NREL is a national laboratory of the U.S. Department of Energy

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Operated by the Alliance for Sustainable Energy, LLC

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

Fixed Point Raptor Migration 

Survey at the National Wind 

Technology Center

April 2010

John A. Du Waldt

Eco-Logic, LLC

Evergreen, Colorado

NREL Technical Monitor: Brenda Beatty

Subcontract Report

NREL/SR-1900-70460

June 2018
NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

National Renewable Energy Laboratory

15013 Denver West Parkway

Golden, CO 80401

303-275-3000 • www.nrel.gov

Fixed Point Raptor Migration 

Survey at the National Wind 

Technology Center

April 2010

John A. DuWaldt 

Eco-Logic, LLC

Evergreen, Colorado 

NREL Technical Monitor: Brenda Beatty 

Prepared under Subcontract No(s). P.O. 194304 

Suggested Citation 

Eco-Logic LLC. 2018. Fixed Point Raptor Migration Survey at the National 

Wind Technology Center: April 2010. Golden, CO: National Renewable 

Energy Laboratory. NREL/SR-1900-70460. 

https://www.nrel.gov/docs/fy18osti/70460.pdf.

Subcontract Report

NREL/SR-1900-70460

June 2018
This publication was reproduced from the best available copy

submitted by the subcontractor and received no editorial review at NREL.

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Final Summary Report 

April 2010 Fixed Point Raptor Migration Survey at the 

National Wind Technology Center 

Prepared for: 

National Renewable Energy Laboratory (NREL) 

NREL Technical Monitor: Brenda Beatty 

1617 Cole Boulevard, MS: 1532 

Golden, CO 80401-3393 

303-275-3114

September 14, 2011 

Revised May 9, 2013 

Submitted by: 

Eco-Logic LLC 

John A. DuWaldt, Principal Investigator 

1484 Belford Court 

Evergreen, CO 80439 

720-849-1740

303 674-1212 (fax) 

Jduwaldt@evcohs.com 
1

TABLE OF CONTENTS 

1.0 Introduction.........................................................................6 

2.0 Background.........................................................................6 

3.0 Objectives..........................................................................78 

4.0 Methods.............................................................................9 

5.0 Results..................................................................................10 

5.1 Analysis of Current Data...............................................11 

5.2 Comparison of Current Data and Historical Data...................22 

6.0 Discussion........................................................................26 

6.1 Resident Raptors Observed on the NWTC in 2010 and 1995.....27 

6.2 Migratory Raptors Observed on the NWTC in 2010 and 1995...27 

6.3 Flight Heights of Raptors in NWTC Airspace......................30 

6.4 Duration of Onsite Raptor Visits......................................31 

7.0 Conclusions...............................................................................32 

8.0 Recommendations...............................................................32 

9.0 References........................................................................35 

 
2

List of Tables 

Table 5.1 April 2010 Numbers of Raptors by Species Observed At the NWTC, By Date

Table 5.2 April 2010 Migrating Raptors Observed Within the NWTC Airspace 

Table 5.3 April 2010 Resident Raptors Observed within the NWTC Airspace 

Table 5.4 Comparison of Migrant Raptor Species Observed at NWTC, April 1995 

 versus April 2010 

Table 6.1 Comparisons of spring Migratory Raptor Observations between Years and Sites 

Table 6.2 Comparison of Observed Raptor Flight Heights in NWTC Airspace to Heights of 

 Onsite Aerial Structures 
3

List of Figures 

Figure 1.1 National Wind Technology Center Location Map 

Figure 5.1 Total Raptor and Vulture (Migrant+Resident) Frequency Ranking for April 2010, 

NWTC Observations 

Figure 5.2 Histogram of NWTC April 2010 Resident Raptor Visit Durations 

Figure 5.3 Histogram of Raptor Flight Height in Relation to Structure Height and Number 

Figure 5.4 DRRMS Total April and Spring Raptor Count 2004-2010 

Figure 5.5 DRRMS Recent Spring and April Migrating Raptors Per 

 Observation Hour 

 
4

List of Acronyms and Abbreviations 

BGEPA Bald and Golden Eagle Protection Act 

BMPs Best Management Practices 

DOE United States Department of Energy 

DRRMS Dinosaur Ridge Raptor Monitoring Station 

EO Executive Order 

HMANA Hawk Migration Association of North America 

m meters 

MBTA Migratory Bird Treaty Act 

met meteorological 

min. minutes 

MOU Memorandum of Understanding 

NREL National Renewable Energy Laboratory 

NWTC National Wind Technology Center 

USFWS United States Fish and Wildlife Service 

wk week 

 
5

 
6

1.0 Introduction

The U.S. Department of Energy’s (DOE) National Renewable Energy Laboratory (NREL) 

conducts a variety of research projects concerning renewable energy technologies at its South 

Table Mountain headquarters in Golden, Colorado; and wind energy research at the National 

Wind Technology Center (NWTC) located approximately 8 miles south of Boulder, Colorado, in 

northern Jefferson County (Figure 1.1). The NWTC provides a location with consistent winds 

and high speed gusts suitable for testing wind turbine technologies. It is operated by the Alliance 

for Sustainable Energy LLC for DOE as the premier wind energy research and testing center in 

the country (DOE, 2002). Currently the NWTC houses a variety of structures on the 114 hectare 

(290 acre) site, including 13 operating turbines and 24 meteorological (met) towers (personal 

communication, B. Beatty, 2010). Although there is a relatively small number of turbines at the 

NWTC compared to many commercial wind farms, the aerial structures at the NWTC pose a 

potential threat to resident and migratory birds, just as other instrument, telephone, and broadcast 

towers, and large-scale wind power generation facilities do (Erickson, et al., 2001; Manville, 

2005). As a result, to understand the site-specific risks, NREL is currently conducting a yearlong survey of avian and bat use patterns at the NWTC, supplemented by mortality surveys. 

That study consists of fixed-point avian use surveys, sitewide transects to assess abundance of 

nesting birds, surveys of springtime migrating raptors, and mortality surveys for birds and bats. 

This report details the results of a migratory raptor field survey conducted at the NWTC during 

the month of April, 2010 as part of the larger study of site use by birds, and the site’s effects on 

birds and bats. 

2.0 Background

Seasonal bird migrations are a world-wide phenomenon affecting a large number of species. In 

the United States bird species and other vertebrate groups exhibit a variety of distinct migratory 

habits thought to have developed as a result of seasonal changes in food availability (Newton 

2009). The United States Fish and Wildlife Service (USFWS) lists 1,007 of the 2,055 North 

American bird species residing in the United States as migratory (Federal Register, 2010; 

Chesser, et al., 2009). A noteworthy portion, and perhaps the best known migrations in North 

America, is the southward fall and northward spring bird migrations of which the Central Flyway 

portion of the North American Flyway is a significant part. North American migrants annually 

fly south to the southern United States, Central America, South America, and the Caribbean to 

overwinter, returning north to their nesting regions each spring. Colorado’s eastern Southern 

Rocky Mountain front is a linear path that raptors follow in the western portion of the Central 

Flyway. 

Some migratory movements are not obvious because they occur at night. Many songbird species 

utilize this method and their activity is known to us through the use of detection tools such as 

radio-tagging, radar, and night lights. Two groups, the diurnal raptors (herein referred to as 

raptors), Order Falconiformes, and the vultures, Order Ciconiiformes (storks, herons, etc.) 

Incertae sedis [of uncertain taxonomic placement] are adapted to daytime migratory flights 

(Newton, 2008; Remsen, et al., 2007). Raptors take advantage of rising sun-warmed air, called 

thermals, or disjunct topographic highs, such as mountain fronts, that force prevailing winds 

upward and aid in migratory flight. 
7

Figure 1.1 NREL National Wind Technology Center Location Map 

 Raptors use spread-wing soaring to gain altitude in currents of rising air, and then glide a 

distance, gradually losing altitude until they encounter the next thermal to regain gliding height. 

This technique minimizes energy-depleting flapping, and although rising and gliding is slower 

than flying by straight-line flapping, it uses only 5 to 25% of the energy consumed by flapping, 

reducing the physiological stress of migration, and reducing the number and duration of 

stopovers for recovery and feeding, which, in turn, reduces predation and other risks (Newton, 

2008). 

The eastern front of the Southern Rocky Mountains in central Colorado (i.e., the Front Range) is 

used annually by raptors residing in the intermountain west, particularly for spring migration 

north from lower latitudes. The eastern mountain front trends north-south and is set-off from the 

adjacent high plains by a precipitous rise of 3,000 feet in one mile. This juxtaposition of a flat 

heating surface (the plains) with the mountainous barrier to prevailing winds forms a corridor for 

raptor migration. It should be noted that a number of raptor species and numerous other bird 

species exhibit a circular or dual-route migration due to seasonal climatic variation (i.e., wind 

and storm patterns differ between spring and fall), changes in prey availability, changes between 

seasons, etc. (Newton 2008, Goodrich and Smith 2008). The result is that a species or group of 

species migrate south by one route and north using another, or they exhibit different 

concentration patterns at the migration terminuses. Specifically, when birds initiate migration 

their numbers often coalesce starting with a broad, diffuse front that tapers from a broader band 

into a narrow stream. One of the better known examples is the Central America landmass which 

is utilized as a narrow land route that affords a non-water crossing from North to South America. 
8

Large water body crossings are very costly to bird species, and many species go to great lengths 

to avoid them (Newton 2008). 

Where geography and/or climate provide thermals for raptors, as along the Front Range, 

dramatic concentrations of raptors occur in the spring as they use the consistent, moving air 

masses to gain altitude for gliding (Newton 2008). The Colorado Front Range exhibits spring 

thermals favorable to efficient northward flight that is not duplicated in the fall. In addition, the 

fall migration organizes as a broad front of individuals and family groups coalescing into a 

concentrated stream south of the northern/rocky mountain states. Similarly, the northern return 

flow through Colorado reflects the overlapping of individual flight paths south of Colorado so 

that favorable daily thermals can be utilized to conserve body fat as birds near their summer 

breeding areas. 

3.0 Objectives

Wind turbines, antenna, and Met towers are known sources of bird collisions and fatalities 

(Manville, 2005; Erickson 2001). Although the NWTC is a research facility, and not a 

commercial wind farm, there are a number of aerial structures present onsite that may pose a risk 

to birds, including migrating and resident raptors. Monahan (1996) conducted an initial raptor 

occurrence study during 1994-1995 to determine species presence and use of the facility. 

Monahan’s team recorded 786 diurnal raptors over a 17 month period, concluding that species 

using NWTC habitat were few in number and any collisions with structures or equipment would 

not affect regional populations. A second study conducted in 2002, compared onsite and offsite 

habitat use by avian species (Schmidt et al., 2003). Schmidt et al. recorded 212 raptor 

occurrences on and near the NWTC during a year-long study. Winter abundance was found to 

exceed summer abundance at the site, which follows the raptor abundance trend for the Boulder 

Valley as a whole (Berry and Bock 1998). Two species, the American kestrel (Falco sparverius) 

and the red-tailed hawk (Buteo jamaicensis), were observed most often at the NWTC during the 

2002 study. The study concluded that raptors are slightly more abundant at the NWTC during 

winter, and show a slight preference for perch structures at the site compared to structures on the 

surrounding landscape. Some evidence of raptor mortality at the NWTC was observed during the 

2002 study (six bird carcasses were recovered during mortality searches in 2001 and 2002, 

including one American kestrel). In addition, the 2002 Sitewide EA reported a rough-legged 

hawk fatality. Since that time, NREL ESH staff reports that the remains of two great horned 

owls (2008 and 2010) and one red-tailed hawk (2008) were found on the NWTC near met 

towers in the western portion of the site (personal communication, B. Beatty 2010). Schmidt et 

al. estimated that approximately 24 avian fatalities per year may occur at the NWTC, but very 

few would be raptors. 

The objective of the current study was to gain further understanding of raptor use of the NWTC 

during spring migration, and to compare the results to data collected in 1994-95 and 2002. This 

was accomplished by diurnal observation of raptor movement in and around the NWTC during 

the month of April, 2010 when raptor migration along Colorado’s Front Range peaks (Hawk 

Migration Association of North America [HMANA] 2010). Observations made during April 

2010 provide information on total numbers, species composition, flying times, heights, and 

directions. In addition, the NWTC observations are compared to the migrant raptor population 

observed at the Dinosaur Ridge Raptor Monitoring Station (DRRMS), an annual ornithological 

monitoring station located 8 miles south of the NWTC. 
9

4.0 Methods

The primary method applied to this study of migratory raptors was a fixed-point survey, a 

technique where recurring, daily observations are made from the same location (Erickson, et al., 

2007). The observer employed standard 8x50 binoculars and a 60X zoom spotting scope to 

make identifications at distance. The observation point, designated observation point 1A as 

shown in Figure 1.1, was located in the northwestern corner of the NWTC between Schmidt’s 

(2003) observation points Number 1 and 2. This location has GPS-defined latitude and longitude 

coordinates: 

N: 39° 54.658’ 

W: -105° 14.008’ 

Observations to locate raptors were conducted Monday through Friday from April 5th through 

April 30th, 2010. Daily observations were conducted for five hour periods during the hours of 

0900 through 1400. This time-frame coincides with daytime air heating and the development of 

thermals necessary for raptor migratory flights. Daily raptor migration behavior in temperate 

latitudes consists of morning roosting until air temperatures increase enough to create rising air 

thermals sufficient for soaring. The majority of daily soaring and gliding trails off in midafternoon when vertical air movement begins to decrease (Newton, 2008; HMANA, 2010). 

April raptor observations were conducted only during weekdays, Monday through Friday. The 

effect of weekday-only observations is an undercounting of spring raptors migrating past the 

NWTC. A simple undercounting assumption is that 6/20th or 30% of the weekly raptor passage 

was not observed due to no weekend counts [six weekend days or six twentieths of the study 

periods occurred during the four weekends in the month of April 2010]. The April 2010 average 

migratory raptor/hour passage observed at the nearby DRRMS was 3.1 raptors/hour on weekdays 

and 9.7 raptors/hour on weekend days (HMANA 2010). The highest daily April raptor count 

was Saturday April 10, when DRRMS recorded 98 raptors, or 22% of the total April tally. The 

large weekend percentage for the monthly tally for April 2010 suggests that a significant number 

of raptors observable from the NWTC were not tallied due to the survey schedule. A review of 

six previous years of DRRMS data from 2004 to 2009 shows that for each year the two days with 

highest April tallies account for 19%, 19%, and 32% of the monthly total. For 2009 the three 

days of highest values equaled 52% of the month’s observed migrants. These percentages 

suggest that missing a single day of the April migration could have a large effect on the monthly 

total. This difference is likely due to weather-mediated fluctuations in daily movement. 

Therefore a 30% correction is applied to the total hawk count estimate for the NWTC, yielding 

an estimate of 491 total raptor passing the site in April 2010. 

Raptor migration observations were tabulated hourly on standard data sheets. Information 

collected included species, number observed, height, direction and distance from observer, as 

well as weather conditions. Instances of raptors passing over or within NWTC air space were 

mapped on 7.5 minute topographic quadrangle reproductions tied to field data notes. Migrant 

raptors were distinguished from non-migrants as those exhibiting south to north flight, especially 

when combined with locations west of the NWTC and/or above or west of the mountain front. 

Field observations were transferred to the NREL natural resource database as Microsoft® Excel 
10

spreadsheets. Original field sheets were transferred to the NREL technical monitor as one of the 

project deliverables. 

5.0   Results

April 2010 raptor observations at the NWTC recorded both migratory and resident raptors. 

Table 5.1 shows the total daily raptor occurrence by date and species abundance. 

5.1 Analysis of Current Data 

Migratory and resident raptors and vultures were observed for 20 consecutive weekdays from 

April 5 to 30, 2010 to gain information on migratory visits to, over, or near the NWTC. In 

addition, this information was compared to raptor migration observations collected during the 

same time period at the DRRMS, a nearby monitoring post (eight miles south of the NWTC) that 

records the spring migration of raptors that pass south to north along the eastern edge of the 

Front Range. 

During 20 days of 0900 to 1400 “mid-day” observations at the NWTC a total of 378 

observations of 10 different raptor species were recorded (Table 5.1). This included 73 raptors 

that could not be identified to species (sum of three unidentified genera: Accipiter, Buteo, and 

Falco ), and 24 that could not be identified to genus (i.e., Unidentified Raptor). The unidentified 

raptors result from observations made during poor lighting and/or at distances that precluded 

species silhouette recognition. Typically this class of observations consists of migratory birds 

that occurred over the mountain front two miles west of the NWTC. An average of 18.9 or 19 

raptor observations per day were recorded from the NWTC, with a range of 3 to 42. It should be 

noted that this average includes both migratory and resident raptors. Resident raptors often made 

multiple appearances daily, particularly a pair of American kestrels and two pair of red-tailed 

hawks. Species frequency ranking for April 2010 NWTC observations is shown in Figure 5.1. 

The most abundant species observed was the turkey vulture (114 observations), followed by the 

American kestrel (85), and red-tailed hawk (65). Seven species were observed fewer than 10

times: golden eagle (7), osprey (3), bald eagle (2), Cooper’s hawk (2), sharp-shinned hawk (1), 

prairie falcon (1), and merlin (1). 
11

 

Table 5.1 April 2010 Numbers of Raptors by Species Observed At the NWTC, By Date (includes 

both Migrating and Resident Raptors)

Species 05 06 07 08 09 12 13 14 15 16 19 20 21 22 23 26 27 28 29 30 Total

Turkey  

Vulture

23 2 35 7 5    2 1 7 7 4 1 11 1    4 2    2    114

Osprey                  1       2 3

Bald Eagle           1 1                   2

Northern

Harrier

                     0

Sharp‐

shinned

Hawk

                 1       1

Cooper’s

Hawk

         1     1                 2

Northern

Goshawk

                     0

Broad‐

winged

Hawk

                     0

Red‐tailed

Hawk

5 1    7 6 4 1 3 5 5 1    3 4 1 3 10    5 1 65

Swainson’s

Hawk

                     0

Ferruginous

Hawk

                     0

Golden

Eagle

1       1 1         1    1 1    1         7

American

Kestrel

2 1 6 6 5 4 1 2 6 11 11 2 4 1 6 4 3 5 5 85

Merlin                1             1

Peregrine

Falcon

                     0

Prairie

Falcon

             1               1

Unidentified

Accipiter

     1                 1             2

Unidentified

Buteo

4 2 4 4 4 2    16 7 2 3    3     7 1    1 3 63

Unidentified

Falco

2    1 1 1                     3         8

Unidentified

Eagle

                     0

Unidentified

Raptor

2    1 2 2 5 2 2 3    4       1           24

Total By

Date

37 7 42 27 26 17 9 23 25 21 24 13 22 12 3 25 18 3 13 11 378

                        
12

  

Figure 5.1 Total Raptor and Vulture (Migrant+Resident) Frequency Ranking for April 2010

NWTC Observations  

Thirty- four percent of identified raptors observed (127 of 378) during the study period (i.e., 

during 95.5 hours of observation, weekdays only) were classified as migrants, an average of 6.5 

per day with a range of 1 to 40. Of the 10 species observed, the most abundant migrant observed 

from the fixed-point study location was the turkey vulture (114), followed by the red-tailed hawk 

(10), golden eagle (3), osprey (2), and one each: sharp-shinned hawk, Cooper’s hawk, merlin, 

prairie falcon, and bald eagle. In addition 97 unidentified raptor migrants were observed 

including: unidentified Buteo (32), unidentified raptor (10), unidentified Accipiter (2), and 

unidentified Falco (2). The unidentified species component of migrant observations is equal to 

36.5 % of all migrants observed. This contrasts with the unidentified portion of resident raptor 

observations, which represents only 20% of total non-migrant observations. The difference in 

these percentages is reasonable given that most of the April 2010 migrant raptors flew along the 

top of the mountain front to the west, at a substantially greater distance from the NWTC than 

most observed non-migrants and thus presented on average, a smaller, less resolvable image. 

During the same time period, DRRMS recorded 437 raptors representing 17 species during 105.5 

hours of observation, including weekends (HMANA 2010). 

114

85

65

7322111

0

20

40

60

80

100

120Number of April Observations

Raptor/Vulture Species
13

During April observations five migrant raptors entered the NWTC airspace during four 

observation events (i.e., three individuals and one pair). These occurrences are listed in Table 

5.2 with presence data including date, time of day, location, heights, behavior and duration. 

These five raptors represent three percent of the migratory raptors observed during the April 

survey; therefore 97% of observed migrating raptors (during this single season) were seen 

outside of the NWTC airspace and did not enter NWTC airspace (this does not account for 

migrants passing through the NWTC airspace on weekends). In 2010, the stream of migrating 

raptors tended to be west of the NWTC, this varies by year. The frequency of migrant passages 

through NWTC airspace in April 2010 averaged 1/week (wk) with an average duration of 4.25 

minutes (min.). Flight height of these migrants ranged from zero to 250 m. 

Table 5.2 April 2010 Migrating Raptors Observed Within the NWTC Airspace 

DATE SPECIES TIME 

(Duration) 

HEIGHT LOCATION and 

BEHAVIOR 

4/9/10 Unknown 

raptor 

1220-1222 

(2 min.) 

250 meters Gliding north to 

south over site 

4/15/10 Coopers 

Hawk 

0900-0910 

(10 min.) 

0-20 

meters 

Perched in 

Ponderosa Pine on 

NW corner of site; 

flies to LaFarge 

property and dives 

on passerines 

4/22/10 Merlin 1148-1149 

 (1 min.) 

60 meters Crossed W edge 

of site flying south 

to north 

4/30/10 Osprey(pair) 1116-1118 

 (2 min.) 

100 meters Flying SE to NW 

over northern third 

of site. Flew 

within 70 meters 

of Site 4.0 turbine 

rotor sweep 

The migrant raptors observed within the NWTC airspace included: one unknown raptor on April 

9; one Coopers hawk on April 15; one merlin on April 22; and a pair of osprey on April 30. Four 

of these five birds crossed the NWTC at altitudes ranging from 60 to 250 meters (m). Flight 

durations within the NWTC airspace ranged from one to two minutes. The April 15 visit by a 

Cooper’s hawk was the longest and lowest onsite visit by a migrating raptor, lasting 10 minutes 

and at heights ranging from 0 to 20 meters. This individual perched on a tree in the pine ridge at 

the northwest corner of the site, then flew to the adjacent LaFarge property west of the NWTC 

western boundary and dove on passerines (i.e., perching birds, or songbirds). This individual was 

the only migratory raptor observed hunting or perching on the NWTC in April 2010. Onsite 

visits and hunting activities vary by year, depending on weather and wind patterns. The pair of 

osprey was observed flying within 70 meters of the utility-scale turbine at site 4.0. 
14

During April 2010 DRRMS observers tallied four bald eagles, seven golden eagles, and two 

peregrine falcons in migration over the I-70/Morrison Hogback viewing station (HMANA 2010). 

Only half of these DRRMS observations occurred on days when the investigator for this study 

was onsite at the NWTC, and none of these migratory birds were observed. However, as 

discussed below, resident golden eagles and bald eagles were observed near the NWTC during 

the April 2010 survey. 

In contrast to the transient individuals shown in Table 5.2 and discussed above, three pairs of 

resident raptors: one pair of American kestrels and two pairs of red-tailed hawks, made frequent 

visits to the NWTC to perch, mate, and hunt. In addition, local turkey vultures periodically flew 

over the site. Seven other unidentified raptors made one to two minute passages of the site, and 

could likely have been the abovementioned residents observed in unfavorable light or other 

identification limiting conditions. Table 5.3 lists these occurrences along with visit duration, 

height, location, and behavior. 

In addition to the near-site nesting raptors discussed above, bald eagles, golden eagles, and 

peregrine falcons were observed and are known to occur in the vicinity of the NWTC during the 

breeding season (Monahan 1996). Monahan (1996) observed all three species in the Eldorado 

Canyon area within 2.5 miles of the NWTC. Fifteen bald eagle and 13 golden eagle transits of 

the site were also observed by Monahan (1996) during a 17-month survey, although none of the 

bald eagle observations occurred during the April 1995 portion of the study. Monahan was 

unable to establish actual numbers of eagles in the vicinity of the NWTC, however two winter 

afternoon roosting surveys at Eldorado Canyon observed eight and 18 eagles in 1995. During 

April 2010 two bald eagles were observed in flight north of the NWTC, and a pair of bald eagles 

was observed nesting in a plains cottonwood stand in the Coal Creek drainage channel 

approximately 2.5 miles northeast of the NWTC. The Coal Creek nesting pair were observed 

only from offsite locations and did not approach the NWTC during April observation periods. 

Local ornithologists report five breeding bald eagle pairs existed in Boulder County during 2008-

2010 surveys, including the Coal Creek pair (Hallock and Jones 2011). A nesting pair also exists 

nearby at Standley Lake, in Jefferson County. Local authorities also report from three to five 

breeding pairs of peregrine falcons currently in the county (Hallock and Jones 2011). 

Table 5.3 April 2010 Resident Raptors Observed Within the NWTC Airspace 

DATE SPECIES TIME

(Duration) 

HEIGHT 

(Meters) 

LOCATION 

and 

BEHAVIOR 

4/05/10 Red-tailed 

Hawk 

1025-1042 

(17min.) 

80 Roosting on

MET tower 

“ “ “ “ 1051-`1055 

(4min.) 

80 Top of MET 

tower 

“ “ Turkey 

Vulture 

1121-1125 

(4min.) 

50 Flying across

south edge of 

site 

“ “ Unknown 1110-1111 120 Flying across 
15

Falco (1min.) northern half 

of site 

“ “ Red-tailed 

Hawk 

1250-1307 

(17min.) 

80 Flies to met 

tower perch; 

joined by 

mate; copulate 

4/05/10 Unknown 

Falco

1334(1min.) 240 Fast glide 

south to north 

across site 

4/06/10 American 

Kestrel 

0910(1min.) Unknown Blown across 

site 

“ “ Red-tailed 

Hawk 

1125-1127 

(2min.) 

10 Hunting low 

on N edge of 

site 

“ “ Turkey 

Vulture 

1130(1min.) 20 Flying along 

E edge of site 

4/06/10 American 

Kestrel 

1220(1min.) 60 Blown across 

site SW to NE 

4/07/10 American 

Kestrel 

0940-1000 

(20min.) 

10-20 Perch on pole; 

flies to MET 

tower 

guywire; 

copulates; 

preens 

4/08/10 American 

Kestrel 

0917-0921 

(4 min.) 

20-60 Perched on 

guy wire 

“ “ American 

Kestrel 

0954-0956 

(2min.) 

20-40 Flies over 

central portion 

of site 

“ “ Red-tailed 

Hawk 

1039-1111 

(32min.) 

100-150 Perched on 

top of MET-2 

tower 

“ “ Red-tailed 

Hawk 

1052-1053 

(1min.) 

25 Soaring east 

side of site 

“ “ American 

Kestrel 

1139-1242 

(3min.) 

10 Flies in along 

S edge of site; 

perches 

guywire with 

mate 

“ “ American 

Kestrel 

1140(1min.) 0-10 Attempted kill 

“ “ Turkey 

Vulture 

1202-1204 

(2min.) 

320 Soaring across 

site 

4/08/10 Red-tailed 

Hawk 

1202-1204 

(2min.) 

100 Same as 

above 

4/09/10 American 0937-1025 30 Perched pair 
16

Kestrel (48min.) on MET-2 

tower 

arms/guywires 

“ “ American 

Kestrel 

1025-1058 

(33min.) 

3-30 Pair perched 

on telephone 

pole; female 

hunts SW 

corner of site 

“ “ “ “ 1136(1min.) 20 Fast flight 

across site 

“ “ Red-tailed 

Hawk 

1305-1310 

(5min.) 

80 Perched on 

MET-2 tower 

“ “ “ “ 1310(1min.) 120 Glides across 

site 

4/12/10 Red-tailed 

Hawk 

1012-1022 

(10min.) 

10 Perched in 

pine ridge 

“ “ American 

Kestrel 

1033-1100 

(27min.) 

20 Pair on 

eastern met 

tower guywire 

4/12/10 “ “ 1158(1min.) 30 Transit 

through site 

4/13/10 American 

Kestrel 

1107-1121 

(14min.) 

0-20 Perched on 

guywire 

4/13/10 “ “ 1345-1347 

(2min.) 

10 Flight across 

site 

4/14/10 American 

Kestrel 

1056-1058 

(2min.) 

10-20 Hunting hover 

SW corner of 

site 

4/15/10 Turkey 

Vulture 

1032(1min.) 50 Soaring along 

S border of 

site 

4/16/10 Red-tailed 

Hawk 

0900-0930 

(30min.) 

80 Perch on top 

of MET-2 

tower 

“ “ American 

Kestrel 

1040-1049 

(9min.) 

15-50 Flies S half of 

site and 

perches on 

telephone pole 

“ “ Red-tailed 

Hawk 

1305-1330 

(25min.) 

0-30 Perched on 

MET-2 tower, 

also observed 

on LaFarge 

water tower 

and SW cell 

tower 

“ “ “ “ 1332-1336 60 Soaring SW 
17

(4min.) corner of site 

“ “ American 

Kestrel 

1310(1min.) 5 Perched on 

NE side met 

tower 

guywire; 

joined by 

mate, fly off 

4/16/10 Red-tailed 

Hawk 

1342-1347 

(3min.) 

60-80 Perched on 

MET-2 tower; 

flies to the 

SW 

4/19/10 Red-tailed 

Hawk 

1013-1029 

(16min.) 

80 Perched 

MET-2 tower; 

sentinel 

“ “ Turkey 

Vulture 

1138-1141 

(3min.) 

20 Soars west 

side of site 

“ “ American 

Kestrel 

1316-1320 

(4min.) 

20 Pair perched 

on MET tower 

“ “ “ “ 1320-1334 

(14min.) 

25 Male 

successful 

hunt; returns 

to female; 

conducts 2nd

hunt 

“ “ Turkey 

Vulture 

1334(1min.) 200 Soars through 

center of site 

4/19/10 American 

Kestrel 

1343-1348 

(5min.) 

30 Perched on 

MET tower 

arm; flies to 

LaFarge 

telephone pole 

4/20/10 American 

Kestrel 

1105-1119 

(14min.) 

20 Perched on E 

side MET 

tower 

guywire; flies 

E side of site 

and leaves 

“ “ American 

Kestrel 

1250(1min.) 5-10 Crosses site 

4/20/10 American 

Kestrel 

1315-1327 

(12min.) 

15 Flew in from 

LaFarge area, 

then observed 

moving 

among met 

towers on east 

side of 
18

NWTC, 

apparently 

hunting 

4/21/10 Red-tailed 

Hawk 

1154-1205 

(11min.) 

2-20 Glides into 

SW corner of 

site and 

captures 

sm.mammal; 

flies to 

telephone pole 

and consumes 

“ “ American 

Kestrel 

1300-1310 

(10min.) 

20 S edge of site 

perched on 

power pole 

“ “ Red-tailed 

Hawk 

1224-1312 

(48min.) 

80 Perched top of 

MET-2 tower 

“ “ Turkey 

Vulture 

1258-1300 

(2min.) 

40 Crosses NW 

corner of site 

4/21/10 Unknown 

Buteo 

1322(1min.) 80 Soaring SE 

corner of site 

4/22/10 American 

Kestrel 

1029-1031 

(3min.) 

10 Hover S edge 

of site, 

hunting 

“ “ American 

Kestrel 

1045-1046 

(1min.) 

40 Transit west 

boundary of 

site 

“ “ American 

Kestrel 

1141(1min.) 10 Hover/stoop S 

edge of site 

“ “ Red-tailed 

Hawk 

1236-1247 

(11min.) 

80 Perch on 

MET-2 tower 

4/22/10 American 

Kestrel 

1340-1343 

(3min.) 

20 Power line 

perch SW 

edge of site 

4/23/10 Red-tailed 

Hawk 

0948-1000 

(12min.) 

10 Perched in 

pine ridge 

“ “ Unknown 

Raptor 

0935(1min.) 10 Perched in 

dead pine tree 

4/23/10 American 

Kestrel 

1015(1min.) 2 Perched on 

mullein S site 

boundary 

4/26/10 American 

Kestrel 

0915(1min.) 20 NE portion of 

site perched 

on met tower 

“ “ Turkey 

Vulture 

0925(1min.) 80 Soaring over 

W half of site 

“ “ American 1031-1034 20 Soaring over 
19

Kestrel (3min.) central portion 

of site 

“ “ Unknown 

Falco 

1055(1min.) 40 Flies across 

NW corner of 

site 

“ “ “ “ 1121-1123 

(2min.) 

100 Soaring 

overhead at 

100+m 

“ “ American 

Kestrel 

1308(1min.) 20 Perched on 

guy wire 

M1.2 tower 

4/26/10 Red-tailed 

Hawk 

1330-1333 

(3min.) 

30 Soaring over 

pine ridge 

4/27/109 Red-tailed 

Hawk 

0907-0923 

(16min.) 

80 Perched on 

M-2 tower 

“ “ “ “ 1125-1128 

(3min.) 

30 Flies in from 

SW to perch 

on M-2 tower 

“ “ “ “ 1152(1min.) Unknown Gliding SE 

corner of site3 

4/27/10 “ “ 1235(1min.) 60-80 Glides in from 

NW, perch on 

M-2 tower 

4/28/10 American 

Kestrel 

0915(1min.) 15 Pair on east 

side met 

station 

guywire 

4/29/10 American 

Kestrel 

1030(1min.) 2 Sunning on 

mullein site 

4.3 

“ “ Red-tailed 

Hawk 

1234-1236 

(2min.) 

10 Hunting low 

at SE corner 

of site 

“ “ American 

Kestrel 

1241-1243 

(2min.) 

10 Flying along 

N edge of site 

“ “ American 

Kestrel 

1259-1301 

(2min.) 

30 Flies through 

structures on 

eastern 

portion of site 

“ “ Red-tailed 

Hawk 

1313-1321 

(8min.) 

80 Perched on 

M-2 tower 

“ “ American 

Kestrel 

1317(1min.) 15 Hunting NW 

corner of site 

“ “ Red-tailed 

Hawk 

1331-1334 

(3min.) 

80 Perched on 

M-2 tower 

4/29/10 Red-tailed 1354-1356 80 Soars over 
20

Hawk (2min.) guard house 

4/30/10 American 

Kestrel 

1005-1007 

(2min.) 

15 Hover hunting 

S of M-2 

tower 

“ “ American 

Kestrel 

1340-1346 

(6min.) 

20 Hover hunting 

over SW 

corner of site 

Resident raptors were observed within the NWTC airspace a total of 528 minutes. (8.8 hours) 

during 20 weekdays in April 2010. This represents 9.2 percent of the 95.5 total hours of raptor 

observation conducted from Station 1A on the NWTC. A total of 82 resident raptor occurrences 

were recorded during the 20 five-hour survey periods or an average of 4 visits per day or 0.8 

visits per hour. Figure 5.2 shows the time distribution of resident raptor visits at the NWTC; an 

inverse relationship illustrating that in April 2010, residents made shorter duration visits much 

more often than long ones. The longest visit intervals were two 48 minute visits, one by an 

American kestrel, the other by a red-tailed hawk. American kestrels were the most frequent 

resident visitors to the NWTC, with 38 visits accounting for 40.7 percent of the total resident

visitor minutes. Twenty-eight red-tailed hawk visits constituted 54.9 percent of the total resident

raptor visit minutes, and in decreasing order the other species time percentages were: turkey 

vulture 2.8, unknown falcon 0.95, unknown Buteo 0.38, and unknown raptor 0.19. 

Flight heights of resident raptors visiting the NWTC are also listed in Table 5.3. Heights range 

from 0 to 320 m for the three resident species (American kestrel, red-tailed hawk, and turkey 

vulture) and three classes of non-specific raptors observed (unknown Falco, unknown Buteo, and 

unknown raptor). Figure 5.3 is a histogram showing the 40 m-increment flight height classes for 

raptors compared to the number of towers and guywires within the height class. Figure 5.3 

double-counts structures and wires from the 41 to 80 and 81 and greater height classes into the 0 

to 40 m class because the taller classes also overlap the shorter classes (a 50 m tower occupies 

space from zero to 40m as well as from 41 to 50 m). The data show that, during this survey, 

raptors made twice as many visits at heights 40 m and under and only one-fifth of the visits at 

heights 80m and greater. The majority of resident raptor flights in NWTC airspace occurred at 

heights similar to the majority of towers, structures and associated guywires. For this analysis 

guywires are treated as separate structures because: 1) they are spatially distinct from towers they 

support by several meters, 2) they are more visually cryptic than tower structures, raising a 

collision concern, and 3) they were utilized frequently for perching by American kestrels. 
21

0

5

10

15

20

25

30

35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 20 25 27 30 32 33 48

Number of Visits

Length of Visit (Minutes)

Figure 5.2 Histogram of NWTC April 2010 Resident Raptor Visit 

Durations

0

10

20

30

40

50

60

0‐40 41‐80 >80

Number of Resident Raptor Flights

and Structures

Height Classes (m)

Figure 5.3 Histogram of Raptor Flight Height in Relation to

Structure Height and Number

Raptors

Towers and Guywires
22

5.2 Comparison of Current and Historical Data

Monahan (1996) reports on 140 hours of migratory raptor observations at the NWTC in April 

1995 that resulted in 290 observations of 14 raptor species. During the same timeframe the 

DRRMS recorded 2319 migrant raptors of 17 different species (Monahan 1996). Turkey vultures

were the most numerous sighting at the NWTC in 1996, comprising 56% of April 1995 migratory

raptor observations. American kestrels were the second most frequently observed species, 

equaling 29% of observed migrants. During the April 2010 NWTC raptor survey a total of 127 

migrating raptors representing ten species were observed during 95.5 hours of observation. For 

the same period DRRMS recorded 437 raptors representing 17 species during 105.45 hours of

observation (HMANA 2010). At the DRRMS site, there were also differences in the number of 

raptors/observation hour in 1995 compared to 2010. In 1995, the raptor frequency was 16.6 

raptors/observation hour, while in April 2010, the rate was 4.14 raptors/observation hour. Factors 

contributing to these variable results are presented in Section 6, Discussion. 

Table 5.4 shows the variability between April 1995 and April 2010 NWTC migratory raptor

observations by species. The table shows that the turkey vulture is the most abundant migrant 

raptor during April in both years, and the American kestrel is second in abundance in 1995, but 

not recorded as a migrant in April 2010. A partial explanation of these results is that 60% of 

migrant American kestrels were observed on weekend days at DRRMS in April 2010 when the 

NWTC observer was not onsite. (Note that Tables 5.1 and 5.4 differ in the number of American 

kestrels reported for 2010. Table 5.1 reports all members of each species observed [migrants and

residents]; whereas Table 5.4 lists only migrants. Residents are individuals who make repeated 

appearances at the NWTC, thus total numbers of American kestrels in Table 5.4 is a subset of 

those totaled in Table 5.1.) As listed in Table 5.4, the next most abundant migrant raptors in 1995 

were red-tailed hawk, Cooper’s hawk, osprey, and unidentified Falco. In 2010 the most abundant 

species by rank after unidentified Buteos were red-tailed hawk, unidentified raptor, golden eagle, 

osprey, unidentified Accipiter, and unidentified Falco. 

Another source of historical data that demonstrate annual variability is the spring migration 

record from DRRMS. Internet sources provided the DRRMS spring raptor migration records for 

six recent years: 2004-2006, and 2008-2010 (HMANA 2010). Figure 5.4 compares the total 

raptor counts for the season, and raptor counts during the month of April during 2004 – 2006 and

2008 – 2010. Figure 5.5 compares the number of raptors counted per observation hour for the 

entire season, and for the month of April, during 2004 – 2006, and 2008 – 2010. Comparing this 

multi-year information with the 2010 migration allows provides additional data regarding the

variability of spring raptor migrations along the Colorado Front Range. 
23

Table 5.4   Comparison of Migrant Raptor Species Observed at NWTC

    April 1995 versus April 2010

Species April 1995 Number 

Observed 

April 2010 Number 

Observed 

Turkey  Vulture 162 60

Osprey 4 2

Bald Eagle 0 1

Northern Harrier 1 0

Sharp‐shinned Hawk 1 1

Cooper’s Hawk 8 1

Northern Goshawk 3 0

Broad‐winged Hawk 2 0

Red‐tailed Hawk 9 10

Swainson’s Hawk 3 0

Ferruginous Hawk 0 0

Golden Eagle 1 4

American Kestrel 85 0

Merlin 1 1

Peregrine Falcon NL* 0

Prairie Falcon 2 1

Unidentified Accipiter 2 2

Unidentified Buteo 3 32

Unidentified Falco 4 2

Unidentified Eagle 1 0

Unidentifed Raptor 0 10

Total Migants 290 127

Total Migrants/Observation Hour 2.07 1.27

*NL – Not listed in Monahan (1996)
24

 Source: Hawk Migration Association of America, 2010 Figure 5.4 DRRMS Total April and Spring Raptor Count 2004 -2010 

0

500

1000

1500

2000

2500

3000

2004 2005 2006 2008 2009 2010

Spring Total

April Total
25

 Source: Hawk Migration Association of America, 2010

Figure 5.5 DRRMS Recent Spring and April Migrating Raptors Per Observation 

 Hour 

0

2

4

6

8

10

2004 2005 2006 2008 2009 2010

Spring Observed

Raptors /Hour

April Observed

Raptors/Hour
26

6.0   Discussion

Wind energy development has been shown to pose a risk to birds, and raptors, in particular, from 

collisions with turbine blades as well as met tower guy wires (Erickson 2001). Raptors, as well 

as other migratory birds, are protected by the federal Migratory Bird Treaty Act (MBTA, 16 

USC 703-71L). Eagles are protected not only by the MBTA, but also by the Bald and Golden 

Eagle Protection Act (BGEPA [16 USC I.S.C. 668a-668b]). In addition, there is an Executive 

Order (EO 13186, 2001) which outlines the Responsibilities of Federal Agencies to Protect 

Migratory Birds. In response to this EO, the US FWS and DOE co-signed a Memorandum of 

Understanding (MOU) regarding the implementation of this EO (DOE and FWS, 2006). 

Because of the collision risks, and the regulatory protection of these species, studies have been 

undertaken at the NWTC to document the use of the site by birds, including raptors. Although 

the NWTC is a research facility, and not a wind farm, the presence of aerial structures onsite 

(turbines and guyed met towers) pose a risk to these species, particularly given the topographical 

overlap with known northward spring migration routes, and known raptor nesting locations in the 

area. The data presented in this report were collected to document site use by spring migratory 

and resident raptors, and their behaviors in relation to onsite aerial structures (i.e., turbines and 

met towers), and for comparison to historic studies and nearby raptor census counts, to provide 

insight regarding collision risks. 

Diurnal raptors and their ecological relatives, owls and vultures, occur at and near the NWTC as 

both migrants and residents (it should be noted that a year-long avian survey at the NWTC 

(2010 – 2011) documented the presence of great-horned owls onsite. The raptor survey in April 

2010 only conducted observations during the middle of the day, and did not resume at dusk to 

survey for owls). The raptor survey conducted at the NWTC in April 2010 provides data 

regarding raptors that were observed flying over the site, as well as those that flew outside site 

boundaries. A similar survey was conducted by Monahan (1996), and the results of both surveys 

are compared in this report. Further, in both 1995 and 2010, raptor counts were conducted at the 

nearby DRRMS site, located 8 miles to the south of the NWTC, and the results of those surveys 

are also discussed in this report for comparison purposes. 

In the results sections (5.1, Analysis of Current Data, and 5.2, Comparison of Current and 

Historic Data), data are compared related to the following topics: 

 Numbers and species of migratory and resident raptors, observed from the NWTC and at 

the DRRMS, in 2010, and in 1995 

 Numbers and species of migratory and resident raptors observed flying within NWTC 

airspace in 2010 and in 1995 

 Flight heights of migratory and resident raptors observed on the NWTC 

 Duration of onsite visits 

 Behavior of migratory and resident raptors observed on the NWTC 

 Migration routes in 2010 and 1995 

A discussion of these comparative results is presented below: 
27

6.1 Resident Raptors Observed on the NWTC in 2010 and 1995

The April 2010 spring raptor migration observations demonstrate that resident species actively 

utilize the NWTC for hunting, perching, mating, loafing, and sentinel posting. In 2010, the 

NWTC was within the breeding territories of two red-tailed hawk pairs, and one pair of 

American kestrels. All three breeding pairs made frequent visits to the NWTC during April 

2010, and resident turkey vultures also regularly flew over the site. Two occupied red-tail hawk 

nests within one-half mile of NWTC were observed during April, but nests of the American 

kestrels and turkey vultures were not observed. These resident raptors were observed within the 

NWTC airspace nearly 10 percent of the 95.5 hours of observation during April 2010, averaging 

4 visits per day or 0.8 visits per hour. American kestrels were the most frequent onsite visitors. 

In 1995, Monahan also noted resident red-tailed hawks and American kestrels perching on the 

NWTC on a regular basis, and also noted perching resident prairie falcons and rough-legged 

hawks. 

In addition to the near-site nesting raptors discussed above, bald eagles, golden eagles, and 

peregrine falcons were observed and are known to occur in the vicinity of the NWTC during the 

breeding season. During April 2010 two bald eagles were observed in flight north of the NWTC, 

and a pair of bald eagles was observed nesting in the Coal Creek drainage channel approximately 

2.5 miles northeast of the NWTC. Local ornithologists report five breeding bald eagle pairs 

existed in Boulder County during 2008-2010 surveys, including the Coal Creek pair (Hallock 

and Jones 2011). A nesting pair also exists nearby at Standley Lake, in Jefferson County. Local 

authorities also report from three to five breeding pairs of peregrine falcons currently in the 

county (Hallock and Jones 2011). In 1995, Monahan observed all three species in the nearby 

Eldorado Canyon area, and also recorded 15 bald eagle and 13 golden eagle transits of the 

NWTC during his 17 month survey. EPA (accessed onsite 2013) reports that Home range sizes 

of bald eagles vary widely depending on the area, season, availability of and distance to food 

resources, and the breeding status of the individual. For example, breeding eagles in 

Saskatchewan utilized home ranges greater than 1,730 acres in size, and average home range 

sizes during breeding season on the Columbia River, OR, was 5,337 acres. A radio-tracked 

immature bald eagle (non-breeding) from the Southwest U.S. was shown to use summer ranges 

of more than 13.6 million acres, each, with winter home ranges of more than 5 million acres. As 

such, the NWTC is within the territories of nearby nesting or winter roosting populations of these 

raptors, even though none were observed onsite during the 2010 survey. 

6.2 Migratory Raptors Observed on NWTC in 2010 and 1995 

The April 2010 spring raptor migration observations showed that migratory species, as well as 

residents, also utilize the NWTC for perching, resting, and hunting, although in 2010, only 5 

migrants were observed passing within the NWTC airspace. For comparison, the April 1995 

raptor survey at NWTC reported 78 raptor overflights of the site (Monahan 1996). The NWTC 

migratory overflight numbers in 1995 were12-times higher than overflight numbers observed in 

April 2010 (0.5 raptor/hour [1995] versus 1 raptor/25 hours [2010]). 
28

A comparison of the numbers and species of raptors observed during spring migration in 1995 

and 2010, from observation points at both the NWTC and DRRMS is shown in Table 6.1. 

Table 6.1 Comparisons of Spring Migratory Raptor Observations between Years and 

 Sites 

NWTC 1995 DRRMS 

1995 

NWTC 2010 DRRMS 

2010 

Hours of observation 140 140 95.5 105.5 

Number of observations 290 2319 127 437 

Number of species 14 17 10 17 

Raptors/observation hour 2.07 16.6 1.32 4.14 

Most abundant observed Turkey vulture Not avail. Turkey vulture Not avail. 

2nd most abundant observed Am. kestrel Not avail. Red-tailed 

hawk 

Not avail. 

3rd most abundant observed Red-tailed 

hawk 

Not avail. Unid. raptor Not avail. 

Onsite data as well as data collected by the Rocky Mountain Bird Observatory since 1995 

demonstrate that large numbers of raptors migrate northward in the spring along the Colorado 

Front Range, although the northward path in relation to the NWTC is variable (i.e., in some years 

large numbers of raptors have been shown to occur to the west or the east of the NWTC, or within 

the airspace of the NWTC). 

A comparison of on- and offsite data from 1995 and 2010 demonstrate variability in numbers, 

species, and locations of spring migrants observed (Table 6.1). In both 2010 and 1995, the 

numbers of species and individuals observed at DRRMS were greater than those observed at the 

NWTC. 

There are several potential explanations in such variability in the numbers of April raptor 

observations at both the NWTC and DRRMS. For example, in 2010, differences in numbers and 

species observed between the NWTC and the DRRMS could be attributable to the lack of 

monitoring on weekend days at the NWTC. In fact, the highest daily April raptor count was 

Saturday April 10, when DRRMS recorded 98 raptors, or 22% of the total April tally. In 

addition, 60% of migrant American kestrels were observed on weekend days at DRRMS in April 

2010 when the NWTC observer was not onsite, while none were observed from the NWTC 

observation point. In 1995, another explanation put forth by Monahan (1996) is that largebodied raptor species that rely heavily on updrafts were frequently observed passing along the 

hogback or the initial ridgetop of the mountain top or further to the west. These topographic 

influences could result in some species preferentially traveling west of the NWTC, and/or the 

hogback depending on weather patterns in a given year. 

At DRRMS, although the same number of species was observed in 1995 compared to 2010, the 

number of observations decreased from 2319 to 437. The Hawk Migration Association of North 

America (HMANA 2010) provides data on the internet for all observations recorded at the 

DRRMS for several years since 2001. From six recent years of recorded data (2004-2006 and 

2008-2010), April observations at DRRMS dropped from a high of 2000 observations to less 
29

than 500 during that timeframe. One possible explanation for variability in counts at DRRMS is 

that, in the mid-1990s, observers at the DRRMS were paid employees, onsite daily from early 

morning until dusk. Starting in 2005, observations were made by volunteers, rather than paid 

employees, and in recent years, there have been fewer volunteers than in earlier years. 

Volunteers would be less likely to occupy the site for such long hours, every day from midMarch through mid-May (personal communication, J. Birek, 2013). 

Another explanation is the annual variability observed in spring migratory raptor routes along the 

Front Range of Colorado. These migration routes can vary from year to year based on wind and 

storm patterns. For example, if the wind patterns in a particular year tend to come from the 

southeast, then raptors normally moving through the eastern plains would be blown westward, 

and tend to “pile up” along the hogback (personal communication, J. Birek, 2013). These 

variable wind and storm patterns can result in migration routes that can concentrate over the 

western foothills, over the hogback, or east of the hogback (including the airspace over the 

NWTC) out to the eastern plains. In 2010, 45% of observed migrating raptors passed the site 

greater than two miles west of the NWTC, while 48% were observed at intermediate distances (0 

to 2 miles) west of the NWTC, and seven percent east. In contrast, Monahan’s April 1995 

observations show the reverse trend: 30% of raptors observed in a distance tally were located at 

greater than two miles west, 6% were observed from 0 to 2 miles west, and 34% were observed 

to the east of the NWTC. 

In addition, on a regional and national basis, trends showing declines in raptor numbers are 

emerging. For example, the average annual number of spring migrants passing DRRMS during 

from 2004 – 2010 was 1570, while Goodrich and Smith (2008) report the annual average at 

DRRMS from 1990 to be 3908 raptors. This is similar to four other intermountain west migration 

monitoring sites; the average annual spring migration count for this set of locations is 3557 

raptors, very similar to the DRRMS annual average. However, in the last three years (2008 to 

2010) total numbers have not reached 1000. In addition, according to analyses conducted by the 

Raptor Population Index (2010) Project, data show that across 28 North American raptor 

monitoring sites from 1995 to 2005: American kestrel numbers are declining at 82% of the sites 

(although this is more prevalent in the eastern US, due to habitat destruction and reduced open 

hunting spaces [personal communication, J. Birek, 2013]); turkey vulture is increasing at 57%, 

red-tailed hawk is decreasing at 65%, golden eagle is decreasing at 50%, Coopers’ hawk is 

decreasing at 54%, and bald eagle is decreasing at 25% of the sites. 

Given that migration is still not completely understood by scientists, it is recognized that there 

are likely other explanations for variability in numbers and species observed at the NWTC and 

DRRMS, such as variable weather patterns, habitat changes, and other factors not yet 

understood. Scientists are still studying migration cues, strategies, mode of flight, weather 

pattern influences, and what guides migration. While this makes generalizations difficult, future 

variability in the numbers of migrants passing through NWTC, whether greater or less than those 

observed in 2010, should be expected. 
30

6.3 Flight Heights of Raptors in NWTC Airspace

During April 2010 observations, a total of five migrant raptors entered the NWTC airspace and 

were observed at heights ranging from 0 to 20 m (0 to 65 ft) while hunting, and 60 to 250 m (197 

to 820 ft) while flying through the area. The Cooper’s hawk, observed hunting, spent 10 min. 

onsite, while the other species passing through the site spent 2 min. or less. Resident raptors 

(American kestrel, red-tailed hawk, turkey vulture, unidentified Falco, unidentified Buteo, and 

unidentified raptor) were observed much more frequently onsite (82 raptor observations totaling 

8.8 hours of time onsite during weekday observations). Flight heights for the resident raptors 

ranged from 0 to 320 m (1050 ft). 

For comparison purposes, the height of the shortest onsite turbines (measured from ground to 

bottom of nacelle) are 9m (30 ft.) with a 3.7m (12 ft.) rotor diameter. The rotor on this size 

turbine reaches approximately half the diameter (i.e., 1.85m, or 6 ft. above and below the nacelle, 

or 7.3m to 10.85m [24 to 36 ft.] above the ground surface [ags]). The height of the tallest onsite 

turbine is 90m (295 ft.) with a 100m (328 ft.) rotor diameter. The rotor on this size turbine 

reaches approximately 50m (164 ft.) above and below the nacelle, or 40m to 140m (131 to 459 

ft.) ags. Met towers range from 30’ high, with one level of guy wire connections and 3 anchors, 

to 443’, with 6 levels of guy wire connections, and 6 anchor points.

Table 6.2 Comparisons of Observed Raptor flight Heights in NWTC Airspace to 

 Heights of Onsite Aerial Structures 

 Migrant 

flight 

heights 

when 

hunting 

(ft) 

Migrant 

flight 

heights 

when 

flying 

through 

the area 

Resident 

flight 

heights 

(ft) 

Shortest 

turbine 

heights 

(ft) 

Range 

of rotor 

diameter 

above 

the 

ground 

Tallest 

turbine 

heights 

(ft) 

Shortest 

met 

tower 

(ft) 

Highest 

met 

tower (ft) 

shortest 

ht. 

0 197 0 30 30 

Highest 

ht. 

65 820 1050 295 443 

shortest 

blade ht. 

ags 

 24 131 

Highest 

blade ht. 

ags 

 36 459 

Levels 

of 

guywires 

 6 

No. of 

anchors 

 3 6 
31

Comparison of observed flight heights of both migratory and resident raptors with heights of 

onsite aerial structures indicates that raptors fly at heights consistent with risks of collision with 

turbine blades or met tower guy wires. 

6.4 Duration of Onsite Raptor Visits 

In the “Analysis of Current Data” section of this report (Section 5.1), Figure 5.2 shows that 

resident raptors made shorter duration visits much more frequently than long ones, with 58 raptor 

visits of 5 min. or less, 15 visits from 5 to 17 min. duration, 6 visits from 20 to 33 min. duration, 

and two raptor visits of 48 minutes. Short visits of resident raptors were likely to be mostly 

flights through or transiting the site, as well as mating and hunting activities, while longer 

duration visits during this survey were primarily perching events which did not involve much 

more flight time than the short duration transits. Raptors were observed perching on tall 

vegetation, poles or posts, trees, met towers, and guy wires. Migrant raptor visits to the site in 

2010 ranged in duration from 1 to 2 min. for those raptors transiting the site, to 10 min. for one 

raptor involved in hunting activities. The small number of migrants observed onsite in 2010 did 

not engage in hunting (other than one Cooper’s hawk), courtship, or territory defense behaviors 

because 1) their movement appeared to be concentrated several miles to the west over 

mountainous terrain, resulting in very few visits to, or overflights of, the site (based on one 

season of observation), and 2) the larger, soaring species do not feed regularly during migration, 

minimizing the need to visit the site. Although larger raptors such as eagles, vultures, and 

ospreys feed infrequently during migrations, small-winged raptors such as the falcons and 

accipiters, feed more regularly. Larger raptors do not rely on powered flight except to reach 

gliding altitude and thus require fewer calories to successfully migrate. Smaller winged, powered 

flight species expend much more energy per unit mass to such an extent that they migrate 

synchronously with song bird prey populations (Newton 2008). 

Superficially, it may seem like less time spent onsite would reduce the risk of collision with 

aerial structures or guy wires. However, raptor behavior must be considered along with time 

spent onsite to understand the overall risk to onsite raptors from structures and guy wires. For 

example, when a raptor sees a prey object, the tendency is to lock eyes on the object while flying 

towards it, and not pay attention to other landscape features. As such, a reduced amount of time 

onsite would not necessarily correlate with a reduced risk of collision. In addition, stochastic, or 

random, events such as guywire collisions are proportional to exposure, such that the probability 

of collision would likely increase in proportion to increases in the number of guywires. 

During this survey, resident raptors engaged in semi-constant use of the NWTC landscape, 

particularly tall towers and guywires. Although no collisions or dead birds were observed from 

collisions with onsite structures in April 2010, from 2008 to 2010 two nocturnal raptors (great 

horned owl [Otis asio]) and one diurnal raptor (red-tailed hawk [Buteo jamaicensis]) fatalities 

occurred on the western portion of the NWTC. In addition, an injured Swainson’s hawk (Buteo 

swainsoni) was found beneath the utility-scale turbine at site 4.2, on the eastern portion of the 

site, and was transported to a raptor rehabilitation facility located in Broomfield, Colorado 

(Beatty, personal communication). 
32

7.0   CONCLUSIONS

Both resident and migratory raptors were observed during the 1995 and 2010 surveys to fly 

through the NWTC airspace or to use onsite habitat for hunting, perching, mating, or sentinel 

posting. In addition, nearby nesting sites and roosting were documented during both surveys for 

bald eagles, golden eagles, and peregrine falcons, in both Jefferson and Boulder counties. While 

no eagles or peregrine falcons were observed using the site during the weekday only surveys in 

2010, they were observed onsite in 1995 (Monahan 1996). Nearby nesting locations of eagles 

and peregrine falcons, coupled with large home ranges and hunting territories, and onsite 

observations outside of survey periods, indicate site use does occur by these large raptors. 

Comparison of onsite data from 1995 and 2010 show that onsite use by raptors is variable on a 

year-to-year basis. Numbers of raptors observed at the nearby DRRMS have also been shown to 

be variable. Factors affecting variability include presence/absence of onsite observers 

throughout the survey periods, fewer hours of onsite observation in 2010 compared to 1995 (i.e., 

95 hours vs 140 hours, respectively), wind and storm patterns, shifts in routes of migration along 

the Front Range, and other less understood factors. Low numbers of migratory raptors (i.e., 5) 

used the site during April 2010, while higher numbers of resident raptors (i.e., 81 separate 

observations) used the site in 1995. 

Both migratory and resident raptors using the NWTC were observed in 2010 to fly through the 

site at heights of onsite aerial structures (turbine rotor swept areas, and guyed met towers). In 

2010, while only 5 migratory raptors were observed onsite, resident raptors were observed using 

the site nearly 10 percent of the observation time. During this time, shorter duration visits 

(transiting the site, on onsite hunting/mating activities) occurred much more frequently than 

longer duration site visits (mostly associated with perching events). While shorter duration visits 

may seem to indicate reduced risk of collision, raptor behavior during hunting activities is more 

likely to increase the risk of collision. 

Although no collisions or dead birds were observed from collisions with onsite structures in 

April 2010, from 2008 to 2010 two nocturnal raptors (great horned owl) and one diurnal raptor 

(red-tailed hawk) fatalities occurred on the western portion of the NWTC, and an injured 

Swainson’s hawk was found beneath one of the utility-scale turbines on the eastern edge of the 

site (Beatty, personal communication). While these numbers may appear low, it should be noted 

that both the Migratory Bird Treaty Act and the Bald and Golden Eagle Protection Act prohibit 

“take” of individuals, and do not address population level effects. 

The results of the 2010 survey represent the April mid-day, weekday portion of Spring raptor 

observations for one year, and because of annual variations, does not represent the norm or 

average springtime raptor presence at the NWTC. That is, numbers of migratory and resident

raptors using or flying over the NWTC could be greater or less than 2010 in future years. 

8.0 Recommendations

As a federal agency, DOE (and therefore, NREL) has an obligation to comply with other federal 

laws (MBTA, BGEPA), Executive Orders (13186), and Agency Directives, and to work 

collaboratively with other federal agencies (e.g., USFWS). In addition, in response to these (and 

other) regulatory requirements, NREL has developed policies and procedures that address 
33

environmental stewardship, protection of natural resources, and compliance with environmental 

laws. These Policies and Procedures include the following: 

 Policy 6-2, Environmental Protection, which commits to onsite continual improvement in 

environmental and sustainability performance regarding environmental stewardship 

practices, 

 Policy 2-7, Sustainable NREL, which stresses managing the impact on the environment 

caused by the placement and general design of NREL structures, and maintaining, 

protecting, and restoring natural environments to sustain natural and native ecological 

systems, 

 Procedure 6-2.2, National Environmental Policy Act Implementation, which requires 

compliance with other federal laws, Executive Orders, etc., and 

 Procedure 6-2.21, Natural Resource Conservation, which commits to avoiding, 

minimizing, and mitigating impacts to natural resources, and to comply with NEPA. 

Further, two Memoranda of Understanding have been signed by DOE that address the protection 

and conservation of natural resources. These include the following: 

 MOU between DOE and FWS Regarding Implementation of EO 13186, “Responsibilities 

of Federal Agencies to Protect Migratory Birds” (OMS 2001), which includes the 

following DOE responsibilities: 

o Implementing management practices that avoid or minimize adverse effects on 

migratory bird populations, and their nesting, foraging, migration, staging, or 

wintering habitats, including the use of effective techniques to minimize the risk 

of collisions with structures, and 

o Consider FWS recommendations and suggested practices regarding energy 

development to avoid or minimize direct and indirect effects on migratory birds 

and their habitats. 

 MOU between the Trustee Council for Natural Resources at Rocky Flats and DOE’s 

Office of Energy Efficiency and Renewable Energy (2009), which commits EERE to the 

following: 

o Manage and operate the NWTC consistent with the Natural Resource 

Conservation Plan (NREL Procedure 6-2.21), and 

o Take appropriate action to mitigate adverse impacts of NREL development 

activities on natural resources at the NWTC. 

These obligations emphasize the importance of implementing procedures at the NWTC to avoid 

and minimize additional mortalities to resident and migratory raptors onsite. Based on the 

results of onsite raptor surveys and the above obligations, recommendations include the 

following: 

o Implement and follow the practices outlined in the FWS Land-Based Wind Energy 

Guidelines (USFWS 2012), including the use of bird diverters or other visibility markers 

on guy wires, 
34

o Conduct onsite monitoring after incorporating the Best Management Practices (BMPs) in 

the FWS Land-Based Wind Energy Guidelines, to evaluate effectiveness 

o Develop an Avian Protection Plan for the NWTC 

o Mow in the vicinity of aerial structures, and remove large rock piles that attract small 

mammal prey species 

o Install alternate raptor sentinel stations offsite in a food rich area to lure raptors away 

from the NWTC. 

The development of the FWS Land-Based Wind Energy Guidelines was a collaborative effort 

among federal agencies (including DOE), wind industry representatives, and conservation 

organizations. These guidelines include BMPs recommending the use of monopole, rather than 

guyed towers (if towers must be guyed, then bird diverters or other visible marking devices 

should be used), using tubular towers to reduce ability of birds to perch and reduce risk of 

collision, placing power lines underground, using the minimum number of met towers, using red, 

or red and white flashing lights, not steady burning bulbs, extinguishing all internal turbine 

nacelle and tower lighting when unoccupied, and removing turbines and guyed met towers that 

are not in use. 

The effectiveness of bird diverters on guywires is currently being studied at numerous sites, and 

current results indicate that the effectiveness is not only site-specific, but also species-specific. 

As such, results indicating that they are or are not effective cannot be applied across-the-board. 

It is recommended that once installed on guy wires at the NWTC that a formalized monitoring 

program be implemented to ascertain the site-specific effectiveness at the NWTC, with the 

species known to use the site. 

If alternate raptor sentinel stations (structures that are higher than nearby nest sites, which allow 

an observation position to assure nest safety) could be located offsite in a food rich area, this 

could serve to lure raptors away from the NWTC airspace, which would help mitigate onsite 

mortalities 

 While the relatively low number of structures in a small area impinges on many fewer breeding 

or wintering raptor territories in comparison to a site like the 68 square mile Cedar Creek II 

Wind farm in Weld County, Colorado, implementing the FWS guidelines would help mitigate 

mortalities for species that currently appear to be declining in number, and would demonstrate 

compliance with regulatory requirements and NREL policies and procedures. 

 
35

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