some cases a site encompasses several relatively discrete breeding locations spread throughout a

drainage (e.g., Whitfield’s data for the South Fork

Kern River in California where several breeding locations occur over the entire 5.6 km study area). Data

were compiled for sites that had documented breeding, suspected or probable breeding, or confirmed

territorial birds (Sogge et al. 1997a). Migrants and

detections with inadequate data to confirm resident

status were omitted from analyses.

All analyses represent a composite of data collected

between 1993 and 1996. This was necessary because

many sites were not surveyed annually and excluding

them may bias population estimates. For sites with

multiple years’ data, the most recent years’ data (with

positive results) were used. This “composite” approach,

however, also has potential biases. Data from 1993 are

treated equally with data from 1996, leaving open

the possibility of overestimating or underestimating

population sizes for sites that were not surveyed in

each year. For example, at several sites flycatchers

were absent during one or more years. However, for

this chapter I considered these sites extant and used

the most recent years’ data (with positive survey

results) for estimating population status. While this

approach may overestimate population abundance,

survey coverage was not complete for any given drainage and local movement of breeding groups could have

gone undetected.

Data on population status are summarized as the

number of territories, pairs, and the total number of

individual flycatchers. Estimates of the number of

territories and pairs were taken directly from the data

source, whereas the total number of individuals represents the sum of territories plus half the number of

documented pairs. A territory is defined as an individual singing in the same location during each of the

survey periods outlined in the protocol. It is the basic

unit the protocol is designed to estimate. Data on the

number of pairs probably represent minimum values,

because documenting breeding status (i.e., unmated/

mated and confirmation of breeding) requires considerable effort above what is required to estimate the

number of territories (Sogge et al. 1997a). Site data

were not always reported consistently across years. In

some cases nearby sites were combined or spilt for

survey or reporting purposes. Thus, there may be

some minor discrepancies with the original reports in

the total number of sites per state. In addition, territory estimates were sometimes reported as a range of

values (e.g., six to seven territories). For the purposes

of this summary, the larger value was used to compile

population estimates.

Several additional attributes were determined for

each site, including the site’s land management status/ownership pattern (e.g., federal, state, private,

etc.), the county and drainage in which the site was

located, the elevation at which the site occurred, and

the general habitat type (e.g., mixed native broadleaf,

monotypic saltcedar).

Results and Discussion ____________

Rangewide Distribution and Abundance

Southwestern willow flycatchers are currently found

in six states from near sea level in southern California to

2700 m (9000 ft) in southwestern Colorado (Table 2-1).

Their current status in southwestern Texas, northern

Sonora, Mexico, and Baja California Norte is not

known due to a lack of survey effort in those areas.

Based on survey data collected between 1993 and

1996, the total known population of southwestern

willow flycatchers is estimated to be 549 territories

(Table 2-1). At least 386 (70%) of these territories were

documented as confirmed or probable breeding pairs.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 5

Southwestern willow flycatchers have been documented at a total of 109 sites on 43 drainages throughout the southwestern U.S. (Table 2-1). The bulk of the

population occurs in Arizona, California, and New

Mexico. Combined, the three states hold 92% of all

territories (Table 2-1). However, the majority of sites

are comprised of small numbers of flycatchers; more

than 70% (n=77) of all sites had only three or fewer

flycatcher territories (Table 2-2). As a proportion of the

rangewide population, those 77 sites contained 22%

only of all flycatcher territories. Many of the smallest

sites apparently lacked breeding pairs. In fact, 17%

(19) of the 109 documented sites were comprised of

single, unmated individuals.

Nearly half (47%) of all flycatcher territories were

distributed among 99 sites containing ten or fewer

territories; 14% of all territories were distributed

among six sites containing 11 to 20 territories; 15% of

all territories were distributed among three sites containing 21 to 30 territories; and 24% of all territories

were found at one site which had an estimated 134

flycatcher territories (Table 2-2). Table 2-3 provides

this breakdown by state and illustrates the widelyscattered nature of the larger sites (>10 territories), in

particular.

Combining site data by drainage provides a population summary at a larger scale and eliminates biases

associated with comparing sites of different sizes,

habitat composition, etc.(Table 2-4). These data indicate that of the 43 drainages where southwestern

willow flycatchers have been documented, 72% (31)

had ten or fewer flycatcher territories; nine percent (4)

of the drainages had between 11 and 20 territories;

12% (5) had between 21 and 30 territories; and seven

percent (3) had more than 30 territories. Additional

survey effort, particularly in California, may add to the

overall number of sites and territories per drainage.

However, given that approximately 13% of sites surveyed rangewide have yielded positive results, it is

highly unlikely that the addition of new sites and

territories will substantially change the distribution

and abundance patterns described above.

During the four year study period, 53% of the southwestern willow flycatcher population occurred at just

ten sites rangewide, and the remaining 47% occurred

in small groups of ten or fewer territories spread out

among 99 sites. The estimates of 549 territories and

109 sites rangewide should be viewed cautiously,

Table 2-1. Population status of the southwestern willow flycatcher by state (1993-1996).

Estimated Estimated Estimated

No. of No. of No. of Estimated No. of

State Territories Pairs SWWF No. of Sites Drainages

New Mexico 223(40)a 175 (45) 398 (42) 25 (23) 9

Arizona 163(30) 113 (29) 276 (30) 44 (40) 11

California 121(22) 80 (21) 201 (21) 22 (20) 13

Colorado 28 (5) 9 (2) 37 (4) 10 (9) 8

Utah 12 (2) 8 (2) 20 (2) 7 (6) 6

Nevada 2 (1) 1 (1) 3 (1) 1 (1) 1

Texas — — — — —

Sonora, MX — — — — —

Baja, MX — — — — —

Total 549 386 935 109 43b

a

 Parenthetical data represent within-column percentages.

b

 The disparity between the number of drainages/state and the total reflects that flycatcher sites were found on portions

of the Colorado, Gila, Virgin, and San Francisco rivers in multiple states (see Table 2-4).

Table 2-2. Frequency distribution for the number of southwestern willow flycatcher territories per site (1993-1996).

No. of Frequency

Territories/ (number of Percent Cumulative

Site sites) of Sites Percent

1 42 38.5 38.5

2 23 21.1 59.6

3 12 11.0 70.6

4 7 6.4 77.1

5 4 3.7 80.7

6 4 3.7 84.4

7 2 1.8 86.2

8 2 1.8 88.1

10 3 2.8 90.8

11 2 1.8 92.7

12 1 0.9 93.6

13 1 0.9 94.5

15 1 0.9 95.4

17 1 0.9 96.3

22 1 0.9 97.2

29 1 0.9 98.2

30 1 0.9 99.1

134 1 0.9 100.0

 Total 109 100.0 100.0
6 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

Table 2-3. Distribution of southwestern willow flycatcher sites by size and state (1993-1996).

No. of Sites No. of Sites No. of Sites No. of Sites

w/ ≤10 Terr. w/ 11-20 Terr. w/ 21-30 Terr. w/ ≥ 31 Terr.

Arizona 41 2 1

New Mexico 21 3 1

California 19 1 2

Colorado 10

Utah 7

Nevada 1

Total 99 6 3 1

Table 2-4. Southwestern willow flycatcher population estimates by state and

drainage (1993-1996).

Total No. of Total No. of Total No. of

Drainage Territories Pairs SWWF

Arizona

Colorado 32 21 53

San Pedro 30 28 58

Salt 22 18 40

Gila 19 8 27

Tonto Ck. 18 11 29

Little Colorado 13 10 23

Verde 11 8 19

Bill Williams 6 2 8

Big Sandy 5 2 7

Santa Maria 4 2 6

San Francisco 3 3 3

Subtotal 163 113 273

California

S. Fork Kern 30 30 60

San Luis Rey 29 24 53

Santa Ynez 23 4 27

Santa Margarita 12 4 16

Colorado 7 6 13

Santa Ana 7 4 11

Mohave 4 4 8

Las Flores Ck. 3 3 6

Pilgrim Ck. 2 1 3

De Luz Ck. 1 — 1

Fallbrook Ck. 1 — 1

San Mateo Ck. 1 — 1

San Onofre Ck. 1 — 1

Subtotal 121 80 201

New Mexico

Gila 156 150 306

Rio Grande 49 21 70

Rio Chama 6 1 7

Coyote Ck. 3 0 3

Nutria Ck. 3 0 3

Rio Grande de Ranchos 2 2 4

Zuni 2 — 2

Blue Water Ck. 1 1 2

San Francisco 1 0 1

Subtotal 223 175 398

Colorado

Gunnison 10 2 12

Beaver Ck. 5 — 5

(con.)
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 7

however. During the study period a number of sites

were either extirpated due to high, sustained levels of

nest predation, cowbird parasitism and, possibly,

drought, or suffered substantial habitat losses as a

result of fire, habitat inundation, agricultural clearing, and clearing for roads and bridges (Table 2-5). In

addition, some sites with relatively large populations

suffered habitat losses or were threatened due to

human activities (see Table 2-5; USFWS 1996, 1997a,

1997b). The scope and magnitude of adverse impacts

and threats combined with the predominance of small

breeding groups vulnerable to stochastic processes,

alone, suggests that the population is probably fluctuating spatially and temporally. Without a more

comprehensive, annual monitoring effort, however,

our ability to track and respond in a management

context to these changes is severely compromised.

Conservation Challenge

The overall distribution of flycatchers—many sites

with small populations and a small number sites

with larger populations—presents a difficult conservation challenge at best. The smallest sites (≤10

territories) comprise nearly half of the total population, but are the most vulnerable to demographic and

environmental stochasticity. In some cases, we know

that reproduction has not been sufficient for these

sites to persist on their own, or to yield emigrants for

colonization of other sites (Muiznieks et al. 1994,

Maynard 1995, Sogge et al. 1997b). However, trends

at these sites should not be used as justification for

shifting conservation efforts away from sites with

small numbers of flycatcher territories. For one, if

this subspecies’ population is comprised of a number

of metapopulations, then some of these smaller sites

probably are regional sources of colonizers, at least in

some years. Second, we do not understand the processes or circumstances under which colonization of

new sites occurs. Shifting management attention

away from smaller sites because they are presumed

to have less conservation potential may result in

losses that further isolate remaining breeding groups

and potentially disrupts regional population dynamics. And third, many opportunities have already been

lost at some of the larger sites, which has considerably narrowed management options (see Table 2-5,

USFWS 1996, 1997a, 1997b). In evaluating the current pattern of small breeding group size and the

potential effects of recent or anticipated habitat and

population losses, I speculated (in USFWS 1997b)

that,

“...variation in individual fitness of flycatchers probably translates to variation in responses to habitat

loss/degradation and subsequent survivorship and

reproductive success. Thus, not all flycatchers are

likely to perish as a result of displacement [due to

habitat loss] and not all flycatchers are likely to fail

to attract mates and breed [after dispersal]. The

more likely result would be a regional phenomenon

of “loss-disperse-decrease” whereby: (1) large habitat patches occupied by the larger breeding groups

are lost either by stochastic (e.g., fire) or deterministic processes (e.g., permitted Federal action); (2)

surviving birds are forced to disperse elsewhere,

most likely into smaller habitat patches; and (3) this

dispersal causes decreases in the probabilities of

Table 2-4. (Con.)

Total No. of Total No. of Total No. of

Drainage Territories Pairs SWWF

Colorado

Anthracite Ck. 4 2 6

Plateau Ck. 3 3 3

Colorado 2 1 3

East 2 1 3

San Miguel 1 — 1

West Ck. 1 — 1

Subtotal 28 9 37

Nevada

Virgin 2 1 3

Utah

Panguitch Ck. 3 3 6

San Juan 3 2 5

Virgin 3 1 4

Panguitch Lk 1 1 2

Swamp Ck. 1 1 2

Yellow Ck. 1 0 1

Subtotal 12 8 20

Rangewide Total 549 386 932
8 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

survival, of obtaining mates, and of reproducing

successfully. This hypothesis is based on the assumption that there is a negative relationship between habitat isolation and flycatcher survival and

reproduction. This phenomenon could actually lead

to a short-term increase in the number of sites

occupied regionally while masking an overall, longterm decrease in population size and fecundity.”

The above quote addressed the specific case of the

Lake Mead inflow on the Colorado River where inundation of 445 ha of occupied willow habitat was

Table 2-5. Impacts documented or anticipated at Southwestern willow flycatcher sites.

No. of

Affected Sites Type of Territories

by State Drainage Impact at Site

Arizona

Roosevelt Lakea Salt & Tonto Ck. Anticipated loss of habitat for 40

up to 45 terr. from inundationb

Lake Mead Inflow Colorado 1100 acres of habitat lost and ≥8

nests losses due to treefall

PZ Ranchc San Pedro Habitat loss due to fire 8

Sanchez Rd.d Gila Habitat loss due to bridge 4

construction; high levels

of cowbird parasitism

Tuzigoot Verde Extirpation; high levels of 2

predation & parasitism

Grand Canyon Colorado <3 sites extirpated; high levels 1-2

of predation & parasitism

Middle Gila Gila ~ 6 miles of occupied habitat ?

lost due to fire

California

Lake Isabellaa S. Fork Kern >700 acres of habitat modified ≤14

and nests lost due to inundation

Santa Ynez Santa Ynez Occupied habitat lost due to ?

agricultural clearing

Colorado

Escalante SWR Gunnison Occupied habitat loss due to fire 10

New Mexico

Gila Valleya Gila Largest known site threatened >134

by flood control efforts

Fort West Ditcha Gila Habitat and nest loss due to 15

intentional clearing

San Juan Pueblo Bridgee Rio Grande Habitat loss due to bridge 13

construction & fire

San Marciala Rio Grande Extirpated 1996, possibly due to 11

drought

Bosque del Apache NWRf Rio Grande Habitat loss due to fire ?

a

 Site is one of ten rangewide with more than ten territories.

b

 Up to 45 territories were anticipated to be lost by USFWS, which was slightly higher than the number documented at both

sites. c

 Up to 18 territories were documented prior to fire.

d

 Only 1 territory was present in 1997 after habitat loss.

e

 However, additional survey effort revealed more territories after fire.

f

 Fire occurred in unsurveyed habitat adjacent to occupied flycatcher area.

anticipated to result in habitat loss, nest loss, and

forced dispersal of flycatchers from at least eight

territories documented in 1996 and, potentially, up to

25 territories. In fact, three of seven nest attempts

at Lake Mead were lost due to treefall in 1996

(McKernan 1997). In 1997, when inundation was

more extensive due to rising lake levels, fewer flycatchers returned. Three breeding attempts were documented; one successfully fledged young, one nest was
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 9

lost to treefall, and one nest disappeared, the cause

unknown.

Dispersal due to habitat loss is not unique to Lake

Mead, but has also been documented at Lake Isabella

on the South Fork Kern River in California (Whitfield

and Strong 1995), at Elephant Butte Reservoir on the

Rio Grande in New Mexico (Hubbard 1987), and is

anticipated to occur at the Roosevelt Lake breeding

sites in Arizona (USFWS 1996). These areas represent

some of the largest known riparian habitat patches in

the Southwest. In some cases the habitat modifications (i.e., inundation) occurred during the breeding

season. Thus, flycatchers were, in all likelihood, forced

to disperse to smaller patches potentially incurring

increased risk of predation, increased competition for

suitable habitat elsewhere, and delayed or foregone

breeding opportunities.

One conclusion drawn from this line of thinking is

that conservation efforts should focus aggressively on

(1) protecting extant sites in order to provide breeding

groups the greatest chance of persistence; and (2) on

vastly increasing habitat options at least drainagewide to provide local options in the event breeding

groups are forced to disperse. In conjunction, research

efforts should focus on identifying dispersal patterns

and capabilities to better understand regional population dynamics and to determine habitat-specific demographic patterns so that local and regional conservation efforts are more effective.

Land Management Status

The data in Table 2-4 point to 43 potential conservation opportunities (river drainages) for the southwestern willow flycatcher. Portions of some of the drainages listed in Table 2-4 are already receiving some

degree of management to benefit the southwestern

willow flycatcher. For example, the Kern River Preserve and Kern River Research Center on the South

Fork Kern River have been cowbird trapping since

Table 2-6. Land management status at southwestern willow flycatcher sites.

Land Management Status

Municipal/

State Federal State Private Tribal County Total

Arizona 27 (62)a 2 (5) 14 (31) 0 1(2) 44

California 15 (71) 1 (5) 5 (24) 0 0 21

Colorado 3 (33) 2 (22) 4 (45) 0 0 9

New Mexico 9 (41) 2 (9) 7 (32) 4(18) 0 22

Nevada 0 0 1 (100) 0 0 1

Utah 4 (57) 0 3 (43) 0 0 7

Totalb 58 (56) 7 (7) 34 (32) 4 (4) 1(1) 104

a Parenthetical values provide within-row percentages for each state.

b

 Parenthetical values represent proportion of the five land management categories.

1993, actively managing and restoring habitat throughout the South Fork, and have maintained a comprehensive monitoring and research program since 1989.

The U.S. Forest Service removed livestock grazing on

the San Luis Rey River in California to benefit both the

endangered least Bell’s Vireo (Vireo bellii pusillus)

and southwestern willow flycatcher. Similarly, on the

upper San Pedro River in Arizona the Bureau of Land

Management removed livestock grazing from the entire Riparian National Conservation Area to benefit

the high degree of biological diversity associated with

that river corridor, including the flycatcher. On the

lower San Pedro River, the Bureau of Reclamation,

U.S. Fish and Wildlife Service, U.S.G.S. Biological

Resources Division, Arizona Game and Fish Department, and The Nature Conservancy have initiated a

comprehensive program of habitat protection, research,

and monitoring to protect Arizona’s largest breeding

group of southwestern willow flycatchers. These programs are being carried out on both public and private

lands demonstrating that cooperation among private

and public entities can yield significant conservation

gains. More of these programs will be necessary as the

current distribution of flycatcher sites is spread out

among mostly federal and private lands.

Data on land management status were available

for 104 of the 109 sites studied. Table 2-6 reveals

that 56% of all southwestern willow flycatcher sites

occurred on lands managed by Federal agencies, while

32% were on private lands. While the threats to some

of these locations are already known and are considerable, a coordinated, rangewide evaluation of the level

of protection currently in place as well as the status of

riparian habitats surrounding currently-occupied sites

has not been completed.

The low overall population status of this subspecies

and the potential isolation of breeding groups due to

habitat fragmentation suggests several paths for

management and research. First, extant breeding

sites should be protected to maintain reproductive
10 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

potential and reduce population isolation. Second,

habitat protection and management efforts should

be focused near existing flycatcher breeding sites

and should be incorporated into comprehensive monitoring programs. Third, an evaluation that identifies, in a spatially-explicit manner, gaps in protection

as well as opportunities for protection and management is critically needed. And fourth, data on dispersal events (i.e., causes and spatial patterns) and

subsequent reproductive success are needed to better

understand population dynamics, including regional

habitat needs over time and space. These efforts will

require considerable resources and a commitment

among the various private and public entities interested in promoting conservation and management

for this species. The data summarized in this chapter

provide a rangewide context from which such an

effort could originate.

Acknowledgments ________________

Many individuals contributed in various ways when

I first attempted to assemble a rangewide database on

the southwestern willow flycatcher in 1995. In particular I would like to thank Mark Sogge and staff at

the Colorado Plateau Research Station for helping

compile the database and for helping ensure that I got

to visit the majority of extant sites. Susan Sferra and

the flycatcher staff at the Arizona Game and Fish

Department reviewed earlier versions of the database.

John and Jane Griffith and Bill Haas shared unpublished data from their California study sites. A number of individuals were instrumental in initiating

statewide training programs that generated much of

the data compiled, including, Susan Sferra, Troy

Corman, Tim Tibbitts, and Mark Sogge in Arizona,

David Leal, Janice Richardson, and Sandy Williams in

New Mexico, Terry Ireland in Colorado, Marilet Zablan

and Ken McDonald in Utah, John and Jane Griffith,

Loren Hays, and Mary Whitfield in California. Debbie

Finch, Susan Sferra, Mark Sogge, Scott Stoleson,

Mary Whitfield, Sandy Williams, and two anonymous individuals reviewed an earlier draft of the

manuscript.

Literature Cited ___________________

U.S. Marine Corps Base Camp Pendleton. 1994. Biological Assessment for riparian and estuarine habitat at Marine Corps Base

Camp Pendelton. Camp Pendelton, CA.

Cooper, C.A. 1996. Summary of 1995 survey results for willow

flycatchers in New Mexico. Final Report. Contract # 96-516.51.

27 p.

Cooper, C.A. 1997. Statewide summary of 1996 surveys for willow

flycatchers in New Mexico. Final Report. Contract #96-516.81.

New Mexico Department of Game and Fish, Sante Fe, New

Mexico.

Griffith, J.T. and J.C. Griffith. 1995. The status of the southwestern

willow flycatcher at the Upper San Luis Rey River, San Diego

County, California, in 1994. Final report to the U.S. Forest

Service, Palomar Ranger District, No. 43-91U4-4-0271.

Holmgren, M.A., and P.W. Collins. 1995. Interim report on the

distribution, breeding status, and habitat associations of seven

federal special status bird species and brown-headed cowbirds at

Vandenburg Air Force Base, Santa Barbara County, California.

Museum of Systematics and Ecology, Department of Ecology,

Evolution, and Marine Biology, University of California, Santa

Barbara, California. Environmental Report No. 3.

Hubbard, J.P. 1987. The Status of the Willow Flycatcher in New

Mexico. Endangered Species Program, New Mexico Department

of Game and Fish, Sante Fe, New Mexico. 29 p.

Kus, B.E. 1996. Status of the least Bell’s vireo and southwestern

willow flycatcher at Marine Corps Camp Pendleton, California,

in 1995. Prepared for the Environmental and Natural Resources

Office, Marine Corps, Camp Pendleton. 52 p.

Maynard, W.R. 1995. Summary of 1994 survey efforts in New

Mexico for southwestern willow flycatcher (Empidonax traillii

extimus). Contract # 94-516-69. New Mexico Department of

Game and Fish, Sante Fe, New Mexico. 48 p.

McDonald, K.P., J. Snider, L.C. Peterson, M. St. Germain, and S.

Staats. 1995. Results of 1995 southwestern willow flycatcher

surveys in the Virgin River drainage and southern Utah. Utah

Division of Wildlife Resources. Cedar City, Utah. Publication No.

95-17.

McKernan. R.L. 1997. Status, distribution, and habitat affinities of

the southwestern willow flycatcher along the lower Colorado

River 1996. Prepared for the U.S. Bureau of Reclamation, Boulder City, Nevada.

Muiznieks, B.D., T.E. Corman, S.J. Sferra, M.K. Sogge, and T.J.

Tibbitts. 1994. Arizona Partners In Flight 1993 southwestern

willow flycatcher survey. Technical Report 52. Arizona Game and

Fish Department, Nongame and Endangered Wildlife Program,

Phoenix, Arizona. 25 p.

Peterson, J.R. and M.K. Sogge. 1996. Distribution and breeding

productivity of the southwestern willow flycatcher along the

Colorado River in the Grand Canyon - 1996. Summary Report.

Grand Canyon National Park, Grand Canyon, AZ, and National

Biological Service Colorado Plateau Research Station/Northern

Arizona University. 30 p.

Pike, J.S. S. Morris, V. Smith, and L.R. Hays. 1995. The status and

management of the least Bell’s vireo and southwestern willow

flycatcher within the Prado Basin, California, 1986-1995. Prepared for the U.S. Fish and Wildlife Service, Carlsbad, CA.

San Diego Natural History Museum. 1995. Empidonax traillii

extimus in California. The willow flycatcher workshop. 17 November 1995. 66 p.

Sferra, S.J., R.A. Meyer, and T.E. Corman. 1995. Arizona Partners

In Flight 1994 southwestern willow flycatcher survey. Final

Technical Report 69. Arizona Game and Fish Department, Nongame and Endangered Wildlife Program, Phoenix, Arizona. 46 p.

Sferra, S.J., T.E. Corman, C.E. Paradzick, J.W. Rourke, J.A. Spencer, and M.W. Sumner. 1997. Arizona Partners In Flight southwestern willow flycatcher survey: 1993-1996 summary report.

Arizona Game and Fish Department. Phoenix, Arizona. Nongame and Endangered Wildlife Program Technical Report 113.

97 p.

Skaggs, R.W. 1996. Population size, breeding biology, and habitat of

willow flycatchers in the Cliff-Gila Valley, New Mexico 1995.

Final Report. New Mexico Department of Game and Fish. 38 p.

Sogge, M.K. 1995a. Southwestern willow flycatcher (Empidonax

traillii extimus) monitoring at Tuzigoot National Monument.

1995 progress report to the National Park Service. National

Biological Service Colorado Plateau Research Station/Northern

Arizona University, Flagstaff, Arizona. 20 p.

Sogge, M.K. 1995b. Southwestern willow flycatcher surveys along

the San Juan River, 1994 - 1995. Final report to the Bureau of

Land Management, San Juan Resource Area. National Biological

Service Colorado Plateau Research Station/Northern Arizona

University, Flagstaff, Arizona. 27 p.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 11

Sogge, M.K. 1995c. Southwestern willow flycatchers in the Grand

Canyon. Pages 89-91. In: E.T. LaRoe, G.S. Farris, C.E. Puckett,

P.D. Doran, and M.J. Mac eds. Our living resources: a report to

the nation on the distribution, abundance, and health of U.S.

plants, animals, and ecosystems. Department of the Interior,

National Biological Service, Washington, DC.

Sogge, M.K., R.M. Marshall, S.J. Sferra and T.J. Tibbitts. 1997a. A

southwestern willow flycatcher natural history summary and survey protocol. National Park Service/Northern Arizona University

Colorado Plateau Research Station Technical Report 97/12. 39 p.

Sogge, M.K., T.J. Tibbitts, and J.R. Petterson. 1997b. Status and

breeding ecology of the southwestern willow flycatcher in the

Grand Canyon. Western Birds 28:142-157.

Sogge, M.K. and T.J. Tibbitts. 1992. Southwestern willow flycatcher

(Empidonax traillii extimus) surveys along the Colorado River in

Grand Canyon National Park and Glen Canyon National Recreation Area. National Park Service Cooperative Park Studies

Unit/Northern Arizona University, Flagstaff, Arizona. 43 p.

Sogge, M.K. and T.J. Tibbitts. 1994. Distribution and status of the

southwestern willow flycatcher along the Colorado river in the

Grand Canyon - 1994. Summary Report. National Biological

Survey Colorado Plateau Research Station/Northern Arizona

University, Flagstaff, Arizona. 37 p.

Sogge, M.K., T.J. Tibbitts, and S.J. Sferra. 1993. Status of the

southwestern willow flycatcher along the Colorado River between Glen Canyon Dam and Lake Mead - 1993. Summary

Report. National Park Service Cooperative Park Studies Unit/

Northern Arizona University, U.S. Fish and Wildlife Service, and

Arizona Game and Fish Department., Flagstaff, Arizona. 69 p.

Sogge, M.K., T.J. Tibbitts, C. van Riper III, and T.J. May. 1995.

Status of the southwestern willow flycatcher along the Colorado

River in Grand Canyon National Park - 1995. Summary Report.

National Biological Service Colorado Plateau Research Station/

Northern Arizona University. 26 p.

Spencer, J.A., S.J. Sferra, T.E. Corman, J.W. Rourke, and M.W.

Sumner. 1996. Arizona Partners In Flight 1995 southwestern

willow flycatcher survey. Technical Report 97, March 1996.

Arizona Game and Fish Department, Phoenix, Arizona. 69 p.

Stranksy, K. 1995. 1995 field survey by the Colorado Division

of Wildlife: southwestern willow flycatcher. Colorado Division of Wildlife, Denver, Colorado.

Tibbitts, T.J., M.K. Sogge, and S.J. Sferra. 1994. A survey protocol

for the southwestern willow flycatcher (Empidonax traillii

extimus). Technical Report NPS/NAUCPRS/NRTR-94/04. National Park Service Colorado Plateau Research Station, Flagstaff, Arizona. 24 p.

Unitt, P. 1987. Empidonax traillii extimus: An endangered subspecies. Western Birds 18:137-162.

U.S. Fish and Wildlife Service. 1996. Biological opinion on the

operation of the modified Roosevelt Dam in Gila and Maricopa

Counties, Arizona. Arizona Ecological Services Office. Phoenix,

Arizona.

U.S. Fish and Wildlife Service. 1997a. Biological opinion on the

Army Corps of Engineers long-term operation of Isabella

Reservoir. California Ecological Services Office, Sacramento,

California.

U.S. Fish and Wildlife Service. 1997b. Biological opinion on lower

Colorado River operations and maintenance - Lake Mead to the

Southerly International Border. U.S. Fish and Wildlife Service

Regional Office, Albuquerque, New Mexico.

Whitfield, M.J. 1994. A brown-headed cowbird control program and

monitoring for the southwestern willow flycatcher, South Fork

Kern River, California, 1994. Prepared for the California Department of Fish and Game. Kern River Research Center, Weldon,

California. 12 p.

Whitfield M.J. and C.M. Strong. 1995. A brown-headed cowbird

control program and monitoring for the southwestern willow

flycatcher, South Fork Kern River, California. California Department of Fish and Game, Bird and Mammal Conservation Program Report 95-4, Sacramento, California. 17 p.

Whitfield, M.J. and K.M. Enos. 1996. A brown-headed cowbird control

program and monitoring for the southwestern willow flycatcher,

South Fork Kern River, California, 1996. Final report to the U.S.

Army Corps of Engineers, Contract DACW05-96-P-0900.
12 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 13

Robert M. Marshall

Scott H. Stoleson

Chapter 3:

Threats

The continued survival of the southwestern willow

flycatcher (Empidonax traillii extimus) is threatened

indirectly by the loss, modification, and fragmentation

of riparian habitat, and directly by factors that impact

the survival and reproductive success of flycatchers.

Because the impact of habitat loss on small populations can be particularly severe, we first discuss some

of the population-level effects that may be influencing

flycatcher population dynamics. We then review some

contemporary cases of habitat loss and discuss other

factors potentially impacting the flycatcher. The effects of brown-headed cowbird (Molothrus ater) parasitism on the southwestern willow flycatcher are discussed in detail in Chapter 8. For additional information

on site-specific threats to the southwestern willow

flycatcher readers should consult Greenwald (1998).

Effects of Habitat Loss and

Fragmentation ____________________

Habitat loss and habitat fragmentation are interrelated processes that affect patterns of species’ abundance and distribution at local and regional scales

(Pulliam and Dunning 1994). Habitat loss is the reduction of the total amount of a particular habitat type in

a landscape. Fragmentation is the apportionment of

the remaining habitat into smaller, more isolated

patches (Wilcove et al. 1986, Saunders et al. 1991).

Habitat loss is often manifested as the conversion of

one habitat type to another (e.g., conversion of a

forested floodplain to agricultural fields). By reducing

the amount of space that can be occupied, habitat loss

reduces the total number of individuals that can

occur at a particular location or throughout a region.

Riparian habitats in the Southwest are naturally

rare and patchy, subject to periodic disturbance and

occurring as widely-separated ribbons of woodland

and forest within a primarily arid landscape. In

Arizona, for example, riparian habitat comprises less

than 0.5% of the landscape (Strong and Bock 1990).

The actual extent of habitat suitable for the southwestern willow flycatcher is much less. Wide-ranging

or highly mobile species that rely on naturally patchy

and ever-changing habitats, such as the flycatcher,

persist at regional scales as metapopulations, or local

breeding groups that are linked together and maintained over time by immigration/emigration and dispersal (Hanski and Gilpin 1991, Pulliam and Dunning 1994). Persistence of local breeding groups is a

function of the group’s size (numbers of individuals),

productivity, survivorship, and the ability of individuals to disperse from one breeding location to

another (Harrison 1991). By isolating habitat patches,

fragmentation reduces the chance of an individual

successfully finding suitable habitat. Searching for

increasingly isolated patches leaves individuals vulnerable to mortality from competition, starvation, or
14 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

predation and can result in delayed or lost of breeding

opportunities. Weins (1996) noted that habitat loss is

probably the most important factor governing population dynamics when the landscape still contains a

high proportion of suitable habitat, but “at a certain

threshold of habitat loss, patch isolation may quickly

come to dominate population dynamics.”

Effects of Small Population Size _____

Demographic Effects

The overall southwestern willow flycatcher population is small with an estimated 549 territories

rangewide (see Chapter 3). Moreover, these territories

are distributed among a number of very small breeding groups and only a handful of relatively large

breeding groups. The small size of flycatcher populations leaves them vulnerable to local extirpation

through environmental stochasticity (e.g., floods, fire,

severe weather events, disease), and demographic

stochasticity (e.g., shifts in birth/death rates and sex

ratios). Even moderate variation in stochastic factors

that might be sustained by larger populations can

reduce a small population below a threshold level from

which it cannot recover. This is especially true with

short-lived species such as the southwestern willow

flycatcher (see Chapter 7).

The persistence of small populations frequently depends on immigration from nearby populations, at

least in some years (Stacey and Taper 1992). The

small, isolated nature of current southwestern willow

flycatcher populations exacerbates the risk of local

extirpation by reducing the likelihood of successful

immigration among populations. McCarthey et al.

(1998) presented data for 36 sites in Arizona where

two or more years worth of survey work had been

completed between 1993 and 1997. They documented

extirpation at ten sites for a loss of 13 territories, and

population declines at an additional 15 sites for a loss

of 56 territories. Of the 25 sites that were extirpated or

that experienced declines, all but four were small sites

comprised of ten or fewer territories. Five of the 36

sites had no change in the number of territories and six

sites saw increases in the number of territories for a

total gain of 38 territories. Overall, at the 36 sites

monitored there was a net loss of 18 territories.

Genetic Effects

Small populations tend to be characterized by low

levels of within-population genetic variation, and

possibly inbreeding. These conditions may lead to

reduced survival, reduced fecundity, lowered resistance to parasites and disease, or physiological

abnormalities (Allendorf and Leary 1986, Hartl 1988).

Low effective population size also threatens small

populations. Effective population size is an index of

the actual number of individuals breeding and the

number of offspring they contribute to the next generation. The effective population size for a species

may be much smaller than the censused population

size because of uneven sex ratios, uneven breeding

success among females, polygyny (e.g., Sedgwick and

Knopf 1989), and low population numbers which

exacerbate the above factors.

Synthesizing recent empirical and theoretical studies on population genetics, Lande (1995) suggested

that the number 500, long held by some in the conservation biology community to represent the minimum

effective population size necessary to maintain a

viable population of any species, is far too small.

Lande contended that effective population sizes should

be much larger (in the range of 5000) in order for a

species to maintain normal levels of potentially adaptive genetic variance to counteract the effects of

random genetic drift. Lande concluded that, because

recovery goals for listed species are often not much

higher than the actual population size at the time of

listing, maintenance of adequate evolutionary potential and long-term genetic viability was doubtful

unless populations were recovered to much larger

sizes. Based on Lande’s hypothesis, and considering

the current status of the flycatcher rangewide, the

effective population size for the southwestern willow

flycatcher may be critically low.

Factors Contributing to Habitat

Loss _____________________________

Water Management

Dams and Reservoirs—Most of the major and

many of the smaller Southwestern rivers support one

or more dams that have severely altered the distribution, extent, and species composition of riparian habitats (e.g., Colorado River, Gila River, Kern River,

Mojave River, Rio Grande, Salt river, San Diego river,

Santa Ana River, Sweetwater River, Tijuana River,

Verde River). For example, Mearns (1907; in Ohmart

et al. 1988) estimated that the lower Colorado River

contained more than 161,900 ha of native riparian

habitat at the onset of the 20th Century (prior to the

construction of any dams). Nearly 100 years later and

with the addition of eight dams and diversions along

the 660 km of river between Pearce Ferry and the

border with Mexico, the U.S. Bureau of Reclamation

(1996) estimated the current extent of native riparian

habitat at approximately 1,800 ha, or one percent of its

former estimated total.

Riparian habitats are modified, reduced, or lost

downstream of dams as a result of changes in flood

frequency and duration. Hydrological cycles below
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 15

dams are modified such that minimum flow events

occur with greater frequency and longer duration

reducing instream flows and lowering watertables. In

some cases, sustained high flows have caused loss of

riparian stands from prolonged inundation. For example, Hunter et al. (1987a) documented the loss of a

120 ha stand of cottonwood-willow at the confluence of

the Bill Williams River and the Colorado River in 1981

after 24 months of continual high flows released from

Alamo Lake. Dams also inhibit annual cycles of floodinduced sediment deposition, floodplain hydration and

flushing, and seed dispersal necessary for the establishment and maintenance of riparian habitats

Despite these modifications, some southwestern

drainages still have the capacity to develop substantial stands of native cottonwood-willow. Several thousand acres of cottonwood and willow developed along

the lower Colorado River below Yuma, AZ after the

floods of 1993 (U.S. Bureau of Reclamation 1996).

However, due to the diversion of Colorado River water

upstream at Moreles Dam, those stands declined from

desiccation. Restoration of flows to the lower portion of

the Colorado River could substantially increase the

extent of riparian habitat on that system simply by

maintaining vegetation that becomes established after natural flood events.

The filling of reservoirs results in the loss of riparian

habitats upstream of dams. For example, the flooding

of Glen Canyon resulted in the loss of southwestern

willow flycatchers, which Behle and Higgins (1959)

considered a common species. Over time, however,

some reservoir inflows have developed extensive deltas colonized by some of the largest stands of riparian

trees and shrubs currently found in the Southwest,

such as at the head of Elephant Butte Reservoir in

New Mexico; at the Salt River and Tonto Creek inflows

to Roosevelt Lake in Arizona; the Gila River inflow to

San Carlos Reservoir in Arizona; the Colorado River

inflow to Lake Mead in Arizona; and the inflow of the

South Fork Kern River at Lake Isabella in California.

In addition, these areas (except San Carlos Reservoir)

support or have supported some of the largest southwestern willow flycatcher populations rangewide

(Hubbard 1987, Whitfield and Strong 1995, Sferra et

al. 1997, McKernan 1997).

However, current water management policies do not

support management strategies to protect and maintain these significant riparian stands. As a result,

occupied southwestern willow flycatcher habitat has

been lost and flycatchers have suffered nest losses or

been displaced. For example, inundation at the inflow

to Elephant Butte Reservoir in New Mexico during

the 1980s resulted in the loss of willow habitat and

displacement of at least 10 flycatcher pairs (Hubbard

1987, T. Schrader U.S. Bureau of Reclamation, pers.

comm.). Approximately 283 ha of willow habitat were

modified (i.e., loss of understory vegetation) due to

inundation at the South Fork Wildlife Area at Lake

Isabella in 1995. That event resulted in loss of flycatcher nests and subsequent decline in the number

breeding flycatchers in the South Fork Wildlife Area

(Whitfield and Strong 1995, USFWS 1997a). Approximately 445 ha of occupied Goodding willow (Salix

gooddingii) habitat at the inflow to Lake Mead were

anticipated to be lost during the 1997 and 1998 growing seasons due to prolonged inundation (USBR 1996,

USFWS 1997b). The number of flycatcher territories

and nesting attempts at the inflow decreased in 1997

with increasing levels of inundation (see Chapter 3) .

And finally, the habitat at Roosevelt Lake, which

supports one of Arizona’s largest flycatcher populations, is anticipated to be lost when inflows are sufficient to fill the newly-created reservoir conservation

space (USFWS 1996). The deltas associated with these

and other reservoirs represent some of the most significant management opportunities available to restore a portion of the extensive riparian habitats

historically found on these drainages. They also represent significant opportunities to conserve the southwestern willow flycatcher and the suite of ripariandependent species found in Southwestern riparian

systems.

Diversions and Groundwater Pumping—Surface water diversions and groundwater pumping for

agriculture, industrial use (e.g., mining), and municipal use are considered major factors in the deterioration of riparian habitats (USFWS 1993, Briggs 1996).

Surface diversions and overdraft of groundwater lower

watertables and reduce surface flows. The Arizona

Game and Fish Department estimated that in Arizona, alone, more than 1448 km miles of formerly

perennial stream are no longer perennial. One of the

most extensive stands of native riparian habitat in

Arizona along the upper San Pedro River is threatened by increased groundwater withdrawal by the

nearby city of Sierra Vista (Davis 1995 [in Briggs

1996]). This threat is particularly ironic in light of the

fact that ten years of livestock removal from the San

Pedro Riparian National Conservation Area has resulted in a dramatic comeback of cottonwood-willow

habitat as well as the return of breeding southwestern

willow flycatchers (Krueper 1993, McCarthey et al.

1998). Similarly, human population growth in the

Verde watershed has raised concerns that central

Arizona’s most important aquatic and riparian corridor, the Verde River, will not support riverine, riparian, and aquatic resources over the long term (Verde

Watershed Association 1998).

The combination of severe drought and upstream

diversion for agricultural use was thought to be the

cause of southwestern willow flycatcher territory

loss or abandonment of at least eight territories along
16 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

the middle Rio Grande in the vicinity of San Marcial,

New Mexico (D. Leal, USFWS, pers. comm., Cooper

1997).

Land-Use Practices

Channelization and Bank Stabilization—Flood

control projects generally shorten, straighten, and

narrow river channels with the aim of producing

unobstructed pathways to convey floodwaters. These

projects can severely reduce the extent of alluvialinfluenced floodplain by cutting off main channels

from side channels and adjacent floodplains and by

reducing meander patterns, which slow stream velocity and dampen the effects of flooding (Poff et al. 1997).

Channelization alters stream banks, typically elevating them well above groundwater levels and thus

preventing the roots of most native riparian shrubs

and trees from accessing groundwater. Overbank flooding necessary to deposit sediments, disperse seeds,

rehydrate floodplain soils, and flush accumulations of

salts, is reduced or precluded. Channel cutting further

reduces water tables adjacent to the river, precluding

seedling establishment because of the increased depth

to groundwater (Szaro 1989). Channelization can increase the intensity of extreme floods, because reductions in upstream storage capacity produce accelerated water flow downstream. Channelization also

reduces the width of wooded riparian habitats, increasing the proportion of edge. Avian species richness

has been shown to increase with the width of wooded

riparian habitats (Stauffer and Best 1980).

Bank stabilization is typically used to protect property and structures from the impacts of flooding.

Various manmade structures are used to protect banks

and reduce the likelihood and impact of floods. Bank

armor, such as rip-rap and levees, can protect stretches

of bank and adjacent riparian vegetation, but can also

lead to eddying and increased scouring of unprotected

banks (DeBano and Heede 1987). In addition, bank

armor reduces over-bank flooding, and consequently

the occurrence of germination and regeneration of

riparian vegetation. Under some conditions, certain

types of flood-control structures can protect or enhance riparian habitat. For example, streamflow separations are used to create low energy flows at the bank.

In so doing, separators can increase sediment deposition and create extensive stillwater areas adjacent to

banks (DeBano and Heede 1987).

The riparian habitat that contains the largest known

population of southwestern willow flycatchers along

the Gila River in southwestern New Mexico is threatened by a combination of bank stabilization structures

and agricultural practices within the floodplain

(Phelps-Dodge Corporation 1995). Much of the floodplain is devoted to agricultural and ranching uses.

Levees are used extensively along the border of agricultural fields to protect from flood damage. Riprapping, earthen dikes, and other structures are used

along channel banks to further minimize flood damage. In some cases, the structures protect occupied

flycatcher habitat. However, the combination of flood

control structures in the channel, appropriation of the

floodplain for agricultural or other uses, and the use of

levees to further protect the land-uses occurring within

the floodplain, has resulted in a system that isolates

most of the floodplain, including existing flycatcher

habitat, from natural flood processes needed to sustain and regenerate extensive new habitats.

Given that 25% of all known southwestern willow

flycatchers breed at this site, the ramifications of

potential habitat loss are substantial. Beyond these

ramifications, however, this scenario points to a problem observed throughout the range of the southwestern willow flycatcher—development within floodplains.

Be it homes, other types of structures, agricultural

lands, or roads and bridges, development within floodplains increases the economic justification for flood

control projects, which generally decreases opportunities for maintenance and restoration of floodplain

processes necessary for the continual regeneration of

riparian habitats (Poff et al. 1997).

Agricultural Development—The availability of

irrigation water, relatively flat land and rich soils has

spawned wide-scale agricultural development in river

valleys throughout the Southwest. For example, more

than 75% of the Mohave, Parker, Palo Verde, and

Yuma valleys on the lower Colorado River has been

converted to agriculture (Ohmart et al. 1986). These

areas formerly contained vast riparian forests captured in early photographs of the area and probably

comprised the most important riparian corridor in the

Southwest. Collections of southwestern willow flycatcher nests made in the vicinity of Yuma in 1902

indicate that the flycatcher was at least locally very

abundant along the lower Colorado River (Huels in

litt. USFWS 1997b). The clearing of floodplain riparian habitat for agriculture continues today. For example, in January 1996, up to 2 km of occupied

flycatcher habitat was lost to agricultural expansion

on the Santa Ynez River in California (USFWS in litt.).

Livestock Grazing—Overgrazing by livestock has

been a major factor in the modification and destruction of riparian habitats in the arid western U.S.

(Fleischner 1996, Ohmart 1996, Dobkin et al. 1988).

Riparian areas are often disproportionately preferred

by cattle over surrounding uplands because of access

to water, abundant and palatable forage, a cooler and

shadier microclimate, and moderate slopes allowing

easy access (Ames 1977, Glinski 1977, Szaro 1989;

Fleischner 1996, Ohmart 1996). On uplands livestock
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 17

act as geomorphic agents. By reducing vegetation

cover and compacting soil, heavy livestock grazing

reduces infiltration and increases runoff, erosion, and

sediment yield, which can destabilize stream channels

and affect the extent and distribution of riparian

habitats (Trimble and Mendel 1995).

Grazing affects riparian vegetation through removal

and trampling (Kauffman and Krueger 1984, Marlow

and Pogacnik 1985). Removal by browsing affects the

structure, spacing, and density of vegetation (Rea

1983, Cannon and Knopf 1984, Kauffman and Krueger

1984, Sedgwick and Knopf 1991). In several studies,

willow canopy coverage was eight to ten times greater

in areas excluded from grazing than in grazed areas

(e.g., Taylor 1986, Schulz and Leininger 1990).

Grazing can also alter the age structure and species

composition of riparian areas. Cattle readily eat shoots

of cottonwood and willow, and heavy grazing can

completely eliminate regeneration of these species

(Glinski 1977, Rickard and Cushing 1982, Boles and

Dick-Peddie 1983, Kauffman et al. 1983, Ohmart

1996). In contrast, cattle tend to avoid less palatable

species such as saltcedar and juniper. Prolonged grazing in a riparian area can act as a selective agent

shifting the relative abundance of plant species over

time (Szaro and Pase 1983, Kerpez and Smith 1987).

Dobkin et al. (1998) found that livestock grazing in

riparian meadows resulted in a loss of perennial flow

and a conversion of obligate wetland plant species and

riparian bird species to upland species. When livestock were removed, perennial flow returned, as did

obligate wetland plant species and an avian community comprised of wetland rather than upland species.

Trampling by livestock contributes to soil compaction, streambank erosion, widening and deepening of

channels, increased runoff, and physical destruction

of vegetation (Kauffman and Krueger 1984, Marlow

and Pogacnik 1985, Szaro 1989, Trimble and Mendel

1995). In turn, unstable stream banks lead to accelerated erosion and increased sediment loads, which can

destabilize floodplains and threaten the persistence of

riparian habitats.

The impacts of grazing on riparian vegetation vary

with the intensity and season of grazing. Late autumn

and winter grazing may have relatively little effect, at

least compared with other disturbances such as flooding (Kauffman and Krueger 1984, Knopf et al. 1988,

Sedgwick and Knopf 1991). However, late spring and

summer grazing typically has severe impacts, and

results in little or no recruitment of riparian vegetation. This produces even-aged, non-reproducing communities of mature cottonwoods and decadent willows, with little understory. Such decadent, park-like

stands, which are common throughout grazed drainages in the Southwest, are not suitable for southwestern willow flycatchers (Kauffman and Krueger 1984,

Knopf et al. 1988, see Chapter 9).

In several studies, Willow Flycatcher numbers increased following the reduction or elimination of

cattle grazing in riparian areas (Taylor 1986, Taylor

and Littlefield 1986, Knopf et al. 1988). Harris et al.

(1987) reported a 61% increase in flycatcher numbers

over five years after grazing was reduced. Recent

removal of livestock from the Riparian National Conservation Area on the upper San Pedro River in

Cochise County, Arizona has resulted in both a dramatic increase in the recruitment of riparian vegetation and in the abundance of avian species reliant on

dense understories, including the southwestern willow flycatcher, which was recently confirmed as a

breeding species on the upper San Pedro (Kreuper

1993, McCarthey et al. 1998).

Low-intensity grazing during the non-growing season may be compatible in certain floodplain systems

(i.e., those in proper functioning condition [USBLM

1993] and containing the full complement of riparian

plant species and successional habitat types). For

example, the Kern River Preserve in Kern County,

California permits occasional, short duration and

highly supervised livestock grazing in a small portion

of the Preserve where meadows interface with riparian forest (R. Tollefson, pers. comm.). Livestock use of

the Preserve, however, is not part of any annual

grazing scheme. Furthermore, use is based on current ecological conditions, permitted at the discretion of and with the supervision of the Preserve

Manager, and only permitted during the non-growing season. In the Gila Valley in southwestern New

Mexico, livestock grazing occurs in irrigated pastures adjacent to the riparian stringers occupied by

the largest known concentration of southwestern

willow flycatchers (Parker and Hull 1994). In that

case livestock forage is provided in the adjacent

irrigated pasture and livestock use the riparian habitat primarily for shade. Neither of these cases represent a typical grazing situation for the Southwest,

however. In the context of riparian management for

the southwestern willow flycatcher, the appropriateness of a particular livestock grazing regime (in the

uplands or riparian areas) should be evaluated based

on current ecological conditions, the ecological potential for an area to support flycatcher habitat in the

absence livestock grazing, and on the potential for

livestock to serve as a magnet for cowbirds.

Although not yet documented for the southwestern willow flycatcher, livestock have been documented destroying - through trampling - willow

flycatcher nests placed low in vegetation (Valentine

et al. 1988). This should be considered a threat at

any site within the southwestern willow flycatcher’s

range where flycatcher nest placement averages 3 m

or less and where livestock are present during the

breeding season.
18 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

Wild ungulates can also adversely impact riparian

habitats, particularly when population densities are

high. Elk (Cervus canadensis) have been shown to

preclude the recovery of willow habitats even after the

cessation of livestock grazing (Case and Kauffman

1997). Where elk and livestock are sympatric, reversing impacts to riparian areas may require more intensive management of both species. Elk occur in areas

currently inhabited by southwestern willow flycatchers, including the higher elevation flycatcher sites in

Arizona, New Mexico, Colorado, and Utah. The extent

to which elk are adversely affecting areas inhabited by

southwestern willow flycatchers is thought to be substantial in certain areas, however, quantitative studies that characterize the nature of impacts (e.g., extent, season, numbers of elk) are lacking.

Phreatophyte Control—In some areas riparian

vegetation is still removed from waterways (streams

and irrigation ditches) by mowing, cutting, rootplowing

or spraying of herbicides. The intent of these practices

is to increase watershed yield, remove impediments to

stream flow, and limit water loss through evapotranspiration (Horton and Campbell 1974). As a consequence, riparian habitat is eliminated entirely or is

maintained as a mosaic of very early successional

patches not suitable for breeding flycatchers. Willow

flycatcher populations (E. t. adastus) at the Malheur

National Wildlife Refuge increased following the elimination of willow cutting and spraying (Taylor and

Littlefield 1986).

Recreation—In the Southwest, campgrounds and

recreational activities are concentrated in riparian

areas because of accessibility, the presence of water,

fishing opportunities, shade, and aesthetic qualities.

These recreational activities include off-road vehicle

use, boating, fishing, hunting, camping, birdwatching,

hiking, swimming, floating, picnicking, and river rafting. The magnitude of such activities can be considerable. For example, Johnson and Carothers (1982)

reported that the Glen Canyon and Lake Mead National Recreation Areas in Arizona received eight to

nine million visitors per year. Recreation can impact

riparian vegetation through damage or destruction of

plants, elimination of seedlings, promoting invasion

by exotic species, increased incidence of fires, indirect

effects from soil compaction, and bank erosion (Johnson

and Carothers 1982).

Disturbance from human recreation can reduce

both the density and diversity of avian communities

(Aitchison 1977, Szaro 1980, Taylor 1986, Riffell et

al. 1996). In riparian areas in Utah, the presence of

willow flycatchers was negatively correlated with

campgrounds (Blakesley and Reese 1988). Food scraps

and garbage in areas of high recreational use attract

larger birds (e.g., jays, ravens) and small mammals

(skunks, squirrels) which prey on bird nests and

recently-fledged young (Johnson and Carothers 1982,

Blakesley and Reese 1988). However, Haas (pers.

comm.) reported a pair of southwestern willow flycatchers successfully fledging young from a nest that

was several meters from a picnic table used frequently on weekends.

Urban Development—Urban development can

result in a multitude of impacts to riparian habitats,

such as the placement of homes and buildings within

floodplains; the development of reservoirs and flood

control structures within natural channels; overdraft

of groundwater supplies and dewatering of streams

and rivers; degradation of plant communities from

heavy recreational use; increases in native and exotic

predators; and improper placement of bridges. Some

of these threats are discussed elsewhere in this chapter. Bowler (1990) documented the loss of riparian

habitats in southern California that resulted from

urban growth. One area of particular importance to

the southwestern willow flycatcher and riparian habitats is the impacts of roads and bridges.

Southwestern willow flycatchers have been directly

affected by roads and bridges that bisect riparian

habitat. For example, an Empidonax flycatcher (probably a willow flycatcher) was killed by an automobile

on a rural road that bisects willow flycatcher habitat

in the White Mountains of Arizona (Sferra et al. 1995).

In the San Juan Pueblo of New Mexico, placement of

a new bridge across the Rio Grande resulted in the

direct loss of habitat that contained two flycatcher

territories (USFWS 1996). In Arizona, construction of

a new bridge across the Gila River resulted in the loss

of approximately one-third of a 1.5 ha riparian patch

that supported four flycatcher territories (USFWS

1996). The number of territories decreased to one

following habitat loss at that Graham County site

(McCarthey 1998).

Placement of roads and bridges may have long-term

effects of reducing overall habitat suitability for the

willow flycatcher. Foppen and Reijnen (1994) and

Reijnen and Foppen (1994) documented reduced breeding success, lower breeding densities, and higher dispersal rates of willow warblers (Phylloscopus trochilus)

breeding next to roads that bisect forested habitat.

Sogge (1995a) noted that the population decline and

changes in the distribution of willow flycatcher territories on the Verde River in Arizona were consistent

with other studies documenting adverse effects of

roads that bisect habitat. However, Sogge (1995a)

noted that the small size of that population coupled

with sustained, high levels of predation and cowbird

parasitism, may also have been factors at that site.

While the small size of sites and small number of

territories involved in the above instances may not

seem to justify conservation attention at first glance,

it is important to keep in mind that these small
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 19

instances of riparian habitat loss are numerous, frequent, and widespread (USFWS 1996, 1997b). At the

minimum, these losses increase habitat fragmentation and reduce the carrying capacity of an area.

Taken across the range of this species, the cumulative

effects of these and other adverse impacts addressed

in this chapter may result in destabilization of regional population dynamics.

Other Factors Contributing to Habitat Loss

Fire—Fire is a critical threat to occupied and unoccupied flycatcher habitat. In June of 1995, a fire on the

Gila River in Pinal, County, Arizona, burned approximately six miles of riparian habitat potentially occupied by southwestern willow flycatchers (USFWS in

litt., USFWS 1997b). In 1996, five flycatcher breeding

sites were degraded or lost altogether to fire, including

two sites on the Rio Grande in New Mexico, one of the

largest flycatcher sites on the San Pedro River in

Arizona (Paxton et al. 1996), and two additional areas

on the Gila River in Arizona where approximately

eight miles of riparian habitat burned. In 1997 a fire

started by an adjacent landowner burned a 32-ha

portion of the Escalante Wildlife Area near Delta,

Colorado (Owen and Sogge 1997). That location comprised one of the largest known breeding sites for

willow flycatchers in Colorado with approximately

seven pairs occupying the site in 1996.

Although fires are known to have occurred in riparian habitats historically, riparian habitats are not

fire-adapted nor are they fire-generated communities. Thus, fires in riparian habitat are typically

catastrophic. Busch (1995) documented that the current frequency and intensity of fires in riparian

habitats is greater than what occurred historically

because: (1) a greater accumulation of fuels due to a

reduced frequency of scouring floods; and (2) the

expansion and dominance in many areas of saltcedar

(Tamarix chinensis), which is highly flammable. The

increased incidence of fire is causing profound alterations in riparian habitats throughout the Southwest. Both saltcedar and arrowweed (Tessaria sericea)

recover more rapidly from fire and are more tolerant

of fire-induced increases in salinity and decreases in

soil moisture than are cottonwood and willow (Busch

and Smith 1993, Busch 1995). Consequently, saltcedar

and arrowweed are becoming increasingly dominant

in low elevation riparian habitats, and cottonwood

and willow less so. On the lower Colorado River

alone, Busch and Smith (1993) and Busch (1995)

documented 166 individual fires that burned more

than 11,800 ha between 1981 and 1990. Given the

rate and extent of loss documented by Busch and

Smith, and that the remaining cottonwood-willow

habitat on the lower Colorado River is virtually

surrounded by saltcedar, the potential for fire to

result in further losses of the remaining cottonwoodwillow habitat is substantial.

Exotic Species—The exotic tamarisk, or saltcedar,

was introduced from Asia as an ornamental and erosion-control agent in the 1800s. It began spreading

rapidly throughout the Southwest during the early

part of the 20th Century (Tellman 1998). Today it has

become dominant along many watercourses replacing

multi-layered, multi-species native communities with

monotypic stands uniform in structure. Hunter et al.

(1987b) estimated that saltcedar dominated 49% of

the area encompassed by riparian habitats in the

Southwest, and occurred as a minor component in

considerably more.

With its deep root system and extended production

of seed from March through October, saltcedar thrives

or persists where surface flow has been reduced or lost

(Warren and Turner 1975, Horton 1977, Minckley and

Brown 1982). The development of reservoirs and the

concomitant change in flood regimes essential to the

establishment of native riparian communities has

enabled saltcedar to replace native broadleaf species.

Furthermore, saltcedar establishment often results in

a self-perpetuating regime of periodic fires. Fires were

uncommon in native riparian communities prior to

invasion by saltcedar, due to high moisture content in

fuels and rapid removal of litter through decomposition and floods (Bradley et al. 1992). Consequently,

native species are fire-intolerant. In contrast, saltcedar

regenerates rapidly after fire (Busch and Smith 1993,

Busch 1995). Areas with saltcedar that are not flooded

regularly build up accumulations of salts in the soil,

rendering the soil inhospitable for reestablishment of

native species (Kerpez and Smith 1987).

Finally, the displacement of cottonwood-willow by

saltcedar, particularly at elevations below 365 m, may

reduce thermal buffering provided by the canopies of

native riparian trees (C. Hunter pers. comm.). The

absence or overall low reproductive success of midsummer breeding birds at elevations below 365 m may

be tied closely to a combination of (1) thermal tolerance of bird eggs being exceeded at ambient temperatures above 42° C (Walsberg and Voss-Roberts 1983);

(2) predictable summer temperatures that frequently

exceed 42° C during June and July (Hunter 1988,

Hunter and Ohmart unpubl. manuscript); and (3) loss

of most cottonwood-willow forests that may have provided effective thermal cover prior to the 1930s (e.g.,

Rosenberg et al. 1991). Hunter (pers. comm.) speculates that anticipated increases in average global

temperature may exacerbate potential problems with

productivity and distribution for mid- to late-summer

breeding species such as the southwestern willow

flycatcher.

In spite of the adverse impacts associated with the

spread of saltcedar, this species is now a naturalized
20 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

component of Southwestern drainages, particularly in

Arizona, New Mexico, southern Utah, and southern

Nevada. There is considerable irony in the fact that

certain saltcedar habitats now provide what appears

to be suitable nesting habitat for the endangered

flycatcher (see Chapter 9)! That irony is reinforced by

the fact that federal agencies responsible for recovering the southwestern willow flycatcher are also expending funds to “control” saltcedar. While saltcedar

control may have some merit in systems for which the

hydrological regime and water quality could truly

support native riparian trees and shrubs, current

control efforts and planning are focused almost exclusively on the symptoms rather than the root of the

problem. Those involved in saltcedar management

should heed Ewel’s (1986) observation that “species

invasions often reflect the conditions of the community being invaded rather than the uniquely aggressive traits of the invader.” In the case of saltcedar,

water management and water quality are the key

factors. Control programs that do not consider these

factors in the design of a restoration program run the

risk of further reducing the biological diversity of an

area, and, possibly, eliminating nesting habitat for the

southwestern willow flycatcher. At a minimum, any

area slated for saltcedar control or management should

be thoroughly surveyed for flycatchers well in advance

of physical alterations so that potential impacts to

flycatchers can be fully evaluated and avoided.

Other exotic species have spread in riparian habitats throughout the range of the southwestern willow

flycatcher. Russian-olive (Elaeagnus angustifolia) is

abundant at middle elevations in New Mexico and

Colorado (Szaro 1989). Where it occurs it is sometimes used for nesting by southwestern willow flycatchers (e.g., Skaggs 1996). Russian-olive appears

to be less invasive than saltcedar and competitively

inferior to native overstory species (Knopf and Olson

1984). Where found in mixed stands with native

species, Russian-olive commonly occurs in less moist

sites along the outer edge of riparian patches (Knopf

and Olson 1984). Russian-olive supports a relatively

high diversity and density of bird and mammal species, and may provide equivalent or better nesting

habitat, although quantitative data are lacking (Knopf

and Olson 1984). In California, giant reed (Arundo

donax) is spreading rapidly. It forms dense monotypic stands unsuitable for flycatchers. Other exotic

trees, such as Siberian elm (Ulmus pumilis) and tree

of heaven (Ailanthus simaruba) occur in riparian

areas within the flycatcher’s range and do not appear

to have any value as nesting substrates for flycatchers. At present their distribution is highly localized,

which suggests that impacts to the flycatcher may be

limited to local changes in riparian community

composition.

Factors Directly Affecting

Flycatchers _______________________

Cowbird Parasitism

Brood parasitism by brown-headed cowbirds

(Molothrus ater) is a major threat to some populations

of the southwestern willow flycatcher (Brown 1988,

Harris 1991, Whitfield 1990, Sogge et al. 1997). The

ecology of the cowbird is discussed in detail in Chapter

8. Originally thought to be commensal with American

bison (Bison bison), cowbird numbers have increased

tremendously with the expansion of livestock grazing,

agriculture, and forest cutting (Laymon 1987, Robinson

et al. 1993, Rothstein 1994). Cowbirds do not raise

their own young, but rather lay their eggs in the nests

of other species thus directly affecting their hosts by

reducing nest success. Cowbird parasitism reduces

host nest success in several ways. Cowbirds may

remove some of the host’s eggs, reducing overall fecundity. Hosts may abandon parasitized nests and attempt to renest, which can result in reduced clutch

sizes, delayed fledgling, and reduced overall nesting

success and fledgling survivorship (Whitfield 1994,

Whitfield and Strong 1995). Cowbird eggs, which

require a shorter incubation period than those of many

passerine hosts, hatch earlier, giving cowbird nestlings a competitive advantage over the host’s young for

parental care (Bent 1960,McGeen 1972, Brittingham

and Temple 1983).

Where studied, high rates of cowbird parasitism

have coincided with southwestern willow flycatcher

population declines (Whitfield 1994, Sogge 1995a, b,

Whitfield and Strong 1995, Sogge et al. 1997), or, at

a minimum, resulted in reduced or complete elimination of nesting success (Muiznieks et al. 1994,

Whitfield 1994, Maynard 1995, Sferra et al. 1995,

Whitfield and Strong 1995). Whitfield and Strong

(1995) found that flycatcher nestlings fledged late in

the season had a significantly lower rate of survival,

and that cowbird parasitism was often the cause of

delayed fledging.

A second brood parasitic species, the bronzed cowbird (Molothrus aeneus), is sympatric with E. t. extimus

in portions of its range. However, except for one

possible instance in the Gila River valley of New

Mexico (Skaggs 1996) and one instance at Roosevelt

Lake in Arizona (Sferra et al. 1995), the southwestern willow flycatcher is not known to be a host of this

species (Lowther 1995). The bronzed cowbird is unlikely to pose any significant threat to E. t. extimus

because it has a very limited distribution within

the range of E. t. extimus, occurs at much lower

densities than the brown-headed cowbird, prefers

open habitats, and tends to prefer larger hosts, especially Icterids (Lowther 1995).
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 21

Predation

For many flycatcher populations, nest predation is

the major cause of nest failure (Chapter 6). Most

monitored populations experience high rates of nest

predation ranging from 14 to 60% (Spencer et al. 1996,

Whitfield and Strong 1995, Sferra et al. 1997, Sogge et

al. 1997). Known or suspected nest predators include

various snakes, predatory birds including corvids,

owls, hawks, grackles and cowbirds, and small mammals including raccoons, ringtails, weasels, and rats

(McCarthey et al. 1998).

Rates of predation may increase in human-altered

landscapes. In the lower Colorado River valley,

Rosenberg et al. (1991) noted increases in great-tailed

grackles, a common nest predator. Increases in the

extent of habitat fragmentation have been correlated

with increased rates of nest predation in both forested

and non-forested habitats (Picman et al. 1993, Askins

1993, Robinson et al. 1995). Whitfield (1990) noted

that predation on flycatcher nests increased with

decreasing distance to edge. Most small bird species in

North America experience moderate rates of nest

predation (30 to 60%) and the southwestern willow

flycatcher, presumably, has adapted to similar rates.

The key factor to determine is whether impacts, such

as habitat fragmentation, are resulting in substantially higher rates of predation.

Parasites and Disease

Parasites and diseases can be critical factors affecting avian survival and reproduction, but tend to be

poorly known (Dobson and May 1986). A variety of

internal and external parasites have been recorded to

affect willow flycatchers (Boland et al. 1989; Chapter

6 and references therein). However, the impact of such

parasites or diseases on flycatchers is unknown.

Environmental Toxins

Where flycatcher populations are in proximity to

agricultural areas, the use of pesticides poses a potential threat. Birds may be affected through direct toxicity or a reduction of their insect prey base. Although no

quantitative data are available, physical deformities

in willow flycatchers may indicate exposure to toxic

compounds. Bill deformities and missing eyes have

been reported from birds at sites in Arizona, Colorado

and New Mexico (Paxton et al. 1997). In addition,

flycatchers may be exposed to potentially toxic compounds on wintering or migration grounds.

In the lower Colorado River area, water management operations may exacerbate potential effects to

flycatcher reproduction by concentrating naturallyoccurring selenium. Selenium and other contaminants

have been found in elevated levels in other birds

within the lower Colorado River area (King and

Andrews 1996). Selenium levels are known to be high

at the Escalante State Wildlife Area in Colorado,

where a willow flycatcher nestling was found with

skull and bill deformities.

Summary _________________________

The above discussion illustrates the wide scope and

magnitude of threats faced by this subspecies

rangewide. The impacts documented during the last

four years, alone, are alarming. Moreover, both small

and large flycatcher populations have been adversely

impacted or remain threatened. Haig et al. (1993)

observed for the red-cockaded woodpecker, an endangered bird that still numbers from one thousand to

several thousand pairs, that,

“...species with such small populations are easily

‘nickel and dimed’ to extinction. That is, loss of a few

small populations does not cause concern, but the

cumulative effects of these losses could be dramatic.

Therefore, a first step to species’ recovery will be to

stop these local extinctions.”

The losses sustained by the flycatcher and current

threats have been the subject of considerable conservation and research, public scrutiny, and litigation.

However, we have yet to witness widescale application

of what Haig et al. termed the “first step.” This is

evidenced by the numerous federal actions that have

resulted in or are anticipated to result in the loss of

flycatcher habitat and the displacement of flycatchers

(USFWS 1997b). The cumulative effect of the threats

and adverse impacts addressed in this chapter is

substantial, and may account for the current low and

relatively isolated population status for this subspecies. The rangewide scope and, in some cases, intense

magnitude of these impacts underscores the critical

need to protect existing flycatcher breeding groups

and their habitat so as to not increase the degree of

isolation among breeding groups. It also reinforces the

concept of habitat conservation and management at

the scale of the drainage (see Chapter 3), with the goal

of decreasing habitat isolation and providing for population movement that results from population phenomena (i.e., emigration, dispersal), stochastic events

(e.g., catastrophic floods, fires), or deterministic events

(e.g., inundation of habitat).

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USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 25

Richard D. Periman

Jeffrey F. Kelly

Chapter 4:

The Dynamic Environmental History of

Southwest Willow Flycatcher Habitat: A

Survey of Changing Riparian Conditions

Through Time

The extent of riparian habitat in the southwestern

United States during the past 100 years appears to

have been reduced by modern land development, urban expansion, and a general increase in human

populations (Finch 1996; Shaw and Finch 1996). Thousands of acres of river flood plain along the Rio Grande

and Colorado rivers have been cleared for agriculture,

construction of housing, and industrial development.

River channels have been diverted and contained by

dams, levies, and other water control structures in

order to provide irrigation and drinking water, and to

protect this investment in development from the destructive forces of periodic flooding (Tellman and others 1997; Wozniak 1995). Additionally, livestock grazing and the suppression of fire in upland ecosystems

have detrimentally affected the overall conditions of

riparian habitat (Thibault and others 1999).

The U.S. Fish and Wildlife Service added the

Southwestern Willow Flycatcher (Empidonax

traillii) to the endangered species list due to the

brood parasitism of Brown-Headed Cowbirds and the

destruction and modification of riparian habitats

(U.S. Fish and Wildlife Service 1995). Cottonwoodwillow riparian ecosystems are essential habitat for

this species. While we can assume that industrial-era

human expansion and land use have been responsible

for Willow Flycatcher habitat loss, the environmental

history of Southwestern cottonwood-willow ecosystems likely did not follow a linear trend of development or degradation.

Southwestern riparian ecosystems follow a dynamic

regime of change and are not ecologically static. Prior

to the installation of flood-control structures, periodic

and catastrophic flooding replenished soil nutrients

and served to “replant” riparian vegetation, while

simultaneously removing large stretches of cottonwood-willow habitat from river flood plains (Whitney

1999). Human environmental manipulation, such as

the burning of vegetation and clearing of land for

agriculture, has been another factor of change to

these already dynamic ecosystems. Although occurring at first on a limited scale, this interaction between

people and the environment of the Southwest has

occurred for at least 12,000 years (Cordell 1984).

Archaeological research shows that not only were

riparian plant and animal species used as sources of

food, medicinal plants, and wood fiber for shelter and

firewood, certain drainages in southwestern New

Mexico were virtually stripped bare of trees during the

eleventh and twelfth centuries A.D. (LeBlanc 1985).
26 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

According to ethnographic accounts, many American

Indian cultures used and affected riparian habitats in

a variety of ways. While some groups burned the

vegetation along rivers and streams to provide clear

hunting areas, others pruned and even planted riparian plants, willows for example, for future use

(Blackburn and Anderson 1990; Fowler 1986 and

1996; Opler 1983a and b).

The overall effects of human land use changed in

both method and intensity with Spanish conquest and

settlement after 1540. This environmental change

greatly intensified after 1846 when Anglo-American

ranchers, farmers, and miners began to settle in the

Southwest (Scurlock 1998; Wozniak 1996). Combined

with the effects of modern development, this longterm, socioecological interaction has influenced and

helped create the riparian conditions we see today.

We provide in this chapter a brief environmental

history of Southwestern Willow Flycatcher habitat.

Our geographical focus is the American Southwest

(i.e., New Mexico, Arizona, and portions Southern

California). We begin by describing the general changes

and development of ecological conditions in the Southwest since the end of the Pleistocene. This description

of the area’s natural history provides a background for

a discussion of human-environmental interactions

with stream-side ecosystems, and importantly, illustrates the dynamic quality of Southwestern ecosystems. Within this context, we discuss Pre-Columbian

and ethnohistorical evidence of riparian land use and

manipulation. This is followed with a survey of historical observations of riparian habitat, first those of

Spanish explorers, then more detailed accounts by

Anglo-American trappers, military officers, and ornithologists. The section on exploration also provides

accounts of direct sightings of Willow Flycatchers and

the collection of vegetation and bird species. We then

review the effects of European and Anglo-American

expansion and settlement on the environment of the

Southwest. Finally, we conclude with a discussion of

how this historical ecological information may be

useful in the management of Southwestern Willow

Flycatcher habitat.

Ecological Conditions During the

Holocene ________________________

By the end of the Pleistocene, c.12,000 to 10,000

years before present (B.P.), vegetation zones in the

Southwest were 900 to 1,400 meters below present

elevations. Sagebrush was more widespread during

early- and full-glacial periods than it is today. Vegetation zones that had been at lower altitudes

during the Pleistocene shifted to higher elevations

during the transition from the glacial to post-glacial

periods (Hall 1985:117). During the early-Holocene

(10,000 to 7,000 B.P.) climatic conditions were

generally more cool and moist than at present. This

cool period was followed by a gradual warming and

drying of the climate beginning approximately

7,000 B.P. and extending to 5,000 B.P. During this

period of the middle-Holocene, average annual temperatures were between one and two degrees Celsius

higher than today’s annual temperatures (Mannion

1991:53-54). Pollen records from sites throughout the

Southwest show that the middle-Holocene was extremely warm and dry relative to any other period.

There was a reduction in woodland and riparian

vegetation, and at alpine localities tree lines appear to have been as much as 70 meters above those

of today. Alluvial valleys were eroded, resulting in

the truncation of many alluvial pollen records (Hall

1985:118). After 5,000 B.P., there was a gradual

increase in moisture levels and a correspondingly

progressive increase in woodland and forest vegetation (Hall 1985:118). During the Little Ice Age, occurring during the late-Holocene from approximately

A.D. 1450 until 1900, annual temperatures were one

to two degrees Celsius cooler throughout the Northern Hemisphere than they are today (Kreutz and

others 1997).

As demonstrated by the arid conditions of the middleHolocene and cooler period of the Little Ice Age, the

Holocene has been a period of climatic variability.

Additionally, these broad-scale fluctuations were accompanied by more periodic climatic patterns. For

example, historical records used to reconstruct more

than four hundred and fifty years of El Niño-Southern

Oscillation (ENSO) effects show that climate and

moisture regimes varied extensively during the lateHolocene (Quinn and others 1987). By studying documented observations of abnormal weather, flooding,

destruction of settlements, reductions in coastal fisheries, and other unusual natural events, Quinn and

others identified moderate, strong, and very strong

El Niño occurrences from 1525 to 1987. High-resolution alluvial sedimentary deposits in Ecuador show

that the current periodicity of ENSO was established

about 5,000 B.P. (Rodell and others 1999:518-519).

In the American Southwest, fire-histories show

that years of increased or decreased regional burning

have been closely related to the extreme ENSO phases

(Swetnam and others 1999). Extensive drought and

fire records, compiled for the Southwest from

dendroclimatological data, show that extreme El Niño

(high moisture) phases result in an accumulation of

fuels. When followed by La Niña drought conditions,

such fuels result in a greater number of fires on a

regional scale (Swetnam and Betancourt 1998:3132-

3136). Such patterns of climatic and moisture change

would have affected levels of runoff and groundwater

accumulation. This oscillating cycle of precipitation
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 27

and fuel accumulation followed by drought and

burning likely affected riparian vegetation as well as

upland forests.

Native American Use of Riparian

Ecosystems _______________________

For at least the past 12,000 years, humans have

been living in and interacting with ecosystems throughout the Western Hemisphere (Cordell 1984; Frison

1991). The American Southwest has an abundance of

archaeological and ethnographic resources that have

the potential to enhance our understanding of riparian history. Archaeological research and excavation

often includes the study of past environments through

the examination of buried botanical remains collected

from archaeological sites and other locations where

pollen might be preserved, such as a peat bog or lake

bottom (Birks and others 1988; Chambers 1993). Studying the remains of plants used by prehistoric and

historic cultures can provide insights into human land

use and environmental effects (cf. Pearsall 1989).

Prehistoric archaeological sites are often associated

with riparian areas because of the presence of water

and high resource availability. Riparian vegetation

was used for firewood, construction material, shade,

and food (Cordell 1984). Frequently, the macrofloral

remains are well preserved in Southwestern archaeological sites, and pollen analysis of site sediments

often reveals willow (Salix spp.), cattail (Typha spp.),

and other riparian plants (Reinhard and others

1991:119; see also Cummings 1995a, b, 1997).

During the Paleo-Indian period (ca. 12,000 B.C. to

5,500 B.C.) and the Archaic period (5,500 B.C. to

A.D. 400), people in the Southwest primarily foraged and hunted for subsistence (Cordell 1984).

With the introduction of maize into the area, between 2,000 B.C. and 1,500 B.C. (Minnis 1992) until

approximately A.D. 400, people continued to follow

a hunting and gathering economy but with an increased dependence upon agriculture. Interestingly,

during the Archaic period land use in some areas

concentrated on upland landscapes that had high

elevational diversity and therefore a greater range

of resources to be exploited throughout the year

(Tainter and Tainter 1996).

Between A.D. 400 and A.D. 600, until the beginning

of Spanish colonization in A.D. 1540, agricultural

groups began to build permanent settlements

throughout what is now Arizona and New Mexico.

Unlike those of earlier, Archaic period groups, these

settlements were in some areas associated with large

river-bottom environments (Tainter and Tainter 1996).

Prehistoric farmers planted crops in riparian areas

after clearing flood plains of vegetation (Cordell 1984;

LeBlanc 1985). Agricultural societies in the Southwest had significant effects on riparian ecosystems.

These societies included: the Patayan cultures located

along the Colorado river in western Arizona; the

Hohokam on the Gila, Verde, Salt, and San Pedro

rivers in south-central Arizona; and the Anasazi of

the Four Corners region in the San Juan, Little Colorado, and Rio Grande river basins. The Mogollon culture, which covered much of southern New Mexico,

southeastern Arizona, and northern Sonora and Chihuahua, Mexico, was associated with the Rio Grande,

Gila, Salt, the Rio Conchos, Rio Yaqui, and Rio de

Sonora rivers (Cordell 1984).

In the southwestern corner of New Mexico archaeological and paleoenvironmental research in the

Mimbres Valley reveals that prehistoric cultures had

a profound effect on riparian ecosystems. A team of

scientists headed by archaeologist Steven LeBlanc

speculates that the Mimbres Valley underwent human-caused degradation during the period from A.D.

1000 to A.D. 1130 (LeBlanc 1985). Centuries of occupation and land use greatly reduced tree-cover in the

river valley’s riparian areas. Before A.D. 1000, cottonwood had been a common source of firewood and roof

timbers but this source became depleted. Examination of wood charcoal shows that people were forced to

use upland trees as substitutes when cottonwood was

depleted. Clearing of the valley bottom for agriculture

also likely affected the riparian plant community.

After the valley was abandoned, most of the original

vegetation returned. However, pollen analysis indicates that at least one species, sycamore (Platanus

wrightii), was eliminated entirely from the ecosystem

(1985:21).

Riparian-related vegetation was utilized as a resource by a variety of American Indian tribes throughout the Southwest and Great Basin. These cultures

systematically tended riparian landscapes through

the use of fire, pruning vegetation, and planting stands

of willows and other plants in new locations (Anderson

1999; Blackburn and Anderson 1993b). The Timbisha

Shoshone of Death Valley, California, used fire to

clear riparian areas of dense willows and various

grasses in order to promote the growth of certain types

of seeds, particularly the white-stemmed blazing star

(Mentzelia albicaulis) (Fowler 1996:40). The Timbisha

routinely removed willows from springs and other

water sources so that water could accumulate both

for wildlife and for their own use (1996:98). As with

pruning and selective harvest of riparian species,

fire-use was localized and used for specific purposes

(Anderson 1999). In order to produce straight willow

stems for use in basketry, willow was pruned and

coppiced by the Timbisha and other Shoshonean

tribes in the Great Basin (Fowler 1986, 1996).
28 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

The Chiracahua Apaches of southwestern New

Mexico and southeastern Arizona used the roots of

tule (Scirpus spp.) and cattail (Typha spp.) for food and

baskets. Sedge (Carex spp.) tubers and rootstocks of

cattail were significant in the diet of the Mescalero

Apaches of southeastern New Mexico and western

Texas (Opler 1983a; Opler 1983b).

For native peoples, riparian ecosystems were

highly productive and therefore attracted a high level

of land use. This manipulation of vegetation likely

occurred over thousands of years in both upland and

lowland stream environments. Many of the streamside areas encountered by European and Anglo-American explorers may have in fact been semi-managed

ecosystems (Blackburn and Anderson 1993b).

Western Exploration and

Settlement _______________________

In 1540, the viceroy of New Spain ordered Fray

Marcos de Niza, who traveled ahead of the Coronado

expedition, to take note of “the nature, fertility and

climate of the land; the trees, plants, and domestic

and wild animals...the rivers, whether they are large

or small” (Hammond and Ray 1940). De Niza neglected to write about the natural world through

which he passed, contrary to the viceroy’s orders.

Alvar Nuñez Cabeza de Vaca, in the chronicles of his

journey from a shipwreck in Florida to New Spain

(Mexico), described crossing the Rio Grande somewhere north of what is now El Paso; he suggests the

presence of settlements and farms on the flood plain

(Cabeza de Vaca 1542). Hernando de Alvarado and

Fray Juan de Padilla, on their way to rendezvous

with Coronado along the Rio Grande, noted the existence of cottonwoods near present day Albuquerque

(Hammond and Ray 1940:183).

In general, the journals and documentation of

Spanish explorers do not mention riparian or other

vegetation in detail. Although Spain sent scientific

expeditions to New Spain between 1785 and 1800,

primarily for the purpose of recording plant and animal life in addition to mineral and other resources,

none traveled through the American Southwest

(Engstrand 1981).

Journals of explorers, trappers, and members of

military expeditions contain information relevant to

identifying historical environmental conditions during following the Spanish Colonial period (1540 to

1821) as well as during and following the Mexican

period (1821 to 1848). American fur trappers entered

the Southwest as early as 1831 (see Gregg 1905). The

records of American military expeditions associated

with the war with Mexico, 1846 to 1848, and the

exploration of possible routes for transcontinental

railways contain the most thorough observations of

environmental conditions prior to the twentieth century (Abert 1962; Bell 1870; Pattie 1905; Pike 1966;

Sitgreaves 1853; Wislizenus 1848).

During his explorations of New Mexico in 1805,

Zebulon Pike observed that, “the cotton tree is the

only tree of this province except some scrubby pines

and cedars [juniper] at the foot of the mountains.

They form borders on the banks of the Rio del Norte

and its tributary streams. All the rest of the country

presents to the eye a barren wild [wilderness] of poor

land, scarcely to be improved by culture, and appears

to be only capable of producing sufficient subsistence

for those animals which live on succulent plants and

herbage” (Pike 1966:47).

In 1826, fur trapper and trader James Pattie remarked that near Socorro, New Mexico, the river

bottoms were thinly timbered and that the only

growth was cottonwood and willow (Pattie 1905:86).

However, during their first night on the Gila River,

Pattie’s group trapped over thirty beavers. The next

day they journeyed along the river, and became “fatigued by the difficulty of getting through the high

grass, which covered the heavily timbered bottom”

(1905:87).

In the 1830s, explorer Josiah Gregg noted that the

aspen along the upper portion of Santa Fe Creek were

as “thick as cottonwoods in the Missouri bottoms”

(Gregg 1905:282). However, the effects of human land

use also were observed by Gregg. Commenting on the

Rio Grande Bosque, he wrote that, “on the watercourse there is little timber to be found except cottonwood, scantily scattered along their banks. Those of

the Rio del Norte are now nearly bare throughout the

whole range of settlements, and the inhabitants are

forced to resort to the distant mountains for most of

their fuel” (1905:159). Similar observations of riparian

vegetation along the Rio Grande were made by Adophus

Wislizenus in 1846. The banks of the Rio Grande and

south of Albuquerque, he observed, were bare of trees,

with only occasional cottonwoods (Wislizenus 1848:34).

Beginning in 1846, American military expeditions

resulted in some of the most detailed environmental

descriptions of the Southwest during the nineteenth

century. As part of Col. Stephen Kearny’s expedition during the Mexican American War, engineer

Lt. William H. Emory produced a detailed journal

that contains numerous descriptions of riparian vegetation in the Southwest. On the Purgatory River,

shortly after entering New Mexico, Emory remarks

that, “the blighted trunks of cotton-wood and locust

trees were seen for many miles along its course, but

the cause of decay was not apparent” (Emory 1848:17).

He also recorded sighting “black locust, the everlasting cotton-wood, willow, wild currents, hops, plums

and grapes, artemesia [sic], Clementis verginiana,
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 29

salix, in many varieties; and a species of angelica”

(1848:17). On the Canadian River, Emory described

the presence of a few cottonwoods (1848:19).

Emory carefully documented the birds he encountered on the expedition. While on the Canadian River

he wrote that, “birds are rare, with the exception of

the cow-bunting [cowbird?], which has been seen in

great numbers on the whole route [from Bent’s Fort] in

a state so tame as to often alight on our horses”

(1848:21). Additionally, he described healthy stands of

willows and cottonwoods on the Cimarron and its

tributary streams (1848:22).

Emory recounted that the area around Santa Fe

was entirely cultivated through use of irrigation and

that the expedition’s horses had to be sent to graze 12

to 30 miles from the city (Emory 1844:35). He remarked that the area near Albuquerque was destitute

of wood and that grass was sparse for the expedition’s

horses; however, he observed significant cottonwood

and willow growth, along with evidence of beavers,

south of Albuquerque (1848:47-48). At Polvadera, the

river was fringed with large cottonwoods, and Emory

observed flocks of geese “blue-winged ducks, plovers,

doves and a few meadow larks” (1848:48). There were

“immence [sic] flights of sand-cranes and geese well

within gunshot of houses and the largest towns”

(1848:48). He mentioned that as he traveled south, the

cottonwood growth become more plentiful (1848:50).

South of Socorro, Emory’s party encountered an

area which he judged to be “the best in New Mexico;

the valley is broader, the soil firmer, and the growth of

timber, along the river, larger and more dense” (Emory

1848:53). Emory noted evidence of irrigation and two

deserted towns in the area that had once been a

successful Spanish settlement. After passing the Fra

Cristobal Mountains, Emory remarked on shooting

three quail and one small lark that resembled a

“sparrow-hawk” (1848:54). Interestingly, at this point

in the journal he made the statement that “game in

New Mexico is almost extinct” (1848:54). In contrast,

when the expedition reached the area located one mile

north of San Felipe, he observed that their camp was,

“well grassed and wooded, and apparently untrodden

by the foot of man” (Emory 1848:55). For the first time

in New Mexico, Emory’s expedition saw abundant

evidence of game (1848:55).

The expedition left the valley of the Rio Grande and

traveled west toward the Gila River. After crossing

over table lands and entering more hilly terrain,

Emory observed a stream with “green trees and luxuriant foliage.... The stream was clear, limed, and cool,

the first but one I had seen since crossing the

Alleghenies, where water could be drunk without

imbibing a due proportion of mud and sand” (1848:57).

As the group progressed further, he noted that, “in the

valley grows cotton-wood, a new variety of evergreen

oak...and a new kind of walnut” (1848:57). Emory

wrote that the sides and banks of the Rio Mimbres

were “covered with a growth of stunted live oak,” and

the valley was “densely covered with cotton-wood,

walnut, [and] ash” (1848:57). He stated that the river

was filled with an abundance of fish “without scales

[Roundtail Chub]” (1848:61-62). The Gila River valley,

according to Emory, was covered with a luxuriant

growth of trees, “chiefly cotton-wood, a new sycamore,

mezquite [sic]...a few cedars, and one or two larch

[Arizona Cyprus?]” (1848:62).

Lt. J. W. Abert, also under Col. Kearny’s command

during this period, traveled in a separate expedition

and kept a journal much like the one recorded by

Emory. Abert recorded specific bird sightings and like

Emory made observations of the vegetation encountered. For example, on the Purgatory River, he observed, “we here saw several flickers, with red lined

wings and tails...the common flicker, and large flocks

of the yellow headed black bird” (Albert 1962:30).

Further down the Purgatory River, he recorded

sightings of “red shafted flickers” (1962:30-31). Like

Emory, Abert also observed the “common cow bird”

(1962:32). Abert described the riparian vegetation he

encountered in much the same manner as Emory

had; he described the campsites on the Canadian

River as “shaded by large cottonwood trees and willow thickets” (1962:35).

In 1850, an expedition led by Captain L. Sitgreaves

surveyed a segment of the Rio Grande, the Zuni River,

and the Colorado River (Sitgreaves 1853). Like Emory

and Abert, Sitgreaves reported on the vegetation he

encountered. More detailed and technical observations, however, were made by S. W. Woodhouse, M.D.,

the surgeon and naturalist of the expedition.

Woodhouse noted that, “From El Paso, passing up the

Rio del Norte, the vegetation alters but little, the

timber being principally cotton-wood, and mezquit

[sic] extending as far as the Jornada del Muerto.”

From the Pueblo of Santo Domingo to Albuquerque,

Woodhouse noted that there was little change in vegetation “with the exception of a few scattered cottonwood trees...or occasionally a few cedars [junipers]”

(1853:35). Significantly, Woodhouse made thorough

and detailed observations of birds, and recorded

sightings of “Trail’s Fly-catcher” and the “Dwarf

Fly-catcher” (1853:74-75). What he calls “Trail’s Flycatcher” is likely the Southwestern Willow Flycatcher

(Empidonax traillii extimus).

Interestingly, when describing the Zuni River,

Woodhouse remarked that they encountered “in but

one place a few poplars (Populus augustifolia), and

near these trees was a beaver dam, in which was

growing cattails (Typha latifolia)” (Woodhouse 1853:36).

He also noticed beaver lodges on the banks of the

Little Colorado, although there was little timber.
30 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

However, he did mention “a species of swamp-willow

(Salix)” (1853:36). As the expedition approached the

San Francisco Mountains, he noted that the cottonwood became more abundant (1853:39). Woodhouse

wrote that the banks of the Little Colorado were

“fringed with cotton-wood trees” and that at its

confluence with the Colorado, he had trouble measuring the velocity of the of the river due to a “dense

growth of willows and weeds” (1853:40).

Woodhouse noted several streams with willows, and

upon finally reaching the “Colorado of the West,” he

remarked, “On the banks of this stream are growing

willows (Salix spp.) of several kinds, one of which, the

Salix augustifolia, affords good fodder for the mules;

they oftentimes, whilst on this stream, had nothing

else, and in fact we thought that we were doing well

when we found this species of willow” (1853:39).

Additionally, he observed that from the place where

they first encountered the Colorado to the mouth of

the Gila (Figures 4-1 through 4-3), “south to the

entrance of the Great Desert,” the vegetation varied

little, being cottonwood, mesquite, and willow

(Woodhouse 1853:39).

The extensive Wheeler railroad surveys, conducted

from 1871 through 1874, produced seven volumes of

information about the West (Wheeler 1878). Volume

VI documents the vegetation encountered in detail

(Figures 4-4 through 4-6). As reported in Volume V,

ornithologist Henry W. Henshaw collected extensively

in western New Mexico, Arizona, Utah, and Nevada.

In all, his expedition procured more than 3,000 birds

(Henshaw 1875). In 1874, his party, which included two other scientists, traveled from Santa Fe

to Fort Wingate, New Mexico, through Arizona to

Fort Crittenden near the Mexican border. From Fort

Crittenden, they returned to New Mexico through

eastern Arizona, then north along the Rio Grande to

Santa Fe.

In his report, Henshaw recorded the species, sex,

location, and the number of birds collected during the

expedition. He stated that, “no birds have been introduced that have not actually been taken or observed by

the expedition. In all cases where we have utilized the

notes of observers other than members of the survey,

attention is called to the fact” (Henshaw 1875:139).

As he traveled through the Gila valley Henshaw

remarked that the avifauna was very much like that of

the Colorado Valley (1875:140). In all, Henshaw collected and documented what were then classified as

fifteen different species of “flycatcher,” including the

Willow Flycatcher (Empidonax traillii), which he referred to as the “Little Flycatcher” (1875:356-358).

Henshaw’s team collected Willow Flycatchers

throughout Arizona, in the White Mountains, at Fort

Apache, Fort Bowie, and Fort Crittenden. A list of

these birds, and observations made by Henshaw, is

provided in Appendix A. Reviewing the results of his

other expeditions, Henshaw remarked that these

birds (Southwestern Willow Flycatchers) were

...exceedingly numerous near Provo, River [Utah] in

willow thickets; sparingly so in eastern Nevada.

Colorado, New Mexico, and Arizona are all included

in the range of this flycatcher; its abundance being

dependent upon the presence or absence of its favorite grounds.... Wherever willows are found growing

in small clumps or fringing the streams, this flycatcher is almost certain to be found common

(Henshaw 1875:357).

During the 1860s and 1870s, U.S. Army surgeon

and ornithologist Elliott Coues collected and recorded

birds and mammals as a member of numerous military expeditions in the West (Coues 1866, 1874, and

1878). Coues describes Empidonax traillii as being

an abundant “Flycatcher of the West” and he observed that at “Fort Whipple, in Arizona, it is the

commonest and characteristic species of its group,”

ranging from the central plains to the Pacific (Coues

1874:252-253).

Dr. Edgar Alexander Mearns recorded his observations of mammals and trees encountered during the

Mexican Boundary Survey of 1892 to 1894. The survey

party covered territory from Fort Worth, Texas, to

San Diego, California, along the present border of

the United States and Mexico. Mearns states that the

streams along the route were lined with trees, including Fremont cottonwood, black willow, box elder, walnut, sycamore, oak, mulberry, and ash (Mearns 1907).

He described cottonwood and willow as associated

with every permanent stream, bordered with a broad

zone of mesquite (1907 261:32-35d). Mearns estimated

that about 160,000 to 180,000 hectares of riparian

vegetation covered the Lower Colorado River flood

plain between Fort Mojave and Fort Yuma (Rosenberg

and others 1991:21).

In 1910, over a period of three months, ornithologist

Joseph Grinnell surveyed the Colorado River from

Needles, California, to Yuma, Arizona. During the

expedition, Grinnell systematically collected and studied the plants and animals of the Lower Colorado

Basin. The party collected 1,374 bird specimens, along

with large numbers of other vertebrates (Grinnell

1914:52). Grinnell described cottonwood and willow

as the dominant vegetation in the flood plain adjacent to the river, while the higher terrace was predominantly covered with mesquite and “salt-bush”

(1914:60-61). He observed that the river landscape

continually changed as floodwaters cut new channels

and cleared areas of vegetation. New sediment was

continually deposited on the inside curve of the river.

Trees tended to grow in a successive pattern. The

youngest cottonwoods and willows growing nearest to

the water’s edge, progressively older and taller trees

growing away from the river.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 31

Figure 4-1. Lower portion of the Colorado River with large areas of mesquite, cottonwood, and willow

represented. Map made from information collected during 1851 Sitgreaves expedition (Sitgreaves 1853).
32 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

Figure 4-2. Middle portion of Gila River. Map made from information collected during 1851 Sitgreaves expedition (Sitgreaves

1853).

Grinnell also described the effects of Laguna Dam,

which was completed in 1909, as already altering large

areas of the Lower Colorado River landscape by 1910.

The vegetation growing along a ten-mile stretch of

the river had been decimated by rising water levels,

while a prodigious amount of sediment was deposited

over the flood plain. As water subsided, the original

riparian vegetation was replaced by “vast mudflats

growing arrowweed. All of this change, of course,

involved the birds and mammals of the area affected,

in addition to the plant life” (Grinnell 1914:61-62).

Grinnell observed and collected Willow Flycatchers

(Empidonax traillii) during the 1910 survey, referring

to the species as the “Traill Flycatcher” (Grinnell

1914). He first spotted several of the birds on the

California side of the Colorado River and again

sighted Willow Flycatchers five miles northeast of

Yuma. Grinnell states that, “On both sides of the river

in the vicinity of Pilot Knob the species [Empidonax

traillii] was frequently observed” and that, “The birds

were never detected away from dense willow growths

close to water” (1914:151).

In California, Grinnell described “Western Traill

Flycatchers” (Empidonax traillii) as being present in

riparian ecosystems throughout the length the state,

east and west of the Sierras.” He stated that the birds

“avoid in major part forested areas including northwest coast belt, the open deserts, and the higher

mountains...this flycatcher exists in summer time

practically wherever its special habitat exists” (Grinnell

and Miller 1944:256). At that time, traillii could be

found within the “Lower and Upper Sonoran zones,

Transition [zones], and even Canadian [zones]. Altitudes of known nestings extend from within a hundred

feet of sea level, for example at Alviso and Palo Alto,

Santa Clara County, up to at least 8,000 feet, in the

neighborhood of Mammoth, Mono County” (1944:256).

The Flycatchers were primarily restricted to willow

thickets, “along streams in broad valleys, in canyon

bottoms, around mountain-side seepages, or at the

margins of ponds or lakes” (1944:257).

In a 1908 study that focused on the San Jacinto

Mountains of southern California, Grinnell and

Swarth described Empidonax traillii as only occurring
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 33

Figure 4-3. Colorado River along Arizona, California, and Nevada boundaries. Confluence of the Virgin River

and the Colorado is show in upper portion of map (Sitgreaves 1853).
34 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

Figure 4-4. Riparian habitat at the head of the Conejos River in south central Colorado. Photo

taken during Wheeler railroad surveys of the 1870s (Wheeler 1878).

Figure 4-5. Braided stream and riparian habitat along the Conejos River in south central

Colorado. Photo taken during Wheeler railroad surveys of the 1870s (Wheeler 1878).
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 35

on the desert side of the mountain range during

spring and in late summer moving to other locations

in the San Jacinto area (Grinnell and Swarth 1913).

Their group collected traillii, which appear to have

been abundant, at Cabezon and Dos Palmos. The

birds were found in thickets of desert willow at

Carrizo Creek and Palm Canon and in other areas up

to 3,000 feet above sea level. Grinnell and Swarth

collected 19 traillii specimens during the survey

(1913:256). Additionally, Grinnell and Kellogg each

mention Empidonax traillii as occurring in the Trinity region of northern California (Grinnell 1916:404;

Kellogg 1916:382).

Euro-American Impacts on

Southwestern Riparian

Ecosystems _______________________

Beginning with Coronado’s entrada (1540 to 1542),

native cultures began to change. Throughout the

world, European contact with indigenous societies

Figure 4-6. Base of the San Francisco Mountains in Arizona with Wheeler survey

party and photographic equipment in the foreground (Wheeler 1878).

has disrupted native cultures in a number of ways

(Ferguson 1992a and b). Coronado’s expedition likely

effected environmental change by pressuring

Puebloan groups to produce food and fuel for his

soldiers (thereby putting additional pressure on

flood plain resources) and by introducing metal

tools with which native people could more effectively clear trees (Wozniak 1995). European contact

with Native Americans also introduced previously

unknown diseases into a native population with no

immunity to European illness. This caused a vast

decline in population and a disruption of the cultural and economic activities focused on resource

production (Ramenofsky 1987), such as burning

undergrowth, harvesting native plants, producing

crops, and managing water, all of which would have

affected vegetational patterns.

By 1598, Spanish settlement affected environmental change on a large scale. Spanish colonists introduced domesticated livestock, intensive ditch and

flood irrigation, a plethora of European plants, use of

metal tools, and the plow. This resulted in tremendous
36 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

The arrival of the railroads during the late 1870s

and early 1880s brought new settlers, resulting in

the expansion of urban areas. This greatly increased

the amount of river land under cultivation (Wozniak

1995). With the railroads, ranchers were able to ship

their cattle and sheep to eastern markets. These new

transportation systems facilitated the availability of

an increased number of industrial and agricultural

technologies, thus allowing the development of even

more arable land, an increase in the number of

sawmills, and other industrial enterprises (Hess 1992;

Worster 1993).

Since the 1890s, numerous rivers and streams in

the Southwest have been dammed, pushed between

levies, and diverted to satisfy the ever-increasing

demands of urban landscaping, industry, drinking

water, and irrigation. Agricultural lands replaced

cottonwood-willow habitat, and although the numbers of range stock have decreased, riparian areas

continue to be impacted by cattle grazing. Additionally, nonnative plants such as salt cedar and Russian

olive, after their introduction early in the twentieth

century, began out-competing native riparian vegetation, in part due to altered hydrological regimes

(Scurlock 1998; Tellman 1997).

Discussion _______________________

This chapter provides a sketch of a vast and complex historical and ecological topic. The dynamic

climatic, vegetational, and human history of the

Southwest demonstrates the remarkable changeability of its environment. Riparian habitats in the Southwest are not now, nor have they ever been, in a state

of equilibrium (Swetnam and others 1999:1201). These

dynamics of change include long-term climatic variation and human-environmental interaction. While

climatic variation, drought, and periodic flooding

continually have changed drainage and vegetation

patterns, humans also have manipulated Southwestern ecosystems since the end of the Ice Age.

Archaeological research shows that not only were

riparian species used for food and medicinal purposes,

but also that certain southwestern drainages were

virtually stripped bare of trees by prehistoric horticulturists. Large areas were cleared for agriculture and

riparian vegetation was used for construction and

firewood (LeBlanc 1985). The archaeological record

contains riparian-related macroremains and pollen,

however many archaeological investigations have not

focused on overall anthropogenic environmental effects. Although the presence of willow and cattail

pollen in archaeological sites does not provide an

indication of past riparian conditions, such evidence

does demonstrate that humans utilized the resources

changes in water systems, the clearing of larger

tracts of land, grazing pressures, and the expansion of

nonnative plants (Scurlock 1998:105-119).

Fur trappers were some of the first Americans to

affect Southwestern riparian areas primarily by trapping beaver. Trappers often traveled in groups, or

“brigades,” that would trap all of the beaver from a

drainage before moving on to the next river system

(Vandiveer 1971; Ross 1956; Karamanski 1983;

Chittenden 1935). The destruction of beaver dams

likely resulted in a narrowing of riparian areas due to

less water retention in certain drainages. This caused

a change in riparian vegetation. The clearing and

regeneration pattern would have been less dynamic.

By the time of the first major cattle drives from

Texas to the California gold fields in 1850, there were

already indications of overgrazing in the Southwest.

Hispanic ranchers were raising tens of thousands of

sheep and cattle, along with goats and horses (Raish

1995; Scurlock 1988; Scurlock 1998). Texan cattlemen

established two major routes from west Texas to

California: one trail crossed New Mexico near Albuquerque and then proceeded west across Arizona to

Los Angeles; the other cattle trail passed along the

region of what is now the Mexican-American border

through southeastern New Mexico, to Tucson, then

to San Diego. Despite the fact that this enterprise

was only marginally successful, due to the extremely

long distances and shortages of grass and water

(Beck and Haase 1989:29-30), the cattle drives had

significant effects on those areas through which they

passed (Emory 1857).

During the 1870s and 1880s, numbers of cattle and

sheep grew dramatically in the Southwest. AngloAmerican ranchers accumulated vast herds numbering in the millions, and the ecological conditions of

rangeland and riparian areas worsened. Native American herds grew and had a lasting effect on the lands

as well (Scurlock 1998:96-97). Topsoil stripped of

vegetation eroded with wind and runoff, which increased stream sediment and the down cutting of

streams (see Abruzzi 1995; Scurlock 1988 and 1998;

Tellman and others 1997; Widdison 1959; Wilson

1988; Wozniak 1995).

Timber cutting also significantly altered the river

bottoms of the Southwest. Cottonwood was cut for

steamboat fuel along the Colorado, while pine and

other soft woods were removed from mountain ranges

(Rosenberg and others 1991). Around the perimeters

of settlements such as Albuquerque, firewood had

become scarce by the time the American military

arrived during the late 1840s (Emory 1848; Rosenberg

and others 1991; Whipple 1856; Wislizenus 1848;

Sitgreaves 1853). Removal of tree-cover resulted in

increased runoff, and subsequently high sediment

loads.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 37

available in these streamside ecosystems. Long-term

burning, pruning, and planting of riparian vegetation

by native peoples likely helped to create current conditions, and knowledge of their techniques may in fact

present a model for future restoration efforts (Anderson 1999). Further review of the ethnographic record

can provide a wealth of information about past Native American land use.

Historical observations made by explorers and biologists shows that the Willow Flycatcher was locally

common in the cottonwood-willow habitats of the southwest in the 1800s. The first-hand observations discussed

here were made by a few individuals traveling along

primary routes, and therefore provide only a sample

image of past riparian conditions. The ability to draw

conclusions about the extent of riparian vegetation

and the distribution of Willow Flycatchers could be

enhanced by further review of archival material at the

state, county, and community level.

It is necessary to discuss the broad spectrum of

historical conditions of riparian systems in the Southwest. Conducting environmental history requires an

interdisciplinary approach incorporating the expertise of multiple scientific and humanistic fields (Worster

1993:156). Ecosystem conditions of the recent and

distant past need to be evaluated using a variety of

information sources (e.g., paleobotany, geomorphology, as well as local historical documents). Additionally, local historical societies, family journals, legal

documents, and photographs could be significant

sources in developing a local-level environmental

history of riparian ecosystem conditions for specific

project areas.

Managing Willow Flycatcher habitat is ecologically and socially complex and challenging. An ecosystem must be studied and evaluated prior to the

establishment of appropriate target conditions and

objectives concerning the restoration of various components (Dahm et al. 1995:225). Managing riparian

ecosystems (i.e., re-establishing Willow Flycatcher

habitat), requires clear understanding of the ecological complexity and linkages between biotic communities and the abiotic environment, recognition of

the dynamic character of ecosystems, and knowledge

of humans as ecosystem components (Christensen

and others 1996:669-670). These ecological factors

need to be understood at the local and regional scale

and temporally over the short- and long-term

(Swetnam and others 1999:1190). The so-called “range

of historical variability” for the Southwest has been

quite broad. Considering the dynamic nature of

Southwestern landscapes, it would be misguided to

designate a specific point in time from which to

establish a desired future condition for riparian restoration (Swetnam and others 1999:1201; Sprugel

1991).

Having knowledge of all the dynamic forces that

helped to shape riparian habitat will give decisionmakers a greater range of opportunities and options

when considering the management and reestablishment of cottonwood-willow vegetation. Understanding these changes in riparian ecosystem conditions,

and dynamics behind them, can provide an invaluable perspective that applies to the re-establishment

and management of Southwestern Willow Flycatcher

habitat within the limits of modern human social and

resource demands.

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Woosley, A. I.; McIntyre, A. J. 1996. Mimbres Mogollon archaeology:

Charles C. Di Peso’s excavations at Wind Mountain. Albuquerque, NM: University of New Mexico Press. 463 p.

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Worster, D. 1993. The wealth of nature: environmental history and the

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Wozniak, F. E. 1996. Human impacts on riparian ecosystems of the

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40 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

Appendix A _____________________________________________________________

WHEELER SURVEY BIRD DATA (Henshaw 1875)

The following lists only those species that are referred to as “Flycatcher” by Henshaw.

Ash-throated Flycatcher (Myiarchus cinerascens)

Where Collected Survey Year Number of Birds

Fort Wingate, AZ 1873 3

Inscription Rock, NM 1873 1

Camp Apache, AZ 1874 2

Camp Bowie, AZ 1874 1

Camp Crittenden, AZ 1874 1

Notes

“Having a distribution nearly coincident with that of the preceding species, though extending somewhat farther

to the north into Utah, Nevada, and Colorado. It is less abundant than the preceding [Cassin’s], and inhabits much

the same style of country, affecting rough, rocky country sparsely wooded, or the brushy creek bottoms, even

extending its range out to a considerable distance on the dry plains” (Henshaw 1875:345-346).

Recorded as the Yellow-bellied Flycatcher (Myiodynastes luteiventris), now known as the

Sulphur-bellied Flycatcher

Where Collected Survey Year Number of Birds

Chiricahua Mountains, AZ 1874 5

Notes

“This peculiar flycatcher appears to be a summer resident of the Chiricahua Mountains, Southern

Arizona, were I obtained a pair of old birds, together with three young . . .” (Henshaw 1875:346-347).

Olive-sided Flycatcher (Contopus borealis)

Where Collected Survey Year Number of Birds

Nevada 1871 1

Near Ft. Garland, CO 1873 5

Rio Grande, CO 1873 2

Camp Apache, AZ 1873 2

Willow Springs, AZ 1874 1

Indian Valley, CO 1874 1

Bowie Agency, AZ 1874 2

Trinchara Creek, CO 1874 1

Conejos River, CO 1874 1

Mount Graham, AZ 1874 2

Black River, AZ 1874 1

Notes

“The Olive-sided Flycatcher appears to be much more abundant through the West generally than at the East, and

in parts of Utah and Colorado has been found by our parties in considerable numbers. It is a highly characteristic

bird of the pine region, ranging from about 7,000 feet up to timber line.”

“We found it almost as numerous in Eastern Arizona, quite far to the south, as in Colorado; but I supposed that

it was only thus present during the migrations. The past season [1873], however, specimens were taken near Camp

Apache in July, which doubtless were breeding, and later, about the middle of August, young and old were secured

near Camp Bowie, within one hundred miles of Mexico. Its replacement, therefore, in this region by Contopus

pertinax would appear to be only partial, and the two breed in the same districts.” (Henshaw 1875:350-351).

Little Flycatcher (Empidonax trailli, var. Extimus)

Where Collected Survey Year Number of Birds

Humboldt River, NV 1871 1

Provo, UT 1872 23

Wahsatch Mountains, UT 1872 1

Fort Garland, CO 1873 5

White Mountains, AZ 1873 1

Camp Apache, AZ 1873 1

Near Camp Apache, AZ 1873 2

Pueblo, CO 1874 1

Camp Bowie, AZ 1874 4
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 41

Fort Garland, AZ 1874 1

Camp Crittenden, AZ 1874 1

Pagosa, CO 1874 1

Notes

“Exceedingly numerous near Provo River in willow thickets; sparingly so in Eastern Nevada. Very quick and

nervous in its movements, constantly crossing and recrossing the river and catching insects.... Colorado, New

Mexico, and Arizona are all included in the range of this flycatcher; its abundance being dependent upon the

presence or absence of its favorite grounds.... Wherever willows are found growing in small clumps or fringing the

stream, this flycatcher is almost certain to be found common, and it is rarely seen in the summer in other

situations. Its habits and notes appear to be identical with those of its eastern analogue, from which it differs

mainly in its paler coloration. The nest is placed in the upright fork of a bush or sapling a few feet from the ground,

and is composed of grasses and fibrous material, rather loosely woven together, and lined with fine grasses. Its

general appearance is much like that of the nest of the Yellow Warbler, D. aestiva, but it is not nearly so compact

nor artistic.” (Henshaw 1875:356-358).

Least Flycatcher (Empidonax minimus)

Where Collected Survey Year Number of Birds

Denver, CO 1873 1

Henshaw’s notes state that the habitat and behavior of this bird are much like that of Empidonax trailli (Henshaw

1875:358-359).

Recorded as the Western Yellow-bellied Flycatcher (Empidonax flaviventris, var. Difficilis), now known as the

Cordilleran Flycatcher (Empidonax occidentalis)

Where Collected Survey Year Number of Birds

Rio Grande, CO 1873 1

Fort Wingate, NM 1873 1

Inscription Rock, NM 1873 1

Willow Springs, AZ 1874 2

Mount Graham, AZ 1874 1

Camp Bowie, AZ 1874 1

Henshaw’s notes state that the habitat and behavior of this bird are much like that of Empidonax trailli(Henshaw

1875:359-360).

Wright’s Flycatcher (Empidonax obscurus)

Where Collected Survey Year Number of Birds

Bull Run, NV 1871 2

Provo, UT 1872 1

Snake Creek, NV 1872 1

Denver, CO 1873 2

Fort Wingate, NM 1873 1

Inscription Rock, NM 1873 1

Camp Apache, AZ 1873 3

South of Camp Apache, AZ 1873 2

Santa Fe, NM 1874 2

Camp Bowie, AZ 1874 2

Fort Garland, CO 1874 3

Henshaw’s notes state that the habitat and behavior of this bird are much like that of Empidonax traillii (Henshaw

1875:360-361).

Hammond’s Flycatcher (Empidonax Hammondi)

Where Collected Survey Year Number of Birds

Beaver, UT 1872 1

Cedar, UT 1872 1

Rio Grande, NM 1873 1

Camp Apache, AZ 1873 5

Gila River, AZ 1873 4

Fort Bayard, NM 1873 1

Navajo Creek, NM 1874 1

Mount Graham, AZ 1874 6

Gila River, AZ 1874 1
42 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

Henshaw’s notes state that the habitat and behavior of this bird are much like that of Empidonax trailli (Henshaw

1875:362-363).

Buff-breasted Least Flycatcher (Mitrephorus fluvifrons, var. Pallescens)

Where Collected Survey Year Number of Birds

Inscription Rock, NM 1873 5

Camp Apache, AZ 1873 1

Bowie Agency, AZ 1874 1

Notes

Little noted for this bird (Henshaw 1875:364).

Red Flycatcher (Pyrocephalus rubineus, var. Mexicanus)

Where Collected Survey Year Number of Birds

Pueblo Viejo, AZ 1873 1

Sonoita Valley, AZ 1874 3

Camp Lowell, AZ 1874 4

Gila River, AZ 1874 1

Notes

Little noted for this bird (Henshaw 1875:365).
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 43

Mark K. Sogge

Robert M. Marshall

Chapter 5:

A Survey of Current Breeding Habitats

The distribution and abundance of a species across

a landscape depends, in part, on the distribution and

abundance of appropriate habitat. If basic resource

needs such as food, water, and cover are not present,

then that species is excluded from the area. Scarcity of

appropriate habitat is generally the key reason for the

status of most rare and endangered species. An understanding of an endangered species’ habitat characteristics is crucial to effective management, conservation

and recovery.

The southwestern willow flycatcher (Empidonax

traillii extimus) breeds in dense riparian habitats in

all or parts of seven southwestern states, from sea

level in California to over 2600 m in Arizona and

southwestern Colorado. Although other willow flycatcher subspecies often breed in shrubby habitats

away from surface water (Bent 1942, McCabe 1991),

E.t. extimus breeds only in dense riparian vegetation

near surface water or saturated soil. Other habitat

characteristics such as dominant plant species, size

and shape of habitat patch, canopy structure, vegetation height, etc., vary widely among sites. Our objective in this chapter is to present an overview of southwestern willow flycatcher breeding habitat, with an

emphasis on gross vegetation characteristics. Although

quantitative studies of habitat have begun in some

areas (e.g., Spencer et al. 1996, Whitfield and Enos

1996, McKernan and Braden 1999, Paradzick et al.

1999), we focus here on qualitative information on

plant species composition and structure. Although

many of the details of vegetation characteristics differ

among breeding sites, we will draw attention to those

common elements or themes that are shared by most

sites. All of the breeding sites described herein are

within the geographic range currently administered

as the southwestern subspecies (E.t. extimus) by the

U.S. Fish and Wildlife Service. Several on-going studies could ultimately change the accepted boundary

designations for E.t. extimus. Thus, some of the breeding sites described may eventually be removed from

E.t. extimus range, while new sites could be added.

Any such changes may provide new perspectives on

southwestern willow flycatcher habitat.

What is “Habitat”?_________________

Birds and bird communities have played a major

role in the development of the concept of habitat, yet

specific definitions of the term habitat are often vague

and/or differ from one another (Block and Brennan

1993). However, a common theme among different

definitions and terms is that “habitat” includes the

physical and biological environmental attributes that

influence the presence or absence of a bird species

(Morrison et al. 1992). Thus, habitat involves many

components in addition to vegetation composition and

structure. Environmental features (climate, food, patch
44 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

size or area), predation, competition, parasitism, disease, disturbance, past history and even chance influence the current distribution and abundance of species (Wiens 1989a, 1989b). Research is usually focused

on those habitat components that are most easily or

reliably quantified and/or considered most likely to

influence the bird community, and no single study can

address all of the factors that may influence bird

species use in a system.

Many factors underlie habitat selection and these

factors do not act equally for all species or even for all

populations of a single species (Wiens 1989a, 1989b).

A species’ morphological and physiological traits allow

it to exploit subsets of resources and, hence, certain

habitats (Morrison et al. 1992). Life-history traits

such as foraging behavior and mating strategies are

also mechanisms that underlie habitat selection in a

species (Hansen and Urban 1992). Proximate factors

such as song perches, nest sites, and the structure and

composition of the vegetation determine whether a

bird settles in a habitat. These are part of a habitat

selection “template” (Wiens 1989a) that results from

both an individual’s genetic makeup and information

learned through experience with different areas and

habitats. Ultimately, the suitability of a particular

habitat is a function of reproductive success and survivorship. Thus, mere occupancy of a habitat does not

imply the habitat is optimal, only that it meets the

selection template for those individuals breeding there.

There has yet to be developed a comprehensive habitat

model for the southwestern willow flycatcher that

enables one to determine which breeding habitats, or

parts of a single breeding patch, are better than others

based on vegetation characteristics alone.

General Vegetation Composition and

Structure _________________________

Southwestern willow flycatcher breeding habitat

can be broadly described based on plant species composition and habitat structure. These two habitat

characteristics are the most conspicuous to human

perception, but are not the only important components. However, they have proven useful in describing

known breeding sites, evaluating suitable survey habitat, and in predicting where breeding flycatchers may

be found.

We have organized habitat descriptions into three

broad types - native vegetation dominated, exotic

vegetation dominated, and mixed native/exotic. These

broad habitat descriptors reflect the fact that southwestern willow flycatchers now inhabit both native

and non-native dominated riparian habitats. Saltcedar

(Tamarix ramosissima) and Russian olive (Elaeagnus

angustifolia) are used as nesting substrates and in

some cases, flycatchers breed where these species

dominate the canopy or occur in nearly monotypic

stands. Data on the most conspicuous plant species at

106 flycatcher breeding sites (Table 5-1) demonstrate

the widespread use of both native and exotic trees and

shrubs.

Narrative descriptions of the general vegetation

types used throughout the southwestern willow

flycatcher’s range are provided below, with a focus on

the dominant tree and shrub components. The habitat

types described include a continuum of plant species

composition (from nearly monotypic to diverse assemblages) and vegetation structure (from simple, single

stratum patches to complex, multiple strata patches).

Because pictures are often more effective than verbal

descriptions at conveying the general nature of a

riparian patch, we include one or more photographs of

each type of occupied breeding habitat. The intent of

the descriptions and photographs is to provide a basic

understanding of the types of habitat occupied by the

flycatcher, not to create a standardized definition or

classification. All known breeding sites are not described or illustrated, so every potential variant is not

shown. However, the sites presented capture most of

the known range of patch floristics, structure, and

size.

Native Vegetation Dominated

Many of the areas used by breeding southwestern

willow flycatchers are dominated by native trees and

shrubs, especially, as one might expect based on the

bird’s common name, willows (Salix spp.). The floristic

and gross structural variation of occupied nativedominated habitats is quite broad. Occupied sites vary

from monotypic, single strata patches to multi-species, multi-layered strata with complex canopy and

subcanopy structure. Overall, low to mid-elevation

sites and high elevation sites differ substantially, and

are treated separately below.

Low to Mid-Elevation Native Sites:

General characteristics: These sites range from

single plant species to mixtures of native broadleaf

trees and shrubs including (but not limited to)

Goodding’s (Salix gooddingii) or other willow species,

cottonwood (Populus spp.), boxelder (Acer negundo),

ash (Fraxinus spp.), alder (Alnus spp.), and buttonbush (Cephalanthus occidentalis). Average canopy

height can be as low as 4 m or as high as 30 m. Gross

patch structure is generally characterized by trees of

different size classes, often forming a distinct overstory of cottonwood, willow or other broadleaf tree

with recognizable subcanopy layers and a dense understory of mixed species. Although some descriptions

of flycatcher breeding habitat emphasize these multispecies, canopied associations, flycatchers also breed

at sites with tall (>5 m or greater) monotypic willow.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 45

Table 5-1. Frequency of occurrence of different types of southwestern willow flycatcher

breeding sites based on whether the tree and shrub components are dominated by native or exotic species, or a mixture of both. Data are for 106 known

breeding sites (as of 1998) from Ahlers and White (1999), Sferra et al. (1997),

McKernan and Braden (1998), Cooper (1997), and USFWS unpublished data.

Native Exotic Mixed

State Dominated Dominated Native/Exotic Total

Arizona 12 3 30 45

California 11 0 8 19

Colorado 8 0 2 10

New Mexico 14 0 10 24

Nevada 0 0 1 1

Utah 4 2 1 7

Total 49 5 52 106

Exotic or introduced trees and shrubs may be a rare

component at these sites, particularly in the understory. In an unusual site along the upper San Luis Rey

River in San Diego County, CA, willow flycatchers

breed in a streamside area dominated by live oak

(Quercus agrifolia), where willows once predominated

but were eliminated by a phreatophyte control program several decades ago (W. Haas, pers. comm.).

Examples

South Fork Kern River at Lake Isabella, Kern

County, CA. Elevation 780 m. (see Whitfield and

Enos 1996). This is one of the largest tracts of monotypic native-dominated flycatcher habitat in the

Figure 5-1. Breeding site at the South Fork, Kern River, CA. Note the canopy height and

breadth of floodplain at this cottonwood-willow dominated site. Photo by Mark Sogge.

Southwest (Figure 5-1). The site includes roughly

500 ha of riparian woodland dominated by a dense

overstory of red willow (Salix laevigata) and

Goodding’s willow, interspersed with open areas often dominated by nettle (Urtica dioica), mule fat

(Baccharis salicifolia), cattails (Typha spp.), and tules

(Scirpus spp.). Canopy height is typically 8 to 12 m.

This site has numerous river channels, sloughs, and

marshes that provide surface water and saturated

soils throughout most of the breeding season (Figure 5-2).

Santa Ynez River, Santa Barbara County, CA. (see

Holmgren and Collins 1995). Willow flycatchers breed
46 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

at several areas along the perennial Santa Ynez River

between Buellton (elevation approximately 150 m) and

the ocean. These species-rich riparian sites (Figure

5-3) are comprised of red willow, black cottonwood

(Populus trichocarpa), and box elder, with dense,

shrubby thickets of willows (Salix lasiolepis and S.

exigua), mulefat, poison oak (Toxicodendron

Figure 5-2. Breeding site at the South Fork, Kern River, CA. Note the dense

tangle of willow understory and small openings directly above surface water.

Photo by Mark Sogge.

Figure 5-3. Breeding site on the Santa Ynez River, CA. Note the structural

complexity and density of the multiple native broadleaf species, and the proximity

to surface water. Photo by Mark Sogge.

diversilobum), and blackberry (Rubus spp.). Beaver

dams pond water in may areas along the river, creating slow-water and emergent marsh conditions.

San Pedro River, Pinal County, AZ. Elevation 600 m.

(see Spencer et al. 1996). Several flycatcher breeding

sites along this narrow riparian system are dominated

primarily by cottonwood and willow, with some ash
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 47

and boxelder overstory. Understory is comprised of

younger trees of these same species, with saltcedar as

a major to minor component in some areas. Overstory

canopy height averages 15 to 20 m. Open water,

marshes and seeps (including cattail and bulrush;

Figure 5-4), and saturated soil are present in the

immediate vicinity.

Gila River, Grant County, NM. Elevation 1,480 m.

(see Skaggs 1996, Stoleson and Finch 1998). The

Figure 5-4. Breeding site on the San Pedro River, AZ. Note the emergent

plants bordering the dense willow and buttonbush-dominated patch. Surface

water is present throughout this site. Photo by Mark Sogge.

Figure 5-5. Breeding site on the Gila River, NM. Note the stringers of riparian

“scrub,” some of which are less than 10 m wide, but in total form a wider

mosaic. The exposed banks are the result of past livestock grazing. Photo by

Rob Marshall.

largest known population of breeding southwestern

willow flycatchers is found in a series of riparian

patches distributed over a 13 km stretch of the Gila

River. Flycatchers breed in two distinct structural

types; riparian scrub and riparian forest. Riparian

scrub (Figure 5-5) is dominated by 4 to 10 m tall

shrubby willows and seepwillow (Baccharis glutinosa)

that grow along the river bank or in old flood channels.

These shrub strips are sometimes less than 10 m wide
48 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

and rarely more than 20 m. Riparian forest patches

(Figure 5-6) were 100 to 200 m wide, and dominated by

trees such as Fremont cottonwood (Populus fremontii),

Goodding’s willow, Arizona sycamore (Plantanus

wrightii) and boxelder. Understory includes young

trees of the same species. Canopy height generally

ranges between 20 and 30 m. Much of this forest

vegetation is sustained by water from the river and

small, unlined water diversions that function much

like a dendritic stream system.

High-Elevation Native Sites

General characteristics: As a group, these sites are

more similar than low elevation native sites. All known

high elevation (1,900 m and above) breeding sites are

comprised completely of native trees and shrubs. Most

sites are dominated by a single species of willow, such

as Coyote willow (Salix exigua) or Geyer’s willow (S.

geyeriana). Average canopy height is generally only 3

to 7 m. Gross patch structure is characterized by a

single vegetative layer with no distinct overstory or

understory. There is usually very dense branch and

twig structure in the lower 2 m, with high live foliage

density from the ground to the canopy. Tree and shrub

vegetation is often associated with sedges, rushes,

nettles and other herbaceous wetland plants. These

willow patches are usually found in mountain meadows and are often associated with stretches of stream

or river that include many beaver dams and pooled

water.

Figure 5-6. Breeding site on the Gila River, NM. Note the openings within the dense

cottonwood and boxelder and the channel with agricultural tailwater in the bottom

foreground. Photo by Rob Marshall.

Examples

Little Colorado River near Greer, Apache County,

AZ. Elevation 2500 m. (see Spencer et al. 1996,

Langridge and Sogge 1997). This 14 ha site is a mosaic

of dense, shrubby Geyer’s willow (Figure 5-7), dense

herbaceous ground cover, and open water. The river

and associated beaver ponds create marshes, wet

meadows and saturated soil conditions. Average willow canopy height is 4 to 6 m. The willow matrix is a

combination of clumps and thin (3 to 5 m wide) strips.

The shrubby vegetation is structurally composed of a

single layer of live vegetation, with dense branch and

twig structure and high live foliage density from ground

level to canopy. Habitat surrounding the broad valley

is primarily ponderosa pine (Pinus ponderosa) and

scattered houses and cabins.

Beaver Creek, Dolores County, CO. Elevation

2,440 m. (see Owen and Sogge 1997). This is a large

site, at least 3,200 m long and 400 to 500 m wide,

located in a broad, wide mountain valley. The shrubby

vegetation (Figure 5-8) is dense, almost monotypic

willow with small amounts of hawthorne (Crataegus

rivularis). Numerous stream channels and associated

beaver ponds create wet or flooded substrates, as well

as openings within the dense vegetation.

Exotic Vegetation Dominated

General characteristics: Exotic plant species such

as saltcedar and Russian olive were not introduced or
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 49

Figure 5-7. Breeding site on the Little Colorado River in the White

Mountains, AZ. Note the dense shrubby appearance of these high elevation willows not yet fully in leaf. Beaver dams retain surface water

throughout the patch during the breeding season. Photo by Mark Sogge.

Figure 5-8. Breeding site on Beaver Creek, CO. Another site where beaver

dams pond surface water. Note density and height of willows and patch

opening in foreground. Photo by Jen Owen (USGS).

widespread in southwestern riparian systems until

approximately 100 years ago. Thus, southwestern

willow flycatchers evolved in and until fairly recently

(from an evolutionary perspective) bred exclusively

within thickets of native riparian vegetation such as

willows, cottonwoods and seepwillow. However, southwestern willow flycatchers have responded to the

widespread loss and modification of native riparian

habitats by nesting within some exotic-dominated

habitats. From the standpoint of flycatcher productivity and survivorship, the suitability of exotic-dominated sites is not known. Flycatcher productivity in

some exotic-dominated sites is lower than in some

native-dominated habitats (Sferra et al. 1997, Sogge

et al. 1997), but other factors such as small patch size

may be more important correlates of productivity at

those sites. The reverse is also true, with some

saltcedar-dominated sites having similar or higher

flycatcher productivity than nearby native sites

(McKernan and Braden 1999, Paradzick et al. 1999).

Thus, there is currently no clear evidence that the

exotic-dominated habitats in which southwestern

willow flycatchers now breed are generally

suboptimal.

Southwestern willow flycatchers do not nest in all

exotic species that have invaded and sometimes dominate riparian systems. For example, flycatchers do

not use arundo (Arundo donax) or tree of heaven
50 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

(Ailanthus altissima). Even in the widespread

saltcedar, flycatchers tend to use only two conspicuous life forms: (a) low to mid-stature saltcedar (3-6 m

tall) found as a component in the understory of a

native cottonwood-willow gallery forest, or (b) tall (6-

10 m) mature stands of saltcedar that have a high

percentage of canopy closure. Thus, willow flycatchers are largely absent as a breeding species throughout most of the saltcedar habitats of the Southwest,

where saltcedar stands are often too short, sparse,

or dry.

Most exotic habitats range below 1,200 m elevation.

As a group, they show almost as much variability as do

low elevation native-dominated sites. Most exotic sites

are nearly monotypic, dense stands of exotics such as

saltcedar or Russian olive that form a nearly continuous, closed canopy (with no distinct overstory layer).

Canopy height generally averages 5 to 10 m, with

canopy density uniformly high. The lower 2 m of

vegetation is often very difficult to penetrate due to

dense branches. However, live foliage density may be

relatively low from 0 to 2 m above ground, but increases higher in the canopy.

Examples

Roosevelt Lake, Gila County, AZ. Elevation 640 m.

(see Spencer et al. 1996, Sferra et al. 1997). Two of the

largest known southwestern willow flycatcher populations in Arizona breed in large, contiguous stands of

dense, mature saltcedar at the Tonto Creek and Salt

River inflows to Roosevelt Lake (Figures 5-9 and 5-10).

The Salt River site is monotypic saltcedar, while the

Tonto Creek site includes a few scattered, large cottonwood trees that emerge above the saltcedar canopy,

Figure 5-9. Breeding site on Salt River inflow to Roosevelt

Lake, AZ. Note dense, tall, monotypic stand of saltcedar with

openings in the patch interior. No surface water was present

when photo was taken. Photo by Mark Sogge.

Figure 5-10. Breeding site on the Salt River inflow to Roosevelt

Lake, AZ. Note the breadth of this floodplain habitat and the

numerous openings interspersed within the dense mature

saltcedar stand. Surface water was present when this photo

was taken in June 1996 by the U.S. Bureau of Reclamation.

Figure 5-11. Breeding site at Topock March, Colorado River,

AZ. This illustrates the dense vegetative structure (often dead

branches) in the lower 3 to 4 m, and the numerous small

branches providing potential nest sites, common to occupied

saltcedar stands. Photo by Mark Sogge.

which averages 8 to 12 m in height. Within the patches,

there are numerous small openings in the canopy and

understory. As is usually the case in such mature

saltcedar stands, there is little live foliage below a

height of 3 to 4 m within the interior of the patch

(although live foliage may be continuous and thick at

the outer edges of the patch), and virtually no herbaceous ground cover. However, numerous dead branches

and twigs provide for dense structure in the lower 2 to

3 m strata (Figure 5-11). In normal or wet precipitation years, surface water is adjacent to or within the

saltcedar patches.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 51

Figure 5-12. Colorado River in Grand Canyon, AZ. Tall, dense saltcedar

borders a backwater channel on the Colorado River. Note the dense live

vegetation from ground to upper canopy along the outer edge of the patch.

Photo by Mark Sogge.

Colorado River in Grand Canyon, Coconino County,

AZ. Elevation 850 m. (see Sogge et al. 1997). The

willow flycatcher breeding sites along the Colorado

River in the Grand Canyon (Figure 5-12) are very

small (0.6 to 0.9 ha), dense patches of mature saltcedar,

bordered on the upslope side by acacia (Acacia greggii)

and along the river’s edge by a thin band of willow.

Saltcedar canopy height averages 8 to 12 m. Live

foliage is dense and continuous along the edge of the

patch, but does not begin until 3 to 4 m above ground

Figure 5-13. Breeding site on Rio Grande, NM. This dense Russian olivedominated patch is bordered by emergent marsh and slough channel

adjacent to the Rio Grande. Photo by Mark Sogge.

within the patch interior. A dense layer of dead branches

and twigs provides for a thick understory below the

live vegetation. These sites have almost no herbaceous

understory due to a dense layer of fallen saltcedar

branches and needles. All patches are no further than

5 m from the river’s edge.

Rio Grande at San Juan Pueblo, Rio Arriba County,

NM. Elevation 1,800 m. (see Maynard 1995, Cooper

1997). Southwestern willow flycatchers breed in

dense riparian vegetation (Figure 5-13) dominated
52 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

by Russian olive. Several large cottonwoods rise

above the Russian olive canopy. The patch is bordered by emergent marsh on one side and the Rio

Grande on the other. Canopy height of the Russian

olive averages 8 to 12 m in height.

Mixed Native and Exotic Habitats

General characteristics: Many southwestern willow flycatcher breeding sites are comprised of dense

mixtures of native broadleaf trees and shrubs mixed

with exotic/introduced species such as saltcedar or

Russian olive. The exotics are often primarily in the

understory, but may be a component of overstory. At

several sites, saltcedar provides a dense understory

below an upper canopy of gallery cottonwoods, forming a habitat that is structurally similar to the cottonwood-willow habitats in which flycatchers historically nested. A particular site may be dominated

primarily by natives or exotics, or be a more-or-less

equal mixture. The native and exotic components may

be dispersed throughout the habitat or concentrated

in distinct, separate clumps within a larger matrix.

Sites almost always include or are bordered by open

water, cienegas, seeps, marshes, and/or agricultural

runoff channels. However, during drought years surface water at some sites may be gone early in the

breeding season. Generally, these habitats are found

below 1,200 m elevation.

Examples

San Pedro River, Pinal County, AZ. Elevation 600

m. (see Spencer et al. 1996). Parts of the extensive

riparian tracts of the lower San Pedro River are

dominated by cottonwood and willow, but include

substantial amounts of dense saltcedar. In some

cases, the saltcedar occurs as a dense understory

amidst a cottonwood, willow, ash or boxelder overstory (Figure 5-14), while in others it borders the

edge of the native vegetation (Figure 5-15). Overall

canopy height ranges from 10 to 18 m.

Verde River at Camp Verde, Yavapai County, AZ.

Elevation 940 m. (see Spencer et al. 1996). Southwestern

willow flycatchers breed here in a mixture of willow,

cottonwood, and saltcedar habitat (Figure 5-16). Most

of the territories are found in a cluster of dense

decadent saltcedar (6 to 8 m tall) bordered by narrow

bands of young willow, which in turn are surrounded

on one side by a large (>50 ha) stand of mature

cottonwoods and willows (15-20 m tall) with little

understory. Although the patch itself is located on a

sandy terrace approximately 4 m above typical summer river level, the Verde River flows along the

eastern edge of the patch and a small intermittently

flowing irrigation ditch provides water to a small pond

adjacent to the saltcedar and willows. Patches of

herbaceous ground cover are scattered throughout

the site, but are absent under the saltcedar canopy.

Figure 5-14. Breeding site on the San Pedro River, AZ. Note

the dense 5 to 6 m tall saltcedar interspersed with the taller

cottonwood overstory. Photo by Renee Netter (USGS).

Figure 5-15. Breeding site on San Pedro River, AZ. Note

the height, density and openings at this mixed native-exotic

site. Surface water is present outside the frame. Photo by

Eben Paxton (USGS).

Figure 5-16. Breeding site on the Verde River at Camp

Verde, AZ. Note the tall cottonwoods and willows mixed

with saltcedar. Photo by Mark Sogge.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 53

Figure 5-17. Breeding site on Virgin River, UT. This dense

mixture of native and exotics is bordered by slough channels

which create openings within the patch. The person in foreground is on a terrace 2 to 3 m higher than the terrain in which

the riparian vegetation is rooted. Photo by Mark Sogge.

Virgin River, Washington County, UT. Elevation

1,100 m. (USFWS unpublished data). Along one portion of Virgin River riparian corridor near St. George,

flycatchers breed in a mixture of dense willow, Russian olive and saltcedar near an emergent marsh

(Figure 5-17). The native trees form a tall (10-12 m)

overstory, which is bordered by a shorter (10-12 m)

band of saltcedar, and a strip of 4 to 8 m tall willow. The

stretch of occupied habitat is approximately 60 m wide

and 100 m long, and is located in an old meander

channel through which the river no longer flows. In

normal and wet years return channels and river flows

seasonally inundate the base of the vegetation.

Patch Size and Shape ______________

The riparian patches used by breeding flycatchers

vary greatly in size and shape. They may be a relatively dense, linear, contiguous stand or an irregularly-shaped mosaic of dense vegetation with open

areas. Southwestern willow flycatchers have nested

in patches as small as 0.6 ha in the Grand Canyon

(Sogge et al. 1997), and as large as 100 ha or more at

Roosevelt Lake (Spencer et al. 1996) and Lake Mead

(McKernan 1997). Most sites fall between these two

extremes, and overwhelmingly toward the smaller

end (probably because large blocks of suitable riparian habitat are uncommon). Flycatchers have not been

found nesting in narrow, linear riparian habitats

where the entire patch is less than approximately 10

m wide, although they will use such linear habitats

during migration.

Except in the extreme smallest cases (such as the

saltcedar patches in the Grand Canyon), all flycatcher breeding patches are larger than the sum

total of the flycatcher territory sizes at that site. This

is because flycatchers, typically, do not pack their

territories into all available space within a habitat.

Instead, some territories are bordered by additional

riparian vegetation that is not defended as a nesting

territory, but may be important in attracting flycatchers to the site and/or in providing an environmental buffer (from wind or heat) and in providing

post-nesting use areas. Based on numerous habitat

use studies (Whitfield and Enos 1996, Paxton et al.

1997, Sferra et al. 1997, Sogge et al. 1997) it is clear

that flycatchers often cluster their territories into

small portions of riparian sites, and that major portions of the site may be occupied irregularly or not at

all. It is currently unknown how size and shape of

riparian patches relate to factors such as flycatcher

site selection and fidelity, reproductive success, predation, and brood parasitism.

Presence of Water _________________

Flycatcher breeding habitats usually include or are

near open water, cienegas, marshy seeps, or saturated

soil. In many cases, nest plants are rooted in or

overhang standing water (Sferra et al. 1997, Whitfield

and Enos 1996). As a general rule, flycatcher territories are seldom farther than a few dozen meters from

water or saturated soil. However, it is critical to keep

in mind that in the Southwest, hydrological conditions

at a site can vary remarkably within a season and

between years. At some locations, particularly during

drier years, water or saturated soil is only present

early in the breeding season (i.e., May and part of

June). At other sites, vegetation may be immersed in

standing water during a wet year, but be hundreds of

meters from surface water in dry years. This is particularly true of reservoir sites such as the Kern River

at Lake Isabella, Tonto Creek and Salt River at

Roosevelt Lake, and the Rio Grande near Elephant

Butte Reservoir. Human-related factors such as river

channel modifications (e.g. by creation of pilot channels) or altered subsurface flows (e.g. from agricultural runoff) can temporarily or permanently dry a

site. Similarly, where a river channel has changed

naturally (Sferra et al. 1997), there may be a total

absence of water or visibly saturated soil for several

years. In such cases, the riparian vegetation and any

flycatchers nesting within it may persist for several

years. However, we do not know how long such sites

will continue to support riparian vegetation and/or

remain occupied by breeding flycatchers.
54 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

Other Habitat Components ________

Other potentially important aspects of southwestern willow flycatcher habitat include distribution and

isolation of vegetation patches, hydrology, prey types

and abundance, parasites, predators, and interspecific competition. Population dynamics factors such as

demography (i.e. birth and death rates, age-specific

fecundity), distribution of breeding groups across the

landscape, flycatcher dispersal patterns, migration

routes, site fidelity, philopatry, and conspecific sociality also influence where flycatchers are found and

what habitats they use. Environmental factors (e.g.

temperature, humidity), may play an important role

in habitat selection, breeding success and persistence,

particularly in lowland desert riparian areas. Most of

these factors are poorly understood, but may be critical to understanding current population dynamics

and habitat use.

Common Factors and Mechanism for

Selection _________________________

Clearly, willow flycatchers breed in widely different

types of riparian habitat across a large elevational

range and geographical area in the Southwest. Breeding patch size, configuration, and plant species composition can vary dramatically across the subspecies’

range. However, certain patterns do emerge and are

seen at most sites. Regardless of the plant species

composition or height, occupied sites always have

dense vegetation in the patch interior. In most cases

this dense vegetation occurs within the first 3 - 4 m

above ground. These dense patches are often interspersed with small openings, open water, or shorter/

sparser vegetation, creating a mosaic that is not uniformly dense. In almost all cases, slow-moving or still

surface water and/or saturated soil will be present at

or near breeding sites during wet or normal precipitation years.

These themes common to flycatcher breeding sites

– dense vegetation and proximity to water – probably relate directly to the underlying mechanisms

driving habitat selection and site suitability. For

example, breeding riparian birds in the desert Southwest are potentially exposed to extreme environmental conditions (Hunter 1988, Rosenberg et al.

1991). Dense riparian vegetation with surface water

or saturated soil may be needed to provide suitable

micro-climatic conditions, therefore limiting the distribution of flycatchers to a subset of available

riparian habitats. Given that willow flycatchers are

one of the latest nesting birds in Southwestern

desert riparian systems (Hunter 1988), their nests

may require substantial buffering against extreme

environmental conditions. Dense vegetation and

surface water may also function to reduce nest

predation and cowbird nest parasitism, both of which

may be important factors in site suitability.

Currently, we can not distinguish the relative

importance of each of the many factors that influence southwestern willow flycatcher habitat use.

The relative importance of particular factors may

vary geographically, and at the local scale males

and females may be selecting for different factors or

habitat characteristics (Sedgwick and Knopf 1992).

All of this complicates our ability to develop quantitative predictions of flycatcher habitat use. Ongoing and future research (e.g., Paradzick et al.

1999, McKernan and Braden 1999, Ahlers and White

1999, others) on local and landscape patch configuration, vegetation characteristics, productivity, and

environmental factors will better determine the

mechanisms responsible for habitat use patterns

and spur development of accurate and comprehensive habitat suitability models for the southwestern

willow flycatcher.

Habitat Suitability _________________

The ultimate measure of habitat suitability is not

simply whether or not a site is occupied. Suitable

habitats are those in which flycatcher reproductive

success and survivorship results in a stable or growing

population. Without long term data showing which

sites have stable or growing populations, we cannot

determine which habitats are suitable or optimal for

breeding southwestern willow flycatchers. Some occupied habitats may be acting as population sources,

while others may be functioning as population sinks

(Pulliam 1988).

What is not Southwestern Willow

Flycatcher Breeding Habitat ________

Cottonwood-willow gallery forests that are devoid

of an understory and that appear park-like do not

provide nesting habitat for southwestern willow flycatchers. Similarly, isolated, linear riparian patches

less than approximately 10 m wide generally do not

provide nesting habitat. However, mosaics made up

of aggregations of these small, linear riparian “stringers” may be used by breeding flycatchers, particularly at high elevations. High-elevation willow patches

devoid of live vegetation structure in the lower strata

(0-2 m from ground) are not used for nesting. Short

stature (< 4 m) saltcedar stands, as well as sparse

stands of saltcedar characterized by a scattering of

trees of any height, also do not provide nesting habitat for flycatchers. See Figures 5-18 – 5-21 for examples of some of the common riparian habitat
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 55

types that are not suitable for nesting southwestern

willow flycatchers.

Migrant willow flycatchers may occur in nonriparian habitats and/or be found in some riparian

habitats unsuitable for breeding. Such migration

stopover areas, even though not used for breeding,

may be critically important resources affecting

local and regional flycatcher productivity and survival. Furthermore, such sites may be appropriate

candidates for restoration efforts designed to create

additional willow flycatcher breeding habitat.

Acknowledgments ________________

Most of the information presented in this chapter

comes from unpublished reports and personal communications. We thank the many authors, researchers, and state agencies for allowing use of their data.

Figure 5-18. Native riparian vegetation that is not suitable

flycatcher breeding habitat. This park-like gallery forest along

a river in Colorado is tall and wide, but devoid of understory

does not provide breeding habitat for the flycatcher. Photo by

Mark Sogge.

Figure 5-19. Native riparian vegetation that is not suitable

flycatcher breeding habitat, along Crystal Creek in the Grand

Canyon, AZ. Such extremely narrow, linear riparian habitats

do not provide breeding habitat for the flycatcher. Photo by

Tim Tibbitts (NPS).

Figure 5-21. Saltcedar-dominated riparian vegetation that is

not suitable flycatcher breeding habitat. This sparse, lowstature saltcedar stand at Roosevelt Lake, AZ does not provide

the tall, dense overall vegetative structure needed by breeding

flycatchers. Photo by Mark Sogge.

Figure 5-20. High-elevation native riparian vegetation that is

not suitable flycatcher breeding habitat. The cropping of

these willow is a result of livestock grazing. The low stature,

low density, and lack of breadth keeps this area from attaining the attributes necessary for flycatcher breeding. Photo

by Mark Sogge.
56 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

Our gratitude to the following agencies, tribes and

landowners who provided access to flycatcher breeding sites highlighted in this chapter: ASARCO, Grand

Canyon National Park, San Juan Pueblo (NM), UBar Ranch (NM), U.S. Bureau of Reclamation, U.S.

Forest Service, and U.S. Fish and Wildlife Service.

We also thank the many individuals who accompanied us to their study sites: particular thanks to Paul

Boucher (US Forest Service), Bryan Brown (SWCA,

Inc.), William Haas (Varanus Biological Services),

Mark Holmgren (U.C. Santa Barbara), Robert

McKernan (San Bernardino County Museum), Dennis Parker, Cliff Stewart (US Forest Service), and

Mary Whitfield (Kern River Research Center). Tracy

McCarthey (Arizona Game and Fish Department),

Susan Sferra (US Bureau of Reclamation), Linda

Sogge, John Rotenberry and two anonymous reviewers provided many useful comments on drafts of this

manuscript. Preparation of this manuscript was

funded by the U.S. Geological Survey and The Nature

Conservancy.

References _______________________

Ahlers, D. and L. White. 1999. 1998 Southwestern Willow Flycatcher study results. Report by the U.S. Bureau of Reclamation

Technical Service Center. Denver, CO. 72 p.

Bent, A.C. 1942. Life histories of North America flycatchers, larks,

swallows, and their allies. Smithsonian Institution United States

Museum Bulletin 179. US Government Printing Office, Washington, D.C. 622 p.

Block, W.M. and L.A. Brennan. 1993. The habitat concept in

ornithology. In: D.M. Power (ed.). Current Ornithology 11: 35-91.

Cooper, C.A. 1997. Statewide summary of 1996 surveys for Willow

Flycatchers in New Mexico. New Mexico Department of Game

and Fish Report. Contract # 96-516.81.

Hansen, A.J. and D.L. Urban. 1992. Avian response to landscape

pattern: the role of species’ life histories. Landscape Ecology

7:163-180.

Holmgren, M.A. and P. Collins. 1995. Interim report on the distribution, breeding status, and habitat associations of seven federal

special-status bird species and Brown-headed Cowbirds at

Vandenberg Air Force Base, Santa Barbara County, California.

Museum of Systematics and Ecology, Department of Ecology,

Evolution and Marine Biology, University of California. Santa

Barbara, California. Environmental Report No. 3.

Hunter, W.C. 1988. Dynamics of bird species assemblages along a

climatic gradient: a Grinnellian niche approach. MS Thesis.

Arizona State University, Tempe, AZ. 103 p.

Langridge, S.M. and M.K. Sogge. 1997. Banding of the Southwestern Willow Flycatcher in the White Mountains - 1997 summary

report. U.S.G.S. Colorado Plateau Field Station/Northern Arizona University report.

Maynard, W.R. 1995. Summary of survey efforts in New Mexico for

the Southwestern Willow Flycatcher (Empidonax traillii

extimus). New Mexico Department of Game and Fish Report.

Contract # 94-516-69.

McCabe, R.A. 1991. The little green bird: ecology of the Willow

Flycatcher. Rusty Rock Press, Madison, Wisconsin.

McKernan, R.L. 1997. Status, distribution, and habitat affinities of

the Southwestern Willow Flycatcher along the Lower Colorado

River: Year 1 - 1996. Report to the U.S. Bureau of Reclamation,

Boulder City, Nevada.

McKernan, R.L. and G. Braden. 1999. Status, distribution, and

habitat affinities of the southwestern willow flycatcher along the

lower Colorado River: Year 3 – 1998. San Bernardino County

Museum Report. Redlands, CA. 71 pp.

Morrison, M.L., B.G. Marcot and R.W. Mannan. 1992. WildlifeHabitat Relationships; Concepts and applications. University of

Wisconsin Press, Madison, WI. 343 p.

Owen, J.C. and M.K. Sogge. 1997. Banding and genetic sampling of

Willow Flycatchers in Colorado - 1996 & 1997 summary report.

U.S.G.S. Colorado Plateau Field Station/Northern Arizona University report.

Paradzick, C.E., R.F. Davidson, J.W. Rourke, M.W. Sumner and

T.D. McCarthey. 1999. Southwestern Willow Flycatcher 1998

survey and nest monitoring report. Arizona Game and Fish

Department, Nongame and Endangered Wildlife Program Technical Report 141. Phoenix, AZ. 98 p.

Paxton, E., S. Langridge and M.K. Sogge. 1997. Banding and population genetics of Southwestern Willow Flycatchers in Arizona -

1997 summary report. U.S.G.S. Colorado Plateau Field Station/

Northern Arizona University report.

Pulliam, H.R. 1988. Sources, sinks, and population regulation.

American Naturalist 132: 652-661.

Rosenberg, K.V., R.D. Ohmart, W.C. Hunter and B.W. Anderson.

1991. Birds of the Lower Colorado River Valley. University of

Arizona Press, Tucson. 416 pp.

Sedgwick, J.A. and F.L. Knopf. 1992. Describing Willow Flycatcher

habitats: scale perspectives and gender differences. Condor 94:

720-733.

Sferra, S.J., T.E. Corman, C.E. Paradzick, J.W. Rourke, J.A.

Spencer and M.W. Sumner. 1997. Arizona Partners in Flight

Southwestern Willow Flycatcher survey: 1993-1996 summary

report. Arizona Game and Fish Department, Phoenix. Nongame

Technical Report 113.

Skaggs, R.W. 1996. Population size, breeding biology, and habitat of Willow Flycatchers in the Cliff-Gila Valley, New Mexico.

New Mexico Department of Game and Fish report. Contract

#95-516-91.

Spencer, J.A., S.J. Sferra, T.E. Corman, J.W. Rourke and M.W.

Sumner. 1996. Arizona Partners in Flight 1995 Southwestern

Willow Flycatcher survey. Arizona Game and Fish Department,

Phoenix. Nongame Technical Report 97.

Sogge, M.K., T.E. Tibbitts and J.A. Petterson. 1997. Status and

ecology of the Southwestern Willow Flycatcher in the Grand

Canyon. Western Birds 28: 142-157.

Stoleson, S.H. and D.M. Finch. 1998. Reproductive success of

Southwestern Willow Flycatchers in the Cliff-Gila Valley, New

Mexico. USDA Forest Service Rocky Mountain Research Station

report to Phelps-Dodge Corporation. 22 p.

Whitfield, M.J. and K.M. Enos. 1996. A Brown-headed Cowbird

control program and monitoring for the Southwestern Willow

Flycatcher, South Fork Kern River, California, 1996. California

Department of Fish and Game, Sacramento. Final report for

contract #FG4100WM-1.

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316 p.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 57

Mark K. Sogge

Chapter 6:

Breeding Season Ecology

The willow flycatcher (Empidonax traillii) breeds

across much of the conterminous United States and in

portions of extreme southern Canada. As might be

expected in such a wide-ranging species, willow flycatchers in different portions of the range exhibit

differences in appearance, song, and ecological characteristics. The intent of this chapter is to provide

information on the breeding-season ecology of the

southwestern subspecies, E.t. extimus. However, most

ecological studies to date have dealt with other willow

flycatcher subspecies. Relatively few studies have

been published on E.t. extimus, and much of what is

currently known is presented in unpublished literature (e.g., agency and consulting firm reports); these

sources are relied upon heavily in this chapter. This

chapter does not address habitat characteristics in

depth, other than for nest sites (refer to A Survey of

Current Breeding Habitats for additional details). Although southwestern willow flycatchers are frequent

victims of nest parasitism by brown-headed cowbirds

(Molothrus ater), this chapter will not address the

topic of parasitism and its effect on breeding ecology

(refer to The Ecology of Brown-headed Cowbirds and

their Effects on Southwestern Willow Flycatchers for

details). Readers interested in more details of willow

flycatcher biology and ecology are encouraged to read

McCabe’s (1991) treatise, which is based on over a

decade of willow flycatchers research in Wisconsin

and includes comparisons with other populations

and subspecies.

Breeding Range and Taxonomy _____

The willow flycatcher is one of 11 Empidonax flycatchers that breed in North America. Although the

Empidonax flycatchers are considered a very difficult

group to identify by sight alone, each has unique

morphological features, vocalizations, habitats, behaviors, and/or other traits that have allowed taxonomists and biologists to characterize each species. The

willow flycatcher differs from most other Empidonax

in lacking a conspicuous eye-ring, and having both a

completely yellow lower mandible and a whitish throat

that contrasts with a pale olive breast. While these

differences may be subtle, the willow flycatcher also

has a distinctive song (often termed fitz-bew; see

below) that clearly separates it from all other bird

species.

The willow flycatcher was first described by J.J.

Audubon, who collected a specimen in the woods along

the Arkansas River in the early 1800s (Audubon 1831)

and named it Muscicapa traillii. Since that time, the

species has undergone a series of name changes and

species/subspecies designations (see Aldrich 1951,

Browning 1993). Prior to 1973, the willow flycatcher

and alder flycatcher (Empidonax alnorum) were
58 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

treated together as the Traill’s Flycatcher (A.O.U.

1957), but subsequent work proved that they do not

interbreed (Stein 1958, 1963), have different vocalizations (Stein 1958), and are genetically distinct

(Seutin and Simon 1988). The American

Ornithologist’s Union (1973) accepted the separation

of willow and alder flycatchers in 1973. McCabe

(1991) reviews the many common names historically

given to the willow flycatcher.

The southwestern subspecies was first described by

Phillips (1948). Unitt (1987) re-evaluated the subspecies taxonomy of the willow flycatcher and recognized four subspecies, each with a distinct breeding

range and differentiated primarily by subtle differences in color and morphology. Browning (1993) performed a similar evaluation and proposed five subspecies, rather than four. Both authors, however,

reconfirmed the validity of E.t. extimus, which has also

been accepted by most authors (Aldrich 1951, Behle

and Higgins 1959, Phillips et al. 1964, Oberholser

1974, Monson and Phillips 1981, Schlorff 1990,

USFWS 1993). Based on Unitt (1987) and Browning

(1993), the breeding range of the southwestern willow flycatcher (Figure 6-1) includes southern California (from the Santa Ynez River south), Arizona, New

Mexico, southwestern Colorado, extreme southern

portions of Nevada and Utah, and western Texas

(although recent breeding records from west Texas

are lacking). Records of probable breeding southwestern willow flycatchers in Mexico are few and restricted

to extreme northern Baja California del Norte and

Sonora (Unitt 1987, Wilbur 1987).

The southwestern willow flycatcher is generally

paler than other willow flycatcher subspecies (Unitt

1987), although this difference is indistinguishable

without considerable experience and training, and

without study skins as comparative reference material. All three western subspecies differ from E.t.

traillii in wing formula (Unitt 1987). Differentiation of

subspecies in the field is not reliable, due to the

subtlety of morphological differences, inconsistent

conditions for comparisons, and the inability to repeat or reassess the identifications of individual

specimens (Hubbard 1999).

Vocalizations _____________________

Willow flycatchers are suboscines, and their songs

appear to be innate, rather than learned (Kroodsma

1984). In fact, even hatching-year flycatchers can sing

(Kroodsma 1984, Sogge 1997). As with most birds,

singing behavior is regulated by hormone levels,

which in turn are influenced by a number of factors

including photoperiod, time of day, and auditory and

visual stimuli from other birds of the same species

(Kroodsma 1984, Catchpole and Slater 1995).

Figure 6-1. Breeding range distribution of the subspecies of the willow flycatcher (Empidonax traillii). Adapted

from Unitt (1987), Browning (1993) and Sogge et al. (1997a).
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 59

The willow flycatcher has a distinct primary song,

often referred to as fitz-bew, that distinguishes it from

all other Empidonax flycatchers, and from other birds

in general (refer to Stein 1963 for a detailed discussion). This is the primary territory advertising song of

male willow flycatchers, and all subspecies sing fitzbew. Singing bouts are usually comprised of a series of

fitz-bews, sometimes interspersed with creet notes,

lasting from less than a minute to over a half-hour.

Breeding males sing to advertise their territory to

prospective mates and other nearby males. Males sing

from a series of song perches throughout their territory, usually from tall perches but sometimes from

within dense vegetation. Weydemeyer (1973) and

McCabe (1991) described willow flycatchers singing

during flight in the evenings, but this has been observed only rarely in E.t. extimus. Migrant willow

flycatchers often sing from tall song perches during

spring migration, in much the way that territorial

birds do (Johnson and Sogge 1997). Sogge et al. (1997b)

found that migrants accounted for up to 64% of the

spontaneously singing flycatchers found each year

along the Colorado River in the Grand Canyon. This

makes it difficult if not impossible to separate territorial birds from migrants based on singing behavior

alone.

Female willow flycatchers also sing, at least in

some circumstances. Seutin (1987) reported female

willow flycatchers singing in response to tape-playback experiments in southern Canada. Although he

clearly established that females sing, the extent of

female song under non-experimental conditions was

unknown. Until recently, it was generally assumed

that females seldom sang, and/or that their songs were

quieter and/or not given from song perches in the way

that males sing (Sedgwick and Knopf 1992). New

research, much of it with banded individuals of known

sex, has shown that female southwestern willow flycatchers regularly sing (though not nearly as often as

males). Female flycatchers sometimes sing more quietly than males and sometimes near the nest (Sogge

et al. 1997b, M. Whitfield unpublished data), but will

also sing loudly and persistently from song perches, as

is characteristic of males (Paxton et al. 1997). The true

extent and function of female song awaits further

research, but it is clearly incorrect to assume that all

loudly singing willow flycatchers are males.

Male willow flycatchers sing most persistently early

in the breeding season, but song rate declines as the

season progresses, particularly once the male finds a

mate and nesting efforts begin. Territorial flycatchers often begin singing well before dawn (as early as

0330 hrs standard time), and song rate is generally

highest early in the morning. Short periods of predawn singing often continue as late as July (Sogge et

al. 1997b). In breeding groups with many territorial

males, song rate may remain high throughout most of

the breeding season. Males may sing up to 60 songs per

minute (H. Yard and B. Brown unpublished data).

Unmated males and males with territories near other

willow flycatchers tend to vocalize more than males in

isolated territories (M. Sogge and M. Whitfield, unpublished data).

Being highly territorial, willow flycatchers readily

sing and/or call in response to broadcast tapes of

willow flycatcher song (Gorski 1969, Tibbitts et al.

1994), which they apparently perceive as an intruding

flycatcher. This ready response to taped vocalizations

forms the basis of standardized survey protocols currently in use (Craig et al. 1992, Sogge et al. 1997a). In

many cases, willow flycatchers that are not vocalizing

when surveyors first arrive at a site begin singing in

response to a broadcast taped song. Territorial breeding males and females, migrants, and (perhaps rarely)

even recently fledged (6-8 week old) willow flycatchers

will respond to tape playback (Sogge 1997, Sogge et al.

1997a). However, much as with the general song

patterns, response to tape playback declines over the

course of the breeding season, and breeding flycatchers may not respond strongly after nesting has begun.

Another common vocalization used by flycatchers is

the whitt call, which is frequently given by both sexes.

Whitts are given as an alarm call and during interactions between flycatchers. Whitts are often the most

common vocalization used during mid- and late breeding season. Many other bird species have similar whitt

calls, so unlike the fitz-bew, the whitt is not generally

considered unique to willow flycatchers.

Foraging and Food ________________

Foraging Behavior

The willow flycatcher, as the name implies, is primarily an insectivore. It is an agile aerialist, capable of

catching flying insects on the wing. It often does so by

darting quickly out on short flights, catching an insect

in its bill, then returning to the same or a nearby

perch. Another common foraging behavior is gleaning,

where they hover to pick insects off of leaves and other

vegetation. Willow flycatchers will also drop to the

ground to capture insects, and females sitting on nests

will sometimes reach out and pluck insects that are

crawling nearby. Larger prey (such as dragonflies or

butterflies) is often beaten against the perch, killing

and softening it prior to consumption. Flycatchers

forage within and above the canopy, along the patch

edge, in openings within their territory, and above

surface water.

Prescott and Middleton (1988) reported that willow

flycatchers in Ontario spent 5 percent of time foraging

and 63 percent sitting, corresponding to a “sit and
60 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

wait” foraging tactic whereby birds can simultaneously

engage in vigilance, food searching and capture, territorial advertisement, and resting. Preliminary studies on southwestern willow flycatchers documented

foraging rates 0 to 4.6 foraging events per minute,

with foraging rate highest early and late in the day,

and during the nestling period (H. Yard and B. Brown,

unpublished data).

Prey Items

All North American Empidonax flycatchers appear

to have generally similar diets during the breeding

season (e.g., predominantly small to mid-sized insects;

Beal 1912). Most available information on specific

prey items of willow flycatchers comes from studies of

subspecies other than E.t. extimus, which demonstrate that the species is somewhat of a generalist.

Overall, wasps and bees (Hymenoptera) are the most

common food items, with beetles (Coleoptera), flies

(Diptera), and butterflies/moths and caterpillars

(Lepidoptera) being other major components (Beal

1912). Vegetable foods such as berries and small fruits

have been reported (Beal 1912, Roberts 1932, Imhof

1962), but overall do not appear to be a significant food

source during the breeding season (McCabe 1991).

A study of diet of adult southwestern willow flycatchers (Drost et al. 1997) found a wide range of prey

taken. Major prey items were small (flying ants) to

large (dragonflies) flying insects, with Hymenoptera,

Diptera, and Hemiptera comprising half of the prey

items. Willow flycatchers also took non-flying species, particularly Lepidoptera larvae. Plant material

was negligible, consisting of a few seeds in several

samples.

McCabe (1991) studied the insects brought by

adults to nesting willow flycatchers (E.t. traillii) in

Wisconsin. He found 33 families of invertebrates in a

total of 214 food items sampled from eight nests. The

most prevalent items were flies (Diptera), butterflies

(Lepidoptera), spittlebugs (Homoptera), and beetles

(Coleoptera). Immature and non-flying adult insects

comprised 30 percent of the total; spiders accounted

for 26 percent of the non-flying food items. This suggests that nestlings are fed similar, if perhaps somewhat smaller, food items to those consumed by adults.

Breeding Chronology______________

A neotropical migrant, southwestern willow flycatchers generally spend only three to four months on

their breeding grounds. The remainder of the year is

spent on migration or in wintering areas south of the

United States (see the Migration and Wintering section). During the relatively short time they are on their

breeding grounds, willow flycatchers must find a territory and a mate, build the nest, lay and incubate

eggs, raise their young, and care for the fledged

young. Figure 6-2 presents a generalized breeding

chronology for the southwestern willow flycatcher

(based on Unitt 1987, Brown 1988, Whitfield 1990,

Figure 6-2. Generalized breeding chronology of the southwestern willow flycatcher (from Sogge

et al. 1997a). Dates for a given stage may vary a week or more at a given site or during a given year.

April Sept May June July Aug

Nest Building

Eggs and Incubation

Chicks in Nest

Fledging from Nest

Arrival

? ? Departure

Migrate North Migrate South
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 61

Skaggs 1996, Sogge 1995, Maynard 1995, Sferra et al.

1997, and Sogge et al. 1997b). Record or extreme dates

for any stage of the breeding cycle may vary as much

as a week from the dates presented. In addition,

flycatchers breeding at higher elevation sites, and

other subspecies in more northerly areas, usually

begin breeding efforts several weeks later than those

in lower, southern areas.

Southwestern willow flycatchers typically arrive on

breeding grounds between early May and early June,

although a few individuals may establish territories

in very late April (Unitt 1987, Maynard 1995, Skaggs

1996, Sferra et al. 1997). Because arrival dates vary

geographically and annually, northbound migrant

willow flycatchers (of all subspecies) pass through

areas where E.t. extimus have already begun nesting.

Similarly, southbound migrants (again, of all subspecies) in late July and August may occur where southwestern willow flycatchers are still breeding (Unitt

1987). Therefore, it is only during a short period of the

breeding season (approximately 15 June through

20 July) that one can assume that a willow flycatcher seen within E.t. extimus range is most likely

of that subspecies.

Nest building usually begins within a week of pair

formation. Egg laying begins (rarely) as early as late

May, but more often starts in early to mid-June.

Chicks can be present in nests from mid-June through

early August. Young typically fledge from nests from

late June through mid-August; later fledglings are

often products of renesting attempts. Adults depart

from breeding territories as early as mid-August,

but may stay until mid-September if they fledged

young late in the season (M. Whitfield and W. Haas,

unpublished data). Almost nothing is known regarding movements and ecology of adults and juveniles

after they leave their breeding sites. Males that fail

to attract or retain mates, and males or pairs that are

subject to significant disturbance (such as repeated

nest parasitism, predation, etc.) may leave territories

by mid-July (Sogge 1995, Sogge et al. 1997b). Fledglings probably leave the breeding areas a week or two

after adults; in Southern Ontario southward migration dates of immatures occurred 15 days later than

for adults (Hussell 1991).

Mating and Territoriality ___________

The southwestern willow flycatcher breeds only in

dense riparian habitats, from sea level in California

to over 2600 m in Arizona and southwestern Colorado (Sogge et al. 1997a). Although other willow

flycatcher subspecies may breed in shrubby habitats away from water, E.t. extimus breeds only in

dense riparian vegetation near surface water or saturated soil. Other characteristics such as dominant

plant species, size and shape of habitat patch, canopy

structure, vegetation height, etc., vary widely among

sites (refer to A Survey of Current Breeding Habitats).

The first flycatchers to arrive at a breeding site are

generally males, which establish a territory by singing and aggressive interactions with other flycatchers. Willow flycatchers are very territorial, and will

sing almost constantly throughout the day when establishing their territory. Females tend to arrive later

(approximately a week or two) and settle on the territory of a male. It is not known exactly what factors a

female uses to select a territory, though it may be

related to some factor of habitat quality or potential

quality of the male. Second year males arrive at about

the same time as females (M. Whitfield, unpublished

data).

Males are usually monogamous, but annual polygyny

rates of approximately 10-15% have been recorded at

the Kern River Preserve in California (Whitfield and

Enos 1996, 1998). Polygyny has also been recorded in

Arizona (Sferra et al. 1997, Sogge et al. 1997b,

Langridge and Sogge 1997, Paradzick et al. 1999).

Polygynous males typically have only two females in

their territory, but there have been several cases of a

male with three and four females in a single year

(Whitfield and Sogge, unpublished data). Preliminary

genetic evidence also suggests that extra-pair copulation occurs, wherein one or more nestlings in a nest

are fathered by a flycatcher other than the territorial

male for that nest (Paxton et al. 1997).

Initial data from studies of color-banded populations in Arizona (Paxton et al. 1997, Netter et al. 1998)

suggest that between-year mate fidelity may be low,

and that during a breeding season some flycatcher

pairs break up and subsequently pair and breed with

other individuals. Whitfield (1980, unpublished data)

also documented two cases where pairs that were

together early in the season broke up and mated with

other flycatchers later that same season. Such pair

“reshuffling” may be related to initial nest failure, but

additional data are needed to test this.

Southwestern willow flycatcher are strongly territorial, and will defend their breeding area from

other flycatchers. Flycatcher territories (defined as a

defended area, per Noble 1939) are often clumped

together, rather than spread evenly throughout a

habitat patch. This has led some authors to label

willow flycatchers as “semi-colonial” (McCabe 1991),

although they do not fit the true definition of a colonial

species and regularly breed at sites with only one or a

few pairs (Sferra et al. 1997, Sogge et al. 1997a and

1997b). The Least Flycatcher (Empidonax minimus)

also tends to breed in groups (Briskie 1994).

Territory size varies greatly, probably due to differences in population density, habitat quality, and
62 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

nesting stage. Early in the season, territorial flycatchers may move several hundred meters between

singing locations, although this has been documented

only at sites with one or two territorial males (Sogge

et al. 1995, Petterson and Sogge 1996, R. Marshall

pers. comm.). During incubation and nestling phases

territory size, or at least the activity centers of pairs,

can contract and be very small. Mapped breeding

territory sizes are 0.06 to 0.2 ha for flycatchers

occupying 0.6-0.9 ha patches on the Colorado River,

AZ (Sogge et al. 1997b), 0.2 to 0.5 ha in a 1.5 ha patch

along the Verde River, AZ (Sogge 1995), and 0.14-2.3

ha along the Kern River, CA (Whitfield and Enos

1996). Estimated territory sizes at the Gila River

near Cliff, NM ranged from approximately 0.2 to >1

ha per territory (Skaggs 1996). Reported territory

sizes of other willow flycatcher subspecies are also

variable; 0.09 ha (Trautman 1940), 0.3 ha (McCabe

1991), 0.4 ha (Walkinshaw 1966), and 1 ha (Gorski

1969). However, only Gorski’s (1969) study was based

on detailed observations of color-banded individuals,

and territory sizes for other studies must be viewed

as approximations.

Territories of polygynous males are often larger

than those of monogamous males (M. Whitfield pers.

comm.). Flycatchers may use a larger area than their

initial territory after their young are fledged, and

utilize non-riparian habitats adjacent to the breeding area. Even during the nesting stage, adult flycatchers sometimes fly outside of their territory (often

through the “air space” of an adjacent territorial flycatcher) to forage for their nestlings.

Site Fidelity _______________________

It is often assumed that most passerine birds, particularly those that are highly territorial, exhibit strong

breeding site fidelity between and within years. Until

recently, this was thought to be the case with the

southwestern willow flycatcher. Repeated annual survey efforts on unbanded willow flycatcher populations

(Sogge 1995, Sferra et al. 1997, Sogge et al. 1997b)

found that the location and boundaries of individual

flycatcher territories were often very similar in successive years, leading to speculation that the same male

was holding the territory each year and that site

fidelity was therefore high.

Evidence gathered during multi-year studies of colorbanded populations (Figure 6-3) shows that although

most male flycatchers return to former breeding areas, southwestern willow flycatchers regularly move

among sites within and between years. Between 1996

and 1997, 29 percent of banded willow flycatchers in

Arizona returned to the breeding site of the previous

year, while 11 percent moved to other breeding areas

within the same major drainage (Paxton et al. 1997).

The remaining 60 percent of flycatchers were not

relocated in 1997, and may have died or moved to

undiscovered breeding sites. Distance moved ranged

from 2 to 30 km, and movements were not always to

the next closest breeding area. Among those flycatchers returning to the same breeding site between

years, 23 percent moved to a different part of the

habitat patch. Distance moved ranged from 20 to 900 m.

There were also two cases of movement (>500 m)

within a breeding site during the course of a breeding

season. Thus, although most returning flycatchers

showed site fidelity to breeding territories, a significant number move within and among sites. The mechanisms controlling the decision to return or move, as

well the adaptive value of movement between sites,

are unknown. Such movement does increase gene flow

among breeding groups, which provides for higher

genetic diversity than if movements did not occur.

In some cases, willow flycatchers are faced with a

situation that forces movement, such as when catastrophic habitat loss occurs. In 1996 and 1997, occupied flycatcher breeding habitat was destroyed by fire

at two sites, one along the San Pedro River in Arizona

(Paxton et al. 1996) and the other along the Gunnison

River in southwestern Colorado (Owen and Sogge

1997). In Arizona, the willow-cottonwood habitat was

completely burned during the breeding season as

nesting was underway, destroying all or most of seven

territories. At least four nests were lost, and all willow

flycatchers abandoned the site within a week after the

fire and were not seen again that year. No willow

flycatchers attempted to breed in the burned area in

1997. Seven displaced flycatchers were resighted in

1997; two had moved to unburned areas within the

breeding site, and five moved to other breeding areas

Figure 6-3. A color-banded southwestern willow flycatcher.

Photo by Michael Moore.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 63

between 2 and 28 km away. In Colorado, virtually all

of the tamarisk (Tamarix ramosissima) and willow

habitat was destroyed, leaving only charred sticks and

a few small scattered live willows. Surprisingly, some

flycatchers returned to the burned areas and attempted

to breed, even in areas without any live vegetation.

However, pairing success and subsequent productivity was negatively affected. Several southwestern

willow flycatcher breeding populations also face potential habitat loss due to flooding from rising reservoir levels. Where and how far these displaced flycatchers will move is uncertain and the subject of

on-going studies (e.g., Paxton et al. 1997).

Nests and Eggs ____________________

Southwestern willow flycatchers build open cup

nests constructed of leaves, grass, fibers, feathers,

and animal hair; courser material is used in the nest

base and body, and finer materials in the nest cup

(Figure 6-4). Willow flycatcher nests sometimes

have 2-15 cm of loose material dangling from the

bottom of the nest. In tamarisk-dominated habitats, nests may be constructed completely of tamarisk

leaves and have no hanging material from the bottom

(Figure 6-5). Nests are approximately 8 cm high and

8 cm wide (outside dimensions), exclusive of any dangling material at the bottom.

Females build the nest, with little or no assistance

from the male, over a period of four to seven days

(although renests are often built in as little as two or

three days). McCabe (1991) conducted detailed studies of nest building E. t. traillii and found that females

brought and added material to the nest every 7 to 10

minutes. Most nests are used only once, although

females will often use some fibers and materials (particularly the lining) from the original nest when constructing a subsequent nest during the same season

(McCabe 1991). There are only a few recorded instances of reuse of the same nest during a breeding

season (H. Yard, B. Brown, and Arizona Game and

Fish Department unpublished data) and no records of

reuse between years.

Typical nest placement is in the fork of a branch with

the nest cup supported by several small-diameter

vertical stems. The main branch may be oriented

vertically, at an angle, or (rarely) horizontally and

stem diameter for the main supporting branch can

be as small as 2 to 4 cm. Vertical stems supporting

the nest cup are typically 1 to 2 cm in diameter. The

nest materials are interwoven among the supporting branches and twigs, such that nests cannot

readily be separated from the branches without destroying the nest. McCabe (1991) studied details of

E. t. traillii nest placement, and found that a network

of main and support branching stems are the key to

nest placement. Main nest support stem diameter

averaged 1.3 cm, and support branches averaged between 2 and 5 cm diameter. Each nest included an

average of five support branches, most of which

angled upward between 40 and 70 degrees (with a

peak at 50 to 60 degrees). Such supporting branch

systems are typical of southwestern willow flycatcher

nests as well.

Nest height varies considerably (from 0.5 to 18 m),

and may be correlated with height of nest plant,

overall canopy height, and/or the height of the vegetation strata that contains small twigs and live growth.

In Arizona and California, flycatchers using mainly

native broadleaf riparian habitats often nest relatively low (usually 2 to 3 m above ground; Sferra et al.

1997, Whitfield and Enos 1996), whereas those using

mixed native/exotic and monotypic exotic riparian

habitats often nest higher (usually 4 to 7 m above

ground; Sferra et al. 1997, Sogge et al. 1997b). However, in any habitat type, nests may be found wherever

the appropriate twig structure and plant cover occurs,

at almost any height and location (near the center or

Figure 6-4. Willow flycatcher nest placed in a willow near

Alpine, AZ. Photo by Mark Sogge.
64 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

on the edge of the nest bush). For example, flycatchers

sometimes nest >10 m high in native-dominated habitats along the San Luis Rey River, CA (W. Haas,

unpublished data) and the Gila River, NM (Stoleson

and Finch 1999). At such sites nest height is linked to

the tree species that dominates the site.

Prior to 1950 the vast majority of southwestern

willow flycatcher nests were found in willows

(Grinnell and Miller 1944, Phillips 1948, Phillips et al.

1964, Hubbard 1987, Unitt 1987). This is not surprising, given that willows were prevalent in streamside

riparian stands in the southwest and that young

willows can provide the dense cover and fine branching structure favored by nesting flycatchers. However,

as the southwest experienced reduction and loss of

native riparian vegetation and the invasion of several

exotic plants, the willow flycatcher adapted to new

host plants and now nests in both native and introduced species.

At high elevation sites in Arizona and southwestern

Colorado, Geyer (Salix geyeriana) and other willows

are used almost exclusively for nesting (Owen and

Sogge 1997, Sferra et al. 1997). Along the Gila River in

Grant County, New Mexico, 76 percent of southwestern willow flycatcher nests were placed in boxelder

(Acernegundo), the dominant understory species, with

the remainder in other native and exotic plants (Skaggs

1996). Saltcedar is the most frequent nest substrate

in Arizona (Brown 1988, Paradzick et al. 1999) and

New Mexico (Hundertmark 1978, Hubbard 1987, S.

Williams pers. comm.), and is also used for nesting in

Colorado, Nevada, and Utah (Owen and Sogge 1997,

Langridge and Sogge 1998, McKernan and Braden

1999; M. Sogge unpublished data). Nests are often

placed in tamarisk even when native vegetation is

present and/or predominant in a territory (Sferra et al.

1997, Owen and Sogge 1997, Paradzick et al. 1999,

USFWS unpublished data). However, not all tamarisk

habitat appears suitable for nesting flycatchers. Willow flycatchers nest in Russian olive (Elaeagnus

angustifolia) at some New Mexico breeding sites

(Hubbard 1987, Maynard 1995, Cooper 1996 and 1997).

In California, most nests are in native vegetation

including willow and stinging nettle (Urtica spp.;

Holmgren and Collins 1995, Whitfield and Enos 1996).

In a very unusual situation along the San Luis Rey

River in San Diego County, California, approximately

90 percent of flycatcher nests were in live oak (Quercus

agrifolia), which became the dominant plant species

adjacent to the river following willow removal in the

1950s (W. Haas, pers. comm.). McCabe (1991) demonstrated somewhat similar switching between nest

substrates in Wisconsin as substrate availability

changed among years. Southwestern willow flycatcher

nests have also been found in buttonbush, black twinberry (Lonicera involucrata), Fremont cottonwood,

alder (Alnus spp.), blackberry (Rubus ursinus),

baccharis (Baccharis spp.) and other plants. Overall,

the plant species appears less important than the

appropriate live foliage density and twig structure.

Figure 6-5. Southwestern willow flycatcher nest placed in tamarisk at Roosevelt Lake,

AZ. Photo by Renee Netter.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 65

Willow flycatcher eggs are buffy or light tan, generally with brown markings in a wreath at the blunt

end but occasionally unspotted (Bent 1942). Eggs are

approximately 18 mm long and 14 mm wide, and

weigh about 1.6 g (McCabe 1991). Clutch size is usually 3 or 4 eggs for first nests, and is typically smaller

in Arizona and New Mexico (usually 3) than in California and elsewhere in the species’ range (usually 4;

McCabe 1991, M. Whitfield unpublished data). The

reasons for these differences are not known, but may

be related to food availability or condition of the

breeding female (Lack 1954). Female flycatchers lay

one egg per day, although some four egg clutches may

take five days to lay.

Females generally do not begin incubating until the

entire clutch has been laid. Incubation generally lasts

12-15 days from the date the last egg is laid. McCabe

(1991) gave a mean incubation period for E.t. traillii

of 14.84 days (n=50 nests), and found that in 97 and

82 percent of three and four egg clutches, respectively, all eggs in a nest hatch within 48 hrs of each

other. He also recorded a 3 percent rate of infertile or

addled eggs. After hatching, females carry the eggshell fragments away from the nest.

Most incubation is by the female, although males

have been recorded incubating in Arizona (H. Yard,

B. Brown, and Arizona Game and Fish Department

unpublished data). Incubating females sit tightly in

the nest cup, with head and tail protruding over the

nest edge. Females spend approximately 50 percent

of the day attending (incubating or shading) the eggs

(H. Yard and B. Brown unpublished data), and incubate throughout the night (Arizona Game and Fish

Department unpublished data). Daytime incubation

and shading bouts last from less than one to more than

60 minutes. Shading females stand on the nest rim or

within the nest cup, positioned to provide shade to the

eggs when the nest received direct sunlight. When

shading during the heat of the day, females often

appear heat-stressed, panting with mouth open.

Nestlings and Parental Care ________

Nestlings hatch out (on day 0) weighing only 2 g,

mostly naked with only sparse gray down and the yolk

sac still visible. Young hatch with the help of an egg

tooth, which is no longer visible after the first week

(King 1955, McCabe 1991). The edge of the bill and

inside of the mouth of nestlings are bright yellow

(Figure 6-6), as opposed to the orange mouth linings of

brown-headed cowbirds (Tibbitts et al. 1994). Recently hatched flycatchers are unable to lift their head

or move about, and motor coordination does not develop until days 2 or 3. Nestlings grow rapidly, reaching about 14 g by day 10. Feather development also

occurs quickly, with most body and flight feathers

emerging from the feather tracts by day 5 or 6, and

feathers unsheathing on days 7 through 10. By days

10 or 11, nestlings are well feathered (with noticeable

buffy wing bars), are able to perch on the edge of the

nest and often actively preen. Wing flight feathers are

unsheathed, although the tail is still very short and

underdeveloped. By day 12, nestlings engage in much

wing flapping in preparation for fledging and flight.

For the first few days after the chicks hatch, the

female performs most of the care of the young. As the

nestlings grow and demand for food increases, the

male brings food to the nest more often and by days

8-10 both parents feed the young about equally.

Only the female broods the young, although both

parents will shade nestlings if the nest is exposed to

full sun. McCabe (1991) presents many details of

parental care in willow flycatchers (E.t. traillii).

Nest attendance decreased with nestling age, with

females spending less than 10 percent of their time

at the nest after day 7. The number of feeding trips

peaked at approximately 30 trips per hour during

days 5 through 10.

Young willow flycatchers usually leave the nest

(fledge) at 12 to 15 days of age, but will fledge prematurely as early as day 10 if a nest is disturbed (e.g., by

a predator or researcher). After fledging, young flycatchers stay close to the nest and each other for 3 to

5 days. Recently fledged birds may repeatedly return

to and leave the nest during this period (Spencer et al.

1996), up to three times per hour (McCabe 1991).

Fledglings stay in the natal area a minimum of 14 to

15 days after fledging, possibly longer. Male and

female adults both feed the fledged young, which beg

loudly (typically a “peep” call) as they perch or move

about in the dense vegetation. The period following

Figure 6-6. Nestling southwestern willow flycatcher. Note the

yellow edge of bill and mouth lining. Photo by Eben Paxton.
66 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

fledging is a time of high energy demand for fledglings,

and parental feeding rates can be as high as 30 nest

visits per hour.

Renesting ________________________

Second clutches within a single breeding season are

uncommon if the first nest is successful, though this

may vary between sites and years. M. Whitfield (unpublished data) has recorded only 5-10 percent

renesting following successful first nesting. Most attempts at double brooding occur if the young fledge

from the first nest by late June or very early July. On

the other hand, willow flycatchers usually attempt

another nest if the first nest is lost or abandoned due

to predation, parasitism, or disturbance. Replacement

nests are built in the same territory, and may be close

to (even in the same plant) or far from (up to 20 m) the

previous nest (McCabe 1991, Sogge et al. 1997b).

McCabe (1991) found no differences in nest placement

parameters between first nests and renests in E.t.

traillii. Females usually begin construction of replacement nests within a day or two following the loss of the

first nest, and replacement nests are usually constructed more quickly than first nests. Replacement

nest building and egg laying can occur (uncommonly)

as late as late-July or early August. Pairs may attempt

a third nest if the second fails (Sferra et al. 1997, M.

Whitfield pers. comm.), and Harris (1991) documented

one female attempting six nests in one season. Clutch

size (and therefore potential productivity) generally

decreases with each nest attempt (Holcomb 1974,

McCabe 1991, Whitfield and Strong 1995).

Post-Breeding Dispersal ___________

Few specifics are known about when breeding pairs

and their young leave their territory after nesting is

completed. Adults that are successful in raising young

may remain at breeding sites through mid-August and

early September. Pairs with unsuccessful first and/or

second nests sometimes abandon their territories midway through breeding season. In at least four cases,

members of unsuccessful pairs have moved to other

breeding sites within the same season and made

second breeding attempts with new mates (Whitfield

1990, Paxton et al. 1997, M. Whitfield unpublished

data). In Arizona, unmated males remained on territory through the early part of the breeding season but

left by mid-July (Sogge 1995, Sogge et al. 1997b). The

exact departure dates of most flycatchers are unknown, and it is not known if post-breeding flycatchers

immediately begin their southward migration, or if

they disperse and explore local riparian systems prior

to heading south.

Competitors ______________________

In order for competition to occur, two or more species

must attempt to utilize the same limiting resource

(Lack 1954, Schoener 1982, Rosenberg et al. 1982).

Individuals of the same species are often assumed to be

competing (intraspecific competition), at least to some

degree, particularly if they establish and defend separate breeding territories. Limiting resources are usually

assumed to be food, nest sites, and/or mates. Interspecific competition should be strongest between closely

related species that utilize resources in similar ways.

Empidonax flycatchers are very similar in morphology and food habits, and so present the most potential

as competitors. Several studies suggest this may be

the case. McCabe (1991), for example, found that

willow and alder flycatchers maintained mutually

exclusive territories at his study site in Wisconsin.

Frakes and Johnson (1982) found similar diet and

foraging behavior, and little territorial overlap, between coexisting willow and western (Empidonax

difficilis) flycatchers in Washington. Johnson (1963)

noted interspecific territoriality among Empidonax

species, as did Beaver and Baldwin (1975). In the

Southwest, however, the willow flycatcher is usually

the only Empidonax flycatcher breeding within the

dense riparian habitats that it favors, and no evidence

has been seen of competition with other flycatchers.

Other less closely-related bird species are less likely

to be significant competitors, even where they may

share some ecological characteristics (such as nest

placement or dietary overlap). McCabe (1991) found

no evidence of intraspecific competition between willow flycatchers and co-occurring species such as yellow warblers (Dendroica petechia) and American

Goldfinches (Carduelis tristis), which utilize similar

habitat and resources.

There is little evidence that food is a limited resource

at southwestern willow flycatcher breeding sites (although food availability may play a role in breeding

site selection). For example, insects are usually abundant in flycatcher breeding patches, and nestling

starvation is rarely recorded in unparasitized nests.

Furthermore, willow flycatchers are to a large degree

dietary generalists, and can select among differing

prey types depending upon availability. Thus, although flycatchers share their breeding habitats with

many other insectivorous birds, competition for food

is probably negligible.

In terms of nest site competition, willow flycatchers

can build nests in a variety of substrates and locations

where suitable branching structure occurs. Several

other birds, including yellow warblers and yellowbreasted chats (Icteria virens), occur in flycatcher sites

and sometimes build nests of similar structure and

placement. However, these species do not exclude
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 67

willow flycatchers from their territories (based on

many examples of overlapping territories), and suitable nest sites are usually abundant and unlikely

limiting, so competition is not likely.

The one resource for which evidence suggests possible intraspecific competition is mates. Male willow

flycatchers are strongly territorial, and establish and

defend territories through singing and aggressive interactions. At many southwestern willow flycatcher

breeding sites, some territorial males fail to secure

mates (Whitfield and Enos 1998, Ahlers and White

1999, Paradzick et al. 1999). This implies that females may be limited in some breeding groups, and

that males are competing for this reproductively critical resource, with some males more effective than

others. On the other hand, at several of these same

sites, some males are polygynous and mate with more

than one female in their territory (Whitfield and Enos

1998, Paradzick et al. 1999). The criteria and mechanism by which females evaluate and select males are

unknown, but could include song (form, rate, volume,

etc.), aggression, or other factors. Females may also

be selecting a patch of habitat or breeding territory,

whereby the male at that location becomes her mate

by default.

Predation and Predators ___________

Predation, particularly during the nesting phase,

is a significant factor in the natural history and

population dynamics of most small birds, and the

southwestern willow flycatcher is no exception. Being a

small bird with an open-cup nest, flycatchers are exposed to a wide suite of potential predators. In fact,

predation can be the single largest cause of nest failure

in some years (e.g., Whitfield and Enos 1996,

McCarthey et al. 1998, Paradzick et al. 1999), and

most of what we do know about predation and flycatchers involves nest predation. Predation events on adults

of most passerine birds are rarely observed, and we

have virtually no data of this kind for the southwestern willow flycatcher.

Potential predators observed at or near willow flycatcher territories include a variety of snakes, and

small and mid-sized mammals such as chipmunks,

weasels, raccoons, foxes, and domestic cats (McCabe

1991, Sogge 1995, Langridge and Sogge 1997, Paxton

et al. 1997, Sferra et al. 1997). Predatory birds such as

corvids (jays, crows and ravens), hawks (especially

accipiters), and owls are regularly found in occupied

flycatcher habitat. Brown-headed cowbirds, found in

virtually every known flycatcher breeding site, effectively function as predators when they remove a flycatcher egg during parasitization events. Cowbirds

have also been documented killing nestling Kentucky

warblers (Oporornis formosus; Sheppard 1996) and

other small songbird chicks (Tate 1967, Beane and

Alford 1990, Scott and McKinney 1994), and may be

acting as predators on southwestern willow flycatcher

chicks (M. Whitfield and Arizona Game and Fish

Department unpublished data).

There are four documented cases of nest predation

on willow flycatchers. In Wisconsin, McCabe (1991)

captured a milk snake (Lampropeltis triangulum)

that was being harassed by adult willow flycatchers

and found it had eaten a complete clutch of four eggs.

Paxton et al. (1997) reported two predation events in

Arizona. In one, a common king snake (Lampropeltis

getulus) ate two nestlings, while a third survived by

jumping out of the nest. At a second nest, an adult

Cooper’s hawk (Accipiter cooperii) took two nestlings

(one at a time) from a nest. As with the king snake

event, one nestling survived by jumping from the nest.

At the Gila River in New Mexico, three nestlings were

taken from the same nest by a Great Horned Owl

(Bubo virginianus; S. Stoleson unpublished data).

Parasites and Disease ______________

Although individuals of virtually all natural bird

populations are exposed to diseases and are hosts to

one or more species of internal or external parasites,

little is known regarding the role of disease and parasites on most species or populations. Historically,

avian parasite and disease literature focused only on

documenting occurrence and development of host lists.

Recently, increasing attention has been focused on the

ecology of bird-parasite interactions (e.g., see Loye

and Zuk 1991). Disease and parasites clearly may

become a significant factor in periods of environmental stress, during particular portions of a life cycle, or

if an exotic/introduced parasite or disease is introduced into a new or naive host (Karstad 1971, Atkinson

and van Riper 1991, van Riper 1991). It remains

difficult, however, to determine the effect of most

parasites on most host species.

The willow flycatcher is host to a variety of internal

and external parasites. Bennett et al. (1982) listed E.

traillii as host for blood parasites such as Haemoproteus, Leucocytozoon, Microfilaria and Tyrpanosoma.

Boland et al. (1989) and Sabrosky et al. (1989) recorded blow fly (Protocalliphora sp.) larvae on nestling

willow flycatchers. Pence (1975) reported Traill’s flycatcher as host to two species of nasal mites. Most, if

not all, avian species (including Tyrannid flycatchers)

are susceptible to viral pox (Karstad 1971), therefore

this disease probably occurs in willow flycatchers

(though specific records could not be found). Although

these sources provide information on the identity and

occurrence of parasites in willow flycatchers, there is

no information on what impact, if any, parasites have

on the infected birds. McCabe (1991) identified mites
68 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

(Ornithonyssus sylviarum) in 43 percent of flycatcher

nests, but noted no obvious impairment of young. He

also recorded blowfly larvae in 32 percent of nests, but

again found no evidence of negative effects to nestling

flycatchers.

There is virtually no published information available on diseases or parasites in the southwestern

willow flycatcher. Recent preliminary examination of

blood samples (C. van Riper and M. Sogge, unpublished data) found that E.t. extimus is host to several

blood parasites, including Haemoproteus sp., Leucocytozoon sp., and Plasmodium sp. (avian malaria). As

with other parasites, nothing is known regarding the

ecological effects of these blood parasites on the

willow flycatcher.

Data Needs _______________________

Although the southwestern willow flycatcher received relatively little research attention prior to 1990,

a number of studies and monitoring programs initiated since that time have provided us with much

useful information relative to the basic ecology of the

flycatcher. New studies are being initiated every

year. Research programs at the Kern River Preserve

(CA), Roosevelt Lake and San Pedro River (AZ), Lower

Colorado River (AZ, CA, NV, UT), Rio Grande (NM),

and the Gila River (NM) are expected to continue for

several more years and will yield valuable data on

long-term patterns. Thus, while we have learned a

tremendous amount in the last 10 years, we are

positioned to discover even more during the next few

years.

Unfortunately, there are still large gaps in our

knowledge and understanding of southwestern willow

flycatcher ecology, including many topics of considerable management and conservation interest (refer to

the Research Needs chapter for a more complete listing of research needs and priorities). We need more

precise delineation of the subspecies’ northern range

boundary, based on morphological and genetic examination of breeding populations. Another issue of key

importance is the relative suitability of the native and

exotic riparian habitats where flycatchers breed. More

information is needed on how breeding patch size,

shape, and landscape features are related to flycatcher

breeding site selection and success. We lack an understanding of how microclimate characteristics influence breeding and nest site selection, especially in low

and mid-elevation riparian areas. Data on nest predators are scant even though nest predation may be

responsible for the majority of lost productivity. We

know very little about the details of and mechanisms

behind flycatcher breeding site selection, site fidelity,

dispersal, and post-breeding movements, yet these

are critical aspects of the flycatcher’s habitat use and

metapopulation dynamics. Hopefully, future research will be directed at these and other important

questions.

Acknowledgments ________________

Most of the material presented in this chapter comes

from unpublished reports and personal communications. I thank the many biologists who have conducted

these studies and shared their data, experience and

insights. In particular, my gratitude to Bryan Brown

(SWCA, Inc.), Charles Drost (USGS), William Haas

(Varanus Biological Services), Tracy McCarthey

(AGFD), Charles Paradzick (AGFD), Jay Rourke

(AGFD), Susan Sferra (USBR), Mike Sumner (AGFD),

Charles van Riper (UGSG) and Helen Yard (SWCA,

Inc.). Many of the data reported here were gathered

by the dedicated and talented staff of the willow

flycatcher banding project at the USGS Colorado Plateau Field Station at Northern Arizona University:

Heather English, Tom Koronkiewicz, Suzanne

Langridge, Therese Littlefeather, Jennifer Luff, Andrew McIntyre, Michael Moore, Renee Netter, Jennifer Owen, Eben Paxton, and J.D. Semones. Special

thanks to Mary Whitfield (Kern River Research Center), whose dedication and energy have provided us

with almost a decade of excellent research on the Kern

River willow flycatcher population. Her extensive

knowledge of the flycatcher and her willingness to

share her data allowed me to fill in many details that

would otherwise have been left blank. Debbie Finch,

William Haas, Rob Marshall, John Rotenberry, Susan

Sferra, Linda Sogge, Scott Stoleson, Mary Whitfield,

and two anonymous reviewers read drafts of this

manuscript and provided many helpful suggestions.

Preparation of this manuscript was funded by the U.S.

Geological Survey.

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Deborah M. Finch

Jeffrey F. Kelly

Jean-Luc E. Cartron

Chapter 7:

Migration and Winter Ecology

The willow flycatcher (Empidonax traillii) is a

Neotropical migrant that breeds in North America,

but winters in Central and northern South America.

Little specific information is known about migration

and wintering ecology of the southwestern willow

flycatcher (E. t. extimus) (Yong and Finch 1997). Our

report applies principally to the species as a whole

and not just to the endangered subspecies. In this

chapter, we describe timing of migration, migration

routes, stopover behavior and ecology, winter distribution, and migration and winter habitats of the

willow flycatcher species complex. We also explore

the topic of possible threats to migrating and wintering flycatchers associated with habitat loss and

pesticide use.

A round-trip journey of 2,000 to over 5,000 miles,

depending on departure locations and destinations of

individuals, is required by willow flycatchers to migrate between their wintering areas and breeding

grounds. Migration and wintering periods account for

over half of the annual cycle of the flycatcher and are

thus important for understanding the ecology, population changes, and distribution of willow flycatchers. Resource limitations, habitat disturbances, and

inclement weather can reduce survivorship of

Neotropical migrants during winter and migration

periods (Rappole 1995, Moore et al. 1995), but data

clarifying these relationships with respect to willow

flycatchers are limited. While most protection and

recovery efforts in the United States emphasize the

breeding grounds, conservation strategies for the willow flycatcher would be incomplete if we neglected

attention to threats and management actions on the

wintering and migration grounds (e.g., Moore 2000).

Population declines of the southwestern willow flycatcher may be related to problems that arise during

any part of the species’ annual cycle.

Migration Ecology _________________

Willow Flycatcher Identification During

Migration

Willow flycatchers are difficult to distinguish from

alder flycatchers (Empidonax alnorum) in the field

during migration (Stein 1963, Pyle et al. 1997) except

by song, and subspecies of willow flycatchers are even

more difficult to distinguish from each other (Hubbard

1987, 1999). Most individuals of willow and alder

flycatchers can be separated in the hand using Stein’s

(1963) formula, which combines several wing measurements and bill length. To demonstrate that the

migrants captured on the middle Rio Grande were

willow flycatchers, Finch and Kelly (1999) applied

Stein’s formula and found that 88.5% of birds identified as willow flycatchers could be confirmed as willow

rather than alder flycatchers (Figure 7-1). Given that

Stein’s formula only separates 90% of birds known to
72 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

be willow flycatchers (Stein 1963), that the presence of

an alder flycatcher has never been confirmed in New

Mexico (Williams 1997), and that young willow flycatchers are less distinguishable from alder flycatchers than adults, Finch and Kelly (1999) concluded that

unconfirmed birds captured along the middle Rio

Grande were also likely to be willow flycatchers.

Reference to specimen skins increases the probability of accurately identifying live individuals of

different subspecies in the hand (Hubbard 1999,

Unitt pers. comm.). Yong and Finch (1999) note,

however, that collecting voucher specimens of endangered species is generally not approved under endangered species permits, limiting the availability of

this option for identifying willow flycatcher subspecies captured in the Southwest. According to Unitt

(1987), 90% of birds in which primary 10 is longer

than primary 5 are the E. t. traillii subspecies and 90%

of birds in which primary 10 is shorter than primary

5 are western (extimus, brewsteri, or adastus). Using

Unitt’s primary-feather criteria to distinguish subspecies, we analyzed 253 middle Rio Grande migrants

for this paper and found that 99 (20%) sorted into the

eastern subspecies (E. t. traillii) class (P10 > P5).

Owing to the difficulty of obtaining reference specimens of endangered species, Yong and Finch (1997,

1999) attempted to separate net captures of all four

subspecies of willow flycatchers during migration using

wing, tail, wing formula (relative length of flight

feathers), coloration of the head and neck and its

contrast with the back, and the contrast between the

breast-band and the throat based on Unitt’s (1987)

analyses, but these measures do not guarantee accurate identifications (Hubbard 1999). Finch and Kelly

(1999) applied principal components analysis of tail

length, wing length, bill length, wing formula, and

tarsus length of migrant willow flycatchers but found

no single or combination of morphological features

that readily distinguished among subspecies.

An unpublished report by Unitt (1997) provides

further assistance and clarification in separating subspecies, and a peer-reviewed publication of his analysis would aid in resolving discrepancies in methods for

identifying willow flycatcher subspecies. At this time,

however, we conclude that in the absence of voucher

specimens or proven methods to verify subspecies

identification, willow flycatcher subspecies should be

lumped together when reporting migration results in

the Southwest.

Migration Routes

The willow flycatcher as a species breeds as far north

as Alaska and winters as far south as northern South

America (Unitt 1997). Stevenson (1957) reported that

Figure 7-1. Bill length plotted against Stein’s I [(length of longest primary minus length of

the 6th primary) - length of the 5th primary - length of the 10th primary )]. The equation of the

line is bill length = 7.95 + 0.15(I). Stein found that this line separated 90% of Willow and Alder

Flycatchers. (Data from the middle Rio Grande, Finch and Kelly 1999).

-5 0 5 10 15

7

8

9

10

11

Stein's I (mm)

Bill Length (mm)
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 73

trans-gulf migration of Empidonax flycatchers was

not uncommon, but adequate support for use of this

migration pattern by the willow flycatcher has not

been established. Spring migration route(s) of the

willow flycatcher from its winter range in the Neotropics

to the United States and Canada are not well-known.

Bond (1979) listed the willow flycatcher as a vagrant

in Cuba, Isle of Pines, and Jamaica in September and

October, and Sturgis (1928) reported it as a spring

migrant through the Panama Canal Zone (a semiCaribbean locale). On the other hand, willow flycatchers were not recorded on Grand Bahama Island

(Emlen 1977), Cuba (Barbour 1923, Davis 1941), Haiti

and the Dominican Republic (Whitmore and Sales

1931), British Guyana (Blake 1950), or the islands off

Venezuela (Voous 1957).

McCabe (1991) suggested a circum-Gulf migration

pattern, in which the alder flycatcher leap-frogged the

willow flycatcher in establishing winter range, based

on studies by Gorski (1969, 1971) and a range map by

Fitzpatrick (1980) that showed the wintering distribution of alder flycatchers as disjunct from that of willow

flycatchers and lying more to the south. Miller et al.

(1957) recorded E. t. traillii, in spring from the Mexican east coast states of Oaxaca, Tamaulipas and

Veracruz, and from Guerrero, Michoacan, Chiapas

and Baja California on the west coast in fall. Rappole

et al. (1979) proposed two different routes for willow

and alder flycatchers based on captures of 181 migrant

willow-alder flycatchers in Texas (48 km north of

Corpus Christi) and 113 in Los Tuxtlas, Veracruz in

fall, and 14 willow-alder flycatchers in Texas and 136

in Veracruz in spring migration. They suggested that

spring migration is a westerly inland route following

the western coastal plain and mid-elevations of the

Sierra Madre Oriental of Mexico northward, whereas

fall migration followed a western Gulf coast route.

In the Southwest, mist-netting studies and flycatcher

surveys show than many willow flycatchers use riparian habitats along rivers and tributaries during migration. Greater effort is needed to survey non-riparian areas to determine extent of willow flycatcher

migration and stopover use of other habitats. According to William Haas (pers. comm.), “many willow

flycatchers in southern California migrate through

scrub (including coastal sage scrub), agricultural valleys that are minimally riparian, and other open

woodlands (especially where Mexican elderberry is

common)”. Willow flycatchers sing during spring migration making spring migrants easier to distinguish

from other Empidonax flycatchers than fall migrants.

Major drainages where migrating willow flycatchers

have been regularly recorded in spring include the

Colorado River (Rosenberg et al. 1991, Sogge et al.

1997; McKernan and Braden 1999), the San Juan

River (Johnson and Sogge 1997, Johnson and O’Brien

1998), and the Green River (M. Johnson unpubl. data).

In New Mexico, the willow flycatcher is known to

migrate regularly in spring and fall along the Rio

Grande (Yong and Finch 1997, Finch and Kelly 1999)

and the Pecos River (Hubbard 1987), and occurs regularly as a migrant in the southwestern-most desert

region and the eastern-most plains region of the state

(Hubbard 1987).

Arrival and Departure Dates

Other subspecies of the willow flycatcher migrate

through the breeding range of the southwestern

subspecies, making accurate identification of arrival and departure dates of extimus difficult (Unitt

1987, Yong and Finch 1997, Hubbard 1999). In

addition to subspecies differences in migration

timing, variation in age, sex, body condition, and

point of departure can influence migration arrival

and departure dates (Yong and Finch 1997). Male

willow flycatchers are reported to arrive on the breeding grounds before females (Bent 1942, Walkinshaw

1966). In the middle Rio Grande valley of New Mexico,

adult willow flycatchers migrated through a week

earlier (August 26) on average than hatching-year

birds (September 2) (Yong and Finch 1997). In addition, spring and fall migration dates of willow flycatchers can vary greatly from year to year (e.g., by

as many as 29 days for spring arrivals in Wisconsin

(McCabe 1991), further complicating interpretations

of subspecies migration periods.

Arrival and departure dates of willow flycatchers

vary in relation to the latitude where the birds are

recorded. Based on literature records, McCabe (1991)

concluded that earliest and latest spring arrival dates

of the willow flycatchers at 30-35 degrees north latitude were April 27 and May 11, respectively, whereas

at 46-50 degrees north latitude, earliest and latest

dates were May 26 and June 14, respectively. Earliest

and latest fall departure dates were August 25 and

October 8, respectively at 30-35 degrees north latitude

and August 27 and September 30 at 46-50 degrees

(McCabe 1991).

In Big Bend National Park, Texas, the willow flycatcher is an uncommon spring migrant from May 11-

May 28 (Wauer 1985). Hubbard (1987) reported that

May 5 was the earliest verified date that willow

flycatcher arrived in New Mexico. Southwestern willow flycatchers usually arrive on breeding grounds

between early May and early June (Muiznieks et al.

1994, Maynard 1995). The typical arrival date of

extimus in southern California and southern Arizona

is the first week of May (Phillips et al. 1964, Unitt

1987), but breeding extimus has been detected at

Prado Reservoir as early as late April (William Haas

pers. comm. citing Loren Hays pers. comm.). In the

Grand Canyon, Arizona, the earliest record of a male

extimus on a breeding territory was May 8, although
74 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

most resident breeding males were detected the third

week of May (Sogge et al. 1997). In south-central

Arizona, some extimus arrive as early as the third

week in April (Paradzick et al. 1999). Yong and Finch

(1997) reported that the dates of first and last captures

of spring-migrating willow flycatchers netted in the

central Rio Grande valley of New Mexico were May 13

and June 8, respectively. We found that spring migration (all subspecies) on the middle Rio Grande peaked

in the first week of June, although flycatcher numbers

and migration schedules varied by year (1994 to 1997)

and by site (Figure 7-2a, b).

Figure 7-2. Number of willow flycatchers captured by date and year at (A) the

Bosque del Apache National Wildlife Refuge (1994-1998) and (B) Rio Grande

Nature Center (1994-1996), New Mexico during spring migration (data from

Finch and Kelly 1999 and unpublished).

The spring migration of extimus is earlier than that

of the more numerous brewsteri (Unitt 1987). Brewsteri

migrants typically appear 10-15 May (Garrett and

Dunn 1981), peaking in numbers through southern

California around 1 June (Unitt 1987) and are still

migrating north until about 20 June, depending on

weather patterns (Garrett and Dunn 1981). Northbound migrants of brewsteri as well as other subspecies

travel through areas where extimus has already begun

breeding. To prevent mistaking other subspecies as

resident extimus breeders, repeated surveys should be

conducted from June 20-July 15 (Unitt 1987).

05/06 05/13 05/20 05/27 06/03 06/10

0

10

20

30

40

50

1994

1995

1996

1997

(A) Bosque del Apache

1998

Number of Birds

05/06 05/13 05/20 05/27 06/03 06/10

0

10

20

30 (B) Rio Grande Nature Center

Week

Number of Birds
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 75

Wauer (1985) reported that the willow flycatcher

was a fairly common fall migrant from July 21-

September 24 each year at Big Bend National Park,

Texas. Fall brewsteri migrants arrive in southern

California by July 18 (Unitt 1987) when resident

extimus pairs are still breeding. Earliest records reviewed by Hubbard (1987) for autumn migrants in

New Mexico were July 27 (Ft. Bayard), July 29

(Cedar Crest) and July 30 (Hachita). Along the middle

Rio Grande, the latest fall capture date for willow

flycatchers was September 16 (Yong and Finch 1997).

Our observations suggest that fall numbers of willow

Figure 7-3. Number of willow flycatchers captured by date and year at (A) the Bosque del Apache

National Wildlife Refuge (1994-1998) and (B) Rio Grande Nature Center(1994-1996), New Mexico

during fall migration (data from Finch and Kelly 1999 and unpublished).

flycatchers, at least in the middle Rio Grande valley,

can vary greatly by year and site (Figure 7-3a, b).

Variation in fall numbers from year to year may be

related to yearly differences in nesting success,

fledgling survival, weather patterns, predation rates,

or a combination of factors. Yong and Finch (1997)

suggest that habitat differences among sites in the

same drainage are most likely to explain site-to-site

variation in migration volume. Habitat probably

does not explain individual differences in migration

timing within each season (Figure 7-2 in Finch and

Kelly 1999).

08/12 08/19 08/26 09/03 09/10 09/17 09/24 10/01 10/08

0

10

20

30

1994

1995

1996

1997

1998

(A) Bosque del Apache

Number of Birds

08/12 08/19 08/26 09/03 09/10 09/17

0

10

20

30 (B) Rio Grande Nature Center

Week

Number of Birds
76 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

Fat Storage and Body Condition During

Migration

Small landbird migrants deposit fat stores before

their long-distance flights to prepare for the high

energy demands of migration. Migratory birds can

deposit fat stores of up to 30-50% of lean body mass

(Berthold 1975). However, small landbird migrants,

especially Neotropical long-distance migrants such as

the willow flycatcher, generally do not deposit enough

fat to fly nonstop between breeding and wintering

areas. In their study of stopover migrants along the

middle Rio Grande, Yong and Finch (1997) reported

that 50% of the 84 willow flycatchers sampled in

spring and fall had no observable subcutaneous fat

stores. Average body mass of migrant willow flycatchers on the middle Rio Grande was 12.7 g and ranged

from 10.3 to 15.9 g. Body mass significantly differed

among fat classes. Using Pennycuick’s (1989) estimator, Yong and Finch computed that average potential

flight ranges for all willow flycatchers captured at

their study sites was 225 km, but flight ranges were

estimated at 257 km to 404 km or higher for flycatchers having fat stores.

Low fat stores could be caused by depletion of fat

during nocturnal migration, which according to Moore

and Kerlinger (1987), can vary in relation to wind

direction and precipitation. Alerstam and Lindstrom

(1990) proposed that stopover migrants may maintain

low fat stores to minimize the energetic costs of flying

with unnecessary weight. Owing to low fat stores,

willow flycatchers may be constrained to feed at stopover sites to make progress toward their breeding or

wintering destination. Based on low recapture rate

(8% of total captures) and the short duration (within

one day) between capture and recapture, Yong and

Finch (1997) concluded that stopover length of willow

flycatchers was relatively brief compared to many

other species. About 70% of willow flycatchers were

captured between 0700 and 0900 (spring and fall

combined) along the middle Rio Grande. Average body

mass of recaptures in the middle Rio Grande valley

was 12.4 g at initial capture and 12.6 g at last capture,

or an average change of 1.6% body mass/day. Body

mass tended to be positively associated with daily time

of capture in spring and fall.

Average body mass of willow flycatchers captured in

spring was significantly higher than that of fall captures along the middle Rio Grande (Yong and Finch

1997). If hatching-year birds weighed less then adults,

then age may explain differences in body mass between fall and spring captures. However, adults and

hatching-year birds did not differ in body mass in fall.

Although body mass did not vary by capture date

within spring and fall migration periods, declines in

wing length through spring and fall and increases in

condition index (mass/wing length-3) during spring

migration suggested that flycatcher condition and

wing size may be related to departure times (and

travel distances) of different groups or subspecies of

flycatchers. Differences in departure times of sexes,

and influences of interactions between sex, age, and

subspecies, may also partially explain variation in

body condition and wing length during spring and fall

migrations.

Habitat Use During Migration

Karr (1976) recorded willow flycatchers in two shrub

habitats of Panama in September, October, January

and April. In Veracruz, Mexico, fall and spring willow

flycatcher migrants were commonly captured in tall,

evergreen tropical forest of various seral stages

(Rappole et al. 1979). Further north near Corpus

Christi, Texas, fall migrants were common in Aransas

River floodplain forest but spring migrants used this

forest type and/or route less frequently (Rappole et al.

1979). At Big Bend National Park, Texas, specimens of

the willow flycatcher were taken at sites with water

present during spring migration (Wauer 1985).

In New Mexico, migrating willow flycatchers have

been recorded in a diversity of habitats. In the CliffGila valley of southwestern New Mexico, migrants use

broadleafed riparian forests with canopies composed of

box-elder (Acer negundo), Fremont cottonwood (Populus fremontii), Arizona sycamore (Platanus wrightii),

and Goodding’s willow (Salix gooddingii), and understories of mostly native species such as Rhus trilobata,

Amorpha fruticosa, shrub willow (Salix spp.), seep

willow (Baccharis glutinosa), Arizona alder (Alnus

oblongifolia) (Stoleson and Finch pers. obs.). Along the

middle Rio Grande, migrating willow flycatchers use

cottonwood woodlands with understories composed of

native and/or exotic shrubs such as Russian olive

(Elaeagnus angustifolia), saltcedar (Tamarix

ramosissima), New Mexico olive (Floresteria neomexicana), willow (Salix spp.), and saltbush (Atriplex spp.);

monotypic saltcedar habitats; irrigation ditches; and

agricultural fields (Yong and Finch 1997). Migrant

flycatchers have been captured in mist nets along

mowed and unmowed water-conveyance channels

dominated by coyote willow (Salix exigua) and seep

willow (Finch et al. 1998, Finch and Kelly 1999).

Hubbard (1987) reported migrant use of a desert

grassland/shrubland site in the southwestern part of

the state, and lake and drier sites in the eastern plains

grasslands (Hubbard 1987).

A greater variety and distribution of habitats,

including non-riparian vegetation, are used by willow

flycatchers during migration than during breeding.

Migration habitats may lack key components important for breeding birds such as the presence of standing water or moist soils and suitable patch size and

structure.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 77

Where native and non-native riparian habitats are

present together, some evidence suggests that migrating

flycatchers favor native vegetation, especially willow

(Yong and Finch 1997, Finch et al. 1998, Finch and

Kelly 1999). Yong and Finch (1997) reported that willow flycatcher capture rates varied substantially among

riparian habitat types during spring migration as

well as in fall. In a study comparing stopover use

among five habitats along the middle Rio Grande,

Finch and Kelly (1999) reported that capture rates of

willow flycatchers were higher in unmowed coyote

willow than in cottonwood, saltcedar, agricultural edge,

and disturbed (previously-mowed) channel willow. A

greater number of willow flycatchers were captured in

disturbed willow along channels than in other habitats

owing in part to greater banding effort in channel

willow. Comparing relative habitat use by willow flycatchers to net hours of effort, Finch et al. (1998) and

Finch and Kelly (1999) found that generally greater

percentages were captured in disturbed and undisturbed willow along channels than expected based on

the percent effort devoted to banding in that habitat. In

contrast, fewer willow flycatchers were captured in

agricultural edge and cottonwood habitats than that

expected by netting effort expended.

Capture rates of migrant willow flycatchers may

vary among habitats in relation to inter-habitat differences in food supplies within and among seasons.

DeLay et al. (1999) found a positive association between the relative abundance of migrant willow flycatchers and the relative abundance of aerial insects

in one year of a two-year study. No relationship was

detected between foliage insects (beetles, plant bugs)

and flycatcher abundance in either year. Arthropod

abundance varied by year, season, and habitat. Habitat differences in arthropod abundance were evident

during the period of spring migration but not in fall. In

spring, mean numbers of arthropods per trap were

highest in willow, intermediate in cottonwood, and

lowest in saltcedar. The results of this study suggest

that spring migration schedules and stopover habitatuse patterns by willow flycatchers may be influenced

by food availability or by cues such as weather that

predict en-route food supplies.

If food supply varies among habitats during migration periods, fat condition and body mass may depend

on how successfully migrants select foraging habitats

with plentiful food during stopover. Yong and Finch

(1997) reported that body mass and fat condition of

stopover willow flycatchers tended to differ by habitat,

with heaviest birds captured in willow. However,

Finch and Kelly (1999) reported that fat scores and

body masses of willow flycatchers did not differ among

native and non-native habitats, nor between disturbed

(mowed) and undisturbed coyote willow during spring

and fall migrations along the middle Rio Grande.

Because sample sizes of flycatchers in these studies

were small, we suggest that the relationships among

habitat, food supply, and body condition during migration be investigated further.

Wintering Ecology_________________

Distribution

Little information is known about the wintering

habits of the willow flycatcher, let alone of the subspecies extimus. Early accounts did not distinguish between this species and the alder flycatcher. Instead,

the two species were collectively described as wintering from south Mexico south through northern Argentina (Meyer de Schauensee and Phelps 1978).

More recently, however, it has been recognized that

the alder flycatcher alone reaches mid-South America

(Howell and Webb 1995), wintering in Brazil, Bolivia,

Ecuador, Peru, and northern Argentina (Stotz et al.

1996). In Central America, E. alnorum occurs only as

a transient during migration (Ridgely 1976, Howell

and Webb 1995). In contrast, E. traillii’s primary

wintering grounds are Mexico from the state of Nayarit

south and all of Central America (Peterson and Chalif

1973, Stiles and Skutch 1989, Howell and Webb 1995).

In Mexico and northern Central America, the willow flycatcher appears to be restricted to the Pacific

Slope (Howell and Webb 1995). From Honduras south,

its distribution is less known, but it appears to be

present on both the Atlantic and Pacific slopes (see

Stiles and Skutch 1989, Howell and Webb 1995). In

Costa Rica, the willow flycatcher winters primarily

on the Pacific Slope, but it may be found on the

Caribbean Slope as well (Stiles and Skutch 1989). Its

occurrence as far south as Panama has been well

documented by Gorski (1969). Whether the willow

flycatcher reaches northern South America has been

debated (Gorski 1971, Stotz et al. 1996). Hilty and

Brown (1986) do not exclude the possibility that in

extreme northwest Columbia the wintering resident

is traillii rather than alnorum. Everywhere else in

Columbia, however, alnorum may replace traillii.

Unitt (pers. com.) somewhat disagrees with this view,

suggesting that the wintering distribution of the

willow flycatcher includes instead all of Columbia

and northwestern Venezuela while the wintering

distribution of the alder flycatcher is disjunct and lies

to the south, mainly in Bolivia and Peru.

On its wintering grounds in southwestern Mexico,

northern Central America, and Panama, the willow

flycatcher is described as fairly common to common

(Ridgely 1976, Howell and Webb 1995). In Costa Rica,

however, it appears to be generally uncommon as a

winter resident (Stiles and Skutch 1989), except perhaps in the northwestern region of Guanacaste (see

Koronkiewicz et al. 1998).
78 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

Based on a recent evaluation of flycatcher museum

skins collected on the wintering grounds, Unitt (1997)

suggested that the different subspecies co-occur rather

than segregate during the winter. The relative proportions of each subspecies to the total population in a

wintering area is likely to vary (Sogge pers. comm.). In

addition, information is needed on whether males and

females winter separately or together.

Habitat

Although varied, the primary wintering habitats for

E. traillii indicate a preference for semi-open brushy

areas often near water. Both Ridgely (1976) and Hilty

and Brown (1986) contrast the habitat preference of

the willow and alder flycatchers with that of other

Empidonax spp. such as the Acadian flycatcher (E. virescens), the latter group being more strongly associated

with woodlands and forests. The list of habitats where

wintering E. traillii has been recorded includes thickets, shrubby clearings, second-growth scrub, woodland and forest edges, coastal lowlands, and riparian

woodlands (Land 1970, Edwards 1972, Peterson and

Chalif 1973, Hilty and Brown 1986, Stiles and Skutch

1989, Stotz et al. 1996). Habitats used by wintering

flycatchers range from humid to semi-arid sites.

In Mexico, wintering flycatchers were found chiefly

in coastal lowlands comprised of edge, open woodland,

or scrubby habitats (Edwards 1972). A strong association between willow flycatchers and standing water

has been described in Panama (Gorski 1969) and Costa

Rica (Koronkiewicz et al. 1998). In Panama, Gorski

(1969) found them in transitional and edge areas,

typically near a wetland. In Costa Rica and Panama,

willow flycatchers were recorded in lagunas and freshwater wetlands, muddy seeps, wet pastures, slow or

meandering rivers, and oxbows, which were often

bordered by fields, dense shrubby vegetation, and

small groves (Koronkiewicz et al. 1998; Koronkiewicz

and Whitfield 1999). Specific habitat elements of flycatcher wintering habitat were standing or sluggish

water, patches of dense shrubs, stringers or patches of

trees, and open to semi-open areas. Dense woody shrubs

1-2 m in height (e.g., Mimosa sp. and Cassia sp.) that

bordered wetlands were commonly used by willow

flycatchers in winter.

Deforestation has been recognized as an important

threat for many Neotropical migratory species (Finch

1991, Sherry and Holmes 1993). In light of its habitat

associations, however, the susceptibility of the willow

flycatcher to deforestation per se remains to be established. In Costa Rica, threats to this bird and its

habitat may instead consist of livestock grazing, chemical use, and alteration of wetlands due to water diversion and encroachment by plantations (Koronkiewicz

et al. 1998).

Behavior

Willow flycatchers defend winter territories for foraging, with winter and breeding territories measuring

approximately the same size (Gorski 1969). Winter

territories may remain relatively constant over the

winter (Koronkiewicz and Whitfield 1999). With the

onset of the dry season in Panama, however, individuals move to another area and establish new territories

(Gorski 1969). Koronkiewicz et al. (1998) noted that in

northwestern Costa Rica, surface waters are more

widespread in the early part of the winter. As the dry

season progresses, the availability of standing water

diminishes, and the distribution of willow flycatchers

may become more restricted.

Further research is needed to determine whether

individual flycatchers return each winter to the same

site or territory. Territoriality suggests that winter

resource(s) may be limiting, at least some of the time,

and therefore, territorial flycatchers may be defending one or more resources such as food, water, or perch

sites. In Costa Rica, willow flycatchers forage within

the dense foliage of woody shrubs (Koronkiewicz et al.

1998). Winter diet consists primarily of small insects,

which they capture after a short aerial pursuit, or a

sally down to the ground, or upward in the foliage

(Koronkiewicz et al. 1998).

In general, the fitz-bew song is rarely heard spontaneously on the wintering grounds (Hilty and Brown 1986,

but see Koronkiewicz et al. 1998). The kit or whit note

is the call usually observed (Ridgely 1976). Willow

flycatchers are apparently more aggressive in winter

than alder flycatchers when confronted with playback

recordings of their own songs (Gorski 1969, 1971).

Possible Threats___________________

Landbird migrants face a variety of obstacles and

threats during migration including inclement weather,

landscape barriers, predators, limited food and water,

and discontinuity of stopover habitat (see Moore 2000).

The probability of a successful migration is likely to be

increased when stopover habitats are managed with

distances between stopovers in mind. Assuming flycatchers can fly nonstop about 225 km on average (up

to 404 km using fat reserves) (estimated by Yong and

Finch 1997), then distances between stopover habitats that exceed 225 km may be difficult for many

flycatchers to negotiate. Fragmentation of stopover

habitats along migration routes may therefore be a

potential threat to successful migration and survival

of willow flycatchers. Finch et al. (1998) and Finch and

Kelly (1999) suggest that increasing cycle length of

vegetation mowing or eliminating mowing and clearing activities along channels and ditches may improve

stopover habitats for migrating flycatchers.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 79

In some habitats, fragmentation that results in

smaller habitat patches can increase predation risk at

songbird nests. It is possible that predation risk for

adult songbird migrants also increases with patch size

although data specific to willow flycatchers during any

part of their annual cycle are lacking. Given that

southwestern riparian habitats are naturally discontinuous, the relationship between predation probability and habitat patch size may be weaker than in

habitats naturally occurring as large blocks.

Invasion of exotic shrubs such as saltcedar and

Russian olive may affect migration success and habitat use patterns if stopover habitat quality is reduced

(Finch and Yong 2000). Although some evidence suggests that migrating willow flycatchers use native

habitats such as willow and cottonwood more frequently than introduced habitats such as saltcedar

and agricultural fields (Finch et al. 1998, Finch and

Kelly 1999), habitats dominated by woody exotics

supply more structural continuity and food resources

for migration than open habitats dominated by grasses

and forbs, or man-made sites of riprap banks and

urban lots (Kelly and Finch 1999).

Birds eliminate water during migration, and stopovers provide the opportunity to replenish lost body

fluids. The presence or absence of available water

may influence migration routes, stopover probabilities, and time spent in stopovers. Willow flycatchers

are commonly reported in migration near water sources.

Water limitations caused by drought, irrigation, and

water management activities may pose a problem for

migrating flycatchers, and ensuring that water is

always present in habitats used by flycatchers may be

the best management solution.

Koronkiewicz and Whitfield (1999) reported that

the primary threat to flycatcher wintering habitat is

loss related to agricultural activities. Winter flycatcher

habitats are susceptible to conversion to man-made

landscapes used as livestock pastures, clearings, and

agricultural plantations. Draining of wetlands used

by wintering willow flycatchers will accelerate conversion to habitats associated with drier conditions. Given

that wintering flycatchers frequently use habitats

associated with standing or slow-moving water, reduction of water and wetlands through irrigation,

draining, damming, and habitat conversion may impact survival of willow flycatchers in winter.

Wintering flycatchers forage in wetlands adjacent to,

or near agricultural areas that are treated with chemicals (Koronkiewicz et al. 1998). They are therefore

potentially exposed to these chemicals through the food

they eat. Willow flycatcher deformities (Paxton et al.

1997) recorded on the breeding grounds may be linked

to toxins ingested during any period of the annual

cycle. Deformities are not reported from all southwestern willow flycatchers suggesting that deformities may be associated with site-specific effects.

Protection of riparian habitats where the flycatcher

winters and migrates needs to be increased (Finch

and Yong 2000). Increased research and awareness

of the flycatcher’s habitat requirements in winter

and migration will improve management efforts. Loss

and degradation of known wintering sites used by

flycatchers should be avoided. Discussions with local

managers and landowners may help to prevent serious habitat losses on the wintering grounds, but

increased funds and incentives are needed before

these concerns are likely to be addressed in a serious

way. Cooperative international ventures involving

governments of multiple countries and coalitions and

nongovernment organizations such as Partners in

Flight, Audubon Society, and World Wildlife Fund

should be pursued to protect and restore wintering

habitats. Along migration routes, denuded river and

ditch banks and wetland edges can be substantially

improved by allowing them to grow riparian vegetation in place of riprap or cement (Finch et al. 1998).

Maintaining or adding water in or adjacent to winter

and stopover riparian habitats is desirable. The presence of water can influence local arthropod abundance. Arthropods are food through the winter and

supply energy for replenishing fat stores during

migration.

Further research on migration routes used by flycatchers is needed to improve efforts to protect and

restore habitats and recover flycatcher populations. A

better understanding of wintering distribution and

habitat use patterns by flycatchers is also needed for

conserving winter habitats. Increased emphasis on

conservation and protection of winter and migration

habitats should not supplant habitat management

goals and activities on the breeding grounds.

Acknowledgments ________________

We thank Bill Haas, Phil Unitt, and Hira Walker for

reviewing this manuscript, Ellen Paul for arranging

the review through The Ornithological Council, and

Nancy Chadwick for proofing the material. Linda

DeLay and Michael Means supplied valuable help in

assembling literature and data on migration.

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82 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 83

Scott H. Stoleson

Mary J. Whitfield

Mark K. Sogge

Chapter 8:

Demographic Characteristics and

Population Modeling

An understanding of the basic demography of a

species is necessary to estimate and evaluate population trends. The relative impact of different demographic parameters on growth rates can be assessed

through a sensitivity analysis, in which different parameters are altered singly to assess the effect on

population growth. Identification of critical parameters can allow managers to focus their efforts on

factors most likely to increase populations.

In this chapter, we describe the demography of

the southwestern willow flycatcher. Our objectives

are to (1) describe basic population characteristics of

the subspecies, (2) summarize and estimate vital

demographic parameters of the flycatcher, (3) use

those parameters in a life table and population projection model, and (4) use an elasticity analysis to

determine which parameters have the greatest impact on population growth rates, and therefore offer

the greatest potential for management. Much of

these data come from the only two well-studied populations of E.t. extimus, in the Kern River Preserve in

California (data from Whitfield 1990, Whitfield and

Strong 1995, Whitfield and Enos 1996, and Whitfield

unpublished) and the Grand Canyon in Arizona

(data from Sogge et al. 1997 and Sogge unpublished).

Information from the Kern River Preserve that lacks

a citation represents previously unpublished data of

M. Whitfield. To provide perspective, we present

additional data from other willow flycatcher subspecies as well as other Empidonax flycatchers.

Populations Characteristics ________

Age Classes

Because of the lack of age-specific plumages, there

are only two discernible age classes for the willow

flycatcher: hatching year (HY) and after hatch-year

(AHY).

Sex Ratio

There is no sexual dimorphism in this species; therefore inferences about sex ratios are necessarily circumstantial. Fledgling sex ratios are likely to be 1:1,

as is true for almost all birds (Clutton-Brock 1986,

Breitwisch 1989). Although facultatively monogamous,

the sex ratio of adults in some populations of this

subspecies appears to be male-biased, as suggested by

the substantial proportion of unmated males observed.

However, the proportion of unmated males, and

therefore the sex ratio, varies greatly among sites. For

example, in the Grand Canyon from 1993-1996, 44% of

territorial males were unpaired (Sogge et al. 1997). In

contrast, at the Kern River Preserve, generally 2-4 of

28-35 males (6-14%) are unpaired in any year, while in

the upper Gila Valley, New Mexico, a maximum of 8 of
84 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

195 singing males (4.1%) may have been unpaired in

1997 (Parker 1997; S. Stoleson and D. Finch, unpublished data). The apparent skewed sex ratio in some

populations may be due to higher mortality among

females (see Survivorship section below). Alternatively, in very small populations, such as the Grand

Canyon, sex ratios skewed in either direction can

result from demographic stochasticity (Burgman et al.

1993). Polygyny has been recorded in this species,

which may result from an excess of females in a

population (Sedgwick & Knopf 1989, M. Whitfield

unpublished data). However, mating systems in this

species may be more complex than previously thought,

as both unmated and polygynous males can occur in

the same population (M. Whitfield, unpublished data).

Population Growth Rates

Few, if any, populations of E.t. extimus have been

monitored for a sufficient time to quantify long-term

population trends. Former populations in Arizona on

the lower Salt River, Santa Cruz River, and lower

Colorado River near Yuma are believed to have been

extirpated. In California, a large population on the

Santa Ana River in San Bernadino County mentioned

by Hanna (1928) is gone although some suitable

habitat remains (R. McKernan, personal communication). Similarly, a population at the mouth of the

Santa Clara river appears to have been extirpated

(M. C. Badger, cited in Unitt 1987).

In the short term, most monitored populations of

the southwestern willow flycatcher have declined or

shown no trend. For example, between 1993 and 1997,

two small populations (3 to 4 pairs) in the Verde

Valley, Arizona, have disappeared. Two small populations in the White Mountains of Arizona have decreased steadily over the past five years (Langridge

and Sogge 1997). Between 1989 and 1993, the population in the Kern River Valley, California, dropped from

44 to 27 pairs (Whitfield 1993), but since then has

remained relatively stable at about 32 to 34 pairs

(Whitfield et al., in review). Similarly, the overall

population in the Grand Canyon has not shown any

consistent trend in the years 1982 to 1997, although

there have been local extirpations and recolonizations

at all the small breeding patches (Brown 1988, Sogge

et al. 1997). Surveys conducted from 1994 to 1999

along the upper Gila River in New Mexico suggest that

flycatcher numbers there may be increasing (Parker

1997, Stoleson & Finch unpublished data).

Fecundity ________________________

Fecundity is the measure of rate of reproduction in

a population. Seasonal (or annual) fecundity is difficult to quantify in the field, especially for species that

can attempt multiple nests in a season (Clobert &

Lebreton 1991). In bird populations, average seasonal

fecundity is the product of the average probability of

an individual breeding in a given season, the average

number of eggs produced per clutch, the average

number of nesting attempts per season, and the overall hatching and fledging success rates of nesting

attempts within a season. We address these components individually below.

Probability of Breeding

Willow flycatchers apparently begin to breed in

their second calendar year, i.e., on their first return to

the breeding grounds (M. Whitfield, unpublished

data, Paxton et al. 1997). At the Kern River, all

females of all age classes appear to breed every year,

perhaps as a result of a skewed adult sex ratio. The

same is likely to be true for other populations as well.

As noted above, in many populations a significant

portion of territorial males may remain unpaired.

Whether or not a male remains unmated during a

season appears to be independent of age (M. Whitfield,

unpublished data).

Clutch Size

The modal clutch size is three eggs throughout most

of the range of E.t. extimus, and varies from 1 to 5

(although one-egg clutches are likely the result of

predation or disturbance). Clutch sizes at the Kern

River appear to be larger than in Arizona or New

Mexico (Table 8-1). Four-egg clutches are common

among first nesting attempts of a season at the Kern

( x = 3.64 ± 0.68 eggs [mean ± SD], n = 96). Elsewhere

in the southwest, clutches of four eggs are rare (Sferra

et al. 1997, Sogge et al. 1997, S. Stoleson and D. Finch,

unpublished data). Among other subspecies of the

willow flycatcher, clutches of four are not uncommon,

and mean clutch size is generally larger than that of

E.t. extimus (Table 8-1).

In general, clutch sizes of other flycatchers in the

genus Empidonax tend to be larger than those of E.t.

extimus (Table 8-1). Clutches of four eggs are common,

and clutches of five eggs occur in several species (e.g.,

Briskie & Sealy 1989, Sedgwick 1993, Bowers &

Dunning 1994). This difference may be related to

habitat or latitude. Unlike the shrub and riparian

woodland inhabiting willow flycatcher, several of the

other Empidonax listed in Table 8-1 occupy closed

forest habitats (Bent 1942). Forest interior birds may

experience lower predation rates, and consequently

may have evolved relatively larger clutch sizes than

sister taxa in more open habitats or edges (Martin

1993, 1995). In general, clutch sizes of birds increase

with increasing latitude (Lack 1968, Klomp 1970).

The southwestern willow flycatcher breeds further

south than any other subspecies of E. traillii (Unitt
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 85

1987), and further south than most of the other species

listed in Table 8-1. Therefore, E.t. extimus may be

expected to have smaller clutches, on average, than

other subspecies or other Empidonax species. It may

be noteworthy that the Kern River population is further north than populations in Arizona and New

Mexico and tends to have larger clutch sizes. The

Acadian Flycatcher (E. virescens), whose breeding

range extends further south in the United States than

any other Empidonax (Bent 1942), also tends to have

smaller clutch sizes than its congeners (Table 8-1).

Clutch size in the southwestern willow flycatcher

does not increase or decrease as a function of age (M.

Whitfield, unpublished data). However, clutch size

does tend to decrease with successive nesting attempts within a year (Holcomb 1974, M. Whitfield,

unpublished data). For example, at the Kern River,

mean clutch size declined from 3.64 for first attempts

to 2.87 for second and 2.67 for third attempts. Least

Flycatchers (E. minimus) show a similar pattern of

decreasing clutch size in successive nesting attempts

(Briskie & Sealy 1989), as do many other passerines

(Rowe et al. 1994, Young 1994).

Number of Nesting Attempts per Season

Willow flycatchers often respond to cowbird parasitism, nest destruction, or other severe disturbance by

abandoning their nests and renesting. Consequently,

females will frequently attempt several nests per

season following the failure of earlier nests (Holcomb

1974, Whitfield 1990, Harris 1991). At the Kern River,

females averaged 1.82 ± 0.89 nesting attempts per

season from 1989 to 1997. Some females in Arizona

and New Mexico have been suspected of initiating up

to four clutches in a season, although this is uncertain

because the birds were unbanded (T. McCarthey, S.

Stoleson and D. Finch, unpublished data). The highest

documented number of nesting attempts within a

season occurred at the Kern River during a year of

intense cowbird parasitism, where one pair built six

nests (Harris 1991). In general, a high incidence of

cowbird parasitism leads to an increased number of

nesting attempts per season because many flycatchers

will quickly abandon and renest. For example, at the

Kern River, females averaged 2.04 ± 0.99 (n=82) attempts per year prior to cowbird trapping, and only

1.66 ± 0.78 (n=118) attempts afterwards.

Other than renesting after nest failure or parasitism, multiple nesting attempts within a season are

rare. McCabe (1991) considered willow flycatchers to

be single-brooded. However, double brooding (raising

a second brood after successfully fledging the first)

has been documented or suspected at several sites in

the southwest (Whitfield 1990, Griffith & Griffith

1995, Sferra et al. 1997, S. Stoleson and D. Finch,

unpublished data). For example, at the Kern River,

3.8% of successful females attempted to raise second

Table 8-1. Clutch sizes of willow flycatchers and other Empidonax species.

Taxona Site Mean N Range Source

E.t. extimus Kern River, CA (1987) NA 16 1-4b Harris 1991

Kern River, CA (1989-97) 3.33 154 1-5b M. Whitfield unpub. data

Grand Canyon, AZ 3 3 3 Sogge et al. 1997

Arizona, statewide 2.34 67 1-4b Sferra et al. 1997

San Luis Rey, CA 2.69 29 1-4b W. Haas pers. comm.

Cliff-Gila Valley, NM 2.67 21 2-4 Skaggs 1996, Stoleson and Finch unpub. data

E.t. adastus WA 3.42 33 3-4 King 1955

E.t. brewsteri Truckee R., CA 2.82 11 2-3 Flett & Sanders 1987

E.t. traillii MI 3.28 92 3-5 Walkinshaw 1966

OH, NE 3.41 91 2-5 Holcomb 1972, 1974

WI 3.59 415 3-5 McCabe 1991

E. difficilis Monterey, CA 4.00 23 NA Davis et al. 1963

E. fulvifrons AZ 3.37 12 2-5 Bowers & Dunning 1994

E. minimus MI 3.95 46 3-5 Walkinshaw 1961

Manitoba 3.92 192 2-5 Briskie & Sealy 1989

E. oberholseri MT 4.00 21 2-5 Sedgwick 1993

E. virescens MI 2.54 66 NA Walkinshaw 1961

MI 2.92 25 2-4 Mumford 1964

a E. traillii = willow flycatcher, E. difficilis = Pacific-slope flycatcher, E. fulvifrons = buff-breasted flycatcher, E. minimus = least flycatcher, E.

oberholseri = dusky flycatcher, E. virescens = acadian flycatcher. b

 Clutch sizes of one are likely the result of disturbance or predation.
86 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

broods. Double brooding is relatively rare in other

Empidonax flycatchers as well, although reported at

least occasionally in most species (e.g., Walkinshaw

1961, Davis et al. 1963, Sedgwick 1993, Bowers &

Dunning 1994, Briskie 1994). Briskie and Sealy (1987)

suggested that the Least Flycatcher may double brood

infrequently because delaying fall migration to raise a

second brood may prevent birds from establishing and

maintaining winter territories. This may apply to

willow flycatchers as well: females seem to double

brood only when the young from their first brood have

fledged by late June or very early July.

Hatching Success

Hatching success of southwestern willow flycatcher

eggs varies among populations. The overall hatching

rate in unparasitized nests for all monitored sites in

Arizona in 1996 was 66% (Sferra et al. 1997). At the

Kern River and San Luis Rey River populations in

California, 63% and 86% of eggs hatched in

unparasitized nests, respectively (Whitfield and Sogge

1999, W. Haas personal communication). Hatching

rates for other subspecies vary widely among populations, from a low of 54.8% (n=272) in Ohio and

Nebraska (Holcomb 1972) to a high of 92.6% (n=67)

in Washington (King 1953). Thus, populations at the

Kern River and in Arizona appear to experience

relatively low hatching success, although comparable rates have been reported for both Acadian

(Walkinshaw 1961) and Dusky Flycatchers (E.

oberholseri; Sedgwick 1993).

Nesting Success

Nesting success for the southwestern willow flycatcher varies greatly among sites (Table 8-2). Areas

with high levels of cowbird parasitism exhibit very low

success (e.g., the Grand Canyon, the Kern River prior

to cowbird trapping). In contrast, other sites experience relatively high levels of nesting success. For

example, at some sites in Arizona and in the Gila

Valley of New Mexico, 50 to 55% of nests successfully

fledged one or more young (Sferra et al. 1997, Skaggs

1996, S. Stoleson and D. Finch, 1999). At the San Luis

Rey River in California, nesting success has reached

70% in an area of intensive cowbird trapping (W.

Haas, personal communication). In general, nesting

success rates for cup-nesting passerines in North

America range from about 38 to 70%, with a median

value of 52% (Nice 1957, Martin 1993). Thus, some

populations of E.t. extimus (listed in Table 8-2) experience poor nesting success compared to other cupnesting songbirds.

Nesting success in Tyrannid flycatchers tends to be

relatively high for the size of the bird, perhaps because

of their aggressive nature (Murphy 1983). Studies of

other willow flycatcher subspecies have generally indicated higher nesting success rates than are typical

for E.t. extimus, from lows of about 40% to almost 70%

Table 8-2. Measures of nesting success in willow flycatchers and other Empidonax species.

% nest No. of Fledglings Fledglings

Taxona Site successb nests per nest per female Source

E.t. extimus Kern River, CA (1987) 15.8 19 0.62 1.25 Harris 1991

Kern River, CA (1989-1997) 36.4 324 1.27 1.44 Whitfield unpub. data

Grand Canyon, AZ 18.0 17 NA 0.70 Sogge et al. 1997

statewide ave., AZ 42.9 163 0.93 0.99c Sferra et al. 1997

Cliff-Gila Valley, NM 55.2 97 NA NA Skaggs 1996; Stoleson & Finch unpub. data

San Luis Rey R., CA (1994) 64.0 11 2.09 NA Griffith & Griffith 1995

San Luis Rey R., CA (1995-97) 66.0 70 1.54 2.45 Haas pers. comm.

E.t. adastus North Park, CO 40.7 27 0.89 NA Sedgwick & Knopf 1988

Malheur N.W.R., OR NA 876 NA 1.81 Sedgwick & Iko 1999

E.t. brewsteri Truckee R., CA 54.5 11 1.27-1.36 1.40-1.50 Flett & Sanders 1987

E.t. traillii MI 69.5 209 NA NA Berger 1967

MI 65.2 92 2.15 NA Walkinshaw 1966

OH, NE 39.5 91 1.11 1.88 Holcomb 1972

WI 68.6 459 2.13 NA McCabe 1991

E. difficilis Monterey, CA 73.9 43 1.92 NA Davis et al. 1963

E. fulvifrons AZ NA 12 2.08 NA Bowers & Dunning 1994

E. minimus MI 56.3 16 3.80 NA Walkinshaw 1961

Manitoba 58.6 273 1.25 NA Briskie & Sealy 1989

E. oberholseri MT 58.3 24 1.60 1.90 Sedgwick 1993

E. virescens MI 59.1 66 1.36 NA Walkinshaw 1961

a

 See Table 8-1 for common names of taxa.

b

 Percentage of nests of known outcome that produced at least one flycatcher fledgling; not Mayfield estimates (Mayfield 1975).

c n = 102 females
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 87

(Table 8-2). Nesting success among other Empidonax

species tends to be somewhat higher yet, usually well

over 55% (Table 8-2). Willow flycatchers may have

somewhat lower success than their congeners because

their riparian habitats tend to be patchy, fragmented,

linear, and with a high proportion of edge, especially in

the Southwest. Birds in such habitats are likely to be

more vulnerable to predation or parasitism than in

more contiguous wooded habitats (Robinson et al.

1995). As mentioned above, most of the other species

in Table 8-2 are species of contiguous forest, which

tend to have higher rates of nesting success than

species that nest in more open or fragmented habitats

(Martin 1993). In addition, most forests in the west

are higher in elevation and further from concentrations of livestock than are floodplain riparian woodlands, so there may be an elevational component to the

difference in nest success as well.

Seasonal Fecundity

Fecundity can be measured as the number of young

fledged on a per nest or per pair basis. The number of

fledglings produced per nest is much more easily

determined. At most sites where data are available,

southwestern willow flycatchers fledge less than one

chick per nest, including nests that fail or are abandoned (Table 8-2). In contrast, studies of other willow

flycatcher subspecies have usually reported more fledglings per nesting attempt, ranging from 1.11 to over 2

(Table 8-2). One exception was in North Park, Colorado, where heavy cowbird parasitism on E.t. adastus

reduced per-nest productivity to 0.89 fledglings

(Sedgwick & Knopf 1988). Because cowbird parasitism often provokes nest abandonment, it can greatly

reduce the average nest productivity. For example, at

the Kern River, an intensive cowbird control program

increased the average per-nest productivity from 1.04

fledglings to 1.72 fledglings (Whitfield et al., in review). The effects of cowbird parasitism on productivity are discussed in depth in the following chapter.

Other Empidonax show higher per-nest productivity

than E.t. extimus as well (Table 8-2).

Because flycatchers may renest multiple times during a breeding season, per-nest productivity does not

necessarily equate with seasonal fecundity. Rather,

the best measure is the number of chicks fledged per

pair per year (Clobert & Lebreton 1991). Recent evidence suggests low levels of mate fidelity in the willow

flycatcher, even within a season (Paxton et al. 1997).

Because female fecundity is much easier to assess

accurately than male fecundity, we will use the number of fledglings per female per year as the measure of

seasonal fecundity. Unfortunately, this rate has been

calculated in very few studies, in part because it

requires having color-banded birds, and because of the

difficulty of following individual females through the

course of an entire breeding season (Pease &

Grzybowski 1995).

Seasonal fecundity of E.t. extimus ranges from a low

of 0.7 fledglings per female in the Grand Canyon to

2.45 fledglings per female (3 year average) at the San

Luis Rey River (Table 8-2). The low rate of reproduction in the Grand Canyon suggests a sink population.

In contrast, the fecundity of the San Luis Rey population, where cowbirds are controlled, exceeds that of

most populations of other subspecies and other

Empidonax species (Table 8-2).

Survivorship ______________________

Data on survivorship for willow flycatchers are

sparse. Information that does exist comes from return

rates of banded birds. Estimates of survivorship based

on resighting or recapture of banded individuals are

necessarily conservative because they do not discriminate between mortality and emigration (Lebreton

et al. 1992, Noon & Sauer 1992). Individuals of E.t.

extimus have been color-banded systematically for

more than three years only at the Kern River Preserve,

although in 1996 the Colorado Plateau Field Station

began a statewide banding effort in Arizona (Paxton

et al. 1997).

Return Rates of Banded Birds

Return rates of banded adult flycatchers from the

two color-marked populations of extimus were very

similar: about 52% for males and 35% for females

(Table 8-3). In contrast, studies of other subspecies

Table 8-3. Return rates of color-banded willow flycatchers in four areas.

Site HYa AHY:malea AHY:femalea Source

Kern River, CA 34.2% (38) 51.7% (29) 33.8% (207) Whitfield unpub. data

Arizona 8% (12) 52.0% (50) 34.0% (48) Paxton et al. 1997

Michigan 1.4% (147) 40.9% (22) 22.6% (31) Walkinshaw 1966

Malheur NWR, OR 13.2% (214) 43.3% (192) 46.8% (211) Sedgwick & Klus 1997

a

 HY = hatch year (from fledging through first breeding season), AHY = after hatch year.
88 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

showed lower return rates. Whether this represents

lower survival rates or greater dispersal due to more

available habitat is unclear. In general, return rates

of female flycatchers were lower than those of males.

Because adult female willow flycatchers are thought

to show high site fidelity (Walkinshaw 1966, Sedgwick

& Knopf 1989, but see Paxton et al. 1997), lower return

rates probably indicate lower survival rates than for

males. Male-biased survival rates are common among

passerines (Breitwisch 1989). The population at

Malheur National Wildlife Refuge, Oregon, is exceptional in that females returned to the site at about the

same rate as males (Sedgwick & Klus 1997). In general, annual survival rates for small migratory passerines range from 0.3 to 0.6 (Dobson 1990, Karr et al.

1990), so the estimates based on resighting probabilities presented in Table 8-3 seem reasonable for birds

the size of willow flycatchers.

Estimates of survivorship for first year birds based

on return rates to their natal region are usually biased

because first year birds normally disperse from their

natal area, and it is impossible to differentiate mortality from permanent emigration (Clobert & Lebreton

1991, Noon & Sauer 1992). Relatively low return

rates for Michigan and Oregon populations (Table 8-3)

almost certainly reflect high rates of natal dispersal.

A relatively high proportion of first year birds returned to the Kern River Preserve (Table 8-3), perhaps

because very little suitable riparian habitat exists in

that region into which birds can disperse, so more

return to their natal area and are detected. For that

reason, return rates of first year birds to the Kern may

be our best estimate of first year survival. The situation is likely to be similar in Arizona, but small sample

sizes (1 of 12 individuals banded in 1996 returned in

1997) make the reported 8% figure suspect (Paxton

et al. 1997). Data are equally scant for related species.

Of 410 fledgling Least Flycatchers banded in Manitoba,

only 4.2% were ever recaptured (Briskie 1994).

The between-season survival rate of fledglings appears to decline as the breeding season progresses.

Whitfield and Strong (1995) found significantly higher

return rates for young fledged before July 21 than for

young fledged afterwards. This pattern has been reported in numerous other avian species as well (e.g.,

Hochachka 1990, Verhulst et al. 1995, Brinkhof et al.

1997). Therefore, there are two reasons why cowbird

parasitism can affect seasonal fecundity even when

birds are successful after abandoning early parasitized nests. Second (or later) clutches will be smaller,

and fledglings from later attempts will be less likely to

survive to breeding age. At the Kern River, observed

return rates for fledglings were greater following the

initiation of cowbird control in 1992, exceeding 0.50 in

1996 and 1997, although this may reflect increased

resighting effort in those years (see Chapter 8).

Lifespan

Apart from a few anecdotal accounts, there is little

information available on the longevity of willow flycatchers. In Oregon, one of 537 birds banded as adults

survived at least eleven years after its first capture

(J. Sedgwick, personal communication). One of 22

males banded as adults survived at least five years in

Michigan (Walkinshaw 1966). For E.t. extimus, one

male survived at least six years after banding, and

three females and a male were still alive five years

after banding at the Kern River (Whitfield and Enos

1996). Similarly, little is known of lifespans of other

Empidonax flycatchers. The maximum known age

for any other species, based on banding recoveries, is

8 years for a Dusky Flycatcher (Sedgwick 1993).

The average life span of a bird can be estimated from

mortality rates using the formula:

 

L m

m = 2 −2

where L = average lifespan and m = the average

annual mortality rate (Gill 1990). Substituting 0.66

for m (based on minimum annual survival = 0.34,

see Table 8-3) yields an average lifespan of 1.02 years;

using 0.47 for m (based on maximum annual survival

= 0.53, see Table 8-3) gives a lifespan of 1.63 years.

Thus, the average lifespan of southwestern willow

flycatchers is likely to be somewhere between 1.02 and

1.63 years.

Lifetime Reproductive Success

Lifetime reproductive success (LRS) has been estimated for very few passerines, in part because of the

logistical difficulties in following individual females

through the course of their lifetimes. Long term studies

of E.t. adastus in Oregon have revealed that females

reared, on average, 3.60 young over their lifespans

(Sedgwick and Iko 1999). LRS varied with whether or

not females were parasitized in their first breeding

year, but was not affected by parasitism in subsequent

years.

Immigration and Emigration

To date, what little is known of movement among

populations of E.t. extimus comes from recent work in

Arizona (Paxton et al. 1997). Rates of immigration and

emigration varied considerably among populations,

perhaps based on the relative degree of isolation of

the population. Overall, of 48 birds that bred in 1996

and were resighted in 1997, 13 (27%) returned to a

different site. It is likely that immigration rates are

lower where available breeding sites are limited, such

as at the Kern River (Paxton et al. 1997). Also, site

fidelity tended to be lower for sites with lower overall
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 89

nesting success, suggesting higher emigration rates

(Paxton et al. 1997). Information on sex-biased dispersal is sparse, but recent data from color-banded

birds in Arizona suggest that males emigrate more

frequently than females, contrary to the normal pattern among passerine birds (Greenwood & Harvey

1982). It is yet unclear whether females disperse

further than males, as in other birds.

Life Table Analyses ________________

A life table summarizes the vital rates of agespecific survivorship and fecundity for a population.

As mentioned above, because of the difficulty in determining fecundity rates for males, life tables are normally female-based. Observed rates of fecundity are

divided in half to indicate the rate of production of

female fledglings, and assume an equal sex ratio at

fledging (Noon & Sauer 1992).

It is evident from previous sections of this chapter

that estimates of demographic parameters for the

southwestern willow flycatcher are available from

very few sites, and that estimates vary considerably,

both among sites, and within sites depending on year

and management practices. For no population (with

the possible exception of the Kern River) is there

sufficient knowledge of demographic parameters to

create a complete life table. Therefore we present two

composite life tables, using estimates of parameters

from various populations. One table is conservative,

and uses minimum estimates of survival and fecundity. It may be thought of as a worst case scenario. The

second table is more optimistic, uses the highest recorded parameter estimates, and represents a best

case scenario. The two tables bracket the likely range

of demographic parameters for the majority of willow

flycatcher populations.

Life Table Parameters

In the conservative life table, we used 0.34 as the

survival rate (px) for both the HY age class, based on

the observed return rate of HY birds at the Kern River,

and for the AHY age class, based on observed return

rates for adult females in Arizona (Table 8-3; Paxton

et al. 1997). The fecundity rate, or maternity function

mx, denotes the expected number of female fledglings

produced by a female of age x. We used the observed

rate of 0.7 fledglings per female per season from the

Grand Canyon (Sogge et al. 1997), divided by two to

account for female offspring only. In the optimistic life

table, HY birds were assigned a survival rate of 0.50

(M. Whitfield, unpublished data), and AHY birds a

rate of 0.47 (Sedgwick & Klus 1997). We used an

optimistic fecundity rate of 1.28 female fledglings per

female per season, based on 1997 data from San Luis

Rey, California (W. Haas, personal communication).

Other parameters in both life tables are x, which

denotes age class expressed in years; px, the probability of survival from age class x to age class x+1; lx, the

probability that an individual aged 0 will survive to

enter age class x; and lxmx, the age-specific reproductive rate. The latter term multiplied by age x is used for

calculating population parameters (see below). Life

tables assumed no reproductive senescence, but were

truncated at eight years, the maximum recorded

lifespan for willow flycatchers. Fecundity and survival

were assumed to be constant after the hatching year.

Life tables are presented as tables 8-4 and 8-5. They

show that for either scenario, a very small proportion

Table 8-4. Life table of the southwestern willow flycatcher

based on conservativea

 parameter valuesb.

x px lx mx lxmx xlxmx

0 0.340 1.000 0.00 0.000 0.000

1 0.340 0.340 0.35 0.119 0.119

2 0.340 0.116 0.35 0.040 0.081

3 0.340 0.039 0.35 0.014 0.041

4 0.340 0.013 0.35 0.005 0.019

5 0.340 0.005 0.35 0.002 0.008

6 0.340 0.002 0.35 0.001 0.003

7 0.340 0.001 0.35 0.000 0.001

8 0.340 0.000 0.35 0.000 0.000

SUM: 0.180 0.272

a

 Conservative scenario uses the lowest value recorded among all

willow flycatcher populations for each parameter. b

 Table parameters are: x = age class in years; px = probability of

survival from age class x to x+1; lx = probability that an individual aged

0 will survive to enter age class x; mx = average number of female

offspring by a female of age x; lxmx = product of lx and mx, the agespecific reproductive rate; and xlxmx = product of lxmx and x (used for

calculation of population parameters).

Table 8-5. Life table for the southwestern willow flycatcher

based on optimistica

 parameter valuesb.

x px lx mx lxmx xlxmx

0 0.500 1.000 0.00 0.000 0.000

1 0.470 0.500 1.28 0.640 0.640

2 0.470 0.235 1.28 0.301 0.602

3 0.470 0.110 1.28 0.141 0.424

4 0.470 0.052 1.28 0.066 0.266

5 0.470 0.024 1.28 0.031 0.156

6 0.470 0.011 1.28 0.015 0.088

7 0.470 0.005 1.28 0.007 0.048

8 0.470 0.003 1.28 0.003 0.026

SUM: 1.205 2.250

a

 Optimistic scenario uses the highest value recorded among all

willow flycatcher populations for each parameter. b

 Refer to Table 8-4 for definitions of parameters.
90 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

of any cohort will survive beyond 3 years of age.

Consequently, females in the 1 and 2 year age classes

make the greatest contribution to reproduction (as

indicated by the lxmx terms).

Population Parameters Calculated from the

Life Table

The life table enables calculation of R0, the net

reproductive rate, and generation time T. The net

reproductive rate is calculated as the sum of the lxmx

terms, and measures the expected production of female fledglings by a female during the course of its

lifetime. Table 8-4 indicates that in the worst-case

scenario, a female willow flycatcher will produce, on

average, 0.18 female fledglings in its lifetime, or

0.36 fledglings of both sexes assuming an equal sex

ratio. This rate is clearly well below the replacement

rate of 1 female fledgling per female, and indicates a

rapidly declining population. The best case scenario

suggests a net reproductive rate of 1.21 female fledglings per female, and indicates a population growing

at a relatively rapid rate.

Generation time T is a measure of the mean interval

between the birth of a female and the birth of her

offspring (Caughley 1977). T is calculated as

 

T xlm

R

x x = ∑ ( )

0

Generation time calculated from the conservative life

table is 1.51 years, and from the optimistic life table

1.87 years. The life table based on optimistic values

yields a longer generation time because the expected

reproductive lifespan of females is greater due to

higher survival rates. In either case, the very short

generation time suggests considerable potential for

rapid population growth as well as considerable vulnerability to rapid population decline.

Population Projection Model _______

Effective management and recovery of a threatened

or endangered species depend on identifying and

correcting the factors that limit population growth.

Demographic modeling can indicate probable population trends under current or future conditions if

model parameters are well known. Sensitivity analyses can be used to indicate which life history components are the most likely to affect population growth

rates, and hence provide the most potential for management. Because the southwestern willow flycatcher

occurs in assemblages of local breeding populations

that occupy dynamic habitat patches scattered across

the landscape, it would be an ideal candidate for a

metapopulation or spatially-explicit model (Hanski 1998).

However, these types of models are data-intensive,

and require reasonable estimates of dispersal and

patch-specific demographic rates, among other data,

to produce meaningful results (Beissinger and

Westphal 1998). Those data do not yet exist; therefore

the use of a spatially structured demographic model

would be premature at this time.

Instead, we present a simple stage-based matrix

projection model for the southwestern willow flycatcher (Caswell 1989, McDonald & Caswell 1993). As

with the life tables, we use estimates of demographic

parameters from various populations. Accordingly,

the model will not represent the dynamics of any

particular population of willow flycatchers, but rather

will provide a crude measure of range-wide population

dynamics. The model was used to estimate lambda (λ),

the finite rate of population growth, which indicates

the factor by which a population grows over the projection interval. A lambda less than one indicates a

declining population, and a lambda greater than one a

growing population.

Model Structure

In the absence of age-specific vital rates (see Life

Table Analyses above), a simple model was created

based on the two life stages that are identifiable in the

field: hatching year and after-hatch year. The population life cycle diagram is presented in Figure 8-1, and

is based on post-breeding censuses and a one-year

projection interval. This life cycle is typical of many

birds (McDonald & Caswell 1993). Post-breeding censuses are used for the model because parameters are

Figure 8-1. A simple life cycle diagram for the southwestern

willow flycatcher used to create a stage-based population

model. P0 = probability of a fledgling surviving through the

following breeding season; P1 = annual survival probability

for adults; F0 = fecundity (i.e., the number of fledglings

produced per female per breeding season) of birds in their

first year of life; F1 = annual fecundity of birds one or more

years old; hy = hatch year; ahy = after hatch year.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 91

Figure 8-2. Hypothetical population trajectories for the southwestern willow flycatcher using conservative (lowest values

among all populations), optimistic (highest values among all

populations), and intermediate (mean of conservative and

optimistic values) parameters for fecundity and survival. Initial

population size was 100 pairs. See text for details of model.

Table 8-6. Values used for demographic parametersa

 in a

conservative, an intermediate and an optimistic

population projection modelb

.

Model P0 P1 mx F0 F1

Conservative 0.34 0.34 0.35 0.12 0.12

Intermediate 0.42 0.41 — 0.38 0.36

Optimistic 0.50 0.47 1.28 0.64 0.60

a See Figure 8-1 for definitions of parameters; maternity function,

mx = number of female fledglings per female (see Tables 8-4, 8-5).

b

 Conservative and optimistic models defined as in Table 8-4;

intermediate model uses means of extreme values.

more easily estimated from field data than is true for

pre-breeding censuses. Pre-breeding censuses also

confound estimates of fecundity and first year survival

(Noon and Sauer 1992). Note that the timing of the

census for the purposes of this model is not meant to

address questions of field methodology.

In the model, P0 is the probability that a fledgling

will survive to the next breeding season. P1 is the

annual survivorship of adults. The annual fecundity

(number of female fledglings per female per year) is

given as F0 for hatch-year birds and F1 for adults. Note

that because this is a post-breeding model, the fecundity value for HY birds indicates the productivity of

birds in their first breeding attempt, after their return

to the breeding grounds but before the census. The

flow of events in the model is (1) birds are censused at

the close of the breeding season, (2) birds survive to the

next season, (3) individuals are aged one year, (4)

survivors breed, and (5) birds are censused again. The

model is completely deterministic, as there are no data

available on the variance of parameters within a

population. Because in this model birds must survive

to the next season before breeding, F values are the

product of the maternity rate (mx) from the life tables

and the stage-specific survival rates P.

The model assumes that: (1) males do not affect

survival and reproduction of females, (2) rates of

survival and fecundity are constant among individuals within a stage class and among years, (3) breeding

occurs in a single birth-pulse, (4) the population is

near a stable age distribution, and (5) parameters are

not density dependent (Noon & Sauer 1992). None of

these assumptions are likely to be strictly true for a

passerine bird. In particular, both survival and fecundity can vary greatly among years at some sites.

However, slight violations of these assumptions are

unlikely to affect the qualitative results of the model

(Noon & Sauer 1992). As with the life tables, we

examined the model using both optimistic and conservative parameters. We also added an intermediate set

of values as perhaps more typical of most populations.

Because data are available for very few populations,

we used simple means of the extreme parameter

values in the intermediate scenario. The vital rates

used for the conservative and optimistic models were

the same as in the life tables (Table 8-6).

Populations were projected over a 25 year time span,

arbitrarily starting with 100 pairs of flycatchers.

Lambda was calculated for each scenario analytically

as Nt /N t-1 at t=25 years, where N is the total population size. For all three scenarios, lambda had stabilized after 20 years.

Population Projection Results

Under the optimistic scenario, the hypothetical population grew exponentially (Figure 8-2). Lambda was

calculated to be 1.11, which indicates an annual increase of 11%. This rapid population growth is based

on actual rates measured in the field, and illustrates

that under optimal conditions, willow flycatchers have

the potential for rapid recovery. In contrast, populations declined to extinction under both the intermediate and conservative scenarios. In the worst-case scenario, lambda was 0.46, which indicates a precipitous

population decline of 54% per year. The population

still declined at an annual rate of 22% under the

intermediate scenario. Estimates of lambda derived

from this model should not be considered as representing the growth rate of any population of willow flycatcher, nor should they be used to predict future

population sizes. However, if the parameter values

used are considered to be representative of the extremes found in flycatcher populations, then the trajectory of any particular population is likely to be
92 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

Table 8-7. Elasticity of lambda to changes in demographic

parameters for three different scenarios. See

Table 8-6 for parameter definitions and values

used in each of the scenarios.

Model P0 P1 F0 F1

Conservative 0.19 0.56 0.07 0.19

Intermediate 0.24 0.28 0.24 0.24

Optimistic 0.24 0.21 0.31 0.24

within the extremes presented. Further, if the intermediate values used are indeed typical of most flycatcher populations, then the prospect for all but a few

populations is bleak.

Elasticity of Demographic Parameters

Elasticity is a measure of the proportionate effect of

a parameter on lambda. Parameters showed different

elasticity among the three scenarios (Table 8-7). Under the conservative scenario, lambda was most

strongly affected by adult survival (P1), and little

affected by other parameters. This suggests that

prolonging the reproductive lifespan of breeding females would have a greater effect on population growth

than would increasing seasonal fecundity, perhaps

since fecundity is low. In contrast, under the optimistic scenario, lambda was most strongly affected by

HY fecundity (F0), and moderately affected by the

other parameters about equally. From Table 8-5 it is

evident that first time breeders make the largest

contribution to reproduction (as indicated by the lxmx

term), and increasing their fecundity would have a

disproportionate effect on population growth. Under

the intermediate scenario, the effect of all parameters

on lambda was similar.

Implications for Management

The effectiveness of management actions can be

maximized by concentrating efforts on those demographic components that have the greatest effect on

population growth rates. However, which demographic

component to manage depends on which scenario is

used. With the conservative scenario, increasing the

population growth rate would be best accomplished by

increasing adult survivorship. Better data are needed

on the causes and timing of adult mortality to accomplish this task. On the other hand, the optimistic

scenario suggests increasing fecundity may be the

best strategy. This might be accomplished by reducing

cowbird parasitism or nest predation. In reality, it is

probably much easier to improve fecundity through

management than it would be to increase survival

rates, in part because this species is a neotropical

migrant with overwinter survival constrained by factors outside of the United States, and therefore difficult to manage. In addition, demographic rates, and

hence the best management strategies, are likely to be

site-specific.

Acknowledgments ________________

We thank W. Haas for graciously allowing us to

use his data, and all the field assistants that helped

to gather the data presented here. Comments by

J.-L. Cartron, C. Hunter, J. Sedgwick, B. Valentine

and members of the Conservation Assessment team

improved this chapter. Writing of this manuscript was

funded in part by the U.S. Geological Survey.

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94 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 95

J. C. Uyehara

Mary J. Whitfield

Lloyd Goldwasser

Chapter 9:

The Ecology of Brown-Headed Cowbirds

and Their Effects on Southwestern

Willow Flycatchers

Brown-headed cowbirds (Molothrus ater) are obligate brood-parasites, that is, female cowbirds lay their

eggs in nests of other species. If the cowbird eggs are

accepted, the host pair may raise the young cowbird,

often at a reduction of the hosts’ reproductive success.

Cowbird females are also known to remove host eggs

and nestlings from nests, which may also affect the

reproductive success of the hosts (Smith 1981, Scott et

al. 1992, Sealy 1992). It is difficult to assess the impact

of parasitism both because reproductive success operates at many levels and because the hosts have defenses against parasitism or cowbird intrusion. In this

chapter, we review the data on the effects of brood

parasitism on reproductive success of the southwestern willow flycatchers (WIFL). We estimate the effects

of brood parasitism on populations of WIFLs and the

degree to which parasitism is a factor in their recovery.

For most of the analyses, we use data from one population along the South Fork Kern River, Kern County,

California.

Cowbird Ecology __________________

In this section, we review the ecology of cowbirds

and point out how the ecological and life history

considerations may affect various management decisions regarding WIFLs. Because cowbirds are a wideranging species, we have focussed our literature review on studies conducted in the southwest, within the

historic range of the Southwestern Willow Flycatcher.

Breeding Biology

The breeding season of cowbirds overlaps with the

initial period of breeding for southwestern willow

flycatchers. In mid-April to early May, female cowbirds arrive on their breeding grounds (Fleischer et al.

1987, Braden et al. 1997), although in some years,

females may arrive earlier (Darley 1983). Cowbirds

begin laying eggs at the end of April and end in midJuly throughout much of the cowbirds’ range (e.g.,

Payne 1976, Yokel 1987, Brown 1994, Braden et al.

1997, Schweitzer et al. 1998). In some areas, cowbirds

may lay in late July (Whitfield, unpub. data). The

length of the breeding season for an individual female

may depend on weather conditions (e.g., Scott and

Ankney 1979), age of the female (Fleischer et al. 1987)

or location. Typically, however, most female cowbirds

are laying eggs in May and June. As the breeding

season progresses, fewer cowbirds are laying eggs
96 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

(Scott and Ankney 1979, Yokel 1987, Holford and Roby

1993). Because WIFLs generally begin nesting at the

end of May, with the highest number of nests initiated

in June, and with some nesting into August (Sogge et

al. 1997), the early nests of WIFLs are at a higher risk

of parasitism than later nests.

During the breeding season, female cowbirds are

egg-laying machines with the maximum number of

eggs laid dependent on the laying rate and breeding

season length. The laying rate of cowbirds may vary by

age and region, but studies show that cowbirds lay

eggs at rates of 0.5 to 0.8 eggs per day (Scott and

Ankney 1979 and 1980; Fleischer et al. 1987, Jackson

and Roby 1992, Holford and Roby 1993).

Female cowbirds are extreme generalists, having

been documented to parasitize over 200 species in

North America (Friedmann and Kiff 1985). Variability in host preference by cowbirds is observed in

individual females and between regions. Individual

females lay eggs in nests of many hosts (Fleischer 1985

and 1986). Between regions of North America, the

primary hosts of cowbirds differ, i.e., hosts that are

parasitized heavily in one region may not be the

preferred host in another region (Hoover and

Brittingham 1993). Thus, the relationship of cowbird

parasitism to an endangered host is not a simple

relationship of cowbird abundance to endangered host

number . So, although cowbirds may be present in an

area with WIFLs or endangered populations of hosts,

the actual parasitism levels of the endangered species

depend on other factors as well.

During the breeding season, cowbird females likely

have two limiting resources: forage and host nests to

parasitize. Because cowbirds are not confined by parental care duties, cowbirds can use one area for

breeding and another for feeding. Throughout the

southwest, host-rich areas tend to be riparian habitat

(cf. Knopf and Samson 1994) and this may be where

female cowbirds concentrate their nest searching activities. In some areas with apparently high densities

of both hosts and food, individual females stay in a

relatively compact home range for a few days or

throughout the season (Table 9-1). Examples of use of

small home ranges are found in prairie habitats in

the Midwest (Elliott 1980), with ephemeral abundant food along a riparian strip in desert (Yokel

1987), and alpine meadows with cattle (Verner, cited

in Rothstein et al. 1987).

When the two resources of host and forage are

spatially separated, female cowbirds split their time

between host-rich areas and concentrated food

sources. Females generally stay in their breeding

areas or host-rich areas in the morning before 1100

hrs and then commute to feeding areas (Raim 1978,

Smith 1981, Dufty 1982a, Rothstein et al. 1984,

Yokel 1987, Thompson 1994, Gates and Evans 1998,

Farmer 1999). Depending on the landscape, the feeding

areas may be adjacent to the breeding areas or disjunct from the breeding areas. Feeding areas favored

by cowbirds have short-grass and high invertebrate

densities or grain seeds. Feeding areas are often associated with anthropogenic factors, e.g., livestock-grazed

pastures, feedlots, dairy farming, golf courses, bird

feeders, and camping grounds (Rothstein et al. 1984,

Rothstein et al. 1987, Thompson 1994, Morris and

Thompson 1998, Goguen and Matthews 1999, Farmer

1999) but generally, cowbirds feed near livestock

(Morris and Thompson 1998).

The distances that cowbirds commute vary across

landscapes (Table 9-1). Feeding sites tend to attract

cowbirds from a great distance, but will tend to have

proportionately lower numbers of female cowbirds

(Rothstein et al. 1984, Rothstein et al. 1987). Recent

research indicates a gradient of cowbird abundance

and parasitism rates from feeding sites. In studies

where cowbirds were feeding near cattle and the cattle

were subsequently moved further from breeding areas, cowbird density or parasitism rates of hosts decreased (Goguen and Matthews 1997, Cook et al.

1997). Therefore, if potential feeding sites are distant

from breeding areas, parasitism rates of ripariannesting species may be lowered.

Later in the day, female cowbirds either roost with

other blackbirds and cowbirds in the evening (e.g.,

Thompson 1994) or return to their breeding areas

(Rothstein et al. 1984) by dusk. Shortly before sunrise,

female cowbirds generally fly straight to a host’s nest

to lay their egg (Scott 1991) and presumably begin

the cycle again. Rarely do females lay an egg later

than 30 minutes past sunrise (Scott 1991).

Female Cowbird Range and Densities

In historic times, cowbirds were found in Arizona,

New Mexico, and California (Perriman and Kelly, this

vol.). By the 1920s, cowbirds were considered abundant along the Colorado and Gila River valleys and the

rivers’ tributaries in Arizona (Wyman and Burnell

1925). In New Mexico in the same time period, cowbirds were considered “fairly common” in the southern

parts of New Mexico (Bailey 1928, cited in Schweitzer

et al. 1998). In southern California, cowbirds expanded their range from the southeastern areas to the

northwest throughout the early 1900s (Laymon 1987,

Rothstein 1994).

In more recent decades, cowbird abundance throughout the southwest has shown no consistent trend,

based on Breeding Bird Survey data (Wiedenfeld, in

press). During 1966-1996, the mean abundances range

from the lowest density category of 0.01-1 birds/survey

to intermediate density category, 4-10 birds/survey

(Sauer et al. 1997).
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 97

Table 9-1. The habitats which female cowbirds used for breeding and feeding sometimes differred.

The area which female cowbirds used primarily for breeding or feeding is listed in

parentheses along with the sample size of banded or radiotracked females. The mean or

range of distances that female cowbirds commuted between the breeding and feeding

areas are listed, when given.

Distance

Breeding between

(ha, sample size) Feeding breeding/feeding Reference

(km)

riparian in desert corrals, bird feeders, 0.6-3.3 from Yokel,

(2.64 ha, n=25) grazed pastures 1987

woodland, open bird feeders, lawns Teather and

woodland, shrub, cattle pastures, Robertson,

swamp woodland, swamp 1985

(9.6 ha, n=7)

campus/creek open lawn areas Darley 1983

(4.5 ha, n=39)

montane forest, bird feeders, corrals 2.1-6.7 Rothstein

riparian et al., 1984

(78 ha, n=5)

campus, New York ca. 0.4 Dufty 1982a

(21.2 ha, n =13)

riparian pastures, golf course 2.8-13.6 Farmer, in press

urban, dairy

forest/shrub sapling short-grass, crops, mean=1.3 Thompson 1994

(*, n=84) feedlots

grassy meadows, not on island 1.3-7 km Smith 1981

shrubs/trees

prairie pastures cattle-grazed pastures 0 Elliott 1980

deciduous forest with mainly grazed pastures mean = 2.27 Gates and Evans,

brush (0.3-6.14) 1998

(ca 8.8, n=19)

oak/juniper cattle-grazed pastures, mean=1.7 Koloszar, in prep

oak/savannah (0-18.1)

(99 ha,n=28)

Cowbirds tend to concentrate in specific habitats

within a landscape, in the ‘edges’ of forest or in larger

open areas within forests (Gates and Gysel 1978,

Brittingham and Temple 1983 and 1996, Coker and

Capen 1995, Annand and Thompson 1997, Niemuth

and Boyce 1997, Evan and Gates 1997, Miller et al.

1998). The size of the opening within forests or the

distance to feeding sites may affect cowbird abundance (Germaine et al. 1997), but the relationship of

cowbird abundance with ‘edge’ transitions in landscapes is not always found throughout North America

(Hahn and Hatfield 1995, Suarez et al. 1997, Bielefeldt

and Rosenfield 1997, Miller et al. 1998). Generally,

cowbirds are found in the highest densities near riparian zones and agricultural-use fields (Anderson et al.

1984, Chase 1997, Cook et al. 1997, Evan and Gates

1997, Farmer 1998). Cowbirds are found in low densities in desert (Sauer et al. 1997), coastal sage scrub

(Farmer 1999 a,b, Braden et al. 1997), and in the

interior of forests with small openings and low anthropogenic influences (Verner and Ritter 1983, Germaine

et al. 1997 but see Hahn and Hatfield 1995). Cowbird

densities in riparian areas traversing deserts can

have low abundances, e.g., Lower San Pedro, Hunter,

unpub data), although if there are anthropogenic factors nearby then densities can be high, e.g., Owens

River near Bishop, California (Yokel 1987; pers obs).

Because southwestern willow flycatchers nest in riparian habitats (e.g., Miller 1951), and because riparian habitat tends to be fragmented or linear
98 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

throughout the southwest (Johnson and Haight 1984),

cowbirds tend to occupy the same breeding habitat as

do WIFLs.

Parasitism rates and effects are hypothesized to be

correlated with abundance of cowbirds. This correlation is the rationale for many cowbird trapping programs. Obviously, with no cowbirds present, there is

no parasitism. With varying densities of cowbirds,

however, the effects on specific hosts depends on many

factors, such as the relative abundance of cowbirds to

hosts (e.g., May and Robinson 1985), the host preferences of cowbirds between areas (Hahn and Hatfield

1995, Hoover and Brittingham 1993), proximity of

other nests near nests of an endangered species (Barber and Martin 1997), overall nest availability (e.g.,

Sedgwick and Knopf 1988), surrounding landscape

use and bird community (e.g., Strausberger and Ashley

1997) and the efficacy of host responses (e.g., Uyehara

and Narins 1995). The regional host preferences of

cowbirds and the parasitism levels of WIFLs within

different host assemblages needs to be researched.

Factors Affecting Cowbird Surveys

Given the sex-specific and temporal aspects of cowbird movements, surveying both breeding areas and

(potential) feeding sites yields different information.

Cowbirds are opportunistic feeders and use ephemeral sources of food, e.g., outbreaks of cicadas (Yokel

1987) within the breeding areas. But if insect or seed

availability is low within a breeding area, cowbirds

commute to feeding sites by afternoon (Yokel 1987;

Thompson 1994). Therefore, censusing cowbirds in

breeding areas before 1100 hrs will yield a more

accurate number of breeding females in an area. Because females spend part of the morning concealed

within the vegetation or under the canopy, (Norman

and Robertson 1975, Payne 1973, Lowther 1979,

Uyehara 1996), detection of female cowbirds is enhanced with playbacks of the female ‘chatter’ call (e.g.,

Dufty 1982b, Rothstein et al., in press).

Throughout its range, male cowbirds tend to outnumber females. This sex bias could be due to the

higher mortality rate of females (Darley 1971), although in one California study, female cowbirds did

not have higher mortality rates than males (Yokel

1987). In the western US, female cowbirds are actively

monogamous (Yokel 1986) and breeding males tend to

be older (Yokel and Rothstein 1991). The breeding

males have consistent morning ranges within which

copulations occur (Yokel 1989). Younger, presumably

unmated, males tend to visit more feeding sites and

travel further than females (Darley 1983, Rothstein et

al. 1987). This difference suggests that trapping males

at feeding sites is not as effective at controlling parasitism as trapping on the breeding grounds (Rothstein

et al. 1987), although more studies are needed.

Cowbird Management Practices

The effect of parasitism or cowbird management

practices on particular hosts has been difficult to

assess. If parasitism is affecting a species, then high

parasitism rates and low nesting success would be

correlated with declining populations (e.g., Trail and

Baptista 1993). However, high parasitism rates and

low nesting success have been reported for species

with relatively stable populations (e.g., Robinson 1992)

and low parasitism rates have been reported for fluctuating populations (McCabe 1991). Thus, the effect of

cowbirds on hosts needs to be assessed for each species

or assemblage of species (e.g., Barber and Martin

1997), host density (e.g., Spautz 1999), and site-specific vegetation characteristics.

For long-term benefits to hosts, increasing suitable

habitat has been advocated (Laymon 1987). Decreased

parasitism levels have been associated with dense

vegetative cover around the nesting heights (e.g.,

Spautz 1999, Farmer 1999b, Uyehara and Whitfield

in press ). Decreased parasitism has also been associated with fewer perch sites from which presumably,

female cowbirds use to scan the adjacent area (e.g.,

Uyehara 1996, Brittingham and Temple 1996, AverillMurray et al. 1999).

To control parasitism in specific host populations,

cowbirds have often been removed, either selectively

(Stutchbury 1997) or through intensive trapping efforts using decoy traps. Selective removal is likely

most effective at low densities of cowbirds (e.g.,

Stutchbury 1997). Intensive trapping can be highly

effective at decreasing cowbird numbers at sites with

a concomitant increase in host numbers (Griffith and

Griffith, in press). However, cowbird removal programs have not always led to increases in nesting

success or host numbers (e.g., Kelly and DeCapita

1982, Braden et al. 1997, Whitfield et al. 1999, Winter

and McKelvey 1999). For WIFL, cowbird trapping

programs have not resulted in an increase in populations at two sites in California (Whitfield et al. 1999,

Winter and McKelvey 1999) In addition, cowbird traps

can cause mortality of nontarget species (Terpening

1999).

To minimize the effects of parasitism, cowbird eggs

in nests have been addled or removed. Because cowbirds also remove host eggs from nests, this strategy

does not eliminate the effects of parasitism.

We present information on parasitism rates at different sites and reproductive success with different

management regimes. In one population in Kern

County, CA, cowbirds were not removed from 1989-

1990. Selective removal of 23 female cowbirds was

used in 1991. Cowbird trapping was instituted in 1993

with increasing number of traps in subsequent years.

In addition, cowbird eggs found in WIFL nests were

shaken to prevent hatching from 1992.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 99

Effects of Parasitism on WIFL _______

Cowbird Abundance and Parasitism Rates

McCabe (1991) hypothesized that, in the case of

linear habitats with low availability of hosts, the

parasitism rates of WIFLs may be higher than in areas

with less fragmentation and higher densities of other

hosts. This hypothesis is supported by two studies of

WIFLs. Of nine parasitized hosts in a coastal central

Californian community, WIFLs are parasitized more

often than expected given the overall parasitism rate

found within the community (Farmer 1999b).

WIFLs occupy territories with structural heterogeneity (Sedgwick and Knopf 1992, Uyehara and

Whitfield in press). Pairs of WIFLs which occupied

territories with more open canopy and less dense

vegetation at the horizontal level around the nest were

more likely to be parasitized than were pairs with

more continuous canopy and dense vegetation around

the nest (Uyehara and Whitfield, in press). The association of parasitism with nests located in more open

areas has been found with other species (Brittingham

and Temple 1996, Burhans 1997, Larison et al. 1998).

The parasitism rates of southwestern willow flycatchers tends to be high throughout their range (cf.

Whitfield and Sogge 1999). Of nine sites where 10 or

more nests were checked, the annual parasitism rates

ranged from 3% at the San Pedro River in Arizona to

a high of 66% at the South Fork Kern River in California. In other parts of the west, high parasitism rates

occur (Sedgwick and Knopf 1988). Also, at Kern, the

relative abundance of cowbirds surveyed in June is

significantly correlated with the parasitism rates of

the population among years (Fig. 9-1), indicating that

a positive relationship exists for this population.

Nest Level

The effects of parasitism at a WIFL nest depend on

the response and subsequent events at that nest (e.g.,

Whitfield 1990). Even in areas with high densities of

cowbirds and relatively low numbers of willow flycatchers, some nests are not parasitized. The WIFLs

at unparasitized nests may present fewer cues near

the nest, or aggressively distract female cowbirds

when cowbirds are further from the nest (Uyehara and

Narins 1995).

Female cowbirds sometimes remove host eggs before 1230 hrs or after 1700 hrs (Scott et al. 1991,

Sealy 1992), usually before parasitizing a nest. For

first clutches of flycatchers from Kern County, CA

(Table 9-2), 52 parasitized nests had a significantly

Cowbird Abundance and Parasitism 

Rates

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

June Point Count

Parasitism Rates

Figure 9-1. Parasitism rates of southwestern willow flycatchers

in a central California population are correlated with female

cowbird abundance (r=0.94, p=0.002). Sites for point counts

(n=60-75 pts/yr) were located 200 m apart along the edge of

riparian habitat near the South Fork Kern River.

Table 9-2. Possible effects of parasitism at the nest level are listed and tested with

data from Kern County, CA. A parasitized nest contained at least one

cowbird egg.

Effect Data from Kern County, CA, 1989-1991

a) Reduced clutch size Mean number of eggs/clutch:

parasitized nests = 3.17

unparasitized nests = 3.60 eggs,

b) Reduced hatching success % Flycatcher eggs hatched

parasitized nests = 22%

unparasitized nests = 63%

c) Reduced fledging rates 28/73 parasitized nests hatched cowbirds,

3/73 parasitized nests fledged flycatchers

d) Overall nesting success % nests fledged one flycatcher

For unparasitized nests only = 54.8%.

All nests, par and unpar = 28.9%.
100 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

lower mean number of eggs than did 97 unparasitized

nests (t-test, t = -2.94, p = 0.004).

The presence of a cowbird egg in the nest is associated with decreased hatchability of flycatcher eggs

(Table 9-2). The reduced hatchability of flycatcher

eggs in parasitized nests may be due to insufficient

incubation by the host if there is a larger egg in the

nest, due to cracking of the flycatcher egg, or due to

desertion of the nest. We have documented the effect

of reduced hatchability at Kern but have no data on the

mechanism leading to the reduced hatching rate.

If a cowbird hatches in a flycatcher nest, WIFLs

rarely are able to successfully raise both their own

young with the cowbird. There is also some evidence

that cowbirds eat or remove all eggs or nestlings from

a nest (e.g., Scott et al. 1992, Sealy 1992, Scott and

McKinney 1994, Arcese et al. 1996, Sheppard 1996),

perhaps to initiate renesting by the host. To our

knowledge, there are no observations of nestling predation by cowbirds in WIFL nests.

The low success at fledging WIFLs in parasitized

nests does not mean that the WIFLs are necessarily

fledging cowbirds successfully. The actual proportion

of cowbird eggs that hatch and subsequently fledge

young in flycatcher nests is low (Fig. 9-2 and Table 9-2).

This low success rate is due partially to the responses

of flycatchers, which can reject the cowbird egg by

burying the cowbird egg (which prevents hatching) or

abandoning the nest. At Kern, of a total of 73 parasitized nests in 1989-1991, 11 eggs were buried within

the nest floor. In 47 parasitized nests, WIFLs initially

accepted the cowbird egg, but of those, 26 nests were

eventually abandoned, probably as a response to some

aspect of parasitism. Overall, for about half of the

parasitized nests, the flycatchers responded so that

the cowbird egg never hatched. Of 28 nests in which

cowbirds hatched, three successfully fledged flycatchers. These nests were likely successful because the

cowbird egg was laid late during incubation.

Nesting success is defined as the percentage of nests

that fledge at least one flycatcher young. Parasitized

nests rarely fledge flycatcher young. Of 28 nests in

which cowbirds hatched, two nests fledged the flycatcher and the cowbird nestling died. One nest fledged

one flycatcher and cowbird each.

Parasitism can reduce reproductive success at all

nesting stages for flycatchers. For other populations of

WIFLs, (not the southwestern willow flycatcher), the

composite nest success is 83% across five studies

(summarized in McCabe 1991, p 114). Thus, the nesting success at Kern is low relative to that of other

subspecies (McCabe 1991, Stoleson et al., this vol).

Paradoxically, if a parasitized nest is subsequently

depredated the flycatcher may renest, which may

increase the flycatchers’ nesting success. Thus, to

better assess the effects of parasitism, we examined

parasitism effects at the level of female reproductive

success per season.

Annual Reproductive Success Per Female

Sedgwick and Knopf (1988) noted that parasitism

rates of nests may not be the best measure of the

effects of parasitism. Willow flycatchers can still nest

after the cowbirds stop laying eggs in mid-July, or a

parasitized female may exhibit rejection behavior and

produce flycatcher fledglings during a season. Rejection behaviors by the flycatchers have the effect of

delaying nesting. At the Kern River Preserve in central California, parasitism followed by rejection of

cowbird eggs caused a delay of about 13 days in the

first egg dates at parasitized nests relative to

unparasitized nests (Whitfield and Sogge 1999).

We tested whether the number of fledglings in a

season differed between parasitized and unparasitized

females in the Kern population. Over all years, parasitized females raised about 1.05 yg annually whereas

unparasitized females fledged 1.93 yg annually. There

was considerable variation in the annual reproductive

success per female among years (Fig. 9-3), so we also

examined the differences for individual seasons. In

1989, parasitized females fledged significantly fewer

young than did unparasitized females, (Mann-Whitney

U test, U=16.50, p=0.007). However, in 1990, there

was no difference in the number of fledglings produced by 18 parasitized and 6 unparasitized females

(Mann-Whitney U test, U=55.0, p=0.94). For 1991,

there were only 2 unparasitized females, so the data

are not amenable to statistical testing. Starting in

Figure 9-2. Of the cowbird eggs laid in southwestern willow

flycatcher nests, only a small proportion hatched or fledged.

0

20

40

60

80

100

120

1989 1990 1991 TOTAL

YEARS

Number of BHCO Eggs 

Eggs

Hatched

Fledged
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 101

0.00

0.50

1.00

1.50

2.00

2.50

3.00

1989199019911992

19931994199519961997

Mean Number of Young

Parasitized

Unparasitized

1993, we hypothesized that more active management

of cowbirds would increase productivity of WIFL

females. Despite increased cowbird trapping efforts,

the number of fledglings per female were not significantly different between parasitized and unparasitized

females for any year between 1993-1997 (Fig. 9-3). To

a certain extent, cowbird management practices increased the reproductive success per individual female, although the main effect may have been to

reduce parasitism rates so that a smaller proportion

of females in the total population were affected by

parasitism.

The difference in significance levels between years

with and without cowbird management (1989-1991)

suggests that other factors besides parasitism are

important in determining annual reproductive success per female. This hypothesis is further supported by

the variation in reproductive success for unparasitized

females over all years (Fig. 9-3) and by studies on other

species (Braden et al. 1997). Logically, to assess the

effect of parasitism, one has to combine the information of parasitism rates with its subsequent effect on

productivity. The productivity of females is the sum of

the number of young fledged in a season multiplied by

the number of seasons that they reproduce. We do not

have the data to assess the impact of parasitism on the

survival of females and their lifetime reproductive

success. On the other hand, we do have data to examine the effects of parasitism on changes in population

size and the year-to-year variation in population

growth.

Population Level

We have presented summaries and data that indicate parasitism reduces productivity when measured

on the level of the nest and on a per-female basis.

Arguably, the most important analysis of the effects of

brood parasitism should be conducted at the population level (e.g., Trail and Baptista 1993, Rogers et al.

1997). If brood parasitism is significantly decreasing

the population growth rate (λ) below the level of a selfsustaining population, then active management may

increase its probability of persistence.

We can test whether the effects of parasitism have

been mitigated by management practices at Kern

County. We analyzed the correlation between parasitism rates and the annual population growth rates for

the Kern population of WIFLs. If parasitism is reducing population growth, we would predict a significant

negative relationship between parasitism rates and

population growth rates.

We used a standard demographic model to calculate

population growth rates. The model makes the following assumptions:

1. Female reproduction is not limited by the availability of males, so it suffices to count only females

(Caswell 1989).

2. There is no immigration or emigration of birds

outside of the study area.

3. Mortality rates can be calculated as the return

rates of banded birds.

We chose a pre-breeding model to include any possible effects of parasitism on (winter) adult survivorship within the same time step as the (summer)

parasitism events. Heuristically, the model starts

with the survey of adults at the end of May. In reality,

some adult females do not arrive until mid-June

(Whitfield, unpub data), but for the purposes of the

model, all adults are treated as arriving by 1 June.

Adults are categorized into two age groups: yearlings

(Second-Year or SY) and older adults (After SecondYear or ASY). Both groups nest and raise young. At the

end of the breeding season, the juveniles, yearlings,

and adults migrate to their wintering sites. Those

individuals that survive return as yearlings (SY) and

older adults (ASY). The yearlings are those birds

hatched the previous year at Kern. The older adults are

now the returning SYs and ASYs of the previous year.

In this standard demographic model, there are four

parameters that contribute to the population growth

rates (Table 9-3):

(a) the reproductive rate of SY adults, or equivalently, the number of fledglings produced by SY

adults multiplied by the return rate of the fledglings the next spring;

Figure 9-3. The mean number of flycatcher young fledged in

parasitized and unparasitized nests. The numbers represent

the number of females in the population that were parasitized

or unparasitized. Beginning in 1993, management practices

designed to minimize parasitism effects were instituted. (See

text for additional details.)
102 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

(b) the reproductive rate of ASY adults, or equivalently, the number of fledglings produced by ASY

adults multiplied by the return rate of the fledglings the next spring;

(c) the survival rate of SY adults

(d) the survival rate of ASY adults

Banding data from 1990 to 1997 indicated that the

average number of young fledged did not differ by

parental age: SY fledged 1.65 yg (n=20) and ASY

fledged 1.66 yg (n=29). We also assumed that the

survivorship of fledglings to yearlings was the same

regardless of parental age, thus simplifying the model

so that a = b. We also did not distinguish between the

survival rates of SY and ASY adults, so that c = d.

Note that our prebreeding model differs from

postbreeding models (e.g., Stoleson et al., this vol). In

our calculation of reproductive rates, the fledglings

produced by adults must survive to the following June.

In a postbreeding model, the reproductive rate of

hatching-year birds (HY) is calculated. This reproductive rate is the product of two factors: a) fledgling

survival to SY and (b) the mean number of fledglings

produced during the following breeding season. In our

prebreeding model, we never calculate a reproductive

rate for hatching year birds because they do not

reproduce until the time step in which they are adults.

Instead, a reproductive rate is calculated for all adults

(Table 9-3).

The overwinter survival of juveniles and adults were

estimated from the data on banded birds, which yields

minimum estimates of survival. The numbers of banded

birds varied across years as did the field effort to

resight birds. In general, the numbers and proportions

of banded adults were higher for more recent years

than at the beginning of the study. There was an

increased field effort to resight and mark adult birds

from 1994.

We used the survival and reproductive rates of

females to calculate the annual population growth

rate, λ. The simplification of a = b and c = d, we were

able to calculate the population growth rate, λ, directly

as λ = a + c.

We looked at the relationship between λ for each

year (June to June) and the parasitism level of the

breeding season (Jun-Aug). We used data from all

years, including the six years of cowbird management.

Cowbird management practices at Kern may reduce

parasitism effects on individual basis, but may not

eliminate all effects. A significant negative relationship between λ and the parasitism rate means that

parasitism is affecting the population growth rate at

Kern.

The growth rates of this WIFL population do tend to

decrease with increasing parasitism rates (Fig. 9-4;

linear regression analysis, r2

= 0.44, p=0.073). Although

the p value is slightly above 0.05, parasitism rates

explain 44% of the variation in the annual population

growth rates. This effect of parasitism rates on λ seems

strong, especially considering that cowbird management occurred in 5 of 8 breeding seasons and that the

statistical power of our analyses is low due to the small

number of years. The slope of the best-fit line gives an

indication of the benefit of reducing parasitism rates.

For every drop of 0.10 (10%) in parasitism rates,

lambda increases by 0.076 for this population.

Table 9-3. Life history parameters of one population of SWFLs in Kern County, California. Banding and nesting biology

of SWFLs were used to calculate the population growth rate, Lambda. Rates are calculated for females only.

We assumed that half of the banded fledglings were female. This number is parenthetically noted after juvenile

survival rates. Rates for adult survival are calculated as the number of banded female adults which were

resighted or recaught the following year. The means for all years and for years with cowbird trapping (1993-

1996) are calculated separately.

Female Juvenile Survival Female

Breeding Fledglings Rate (banded Adult Survival Reproductive

Year per adult female nestlings) (banded adult females) Rate λ

1989 0.57 0.38(8) 0.00(2)* 0.25 0.63

1990 0.48 0.09(11.5) 0.43(7) 0.05 0.47

1991 0.60 0.00 (4.5) 0.63(8) 0.00 0.63

1992 0.89 0.20 (10) 0.38(8) 0.17 0.55

1993 0.56 0.36(14) 0.54(13) 0.24 0.74

1994 1.00 0.27(17.5) 0.35(20) 0.25 0.62

1995 1.00 0.50(14) 0.42(19) 0.50 0.92

1996 1.27 0.59(19.5) 0.63(19) 0.74 1.39

Mean (All yrs) 0.67 0.30 0.42 0.28 0.74

Mean (Trap yrs) 0.96 0.43 0.49 0.41 0.90

*Neither of the two adult females banded in 1989 returned in 1990. Because of the low number of banded birds for 1989, we used the

overall mean of 0.42 for adult survival rate to calculate lambda for 1989-1990.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 103

The regression analysis suggests that the recovery

of the Kern population

(λ ≥ 1) necessitates a very low parasitism level. The

best-fit line for the data reaches a self-sustaining

population (λ = 1) at the low parasitism rate of 8.4%,

with population growth likely only to increase if parasitism levels are maintained below 8.4%. Note that the

8.4% parasitism rate is only a guideline, as it is based

on data from a relatively small number of years.

Using this regression analysis, we produced guidelines for parasitism levels for a self-sustaining or

recovering Kern population, assuming that the range

of rates remain within the parameters for 1989-1997.

For the years without cowbird management (1989-

1991), the high levels of parasitism and subsequent

low λ values indicate that the Kern population was not

a self-sustaining population for those years.

The guidelines we have produced are applicable to

the Kern population or a population with similar rates

for survival and nesting success. Populations with

different parameters of nesting success and overwinter survival will have different population growth

rates for the same parasitism levels. We cannot compare survival rates because we are unaware of any

data on survival rates in other WIFL populations.

However, the range of nesting success of the Kern

population appears comparable to other populations

(cf. Stoleson et al., this volume). The nesting success

for the Kern population from 1989-1997 ranges from

0.48 to 1.27 female fledglings per female (Table 9-3) or

0.96 to 2.5 fledglings per female. Stoleson et al. (this

vol) report comparable values of 1.25-2.45 fledglings

per female for other WIFL populations. One final

analysis may indicate how responsive the Kern population would be to continued cowbird management.

Elasticity Analyses and Cowbird

Management

The biology and data for rates of survival, reproduction, and parasitism show a pattern that warrants

further analysis in planning the recovery of an endangered species. At Kern, parasitism rates have been

lowered with cowbird management. Is continued management of cowbirds likely to contribute significantly

to population recovery? An indirect answer to this

question can be obtained using elasticity analysis of

the data (e.g., Stoleson et al., this vol.). Elasticity

values predict how a change in each parameter of our

prebreeding model would affect the population growth

rate.

Cowbird management may directly affect reproductive rates (along with other factors), but have less or no

effect on the survival rates of adults. If survival rates

are predicted to have a major effect on the population

growth then continued cowbird management will have

small effects on population growth. Conversely, if

changes in reproductive rates of SY or ASY adults

have large effects on population growth, then cowbird

management as a means of enhancing reproductive

success is an efficient option for recovery.

The results of the elasticity analyses support the

findings by Stoleson et al. (this vol.) and the efficacy of

cowbird management practices (Table 9-4). With no

cowbird management, the reproductive rates were low

(Table 9-3), and the strongest effect to a change in

population growth rate was survival of adults. For

example, from Jun 1989-Jun 1990, a 0.01 increase in

population growth rates could be accomplished if ASY

survival rates increased by 0.025. The elasticity measurements for 1991 show that with poor reproductive

success, the population growth rate is most responsive

to an increase in adult survivorship. In 1996, the

elasticities were more evenly divided. An increase in

any one parameter will have about the same effect on

the population growth rate, although increasing reproductive success of SY adults will have the most

beneficial effect on population growth.

However, cowbird management practices will likely

affect reproductive rates of both age-classes simultaneously. When reproductive success is equivalent for

females of both age-classes, then we can calculate the

reproductive success necessary to produce a self-sustaining population using the mean values during active cowbird management (1993-1996). The overall

lambda is 0.90 for the years with active cowbird

Figure 9-4. The population growth rates of the Kern population

is significantly associated with parasitism rates. Solid circles

represent years with no cowbird management (1989-1991) and

open circles represent years with cowbird management (1993-

1996).
104 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

management (Table 9-3). This indicates that even

with the high survival and reproductive rates recorded for these years, this population needs to increase either productivity or overwinter survival to

persist over the long term. A self-sustaining population with cowbird trapping requires a 10% increase in

λ or assuming adult survival rates remain constant,

then the overall reproductive rate of 0.51 will allow

this population to become self-sustaining. Because

reproductive rates are the product of nesting success

and return rates of fledglings of the previous season

and assuming that return rates of fledglings are not

amenable to cowbird management, then, on average,

a self-sustaining population at Kern requires 1.18

female fledglings per female or an average nest success of 2.36 yg per female. On average, any increase

over 2.36 yg per female will contribute to population

recovery.

Summary _________________________

The brown-headed cowbird parasitizes nests of the

southwestern willow flycatcher (Empidonax traillii

extimus). The main breeding season of cowbirds (May

to mid-July) overlaps with the initial nesting period of

southwestern willow flycatchers (Jun-Aug). Parasitism rates of willow flycatchers tends to be high, possibly because of the forest structure and landscape

influences in current nesting areas. Parasitism rates

vary with cowbird abundance, across years, and at

different sites. The effect of parasitism on willow

flycatchers can depend on many factors: cowbird abundance, the overall parasitism rate of nests, the timing

of cowbird parasitism, and the events subsequent to

parasitism, e.g., cowbird egg burial, nest desertion,

renesting, and predation.

For one central California population of southwestern willow flycatchers, we have documented the

effects of cowbird parasitism on a per nest, per female, and population basis. About 1/3 of parasitized

nests reached the stage where cowbirds hatched, the

stage which is considered to lead to the highest

reproductive loss for flycatchers. Of the nests in

which cowbirds hatched, 41% fledged cowbirds. Flycatcher response to parasitism, however, ameliorated the effects of parasitism in about half of all

parasitized nests.

Per individual female, the major effects of parasitism were to delay nesting, decrease hatchability of

flycatcher eggs in parasitized nests and decrease clutch

size either because of removal by cowbirds or later

clutches tend to be smaller. Except for decreased

hatchability, these effects have been documented

throughout the range of willow flycatchers. Due to the

small but cumulative effects of parasitism, the annual

reproductive success of females is influenced by parasitism, if parasitism is the major cause of nest failure

for that year.

For one population in Kern County, California, increasing parasitism rates decreases population growth

rates. Parasitism can explain 44% of the variation in

population growth rates. Our analysis shows that

parasitism rates must be low for this population to

persist or increase. Given the life history parameters

at Kern, population growth may be dependent on

active cowbird management to decrease the effects of

parasitism. In order for this population to become selfsustaining or increase, females need to fledge an

average of 2.4 young. We provide evidence of the

detrimental effects of parasitism on southwestern

willow flycatcher recovery and document the extent of

the benefits of cowbird management. Although our

conclusions are based mainly on one population of

WIFL, this population appears to be similar enough to

others that our conclusions may apply to other populations as well.

Table 9-4. Elasticities of the demographic parameters. From 1989-1991, no

cowbird management was attempted. In 1992, selective cowbird

shooting at the nest eliminated 21 female cowbirds. From 1993,

cowbird traps were placed throughout the breeding and feeding

areas.

Parasitism

Year Rate Lambda E(a) E(b) E(c) E(d)

1989 0.50 0.67 0.11 0.22 0.22 0.44

1990 0.55 0.48 0.01 0.09 0.09 0.80

1991 0.80 0.63 0.00 0.00 0.00 1.00

1992 0.69 0.55 0.10 0.22 0.22 0.47

1993 0.38 0.78 0.09 0.21 0.21 0.48

1994 0.16 0.59 0.17 0.24 0.24 0.35

1995 0.19 0.92 0.30 0.25 0.25 0.21

1996 0.11 1.37 0.29 0.25 0.25 0.21
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 105

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USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 107

Deborah M. Finch

Janie Agyagos

Tracy McCarthey

Robert M. Marshall

Scott H. Stoleson

Mary J. Whitfield

Chapter 10:

Management Recommendations

This chapter was developed over a series of meetings

using a group-consensus process. Our recommendations are based on published results, on information

compiled in the previous chapters, on expert opinion,

and on unpublished data of conservation team members. This chapter is available as temporary guidance

until the Recovery Plan for the southwestern willow

flycatcher is published in the Federal Register. A draft

Recovery Plan has been prepared by the Technical

Subgroup of the Recovery Team and is under current

review by the U.S. Fish and Wildlife Service and by

Implementation Subgroup Members of the Recovery

Team. The Technical Subgroup reviewed this Assessment management chapter to aid in drafting the

Recovery Plan. Several members of the temporary

Conservation Assessment Team were also members of

the ongoing Recovery Team. Given that the draft plan

was assembled over a 2-year period requiring more

than 20 Recovery Team meetings, its guidance will be

much more exhaustive and up-to-date than the recommendations identified herein. Upon publication of the

final Recovery Plan, this chapter will be obsolete and

should not be used in place of, or to contradict, the

Plan.

To initiate discussion for this chapter, we first listed

actual and potential threats to the survival and reproduction of the southwestern willow flycatcher, then

listed potential ways to mitigate or eliminate threats.

In some cases, insufficient research limited our understanding of how perceived threats actually harmed

flycatchers or their habitats, or what steps to take to

mitigate the threat. For example, lack of published

knowledge of the range and habitats used by flycatchers on the wintering grounds constrained our discussion of management recommendations on this topic.

We also describe methods to improve and restore

willow flycatcher habitats as well as ways to distribute

information and educate people about how to protect

and recover flycatcher populations. We include specific sections addressing potential threats to willow

flycatchers from biological factors, invasive exotic

plants, catastrophic fire and management activities,

as well as methods for habitat and watershed restoration and improvement. Many of our recommendations

may also be of use in conserving and protecting populations of other sensitive bird species that occupy

riparian ecosystems, such as the Yellow-billed Cuckoo

(Coccyzus americanus) or the endangered Least Bell’s

Vireo (Vireo bellii pusillus).

Throughout the chapter, we refer to WIFL habitats

as potential, suitable, or occupied. Potential habitats

are defined as sites that lack one or more habitat

component(s) that WIFLs require but that can be

manipulated to make the site suitable for occupancy.

Potential habitat types include those that are naturally regenerating and close to suitable, and those that
108 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

need more time or active improvement before suitability is achieved. Unoccupied suitable habitats are defined as unoccupied sites that are or appear suitable

for WIFL occupancy without manipulation. Unoccupied suitable habitats are similar to occupied habitats

except that they lack WIFLs. Occupied habitats are

suitable without saying and are those where evidence

of WIFL breeding, such as observations of territorial

males, breeding pairs, mating behavior, carrying of

twigs and food items, nests, and fledglings, have been

recorded and verified.

Reducing the Probability of Biological

Threats___________________________

Brown-headed Cowbird Parasitism

Brown-headed cowbirds frequently lay eggs in willow flycatcher nests; cowbird parasitism substantially

reduces the nesting success of flycatchers and is often

a significant biological threat to flycatcher productivity. Adult cowbirds can reduce host nesting success by

stimulating nest desertion, and removing, piercing, or

depredating host eggs. Cowbird nestlings frequently

outcompete host nestlings and fledglings for food,

causing host chick starvation, and occasionally nudge

host nestlings out of the nest. To prevent loss of host

eggs and nestlings caused by cowbirds, a series of

preventative steps are strongly recommended.

Monitoring—We recommend that the potential for

threat to the southwestern willow flycatcher from

cowbird parasitism be monitored at all occupied sites.

Monitoring consists of determining cowbird presence

or abundance via surveys, population counts, or radiotracking (Verner and Ritter 1983, Beezley and Rieger

1987, Rothstein et al. 1987, Whitfield in press); surveying flycatcher nests for presence of cowbird eggs

and nestlings (Harris 1991, Whitfield 1995); and determining cowbird parasitism rates at WIFL nests

(Whitfield 1990, Whitfield in press). If parasitism

rates exceed the threshold of 10% (Whitfield in press),

then cowbird trapping should be initiated along with

an analysis of WIFL productivity. Because parasitism

rates vary with site, year, patch size, and population

size of flycatchers and cowbirds (Robinson et al. 1993,

1995), sites should be monitored for more than one

season. It is possible that the trapping threshold may

vary in relation to site conditions and host nesting

success. If cowbird parasitism does not exceed the predefined threshold (10% threshold being conservative),

continue monitoring.

If sites have not been monitored for cowbird parasitism, trap cowbirds if more than three willow flycatcher

territories are present. If cowbirds are not present at

WIFL-occupied sites, continue surveying for both flycatchers and cowbirds in subsequent years.

Control Program—There are short-term and longterm aspects to a cowbird control program (Schweitzer

et al. 1996). Cowbirds are known to be attracted to

riparian habitats that have been fragmented into

smaller patches, narrow, linear corridors, and edge

habitats (Robinson et al. 1993, 1995). Management

practices over the long term should emphasize:

• Reducing phreatophyte removal, wildfire, water

loss, and exotic plant invasion.

• Increasing habitat patch sizes and migration corridors, and reducing the extent of edge.

• Educating human communities about cowbird

attractants, including types of birdfeeder seed

(e.g., millet) that attract cowbirds.

Over the short term, we recommend implementation

of the Griffith Brown-headed Cowbird Trapping Protocol (Griffith and Griffith 1996) to control cowbird

numbers. During the trapping effort, cowbirds and

parasitism rates should be monitored over multiple

years to determine if trapping is having the desired

effect of reducing parasitism rates (Robinson et al.

1993, Whitfield in press). For trap-shy birds, other

methods of cowbird removal such as shooting may be

needed (Schweitzer et al. 1996). In addition to trapping, cowbird attractants such as livestock should be

removed from WIFL breeding sites. Other attractants

include trash, food, agricultural fields, bird feeders,

plowed fields, livestock feedlots, dairies, and pack

stations. Possible actions to reduce the probability of

attracting cowbirds include removing attractants,

covering trash, and scheduling more frequent trash

pickups. If attractant removal is not possible, use attractants as sites for trapping. Removing attractants

from lands adjacent to occupied sites is also worthwhile,

although the feasibility of attractant removal will need

to be evaluated on a site-specific basis. Attractants that

are feasible to move include cows, trash, and birdfeeders; non-feasible removal may include stationary

attractants such as cropfields and feedlots.

At trap sites, space traps according to habitat size

and landscape features, distribution of WIFLs and

cowbirds, and available finances (refer to Griffith and

Griffith 1996, in press). Where cowbirds are concentrated in known feeding sites in close proximity to

WIFL sites, consider trapping off site and in conjunction with on-site trapping. We do not define a distance

for off-site trapping because of site-specific variation

in cowbird commuting distances, habitat use, and

landscape pattern (Stephen Rothstein, pers. comm.,

Frank Thompson pers. comm.). Note that permission

to trap on private lands will be needed. Federal

landmanagers adjacent to private lands having WIFL

sites will need to work with private landowners to trap

cowbirds. Initiating cooperative efforts to trap cowbirds in mixed-ownership lands is a responsibility of

the federal, state, or municipal agency.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 109

Multiple years of trapping are recommended. Trapping efforts may be reduced or stopped if all of the

following are observed:

• Significant reduction of cowbird numbers based

on cowbird trapping rates.

• Significant reduction of parasitism rate on WIFL

nests.

• WIFL population shows a significant upward

trends at the site(s).

Trapping should be renewed at previously-trapped

sites if parasitism rates ≥10%. A depiction of the

feedback loop for initiating trapping is given in

Figure 10-1.

To trap cowbirds, we recommend using the Griffith

trap design, trap size, and protocol. Trap size can be

reduced by half when finances are limiting, at remote

sites, or when cowbird densities are low. Use the

recommended trap size when cowbird densities are

high and/or at feeding lots where cowbirds are concentrated. Alternatives for trap materials include PVC

to reduce trap weight; shadecloth to reduce heat stress;

or plywood for shading when windy. Predator control

mechanisms may need to be added to the trap design

to deter raccoons, weasels, snakes, and other predators. Traps placed in or adjacent to livestock-occupied

pastures generally have good success in trapping cowbirds, but they need to be protected from livestock

damage.

When to stop cowbird trapping depends on sitespecific conditions and whether WIFL populations are

recovering at each site. Maintenance trapping may be

needed over prolonged periods of time even when

increased WIFL nesting success is detected. WIFL

populations should show significant increases before

they can be considered locally recovered.

Predators

Willow flycatchers, like most songbirds, have opencup nests that can be readily accessed by a variety of

natural predators. These can include small mammals

such as raccoons, skunks, squirrels, and packrats;

birds such as hawks, owls, roadrunners, and corvids;

tree-climbing snakes such as racers; lizards; and domesticated and feral cats and dogs. Nest predation

usually explains the greatest proportion of nest failure in local flycatcher populations (Whitfield 1990).

Adult flycatchers can be captured on the wing by

many raptors such as falcons, accipiters, and possibly

owls. Other predators can catch adult flycatchers at

their nests or at their singing and foraging perches.

To reduce predation rates, the following steps are

recommended:

• Control presence of predator attractants such as

trash and food.

• Use sensitive techniques (e.g., avoid tree-climbing, minimize time at nest, avoid touching or

moving nest and young) when conducting nest

monitoring.

• Educate public about cats and dogs as predators

of birds.

Figure 10-1. Cowbird trapping feedback loop.
110 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

• Trap domestic feral cats, when they are perceived

as a problem.

• Reduce predator intrusion into flycatcher habitat

at edges and along trails by increasing habitat

patch size, reducing patch isolation, and closing

trails.

• If possible, identify predators during nest

monitoring.

• If monitored nests have high predation rates,

develop and implement feasible predator control

measures in the local area.

Evaluate nest predation signs to determine kinds of

predators. Identity of nest predators is valuable information for deciding how to reduce predation levels, if

necessary. However, effective control measures and

strategies still need to be designed for many identified

predators. Nest observers should be careful to avoid

attracting mammalian predators to the nest. Such

predators are known to follow humans or their scent

trails. To mitigate this problem, nest observers should

vary their pathways to nests and take steps to eliminate human scent using napthalene or other scent

removers.

Beaver

If beaver are present at occupied, suitable, or potential WIFL sites, managers should determine if they

are benefiting WIFLs (e.g., beaver dams may be creating backwater, a habitat condition associated with

WIFL occupancy) or damaging habitat by removing

vegetation. Beaver damage is more likely to be a

problem at sites where riparian habitat is linear and

habitat patches are small or where their dams cause

habitats to become inundated (e.g., high elevation

sites). If beavers are determined to be beneficial to

WIFLs, beavers should be left in place and the site

should be monitored to assess future conditions. If

beavers are determined to be detrimental to WIFLs

(e.g., by removing essential WIFL habitat), consider

active reduction or removal of the beaver population.

Site-specific analyses are needed to make

determinations.

As part of a habitat restoration program, beaver reintroduction may be a useful tool to enhance a site,

creating conditions such as still water that encourage

colonization by flycatchers.

Parasites and Disease

There is little evidence that southwestern willow

flycatchers have problems with parasites or disease.

In addition, management capability to address such

problems may be limited. Determining whether

parasites or disease are a significant problem is the

first step in deciding how to address them. In cases

where other bird species are being studied in an area

occupied by willow flycatchers, we suggest that they be

intensively examined for evidence of parasitism or

disease. Any evidence of parasitism detected by WIFL

nest surveyors should always be recorded. Field forms

should be modified to include space for documenting

evidence of parasites or disease. If flycatchers are

found dead at nests or elsewhere, their carcasses

should be collected and analyzed for parasites or

disease. To evaluate incidence of botfly larvae, we

recommend that nests be collected and analyzed by

investigators having permits to do so.

Genetics

Southwestern willow flycatchers may be facing

problems typically associated with small populations

such as genetic isolation and inbreeding. To evaluate

presence of genetic problems, the incidence of deformities will need to be documented through the collection

of tissue samples. This should be conducted by qualified scientists. Reports of possible genetic problems

and specimens should be sent to the U.S. Fish and

Wildlife Service. Tissue analyses can be referred to

the U.S. Fish and Wildlife Service’s Disease Lab in

Madison, WI. The best approach to reducing the

probability of genetic isolation and inbreeding is to

apply habitat restoration and population recovery

methods to increase WIFL abundance and expand its

distribution.

Mitigating Loss of Native Habitat Due

to Invasion by Exotic Plants_________

Invasive exotic plants such as salt cedar (Tamarix

ramosissima, T. chinensis), Russian olive (Elaeagnus

angustifolia), Siberian elm (Ulmus pumila), tree-ofheaven (Ailanthus altissima), white mulberry (Morus

alba) and giant reed (Arundo donax) are replacing

native vegetation along many rivers and streams in

the Southwest (Campbell and Dick-Peddie 1964,

Robinson 1965, Ohmart et al. 1977, Dick-Peddie 1993,

Crawford et al. 1993, Ohmart 1994). Habitat changes

resulting from the spread of salt cedar can influence

bird species composition and use (Hunter et al. 1987,

1988). Whether encroachment of riparian habitats by

exotics has had a negative effect on willow flycatchers

is debatable. At some monotypic and mixed salt cedar

sites, for example, WIFL nests have been found and

many of these nests have successfully fledged WIFLs.

At higher elevations, salt cedar thickets are often the

only habitat with suitable structure available for the

WIFL to nest in. Yet, effects of salt cedar invasion of

riparian zones, particularly at lower elevations occupied by cottonwoods, may be more negative than

positive owing to increased fuel loading, increased

frequency of catastrophic fires related to salt cedar
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 111

flammability, loss of native plant communities to type

conversion, and loss of flycatcher habitat to wildfire.

Anderson et al. (1977) noted that 21 of the 25 tamarisk

stands they studied had burned in the prior 15 years.

When dense tamarisk thickets burns, the fires are

typically fast moving and intense. For example, during just 3 years, recent fires totaled 1,000 ha of riparian habitat along the Lower Colorado River—a substantial amount considering only about 6,200 ha of

suitable bird habitat currently exists along this river

(U.S. Bureau of Reclamation 1999).

With emphasis on salt cedar, we recommend the

following steps be initiated to avert catastrophic fire

risk and prevent further loss of native plants due to

exotic plant invasion while maintaining WIFL breeding sites where exotics are already present:

• Justify the need for exotic plant control at a

particular site prior to taking action.

• Develop a watch list of exotic plant species in

riparian ecosystems with focus on rate of spread,

WIFL use, and effects on native plant species and

ecosystems.

• In occupied sites, leave exotics as is, unless exotics are significantly increasing and detrimentally

altering habitats.

• If exotics are encroaching on previously occupied

sites, consider removing exotics and restoring

sites.

• If exotics encroach on occupied sites that were

exotic-free, eradicate exotics without disturbance

(i.e., during the non-breeding season). Evaluate

symptoms and address causes of exotic encroachment. If conditions are not appropriate for restoration of native plants, then remove of exotics

may not be of strong benefit.

• Monitor effects of increasing presence of exotics at

occupied sites. If signs of negative effects on WIFL

numbers or nesting success are detected, then

remove invasive exotics. For example, type conversion and structural alterations of habitat may

signal a decline in habitat quality that could potentially influence WIFL populations or productivity.

• In suitable and potential habitats dominated by

native plants, suppress encroachment of exotics.

• In suitable habitats dominated by exotics, survey

for WIFL at least 3 years prior to removal of

exotics. Removal should be conducted in incremental blocks of no more than 25% annually at

surveyed sites.

• Evaluate potential for restoration success by investigating measures of watertable depth, salinity, geomorphology, and hydrology. If sites are

amenable for restoration, exotics may be removed.

If not amenable, management of exotics is not

cost-effective and may be detrimental to other

animal species.

• Several aspects should be considered with respect

to biological control of exotics (sensu DeLoach

1997):

— Removal of exotics may be detrimental if the

site is not capable of replacing salt cedar with

natives or if erosion is increased as a consequence of removal.

— A more comprehensive approach that includes

restoration is needed before implementing biological control.

— Adequate field testing is needed before evaluations of biocontrol agents can be considered

completed.

— Managers must first evaluate whether or not

exotics should be removed from WIFL habitat;

once the decision is made to remove exotics,

managers need to decide what method of control is most appropriate.

— If biocontrol is considered to be the best method

of control, then adequate laboratory tests and

isolated field experiments are needed prior to

full release.

— State of knowledge of biotechnology is currently inadequate to recommend full release.

In addition to salt cedar, the following exotic species should be considered when designing removal

programs:

• Giant Reed—Removal is beneficial because native vegetation will replace it and because WIFLs

have not been documented to use reed for nesting.

• Russian Olive—The distribution of Russian Olive is limited to New Mexico, Colorado, and northeastern Arizona. This exotic can provide habitat

structure and nest trees for WIFLs, and WIFLs

are known to nest in it. Detrimental effects of this

exotic to WIFLs are unknown.

• Tree of Heaven—This exotic occurs locally in

Arizona, California, and New Mexico. Very little

is known about the relationship between WIFLs

and Tree of Heaven. This exotic is not highly

concentrated in riparian habitats and detrimental effects to WIFLs, if any, are unknown. We

recommend that it be included on a watch list.

• Siberian Elm and White Mulberry—Concern

about these exotics are similar to that reported for

Tree of Heaven. Detrimental effects to WIFLs, if

any, are unknown. We recommend that they be

included on a watch list.

Reducing the Threat of Catastrophic

Fire ______________________________

In 1997, six WIFL sites in Arizona were destroyed

by fires. This catastrophe alerted managers to the

need to have better plans in place for preventing
112 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

and responding to unexpected wildfire events. Before

a fire occurs, we urge the following steps be taken for

all occupied breeding WIFL sites:

• Prepare a site-by-site fire management plan for

each occupied site in coordination with local firefighters. These plans should include steps for

preventing fires, as well as methods for protecting

willow flycatchers and their habitats if a fire

occurs.

• Refer to the White Canyon Fire Biological Opinion (U.S. Fish and Wildlife Service, Ecological

Services, Phoenix, AZ) as an example of a responsible fire management plan.

• Identify water sources that are not near or in

occupied WIFL habitat in the fire plan.

• Erect fire prevention signs.

• Restrict use of campfires and camping in high risk

areas.

• Reduce fuels adjacent to occupied sites using tools

such as fuel breaks, mechanical clearing, prescribed burning except in salt cedar, herbicides.

• Host training sessions and implement other

measures to educate fire-fighters about WIFL

resource values and locations. This will ensure

that flycatcher protection is included in the fire

plan.

• Identify who needs to be trained.

• Seasonal grazing is not recommended as a fuel

reduction method in occupied WIFL sites because

predominant fuels are woody materials, i.e., not

primary livestock forage (also see livestock management criteria). But livestock grazing may be

appropriate in adjacent uplands where fuel loads

can lead to fire spread to riparian zones.

When a fire event does occur in habitat occupied by

willow flycatchers, the “fire management plan” should

immediately be implemented. Emergency consultation with U.S. Fish and Wildlife Service should be

initiated to ensure that destruction of WIFL habitat is

avoided. Care should be taken when establishing fuel

breaks during the fire. The potential costs of using fuel

breaks are: fragmentation of WIFL habitat, increased

erosion, establishment of a potential travel corridor

for predators or access point for recreationists, destruction of potential or suitable habitat, and invasion

by exotic or undesirable plants. Alternatively, the

benefits of fuel breaks include the installation of a fire

barrier that limits fire spread, the creation of a firefighting attack point, and the minimization of direct

fire threats to WIFL-occupied habitats.

After a fire event, habitats may need to be restored.

If WIFL habitats have been destroyed, fire rehabilitation efforts should be implemented with WIFL habitat

requirements in mind. Emergency consultation can be

included after post-fire rehabilitation plans have been

initiated.

Reducing Potential Threats Caused by

Management Activities ____________

Pesticides

Pesticide use by landowners and agencies in areas

near occupied WIFL habitats should be evaluated

periodically. Water quality tests can be conducted to

determine if pesticides are entering the ecosystem.

Visible pesticide effects (e.g., plant or arthropod responses) at WIFL and adjacent sites should be documented. Any deformities or abnormal behavior of

WIFLs or co-existing birds should be reported by nest

surveyors and migration banders to the U.S. Fish and

Wildlife Service. Deformities in California WlFLs have

been documented with photographs, suggesting that

pesticides may pose a threat to WlFLs as well as other

associated fauna. If pesticides are perceived to be a

problem, a pesticide reduction plan and public education efforts should be implemented. To verify pesticide

contamination, abandoned WIFL eggs and individuals of surrogate species can be tested further. Pesticide

use in a region can possibly be inferred based on

distribution of sales.

Livestock and Other Ungulates

In potential and suitable but unoccupied WIFL

habitats, site conditions should be evaluated prior to

exclusion of livestock. Changes in livestock rotation

schedules and the timing and period of pasture use can

go a long way toward restoring riparian habitats to

benefit WIFLs. If site conditions are suitable (see

criteria below), controlled grazing can be permitted

during the dormant season of woody species. When

grazing is allowed, vegetation should be monitored to

determine if the site is undergoing unusual damage

from grazing. To allow regeneration of habitat, we

recommend that ungulates be excluded during the

growing season (at minimum) of woody species. To

restore degraded or overgrazed riparian habitats, it

may be desirable to exclude cattle altogether. To allow

potential habitat to progress to a stage that is suitable

for WIFL occupancy, livestock removal is appropriate.

However, if priorities for livestock exclusion from

potential habitats must be established, then those

habitats adjacent to or near occupied WIFL habitats

are higher priorities for protection from grazing than

potential habitats that are at a great distance from

WIFL occupied sites.

If breeding WlFLs occur at the site, we recommend

complete exclosure to all livestock and other ungulates year-round. For all excluded sites, managers

should conduct frequent inspections to identify trespass livestock. In occupied sites, remove trespass

livestock by drawing them out using attractants

(hay, mineral blocks) rather than herding. If livestock
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 113

cannot be attracted out of the occupied area, wait until

September 1 (post-WIFL breeding season) before

driving livestock out.

Under proper management, livestock presence may

sometimes be compatible with habitat quality. Therefore, exceptions to year-round livestock removal from

occupied habitats on public lands should be available

if the livestock owner can demonstrate to the permitting agency that grazing during the nongrowing season of woody species at a specific site does not adversely affect WIFL habitat structure and composition.

The following documentation should be supplied to

justify exceptions:

• Dated pictures of habitat conditions before and

after grazing at specified photo points repeated at

the same time each year.

• On-site demonstration visits.

• Data of measured vegetation before and after

grazing. Measurements should be taken of stem

densities, foliage height diversity, canopy cover,

plant species composition, and aerial extent (patch

size) of habitat.

The timing, duration, and intensity of grazing should

not preclude recruitment or adversely affect existing

regeneration of riparian plants. If time is not available

to monitor vegetation, then grazing should not be

permitted. These criteria for excepting livestock exclusion apply to all 3 habitat levels (occupied, suitable,

potential). Managers and stockraisers alike must

maintain an open mind when working together and

avoid being influenced by biased information or unsubstantiated opinions. By building trust and cooperation, effective and honest decisions can be made.

Recreation

We recommend that recreational impacts at occupied, suitable, and potential sites be evaluated regularly to detect any habitat damage, cowbird presence,

or other factors that may impact WIFL. Activities such

as camping, hiking, fishing, boating, biking, photography, and driving vehicles are known to have varying

impacts on nesting birds, depending on the intensity,

timing, location, noise level, predictability, and type of

disturbance as well as the species, abundance, and

habituation level of birds (Knight and Gutzwiller

1994). The following preventative steps are recommended to mitigate negative effects of recreational

activities:

• Close areas to off-road vehicles year round in

potential, suitable, and occupied habitats.

• Exclude human access from occupied sites; use

“Area Closed” signs.

• Fence off occupied habitat; do not allow entry

during breeding season.

• If area closures are implemented, ensure that

closure orders are written to allow entry by authorized personnel (e.g., researchers, surveyors, etc.)

• Occupied habitats that are closed during the

breeding season should be open for day use only in

the non-breeding season. Campfires should not

be permitted at any time.

• Avoid construction of new campground or day use

facilities in occupied, suitable, or potential sites.

• Evaluate if recreational impacts are occurring to

habitat during the non-breeding season. Limit

use with permits if needed. Implement yearround closure to recreation if warranted based on

analysis of impacts.

• No product harvest within occupied or suitable

habitat during the breeding season. The demand

for willow and cottonwood seedlings may be met,

but permit product harvest only outside of the

breeding season and only where it will have beneficial results (e.g., increased vigor and resprouting

in decadent stands).

• To avoid attracting predators and cowbirds, provide adequate trash receptacles and frequent

trash pick-up in developed campgrounds and

dispersed campsites adjacent to or near WIFL

occupied sites.

• Use interpretive signs with a message such as

“prevent fires to avoid destruction of wildlife

habitat”.

• Prohibit construction of new roads or trails in or

adjacent to occupied, suitable, or potential WIFL

habitat.

• For WIFL habitats accessible by boats, use speed

limits, buoys, and closures to restrict boating use

and access. Nests located close to water level can

be disturbed by waves from boats. Also, fishing

lines and lures may disturb nests and/or birds.

• Work with the local community to find alternative recreation areas away from occupied, suitable, or potential areas.

Water Management

Southwestern willow flycatchers occupy breeding

habitats associated with water. Breeding sites are

typically found near still or slow-moving water. Managing for the presence of water is a critical factor in

sustaining occupied flycatcher habitats and in encouraging recolonization of potential and suitable

habitats. Regulated stream flow from dams, levees,

and channelization is thought to be one of the most

important factors explaining the decline of cottonwood and willow woodlands in riparian ecosystems

(Rood and Heinze-Milne 1989, Fenner et al. 1985,

Rood and Mahoney 1990). Given that such water

manipulation and demand can be extreme in the
114 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

Southwest (Brown et al. 1977, Fenner et al. 1985,

Crawford et al. 1993), close monitoring of in-stream

flow is critical to ensure sufficient water for sustaining and regenerating willow flycatcher habitat. To

protect water resources, implementation of a watermanagement strategy that accounts for habitat needs

of flycatchers is urged. Components of such a strategy are outlined as follows:

• Enforce existing laws to minimize illegal water

withdrawal.

• Evaluate effects of goundwater withdrawal and

pumping on riparian habitat. If a problem exists,

work with water users to mitigate water loss by

using financial incentives, public education, etc.

• Where applicable or possible, maintain or acquire

instream flow water rights or work with water

rights holders to increase instream flow.

• Evaluate alternative methods to diverting water

from riparian areas. Although it has its own

drawbacks, pumping may be a worthy compromise if it results in greater instream flow.

• Work with users to maintain, increase, and create

WIFL habitat.

• Eliminate phreatophyte control at occupied sites

and minimize control at suitable and potential

sites.

• Along established earthen ditches, encourage

vegetative growth by avoiding mowing and clearing. Evaluate mowing cycles.

• Work with agencies engaged in phreatophyte

control to minimize disturbance to suitable and

potential habitat.

• Evaluate dredging plans for waterways, including rivers, streams, ditches, ponds, and lakes to

minimize habitat damage. Work with flood control agencies to minimize habitat damage and

evaluate management plans.

• Develop public education on water uses (e.g.,

switching to drip systems rather than flood or

sprinkler irrigation).

• Develop plans to minimize destructive effects of

catastrophic floods, including those caused by

poor riparian conditions. Do this by emphasizing

improvement or restoration of healthy riparian

habitat (refer to habitat restoration section).

Small-scale flood events may be desirable to create backwater habitat for WIFL and to control

salinity. Identify what can be done to recover

habitat after destruction occurs. Remember that

healthy riparian systems are capable of sustaining high runoff events.

• Develop plans to minimize impacts to WIFL habitat at dams and impoundments.

• Avoid dam construction and operations that will

inundate WIFL habitat.

• Evaluate potential for creating WIFL habitat

below dams by releasing water to mimic natural

hydrology and water conditions conducive to

WIFL use.

Mining

Proposed mining (e.g., sand, gravel) sites in riparian

areas of public lands should be surveyed for WIFLs

and habitat suitability and potentiality prior to mine

development. If habitat is occupied, suitable, or potential, alternative sites should be selected, whenever

possible. Where mining is ongoing, managers should

develop a mitigation plan to minimize disturbance to

WIFL habitat during mining operations. After mining

is completed, a reclamation plan that requires restoration of WIFL habitat should be developed and

implemented.

Direct Disturbance by Management

Construction and maintenance of man-made structures in the vicinity of WIFL habitats is likely to

disturb birds while they are nesting, and steps should

be taken to minimize or eliminate this disturbance.

Habitat maintenance or maintenance of fences,

powerlines, dams, roads, trails, facilities, and houses

that occur in or adjacent to occupied sites should

preferably be scheduled during the non-breeding season with minimal damage to habitats. If damage

occurs, habitats should be restored. If emergency repairs are needed, disturbance to nesting birds should

be minimized.

Upland management activities such as grazing,

mining, development, wood-cutting, offroad vehicle

use, prescribed fires, and road construction may sometimes have a downslope effect on riparian zones,

through increased soil erosion, increased runoff, runoff of contaminated water, and reduced vegetation

protection. Effects of upland management activities

on riparian habitats in watersheds that have occupied, suitable, or potential WIFL habitat should be

evaluated periodically. A plan to minimize effects on

riparian habitats prior to implementation of upland

management or while management activities are ongoing should be developed.

Working with Private Landowners

Lands owned privately play an important role in

maintaining WIFL populations. The largest known

population of the southwestern willow flycatcher, for

example, is on a private ranch leased from Pacific

Western Corporation in the Cliff-Gila Valley of New

Mexico. Working cooperatively with private landowners to maintain and enhance riparian habitats,
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 115

especially but not exclusively those occupied by WIFL,

is a high priority. To assist landowners in making

informed decisions about WIFL habitat, agency representatives should provide oral and written information

to them about methods and goals for improving riparian systems to healthy states. Walking tours to riparian sites on private properties provide time for establishing a relationship with the landowner. Developing

mutual trust is an important goal in establishing a

cooperative relationship. The collaborative development of conservation agreements and plans to prevent

damage to WIFL habitats on private lands while

maintaining landowner livelihoods should be an objective. The establishment of conservation easements is

also a worthy investment.

Acquiring lands with WIFL habitats using a similar

land base for exchange should be a high priority for

land-managing agencies or conservation groups such

as The Nature Conservancy. Large parcel ownerships

are more desirable than small parcel ownerships.

Land transactions that result in subdivisions should

be avoided.

Habitat and Watershed Restoration

and Improvement _________________

Unhealthy or damaged riparian areas can be improved using a variety of restoration techniques. We

emphasize, however, that protection of existing habitat is the soundest, most cost-effective management

approach. In areas managed for livestock, a high

degree of flexibility in livestock operations is beneficial. Changes in livestock rotation schedules, timing

and period of pasture use, method of herding, and type

of livestock can go a long way toward restoring riparian habitats.

Replanting lost vegetation is not a substitute for

habitat protection. In addition, conditions may not be

suitable for revegetation efforts if the site has been

irreparably damaged, if nonnegotiable factors limit

the extent of restoration possible, or if the site was

never conducive to vegetation growth in the first place.

We discourage revegetation (plantings) if other kinds

of positive management (e.g., stock removal) are available and appropriate. Plantings, however, may be

appropriate when the following conditions are met:

• The seed sources are native.

• Revegetation is necessary to control water.

• Plantings are used to jump-start habitat restoration (e.g., by creating multi-layered structures or

accelerating natural processes).

• Plantings are used to control exotic plants and

prevent them from returning.

• Plantings are needed to prevent erosion and stabilize stream banks.

If the decision is made that revegetation is desirable

but the area is not initially suitable because of the

presence of exotics, it may be appropriate to supplement with more water, provided this will allow the site

to support WIFLs.

Sites need to be evaluated for conditions appropriate for WIFL habitat restoration based on factors

such as soils, watertable, water quality, geomorphology, elevation, genetic stock of vegetation, floodplain

characteristics, lower gradients, and historical

records. Conditions are suitable for habitat restoration when:

• Adequate surface water and ground water are

present (i.e., surface water is present until the

end of May). Suitable conditions can include moist

areas with potential to restore surface flow.

• A natural or simulated flooding regime exists.

• The site is in close proximity to occupied or historically-used WIFL habitat.

• There is commitment to long-term management

at the site.

• The site can become relatively self-sustaining

over time.

Approaches for creating suitable conditions for WIFL

colonization and occupancy include:

• Creation of slow water conditions (through Section 7 consultation) by excavating to groundwater

(e.g., Gila National Forest; Kern River Preserve);

by controlled inundation (e.g., Bosque del Apache

National Wildlife Refuge); or by using beavers to

dam small pools of water where appropriate.

• Appropriating instream flow.

• Acquiring habitat with water rights.

• Establishing farming landbanks to discourage

use of flood control structures and enable meander patterns of stream.

• Working with private landowners to establish

crop rest-rotation areas.

• Establishing conservation easements.

• Maintaining vegetation along stream banks to

distribute flood flows across floodplain and to

slow water velocity. This will help to re-hydrate

the floodplain and enhance further plant germination and growth.

Wintering and Migration

Habitats __________________________

The dearth of information on where and what habitats WIFLs use in winter and migration limits our

ability to make recommendations. When more information is available on the wintering range and habitats of WIFLs, it will be easier to identify threats and

solutions on the wintering grounds. Information can
116 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

be solicited from investigators who study birds in

Latin America by posting requests on web sites and

newsletters published by organizations such as the

Ornithological Society of North America, the National

Audubon Society, and The Nature Conservancy. Partnerships with Latin American organizations should

be encouraged to survey habitats for the presence of

wintering flycatchers, and to gain new information on

threats and habitat use. Involving Partners in Flight,

the international, interagency coalition for conserving

Neotropical migratory birds, in winter surveys would

also be beneficial.

A bird survey program administered by Mexican

agencies is needed, and North American organizations can assist Mexican biologists in developing this.

Possible breeding sites in northern Mexico also need to

be inventoried (e.g., Rio Santa Tomas; Santa Cruz;

San Pedro), and more information is needed on migration routes and habitats through Mexico.

Rivers known to be used by migrating willow flycatchers include the Colorado River, the Gila River,

and the Rio Grande. The time needed for migration

consumes more than a quarter of the annual cycle of

willow flycatchers. Individuals must stop periodically to refuel their energy reserves, and therefore,

habitats that sustain an abundant food supply of

arthropods may represent higher quality habitat to

migrating flycatchers than habitats with depauperate arthropod faunas. Habitats in close proximity to

water may enable flycatchers to replenish water that

was lost during flight. Exposure to inclement weather

and predators can also be mitigated during migration

if suitable habitat is available for cover. Habitat

protection along major migration routes should be

emphasized more than it has been in the past. For

example, practices such as mowing phreatophytes to

improve stream channels and water flow is likely to

reduce quantities of WIFL migration habitat. We

recommend that mowing cycles be modified to allow

a longer growing period of channel vegetation and to

retain some vegetation at intervals along each channel. Rather than mowing every year, consider mowing every 3 years.

Length of stopover time, body fat condition, and

captures rates are thought to be relevant measures of

quality of migration habitats. According to Yong and

Finch (1997) unmowed coyote willow along the middle

Rio Grande was used more frequently and by fatter

willow flycatchers during migration than mowed

willow, cottonwood, agricultural fields, or Russian

olive. To ensure successful migration by willow flycatchers, we recommend that steps be taken to protect and enhance willow thickets along southwestern

drainages used by migrating willow flycatchers.

Information and Education _________

We recommend that Partners In Flight (PIF) state

working groups take the lead on developing information and education (I&E) materials about the southwestern willow flycatcher. PIF state working groups

have I&E committees already in place that can do this

work. In addition, individual agencies and conservation

organizations are encouraged to develop I&E materials

on the willow flycatcher. Some ideas for I&E materials

include slideshows and scripts that are duplicated and

sold at cost; videos of WIFL, their habitats, and interviews with WIFL experts; brochures; posters; newspaper and magazine articles; interpretation signs at

campsites; interpretive talks; and a paragraph on

WIFL at the PIF web site. Funding sources need to be

developed for I&E materials and for research reports.

Scientists are encouraged to promptly publish WIFL

results and distribute reports and reprints to their

constituencies. Progress reports and updates prepared

by agencies and conservation groups should be widely

circulated to other organizations and interested parties. A list-server for WIFL discussions and news

updates can be established on the Internet. New

publications such as this Conservation Assessment

should be marketed and distributed to pre-established

mail lists. Drawings and photos help to make technical

documents more user-friendly. Information-sharing

sessions should be held periodically to keep interested

parties updated on new developments in the WIFL

arena. We also recommend that a symposium devoted

to the WIFL (or endangered riparian bird species in

general) be sponsored by agencies or professional

societies and a proceedings of the symposium be published and circulated to WIFL mail lists.

Managers need more research and technical information to effectively manage WIFLs and their habitats. Technical information can be supplied through

consultations, publications, WIFL training sessions,

and “show-me” tours. One of the most significant

publications that managers could apply on the ground

is a recovery plan for the southwestern willow flycatcher (now in progress). Finally, new knowledge and

methods to protect and recover WIFL are needed;

more specifics are identified in the next chapter.

Acknowledgments ________________

We owe Susan Sferra, and Mark Sogge a debt of gratitude for their consultations during the preparation of

this manuscript. We thank Jeff Kelly and Sheldon

Plentovich for their comments at initial meetings, and

we appreciate reviews of the manuscript by Chuck

Hunter, Richard Periman and Sartor Williams III.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 117

Literature Cited ___________________

Anderson, B. W., A. Higgins, and R. D. Ohmart. 1977. Avian use

of salt cedar communities in the Lower Colorado River Valley.

Pp. 128-136 In: R. R. Johnson and D. A. Jones, tech. coords.,

Importance, preservation, and management of riparian habitat: A symposium. USDA Forest Service, Rocky Mountain

Forest and Range Experiment Station, Fort Collins, CO. General Technical Report RM-43.

Beezley, J. A. and J. P. Rieger. 1987. Least Bell’s Vireo management

by cowbird trapping. Western Birds 18:55-61.

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preferences. A guide to the most attractive bird foods. National

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endangered Least bell’s Vireo: A management success story. In:

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systems. Condor 90:113-123.

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avifauna of western riparian habitats. Pp. 273-285 In: J. R. Jehl,

Jr. and N. K. Johnson, eds., A century of avifaunal change in

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109:253-268.
118 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 119

Scott H. Stoleson

Janie Agyagos

Deborah M. Finch

Tracy McCarthey

Jamie Uyehara

Mary J. Whitfield

Chapter 11:

Research Needs

Until the southwestern willow flycatcher was proposed for listing as an endangered species in 1993, it

was subject to relatively little scientific scrutiny, in

part because it is drab, prone to inhabiting dense, wet

thickets, and difficult to identify and observe. Consequently, it remained one of the lesser-known of North

American birds. Since that time, it has become clear

that significant gaps exist in our understanding of its

status, distribution, ecology, and management. Recently, many questions about the subspecies have

arisen due to controversy over management actions

and resulting litigation. New information that addresses these specific questions is essential to effect

recovery of the bird. In this chapter we present, in

annotated outline form, what we consider to be important areas for further research. This list of research

needs is not intended to be exhaustive, and has not

been prioritized. Refer to the southwestern willow

flycatcher Recovery Plan for research priorities.

Historical and Current Status of

Habitat ___________________________

The greatest threat to the continued existence of

the southwestern willow flycatcher is the loss and

degradation of its riparian habitat throughout the

Southwest (Chapter 4). A better understanding of

the patterns and changes of distribution of riparian

habitats will enable a greater understanding of the

causes and patterns of population declines in the

flycatcher.

• Review historical and archival records (e.g., historical photos, agency management records, military records, journals and diaries) and other literature to assess the historical extent and

condition of riparian areas in the Southwest,

especially where willow flycatcher populations

were known to have existed.

• Document and map the temporal and spatial

patterns of habitat loss in the Southwest, and

determine the causes of historical habitat loss.

Cross-check historical records with paleobotanical data (e.g., pollen, phytolith, macrobotanical

analyses, packrat middens).

• Correlate changes in willow flycatcher numbers

and distribution to changes in riparian habitats

over time.

• Research and document the current distribution

and extent of riparian vegetation in the Southwest, to determine (1) the location and amount of

currently available habitat, and (2) the types and

condition of extant riparian habitat, including

micro and macroclimatic condition and landscape

context.
120 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

Status on the Breeding Ground _____

Although there is a concensus that population levels

of the southwestern willow flycatcher have declined in

this century, questions remain about its historical

range and distribution. The geographic extent of the

breeding range of E. t. extimus is unclear, particularly

where it intergrades with the ranges of E. t. brewsteri

and E. t. adastus (see Fig. 7-1). Sound management

and recovery require a better understanding of both

the historical and current breeding range as well as

the current status of the southwestern subspecies to

ensure that efforts target the correct subspecies.

• Analyze museum specimens using morphometric

methods to assess (1) species level limits (alder vs.

willow flycatchers) among populations, and (2)

sub-specific level limits among populations. In

particular, efforts should be made to determine

whether birds breeding in western Texas, northern New Mexico, southern Colorado, Utah, and

Nevada, and at higher elevations in Arizona and

New Mexico, are E. t. extimus.

• Analyze museum specimens and field captures

using genetic methods to assess (1) species level

limits among populations, and (2) sub-specific

level limits among populations. In particular,

efforts should be made to determine whether

birds breeding in western Texas, northern New

Mexico, southern Colorado, Utah, and Nevada,

and at higher elevations in Arizona and New

Mexico, are E. t. extimus.

• Identify transitional areas between the ranges of

E. traillii subspecies, using genetics, morphology,

and vocalizations.

• Review historical and archival records to document the historical distribution and abundance of

willow flycatchers in the Southwest.

• Refine our knowledge of the current distribution

and abundance of E. t. extimus throughout the

Southwest.

• Determine the breeding status of E. t. extimus in

Mexico using information from surveys.

• Identify and locate suitable riparian habitat not

currently occupied by flycatchers, in order to

determine the degree of habitat saturation by

flycatchers.

Threats to the Southwestern Willow

Flycatcher ________________________

The various threats to the southwestern willow

flycatcher, described in Chapter 4, are imperfectly

understood. Many must be considered merely potential threats because their actual impacts on flycatchers or their habitats are unknown at this time. A

better understanding of those threats and their interactions with flycatchers and human activities is

required to mitigate for the threats effectively. Also,

it is critical to understand that many of these threats

are interrelated.

Effects of Small Population Size

Populations composed of few individuals are vulnerable to genetic problems, particularly when they exist

as fragmented subpopulations. Small populations tend

to lose much of their genetic variability through drift,

making them less able to adapt to changing environments. In the extreme case, very small populations

may be prone to inbreeding depression. Recently, a

study of population genetics in southwestern willow

flycatchers in Arizona was initiated by the USGS

Colorado Plateau Field Station (Paxton et al. 1997).

This program should be expanded to the whole range

of the flycatcher to address the following genetic questions. Considering the very small size and fragmented

distribution of southwestern willow flycatchers, these

topics should be considered high priority.

• Characterize the genetic variability and relationships within and among breeding populations.

• Assess the extent of gene flow among populations.

• Determine if inbreeding depression occurs in

flycatchers.

• Determine the effective population size (Ne) of

E.t. extimus.

• Determine whether the 2% observed rate of physical deformity in southwestern willow flycatchers

is genetically based, by examining pedigrees of

marked birds.

• If warranted, explore tools or techniques to increase genetic variability, e.g., cross-fostering.

Habitat Threats and Considerations

• Quantify habitat characteristics of Willow flycatcher breeding sites across entire range, in

terms of (1) patch area and shape, (2) proximity to

water, (3) stand age and successional status, (4)

vertical structure, (5) plant species composition,

(6) landscape matrix, (7) climate and microclimate variables, and (8) topography.

• Characterize ‘suitable’ breeding habitat and qualitatively describe variation in ‘suitable’ habitat

across range (in more detail than in Sogge et al.

1997).

• Assess why willow flycatchers are absent from

potentially suitable native habitats, especially

with respect to microclimate.

• Compare placement of nest sites in relationship

to substrate availability and in association with

other habitat features, such as patch size and

stand characteristics.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 121

• Evaluate the impacts of phreatophyte mowing

regimes on willow flycatcher habitat suitability.

• Evaluate the direct and downstream effects of

sand and gravel operations on hydrology and

habitat, using results from the literature or by

conducting field studies.

• Evaluate the impacts of beaver on willow flycatcher habitat in the southwest, through a

compilation of historical records and an assessment of current beaver distribution in different

drainages.

Water Management—Water is a scarce resource

in the southwest, and the health of riparian habitats

is directly impacted by water management practices.

Much remains to be learned about how water management affects riparian vegetation, and how water management can be altered to benefit riparian habitat and

the willow flycatcher.

• Compile information on the effects of water management (e.g., dams, diversions, impoundments)

on riparian habitat, using sources such as aerial

photos and previous studies.

• Compare dammed and undammed drainages with

respect to habitat features.

• Evaluate the effects of groundwater withdrawal

or pumping on riparian habitat.

• Evaluate the effects of surface water movement

(e.g., ditching, irrigation) on riparian habitat.

• Identify management regimes that favor native

plant species and natural patterns of succession.

• Evaluate the impacts of dredging and channel

bed alterations on riparian habitat.

• Determine how willow flycatchers respond to catastrophic habitat loss (e.g., flooding of Roosevelt

Lake, Lake Mead).

Livestock Grazing—Livestock grazing in riparian

areas and adjacent uplands has been identified as a

major cause of the loss and degradation of riparian

habitats. However, because grazing practices can vary

considerably in terms of timing, location, and intensity, the effects of different grazing regimes on vegetation and flycatchers are also likely to vary. Livestock

grazing remains one of the principle uses of public and

private lands within the range of E. t. extimus. As a

result, management policies that affect grazing are

likely to have severe economic and political implications. Robust data on the effects of different grazing

regimes on flycatchers and their habitats is essential

for effective, intelligent, and defensible management

of flycatcher habitat.

• Evaluate the impact of different grazing regimes

(timing, livestock density, duration of grazing)

on flycatcher ecology, e.g., reproductive success,

population size, nest placement, productivity,

predator assemblage, cowbird parasitism rates.

• Evaluate the direct and indirect effects of grazing

on or adjacent to riparian habitats both occupied

and unoccupied by flycatchers.

• Evaluate the effects of livestock grazing in upland

habitats adjacent to riparian breeding habitat on

flycatchers and their habitat.

• Evaluate the effects of livestock grazing upstream

from riparian breeding habitat on flycatchers and

their habitat.

• Compare the impact of different grazing regimes

on plant community structure, particularly the

incidence of native vs. exotic species.

• Determine the effects of livestock grazing on fuel

load, and whether such effects impact flycatchers

either positively or negatively.

• Use artificial nests to quantify the impacts of

trampling and direct disturbance by livestock.

Recreation—Recreational activities tend to be concentrated in riparian areas in the Southwest, and

therefore are likely to create significant conflicts with

flycatcher conservation (Johnson and Carothers 1982).

We currently know little about the direct or indirect

effects of recreation on flycatchers.

• Assess the effects of human activity in and around

riparian habitats on habitat quality, e.g., effects

on vegetation structure, erosion, and introduction of exotics.

• Assess the impacts of human activity in and

around riparian habitats on the settlement and

productivity of flycatchers, e.g., possible changes

in rates of nest abandonment.

• Assess the impacts of the presence and density of

roads on flycatcher habitat use and productivity.

Fire—Fire has become an issue of concern in southwestern riparian systems, especially with the recent

loss of some occupied willow flycatcher sites. Research

on fire may help in the development of management

strategies that minimize future losses to burning.

• Summarize the historical role of fire in riparian

systems.

• Characterize fire sources and risks.

• Compare fire risks in exotic vs. native stands.

• Determine whether fuel loads present a fire hazard, and if so, determine what tools can be used to

effectively reduce fuel loads.

• Determine the short- and long-term response of

flycatchers to different types of fire, e.g., catastrophic, prescribed burnings, and patchy fires.

• Determine the effectiveness and ecological ramifications of fire breaks, e.g., do breaks significantly

increase fragmentation of riparian habitats.

Exotic Vegetation—Much riparian habitat in

the southwest has been invaded by exotic plants,

particularly tamarisk. At the same time, many extant
122 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

populations of the southwestern willow flycatcher

nest in pure or mixed stands of tamarisk, while

others breed in other exotic species, such as Russian-olive. A better understanding is needed of the

relative quality of exotic plants as willow flycatcher

habitat and nest substrate, as well as the potential

for restoring native vegetation.

• Compare willow flycatcher nesting success in

exotic vs. native plant communities (e.g., tamarisk, Russian-olive, willow, box-elder, oak).

• Compare insect prey abundance and flycatcher

foraging efficiency in exotic vs. native plant communities, and in relation to site-specific nesting

success and flycatcher abundance.

• Evaluate quality of tamarisk habitat in relation

to structure, density, canopy cover, and presence

of native species.

• Identify water and land management options

that favor native vegetation.

• Evaluate biocontrol of tamarisk in terms of (1)

likelihood that native species will return to treated

sites, (2) rate of movement of biocontrol insects,

(3) responses of target and nontarget plants to

biocontrol, (4) control mechanisms and containment procedures for biocontrol insects, and (5)

responses of willow flycatchers to vegetation

changes resulting from biocontrol.

Habitat Restoration—Because habitat loss constitutes the major threat to the flycatcher, riparian

habitat restoration should prove to be a vital tool for

recovery efforts. However, riparian restoration efforts

have differed in their goals, methodologies, and levels

of success. Very few have been conducted specifically

to restore wildlife habitat for a particular species.

Recently, restored riparian sites in California were

colonized by least Bell’s vireos (Vireo bellii pusillus;

Kus 1998), although it remains to be seen whether the

sites can persist without management (B. Kus, personal communication). As noted in the discussion on

habitat (Chapter 9), not all riparian areas provide

suitable habitat for the willow flycatcher. Thus, research is urgently needed to assess the appropriateness and efficacy of restoration techniques for flycatcher recovery.

• Summarize and evaluate results of past restoration efforts.

• Evaluate restoration techniques in terms of their

overall efficacy.

• Compare restoration techniques, e.g., “natural”

flooding events vs. managed restoration.

• Monitor colonization by willow flycatchers on restored sites with respect to rate and persistence.

• Determine efficacy of restoration projects with

respect to colonization by willow flycatchers or

population growth rates of willow flycatchers.

• Compare the efficacy of restoration methods

for establishing Willow flycatcher populations

as a function of distance to existing flycatcher

populations.

• Review success of compliance “mitigation” sites

in terms of regeneration and flycatcher use.

• Evaluate the efficacy and feasibility of reinstating “natural” hydrological regimes of disturbance

and plant succession to willow flycatcher breeding areas.

• Determine whether successful restoration techniques are compatible with ongoing management

regimes.

• Evaluate if modifying management practices can

promote habitat restoration; review those modifications that have worked in the past and evaluate

those not yet tried.

Factors Directly Threatening Flycatchers

Cowbird Parasitism—Some willow flycatcher

populations suffer heavy cowbird parasitism, while

for others parasitism appears to be a minor problem.

Work is needed to determine which factors of landscape, habitat, avian community structure, or land

use affect cowbird abundance and parasitism rates.

Cowbird control through trapping has been proven to

be effective in reducing rates of parasitism in some

willow flycatcher populations (e.g., Whitfield and

Strong 1995) as well as in populations of other small

passerines. The methodology for trapping cowbirds is

well established, but it is a labor-intensive and costly

strategy. Questions about when, where, how long, and

whether to trap must be answered if we are to maximize its effectiveness as a management tool, and to

make the best use of scarce management funds.

• Determine if willow flycatchers are a preferred

host of cowbirds at particular sites throughout

the Southwest, compared to other host species.

• Determine how parasitism rates on willow flycatchers vary with host assemblage and density.

• Because cowbird abundance has been associated

with livestock, characterize the relationships between flycatcher parasitism rates, cowbird abundance in feeding and breeding areas, and grazing

regimes (e.g., dispersed vs. congregated, upland

vs. riparian).

• Evaluate whether cattle dispersion and grazing

characteristics are linked to cowbird travel distances between feeding and breeding sites in the

Southwest.

• Quantify cowbird abundance throughout the

Southwest (e.g., using Breeding Bird Surveys) to

determine where areas of high cowbird density

overlap with flycatcher breeding sites.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 123

• Establish scientifically-based criteria to either

implement or discontinue cowbird trapping in

flycatcher breeding habitat.

• Evaluate the effectiveness of cowbird trapping in

different habitats and regions by monitoring its

effects on flycatcher productivity and population

trends over time.

• Compile a list of sites where Animal and Plant

Health Inspection Service blackbird control occurs, and evaluate its effects on cowbird numbers.

Because female cowbirds produce at least 30 to 40

eggs per breeding season and parasitize many species

(Rothstein et al. 1986), the availability of alternate

hosts in a community may affect the risk of parasitism

on flycatchers (e.g., Barber and Martin 1997). Therefore, the following research questions need to be addressed:

• How does the relative timing of nesting phenology among species affect parasitism rates in

flycatchers?

• How do cowbirds select hosts to parasitize?

• What management actions can reduce parasitism

rates?

• How do landscape and structural features of the

habitat (e.g., distance to edge, density of cover,

vegetation structure, availability of perches) affect rates of cowbird parasitism?

• Determine the impacts of parasitism on juvenile

and adult survival in flycatchers.

Predation—Predation may constitute a major

threat to the southwestern willow flycatcher. Nest

predation has been identified as a major cause of nest

failure in most populations studied. Further, the risk

of predation may be increasing as riparian habitats

are further fragmented by human activities. Yet, because our knowledge of predators and predation on

flycatchers is limited to anecdotal accounts, we lack

sufficient data to manage predation effectively.

• Identify potential and documented predators on

E. t. extimus rangewide.

• Identify the temporal and spatial distribution of

predators within riparian habitats.

• Characterize predation rates and types relative

to landscape and habitat features.

• Summarize geographic and temporal variation in

predation patterns.

• Evaluate methods to minimize predation impacts

through management.

• Determine the extent of cowbirds as predators on

flycatcher eggs and nestlings.

Parasites and Disease—

• Summarize ectoparasite and disease-related information from netting and banding stations.

• Expand data collection of nest and blood parasites.

• Assess the effects of parasitism on willow flycatcher survival and productivity.

Environmental Toxins—Agricultural development in the southwest tends to be concentrated in

floodplain areas, suggesting potential threats to the

flycatcher and its habitat from associated pesticides,

herbicides, and other toxins.

• Investigate the incidence, range, and prevalence

of toxins at flycatcher breeding, stopover, and

wintering locations.

• Analyze available specimens and nonviable eggs

for toxins.

• Analyze other sympatric riparian birds (specimens and nonviable eggs) for toxins.

Winter and Migration______________

Willow flycatchers migrate from their summer breeding areas to winter in the tropics. However, little is

known about where they go and what they do once they

leave their breeding grounds (see Chapter 5). Gathering such information for this endangered subspecies is

hampered by difficulties in identifying non-singing

Empidonax species in the field, let alone subspecies.

Such information is essential, however, because conservation measures on the temperate breeding grounds

will be futile if the taxon’s requirements are not also

met during migration and on the wintering grounds.

• Determine the winter range of E.t. extimus, through

the use of (1) museum specimens from Mexico and

Central America, (2) surveys conducted on potential wintering grounds, and (3) information exchange with colleagues in Latin America.

Once the winter range is identified:

• Characterize habitat use and quality.

• Determine if, and how, winter habitat is at risk.

• Identify key areas for protection.

• Develop methods for protecting habitat and birds.

• Identify factors limiting winter survival of flycatchers, e.g., food, weather, contaminants, habitat loss or degradation, catastrophic events, disease, and predation.

Migration is a critical time for migratory songbirds,

especially in terms of energy expenditure and physiological stress. However, migration ecology remains

poorly known for most species, including the southwestern willow flycatcher.

• Identify migration corridors, if any, by analyzing

existing survey data.

• Determine subspecies patterns in migration routes

and timing, if possible.

• Determine sex- and age-specific patterns of

migration.
124 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

• Determine habitat use and influence of exotic

vegetation during migration.

• Evaluate stopover time with respect to habitat

quality, bird condition, and food supply.

• Examine condition of migrants as a function of

age and in relation to stopover time and subsequent probability of survival.

• Examine the relationship between stopover

time and bird condition with the size and degree of patchiness of habitats and other landscape features.

Breeding Biology and

Demography _____________________

Measures of fecundity and mortality are essential

for assessing the health of any population. Much more

work is needed to develop a better understanding of

the factors that affect nesting success, survival, and

dispersal. Many aspects of breeding biology and demography can only be determined by observing individual birds, which requires the use of color-banding.

• Expand the current Arizona banding program to

create a coordinated, rangewide program.

• Quantify rates of site fidelity, both within and

among years.

• Determine rates of recruitment among different

populations.

• Determine the age structure of populations.

• Estimate rates of survivorship for different age

classes and among different populations.

• Determine the effects of age on nesting success,

yearling and adult survival.

• Identify source and sink populations.

• Estimate rates of immigration and emigration, in

terms of (1) distances moved, (2) relationship to

patch size and isolation, and (3) source and sink

populations.

• Characterize the metapopulation structure of

E.t. extimus.

• Characterize nesting success in terms of (1) number of nesting attempts per season, (2) fledging

success over a season, (3) variation among years

and among habitats, and (4) seasonal fecundity

using the Pease-Grzybowski model (Pease and

Grzybowski 1995).

• Determine the relation between nesting success and (1) arrival time on the breeding grounds,

(2) movements within and between years, and

(3) any habitat or cowbird manipulations.

• Determine the relation between nesting success

and habitat or landscape features listed under

habitat threats and considerations.

• Assess the relative impacts of predators versus

brood parasites on fecundity.

• Determine the rates and causes of nest desertion.

Behavior

Many aspects of willow flycatcher behavior remain

poorly known, but have important implications for

monitoring, conservation and management. In particular, a better understanding of the flycatcher’s

mating system would help to answer questions about

effective population size and other consequences of

small population size in this subspecies.

• Determine the frequency of extra-pair copulations (by sex) to determine mate fidelity, estimates of male fecundity, and genetic relations

within and among populations.

• Assess the parentage of family groups to determine the incidence of extra-pair copulations.

• Assess the relative nesting success of polygynous

vs. monogamous pairs.

• Correlate mating strategy (floater vs. territorial

male) with arrival time on breeding grounds.

• Correlate mating success with patch size, population size, habitat quality, and distance to source

population.

• Determine the presence, size, and age of floaters

in relation to population size, quality and range of

habitats.

• Use data on the genetics of floaters to assess (1)

relatedness to nestlings in broods resulting from

extra-pair copulations, and (2) the lifetime reproductive success of floaters.

• Characterize nest defense in willow flycatchers,

in terms of (1) whether birds actively defend

nests, (2) whether nest defense is successful, and

(3) how variable nest defense is with respect to

age, sex, habitat, nest site, population size of

patch, and the type of predator or intruder.

Food Habits

Many of the details of willow flycatcher food habits

remain poorly known. A better understanding of the

prey choice and foraging behavior is needed because

insect prey abundance may be adversely affected by

land use and water management practices, and because food has the potential to limit reproductive

success and population growth.

• Characterize the diet of southwestern willow flycatchers through (1) fecal analyses, and (2) by

direct observations of foraging birds.

• Quantify food delivered to nests, in terms of

frequency of food delivery and differences between sexes and among seasons.

• Identify differences between prey eaten by adults

and prey delivered to nestlings, in terms of insect

size, type, or palatability.

• Assess foraging efficiency in relation to habitat

type (exotic vs. native).
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 125

• Determine insect availability and type (taxon or

size class) in different habitat types, including

exotic vegetation. Identify any variation due to

the effects of season or weather.

• Assess the existence and degree of diet overlap

with other bird species to identify the potential for

interspecific competition.

• Determine if willow flycatcher nest success or population trends vary in relation to food availability.

• Determine if food is limiting within or among

years. Potential indicators of food limitation include (1) the frequency of nestling starvation, and

(2) adult foraging behavior, as measured by the

duration of foraging bouts or the percentage of

daily activity spent foraging.

Vocalizations

Recent work has documented singing in females and

fledglings (Seutin 1987, Sogge 1997), demonstrating

that our knowledge of willow flycatcher vocalizations

remains incomplete. Increased research on flycatcher

song should be considered a high priority for several

reasons. Survey protocols, used for assessing numbers

and status of birds within populations, are based on

singing. At least two current studies report the possibility that song may vary among regions or subspecies

(Travis 1996, J. Sedgwick personal communication).

• Characterize the daily and seasonal patterns of

song.

• Quantify the frequency, timing, and intensity of

female singing.

• Evaluate female territorial defense by singing or

chasing behavior.

• Compare singing behavior between sexes in relation to concealment and height of perch.

• Evaluate the role, intensity, and frequency of

song in migrating willow flycatchers relative to

resident birds.

• Evaluate the relation between song quality and

male quality and nesting success.

• Identify and characterize local and regional dialects in vocalizations, and determine if dialects

are related to subspecies or geography.

• Characterize variation in song and singing behavior, through the use of sonographs, (1) in

relation to social behavior (e.g., floaters, polygyny),

(2) between sexes and age classes, (3) among

habitats, nest sites, or proximity to edge, and (4)

with respect to cowbird abundance and parasitism rates.

Monitoring and Surveying _________

Most of our knowledge of willow flycatcher status,

distribution and reproductive success comes from

survey and nest monitoring programs. These methodologies have been developed for this particular

taxon only very recently, and are still evolving. Further work is needed to assess and improve the accuracy and reliability of data collected from survey and

monitoring efforts.

• Expand rangewide survey efforts to fill gaps in

current knowledge of flycatcher distribution.

• Determine the effectiveness of survey protocol in

detecting accurate numbers of willow flycatchers,

by comparing different census and nest monitoring results to survey results.

• Determine the optimal number of surveys per

site.

• Compile and analyze range- and state-wide survey data to address questions about (1) population trends in time and space, and (2) distributional statistics. If there are declines, determine

their causes.

• Develop a systematic population monitoring program using surveys repeated annually.

• Develop a standardized nest-monitoring protocol

by coordinating the methodologies, reporting, data

management and data analysis of state wildlife

agencies and the US Fish and Wildlife Service.

These data would be used to analyze regionwide

trends and patterns in productivity over time.

• Use winter nest searches to document willow

flycatcher breeding at unmonitored sites, or where

breeding could not be determined during summer

monitoring efforts.

• Assess the impacts of human observers on nesting success.

• Assess the impacts of color bands on willow flycatcher survival and mating success.

• Compare and correlate survey trends with trends

in nest success and cowbird densities.

• Monitor habitat response to beaver reintroductions.

Population Viability Analysis _______

In Chapter 7, a population projection model was

presented to show possible fates of a hypothetical

flycatcher population based on different sets of assumptions about demography. That model was simple

and deterministic. The more in-depth process of modeling and analyzing the various deterministic and

stochastic forces that determine the fate of a population is called population viability analysis (PVA: Boyce

1992). A PVA can provide quantitative estimates of

likely population trajectories under current conditions, as well as test the likely effects of different

management actions. Repeated analyses through the

species recovery process can provide a gauge of the

progress of recovery, and may provide a quantitative

criterion for de-listing. However, a PVA model is
126 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

data-intensive and its results are only as good as the

data used to obtain them. Before any attempt can be

made to assess the viability of the southwestern willow flycatcher population, many of the research needs

outlined in this chapter need to be accomplished.

• Use the data outlined in previous sections to

conduct a population viability analysis, taking

into account (1) variation in vital rates among and

within populations, (2) stochastic events such as

fires and floods, and (3) the spatial distribution of

flycatcher populations (metapopulation).

• Based on PVA results, prioritize where management efforts should be focused for (1) sustaining

current population, and (2) encouraging species

recovery.

Multiple Species Considerations____

Riparian areas in the Southwest support some of the

most diverse and densely populated wildlife communities in North America (Hubbard 1977, Knopf et al.

1988). Many obligate riparian and riverine species in

the region are considered threatened, endangered, or

sensitive, frequently due to the same factors that have

impacted the willow flycatcher. Conservation efforts

may be most efficient and cost-effective when they

benefit multiple species.

• Identify other sensitive, threatened, or endangered species that share occupancy patterns and

distributions with E.t. extimus (i.e., a mini-GAP

analysis with a riparian focus).

• Evaluate the impacts of flycatcher management

on other species, and identify management actions that benefit other sensitive, threatened or

endangered species. Develop criteria for rating

riparian habitats based on multi-species benefits,

or concentrations of threatened or endangered

species (to be used to prioritize protection efforts).

• Refer to state Partners in Flight efforts, State

Heritage reviews, TNC Bioreserve program, and

state-wide GAP analyses, to help prepare sensitive species management plans based on threatened and endangered species priorities.

General Recommendations ________

• Because many occupied flycatcher sites occur on

privately-owned lands, more research should be

conducted on private lands in collaborative efforts with landowners.

• Encourage more coordinated efforts in research,

monitoring, and data sharing.

• Research to date has revealed that many aspects

of flycatcher biology vary considerably among

sites. Therefore, interpretation of local research

results should take this variation into consideration. Research should be conducted at a broader

range of sites to better understand the extent and

causes of variation.

Summary _________________________

The research needs outlined in this chapter demonstrate extensive gaps in our knowledge of many areas

of southwestern willow flycatcher biology. It is unlikely there is enough time or funding to accomplish all

of these suggested studies, however. Nor are all of the

research needs are of equal importance for recovering

the species. The Recovery Plan for the southwestern

willow flycatcher suggests that the top priority areas

for research are the distribution, abundance, demography, and limiting factors of flycatcher populations.

These baseline data are necessary to provide a knowledge base to begin the recovery process. Topics of

secondary priority include assessments of flycatcher

habitat and factors that affect habitat, such as dam

and water management, grazing, and cowbird management, as well as work on migration and wintering

distribution and ecology and subspecific taxonomy.

References _______________________

Barber, D. R., and T. E. Martin. 1997. Influence of alternate host

densities on Brown-headed Cowbird parasitism rates on Blackcapped Vireos. Condor 99: 595-604.

Boyce, M. S. 1992. Population viability analysis. Annual Review of

Ecology and Systematics 23: 481-506.

Hubbard, J. P. 1977. Importance of riparian ecosystems: biotic

considerations. Pp. 14-18. In: Importance, preservation and management of riparian habitat: a symposium. General Technical

Report RM-43 (R.R. Johnson, D.A. Jones, technical coordinators).

Fort Collins, Colorado, USDA Forest Service, Rocky Mountain

Forest and Range Experiment Station.

Johnson, R. R., and S. W. Carothers. 1982. Eisenhower Consortium

Bulletin, 12: Riparian habitats and recreation: Interrelationships and impacts in the Southwest and Rocky Mountain region.

Fort Collins, Colorado, USDA Forest Service, Rocky Mountain

Forest and Range Experiment Station.

Knopf, F. L., R. R. Johnson, T. Rich, F. B. Samson, and R. C. Szaro.

1988. Conservation of riparian ecosystems in the United States.

Wilson Bulletin 100: 272-284.

Kus, B. E. 1998. Use of restored riparian habitat by the endangered least Bell’s vireo (Vireo bellii pusillus). Restoration Ecology 6: 75-82.

Paxton, E., S. Langridge, and M. K. Sogge. 1997. Banding and

population genetics of Southwestern Willow Flycatchers in Arizona: 1997 Summary Report. Flagstaff, USGS Biological Resources Division, Colorado Plateau Field Station, Northern Arizona University.

Pease, C. M., and J. A. Grzybowski. 1995. Assessing the consequences of brood parasitism and nest predation on seasonal

fecundity in passerine birds. Auk 112: 343-363.

Rothstein, S. I., D.A. Yokel, and R. C. Fleischer. 1986. Social

dominance, mating and spacing systems, female fecundity, and

vocal dialects in captive and free-ranging Brown-headed Cowbirds. Current Ornithology 5: 127-185.

Seutin, G. 1987. Female song in Willow Flycatchers (Empidonax

traillii). Auk 104: 329-330.

Sogge, M. K. 1997. Primary song by a juvenile Willow Flycatcher.

Journal of Field Ornithology 68: 630-631.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 127

Sogge, M. K., R. M. Marshall, S. J. Sferra, and T. J. Tibbitts. 1997.

National Park Service Technical Report, NPS/NAUCPRS/NRTR97/12: A Southwestern Willow Flycatcher Natural History Summary and Survey Protocol.

Travis, J. R. 1996. Song types of willow flycatchers in New Mexico.

Report to New Mexico Department of Game and Fish, Contract

No. 95-516-67. Santa Fe.

Whitfield, M. J., and C. M. Strong. 1995. Bird and Mammal Conservation Program Report, 95-4: A Brown-headed Cowbird control

program and monitoring for the Southwestern Willow Flycatcher,

South Fork Kern River, California. Sacramento, California Department of Fish and Game.
128 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 129

Appendix:

Author Biosketches and Addresses

Janie Agyagos received her B.S. from Arizona

State University in 1993. She is currently the Wildlife Staff Officer for the Beaver Creek/Sedona Ranger

District on the Coconino National Forest. In 1997,

as part of the Forest Service’s Seven Species effort,

she led a team that reviewed all Forest Service

activities near occupied and suitable southwestern

willow flycatcher sites. That team then developed

short- and long-term management actions and guidelines to protect the flycatcher and its habitat on

National Forest Systems lands throughout Arizona

and New Mexico. Janie’s duties as District Wildlife

officer include managing for and protecting 50

threatened, endangered and sensitive species, including the southwestern willow flycatcher.

Jean-Luc E. Cartron is Director of Research at

Hawks Aloft, Inc., and Research Assistant Professor

at the University of New Mexico. He received a

Ph.D. in Biology from the University of New Mexico.

His research experience in Conservation Biology

includes a 7-year study on ospreys in the Gulf of

California, Mexico; a study on the impact of electrocutions by power poles among raptors in northwestern Chihuahua, Mexico; a study on the distribution

and productivity of nesting ferruginous hawks in

relation to prairie dogs in central and western New

Mexico; and a study of noise disturbance impacts on

the distribution and reproductive success of golden

eagles in northwestern New Mexico. As a Research

Wildlife Biologist at the Rocky Mountain Research

Station from 1997 until 2000, he coedited with

Deborah M. Finch a conservation assessment for the

cactus ferruginous pygmy-owl. His research interests include life history theory, evolutionary biology, and raptor and Threatened and Endangered

species conservation.

Deborah M. Finch is a Research Wildlife Biologist

and Project Leader with the USDA Forest Service’s

Rocky Mountain Research Station in Albuquerque.

She is also Research Associate Professor in the Department of Biology at University of New Mexico and

Adjunct Professor at Northern Arizona’s School of

Forestry. She received her B.S. in Wildlife Management from Humboldt State University, California, her

M.S. in Zoology from Arizona State University, and

her Ph.D. in Zoology from the University of Wyoming.

Deborah’s research interests include Neotropical migratory bird ecology and reproduction, bird migration,

endangered species, riparian and grassland ecosystem ecology and conservation, and relationships between commodity use and natural resources. Deborah

has published more than 80 articles and journal papers and is an active member of several professional

societies. Since 1998, she has served as Leader of the

Technical Subgroup of the Recovery Team for the

Southwestern Willow Flycatcher.

Lloyd Goldwasser received his A.B. degree in mathematics and music from the University of California,

Berkeley, in 1976, and completed a masters degree in

mathematics there in 1979. He completed his Ph.D. in

zoology at UC Berkeley in 1987. He is currently working on California salmonids for the National Marine

Fisheries Service (NOAA) at the Tiburon/Santa Cruz

laboratory. He has also modeled the populations dynamics of spotted owls and corals, and has worked on

the ecological effects of invasions by non-native species of snails in California estuaries and of plants in

California grasslands.

Jeff Kelly is a Research Wildlife Biologist at the

Rocky Mountain Research Station in Albuquerque.

He received a Ph.D. in Zoology from the Department
130 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000

of Biology at Colorado State University in 1996, a

M.S. in Zoology from Oklahoma State University in

1991, and a B.S. in Wildlife Ecology from the University of Maine in 1987. Jeff’s research focuses on avian

ecology and current projects include: stopover biology of Neotropical migrants, distribution, abundance and habitat use of grassland birds, avian

biodiversity of golf courses, and effects of riparian

restoration on bird community structure.

Rob Marshall is Conservation Sciences Program

Manager for The Nature Conservancy’s Arizona

Chapter in Tucson. Rob oversees the Chapter’s research, monitoring, and restoration programs as

well as the Chapter’s ecoregional and site conservation planning projects. He is currently leading a

binational, interdisciplinary effort to identify conservation priorities and management needs in the

Sonoran Desert Ecoregion. Rob received a B.A. in

Human Ecology from College of the Atlantic and an

M.F.S. in Wildlife Ecology from the School of Forestry at Yale University. His academic training is in

avian ecology, wetlands, and wildlife management.

Prior to joining the Nature Conservancy Rob worked

on endangered species management and research

projects for the U.S. Fish and Wildlife Service, U.S.

Forest Service Rocky Mountain Forest and Range

Experiment Station, Cornell Laboratory of Ornithology, and Manomet Bird Observatory.

Tracy McCarthey received her B.S. degree from

Arizona State University in 1990, then completed

two years of graduate work at the University of

Arkansas. She is currently the Southwestern Willow

Flycatcher Project Coordinator at the Arizona Game

and Fish Department. Her current responsibilities

include: administration of a willow flycatcher survey

and nest monitoring project with over 30 hired employees, producing willow flycatcher management

recommendations, reviewing critical documents, and

synthesizing data from annual, state-wide willow

flycatcher surveys.

Richard D. Periman is a Research Archaeologist

with Rocky Mountain Research Station’s Cultural

Heritage Research Unit, where he has worked since

1995. He is currently writing his Ph.D. dissertation in

Environment Science and Technology, under the Department of American Studies at the University of

New Mexico. His research topic is the development of

an interdisciplinary approach for identifying, quantifying, and interpreting long-term landscape history

and human-induced environmental change. This study

focuses on the nature and extent of human-environmental interaction in order to understand better the

socio-ecodynamics of human activities and their possible continuing influence on the landscapes we see

today. He received a M.A. in medical anthropology

from the University of Montana in 1987 and a B.A. in

anthropology from the same university in 1985. From

1987 to 1995, Richard worked as the Heritage Resource Manager and Tribal Governments Coordinator

on the Deerlodge National Forest, in the Forest Service’s

Northern Region. He has conducted research ranging

from the socioculture aspects of malaria control to the

effects of historic mining on landscape development.

Mark K. Sogge is an Ecologist with the Colorado

Plateau Field Station, Flagstaff, Arizona; a unit of the

U.S. Geological Survey Forest and Rangeland Ecosystem Science Center. He has worked as a biologist for a

variety of federal agencies since 1983. He graduated

from San Jose State University, California with a B.A.

in Biochemistry, and obtained an M.A. in Zoology from

the University of California at Davis. His research

interests include avian monitoring programs and protocols, southwestern riparian bird communities, and

the ecology of endangered birds. He first studied willow flycatchers in 1988 in the northern Sierra Nevada

and southern Cascade Mountains. Since 1992, he has

been coordinating and conducting southwestern willow flycatcher studies throughout the southwest, and

in portions of its wintering range in Central America.

His research emphasis has been on flycatcher demography, breeding ecology, population genetics, and winter ecology and conservation.

Scott H. Stoleson is a Research Wildlife Biologist

with the Rocky Mountain Research Station in Albuquerque, New Mexico. He received a B.A. in Biological

Science from Dartmouth College, and a Ph.D. in Wildlife Ecology from Yale University. His research interests include breeding biology and demography of

birds, parrot biology and conservation, population

modeling, riparian conservation, conservation of nonforested tropical habitats, the biology of invasive exotic species, endangered species management, and

mycology. He has worked on birds and other wildlife in

the western U.S., Costa Rica, Panamá, Ecuador, and

Venezuela. His current research examines the breeding biology and habitat associations of southwestern

willow flycatchers and other riparian birds in the Gila

River Valley of New Mexico.

J.C. “Jamie” Uyehara received her B.S. degree

from University of California at Davis in 1981, and

then worked in positions involving exotic animal

management. Realizing that more information was

needed in order to manage and recover endangered

birds, she entered the graduate program at UCLA.

Her dissertation was “Correlates and field experiments of nest searching behavior in a brood parasite,

the brown-headed cowbird” (1996). Currently, she is

working as a wildlife biologist with the Los Padres
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 131

National Forest in central and southern California.

She still believes that research and good analyses are

needed when managing for endangered species, but

time is a luxury for both the endangered species and

the land managers.

Mary J. Whitfield is the research director at the

Southern Sierra Research Station. She obtained a B.S.

degree in Wildlife and Fisheries Biology from U.C.

Davis, and a M.S. degree in Biology from California

State University, Chico. Mary has worked on a longterm breeding ecology study of the southwestern

willow flycatcher since 1989. In addition, she has

studied the effects of a brown-headed cowbird trapping program (started in 1993) on the reproductive

success of the southwestern willow flycatcher. For the

last two winters, Mary has also worked on a wintering

willow flycatcher distribution study in Central America.
132 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000
Name Affiliation/Address

Janie Agyagos Coconino National Forest e-mail: jagyagos@fs.fed.us

Sedona Ranger District

P.O. Box 300

Sedona, AZ 86339

Jean-Luc E. Cartron Hawks Aloft, Inc. e-mail: jcartron@rt66.com

3101 Dakota NE

Albuquerque, NM 87110

Deborah Finch Rocky Mountain Research Station e-mail: dfinch@fs.fed.us

2205 Columbia SE

Albuquerque, NM 87106

Lloyd Goldwasser National Marine Fisheries Service e-mail: lloyd.goldwasser@noaa.gov

Southwest Fisheries Science Center

3150 Paradise Dr.

Tiburon, CA 94920-1211

Jeffrey Kelly Rocky Mountain Research Station e-mail: jkelly@fs.fed.us

2205 Columbia SE

Albuquerque, NM 87106

Rob Marshall The Nature Conservancy e-mail: rmarshtnc@aol.com

300 E. University Blvd., Suite 230

Tucson, AZ 85705

Tracy McCarthey Arizona Game and Fish Department e-mail: tmccarthey@gf.state.az.us

2221 W. Greenway Rd.

Phoenix, AZ 85023

Richard Periman Rocky Mountain Research Station e-mail: rperiman@fs.fed.us

2205 Columbia SE

Albuquerque, NM 87106

Mark Sogge U.S. Geological Survey e-mail: mark.sogge@nau.edu

Forest and Rangeland Ecosystem

 Science Center

Colorado Plateau Field Station

P.O. Box 5614

Flagstaff, AZ 86011

Scott Stoleson Rocky Mountain Research Station e-mail: sstoleson@fs.fed.us

2205 Columbia SE

Albuquerque, NM 87106

Jamie Uyehara Los Padres National Forest e-mail: jwilson@fs.fed.us

6755 Hollister Suite 150

Goleta, CA 93117

Mary Whitfield Kern River Research Center e-mail: wifl@lightspeed.net

P.O. Box 1316

Weldon, CA 93283
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and technology to improve management, protection, and use of the

forests and rangelands. Research is designed to meet the needs of

National Forest managers, Federal and State agencies, public and

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