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

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.

Subcontract Report
NREL/SR-1900-70460
June 2018

Contract No. DE-AC36-08GO28308

 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

Suggested Citation
Du, Waldt, John A. 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.

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.

National Renewable Energy Laboratory Subcontract Report

15013 Denver West Parkway NREL/SR-1900-70460
Golden, CO 80401 June 2018
303-275-3000 • www.nrel.gov
Contract No. DE-AC36-08GO28308
 This publication was reproduced from the best available copy
submitted by the subcontractor and received no editorial review at NREL.

NOTICE

This work was authored by the National Renewable Energy Laboratory, operated by Alliance for Sustainable
Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding
provided by the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy. The views
expressed in the article do not necessarily represent the views of the DOE or the U.S. Government.

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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
 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

1 
 
 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

2 
 
 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

3 
 
 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

4 
 
5 
 
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 year-
long 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.

6 
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.

7 
 
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.

8 
 
  
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 mid-
afternoon 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

9 
 
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).

10 
 
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   23  2  35  7  5    2  1  7  7  4  1  11  1    4  2    2    114 
Vulture 
Osprey                                1        2  3 
Bald Eagle                    1  1                    2 
Northern                                          0 
Harrier 
Sharp‐                                 1        1 
shinned 
Hawk 
Cooper’s                  1      1                  2 
Hawk 
Northern                                          0 
Goshawk 
Broad‐                                         0 
winged 
Hawk 
Red‐tailed  5  1    7  6  4  1  3  5  5  1    3  4  1  3  10    5  1  65 
Hawk 
Swainson’s                                          0 
Hawk 
Ferruginous                                          0 
Hawk 
Golden  1        1  1          1    1  1    1          7 
Eagle 
American    2  1  6  6  5  4  1  2  6  11  11  2  4  1  6  4  3  5  5  85 
Kestrel 
Merlin                            1              1 
Peregrine                                          0 
Falcon 
Prairie                          1                1 
Falcon 
Unidentified          1                  1              2 
Accipiter 
Unidentified  4  2  4  4  4  2    16  7  2  3    3      7  1    1  3  63 
Buteo 
Unidentified  2    1  1  1                      3          8 
Falco 
Unidentified                                          0 
Eagle 
Unidentified  2    1  2  2  5  2  2  3    4        1            24 
Raptor 
Total By  37  7  42  27  26  17  9  23  25  21  24  13  22  12  3  25  18  3  13  11  378 
Date 
                                           
 

11 
 
  
120

114
100
Number of April Observations

80 85

60 65

40

20

7 3 2 2 1 1 1
0

Raptor/Vulture Species

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).

12 
 
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 HEIGHT LOCATION and
(Duration) BEHAVIOR
4/9/10 Unknown 1220-1222 250 meters Gliding north to
raptor (2 min.) south over site
4/15/10 Coopers 0900-0910 0-20 Perched in
Hawk (10 min.) meters Ponderosa Pine on
NW corner of site;
flies to LaFarge
property and dives
on passerines
4/22/10 Merlin 1148-1149 60 meters Crossed W edge
(1 min.) of site flying south
to north
4/30/10 Osprey(pair) 1116-1118 100 meters Flying SE to NW
(2 min.) 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.

13 
 
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 HEIGHT LOCATION
(Duration) (Meters) and
BEHAVIOR
4/05/10 Red-tailed 1025-1042 80 Roosting on
Hawk (17min.) MET tower
“ “ “ “ 1051-`1055 80 Top of MET
(4min.) tower
“ “ Turkey 1121-1125 50 Flying across
Vulture (4min.) south edge of
site
“ “ Unknown 1110-1111 120 Flying across

14 
 Falco (1min.) northern half
of site
“ “ Red-tailed 1250-1307 80 Flies to met
Hawk (17min.) tower perch;
joined by
mate; copulate
4/05/10 Unknown 1334(1min.) 240 Fast glide
Falco south to north
across site
4/06/10 American 0910(1min.) Unknown Blown across
Kestrel site
“ “ Red-tailed 1125-1127 10 Hunting low
Hawk (2min.) on N edge of
site
“ “ Turkey 1130(1min.) 20 Flying along
Vulture E edge of site
4/06/10 American 1220(1min.) 60 Blown across
Kestrel site SW to NE
4/07/10 American 0940-1000 10-20 Perch on pole;
Kestrel (20min.) flies to MET
tower
guywire;
copulates;
preens
4/08/10 American 0917-0921 20-60 Perched on
Kestrel (4 min.) guy wire
“ “ American 0954-0956 20-40 Flies over
Kestrel (2min.) central portion
of site
“ “ Red-tailed 1039-1111 100-150 Perched on
Hawk (32min.) top of MET-2
tower
“ “ Red-tailed 1052-1053 25 Soaring east
Hawk (1min.) side of site
“ “ American 1139-1242 10 Flies in along
Kestrel (3min.) S edge of site;
perches
guywire with
mate
“ “ American 1140(1min.) 0-10 Attempted kill
Kestrel
“ “ Turkey 1202-1204 320 Soaring across
Vulture (2min.) site
4/08/10 Red-tailed 1202-1204 100 Same as
Hawk (2min.) above
4/09/10 American 0937-1025 30 Perched pair

15 
 
 Kestrel (48min.) on MET-2
tower
arms/guywires
“ “ American 1025-1058 3-30 Pair perched
Kestrel (33min.) on telephone
pole; female
hunts SW
corner of site
“ “ “ “ 1136(1min.) 20 Fast flight
across site
“ “ Red-tailed 1305-1310 80 Perched on
Hawk (5min.) MET-2 tower
“ “ “ “ 1310(1min.) 120 Glides across
site
4/12/10 Red-tailed 1012-1022 10 Perched in
Hawk (10min.) pine ridge
“ “ American 1033-1100 20 Pair on
Kestrel (27min.) eastern met
tower guywire
4/12/10 “ “ 1158(1min.) 30 Transit
through site
4/13/10 American 1107-1121 0-20 Perched on
Kestrel (14min.) guywire
4/13/10 “ “ 1345-1347 10 Flight across
(2min.) site
4/14/10 American 1056-1058 10-20 Hunting hover
Kestrel (2min.) SW corner of
site
4/15/10 Turkey 1032(1min.) 50 Soaring along
Vulture S border of
site
4/16/10 Red-tailed 0900-0930 80 Perch on top
Hawk (30min.) of MET-2
tower
“ “ American 1040-1049 15-50 Flies S half of
Kestrel (9min.) site and
perches on
telephone pole
“ “ Red-tailed 1305-1330 0-30 Perched on
Hawk (25min.) MET-2 tower,
also observed
on LaFarge
water tower
and SW cell
tower
“ “ “ “ 1332-1336 60 Soaring SW

16 
 
 (4min.) corner of site
“ “ American 1310(1min.) 5 Perched on
Kestrel NE side met
tower
guywire;
joined by
mate, fly off
4/16/10 Red-tailed 1342-1347 60-80 Perched on
Hawk (3min.) MET-2 tower;
flies to the
SW
4/19/10 Red-tailed 1013-1029 80 Perched
Hawk (16min.) MET-2 tower;
sentinel
“ “ Turkey 1138-1141 20 Soars west
Vulture (3min.) side of site
“ “ American 1316-1320 20 Pair perched
Kestrel (4min.) on MET tower
“ “ “ “ 1320-1334 25 Male
(14min.) successful
hunt; returns
to female;
conducts 2nd
hunt
“ “ Turkey 1334(1min.) 200 Soars through
Vulture center of site
4/19/10 American 1343-1348 30 Perched on
Kestrel (5min.) MET tower
arm; flies to
LaFarge
telephone pole
4/20/10 American 1105-1119 20 Perched on E
Kestrel (14min.) side MET
tower
guywire; flies
E side of site
and leaves
“ “ American 1250(1min.) 5-10 Crosses site
Kestrel
4/20/10 American 1315-1327 15 Flew in from
Kestrel (12min.) LaFarge area,
then observed
moving
among met
towers on east
side of

17 
 
 NWTC,
apparently
hunting
4/21/10 Red-tailed 1154-1205 2-20 Glides into
Hawk (11min.) SW corner of
site and
captures
sm.mammal;
flies to
telephone pole
and consumes
“ “ American 1300-1310 20 S edge of site
Kestrel (10min.) perched on
power pole
“ “ Red-tailed 1224-1312 80 Perched top of
Hawk (48min.) MET-2 tower
“ “ Turkey 1258-1300 40 Crosses NW
Vulture (2min.) corner of site
4/21/10 Unknown 1322(1min.) 80 Soaring SE
Buteo corner of site
4/22/10 American 1029-1031 10 Hover S edge
Kestrel (3min.) of site,
hunting
“ “ American 1045-1046 40 Transit west
Kestrel (1min.) boundary of
site
“ “ American 1141(1min.) 10 Hover/stoop S
Kestrel edge of site
“ “ Red-tailed 1236-1247 80 Perch on
Hawk (11min.) MET-2 tower
4/22/10 American 1340-1343 20 Power line
Kestrel (3min.) perch SW
edge of site
4/23/10 Red-tailed 0948-1000 10 Perched in
Hawk (12min.) pine ridge
“ “ Unknown 0935(1min.) 10 Perched in
Raptor dead pine tree
4/23/10 American 1015(1min.) 2 Perched on
Kestrel mullein S site
boundary
4/26/10 American 0915(1min.) 20 NE portion of
Kestrel site perched
on met tower
“ “ Turkey 0925(1min.) 80 Soaring over
Vulture W half of site
“ “ American 1031-1034 20 Soaring over

18 
 
 Kestrel (3min.) central portion
of site
“ “ Unknown 1055(1min.) 40 Flies across
Falco NW corner of
site
“ “ “ “ 1121-1123 100 Soaring
(2min.) overhead at
100+m
“ “ American 1308(1min.) 20 Perched on
Kestrel guy wire
M1.2 tower
4/26/10 Red-tailed 1330-1333 30 Soaring over
Hawk (3min.) pine ridge
4/27/109 Red-tailed 0907-0923 80 Perched on
Hawk (16min.) M-2 tower
“ “ “ “ 1125-1128 30 Flies in from
(3min.) 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 0915(1min.) 15 Pair on east
Kestrel side met
station
guywire
4/29/10 American 1030(1min.) 2 Sunning on
Kestrel mullein site
4.3
“ “ Red-tailed 1234-1236 10 Hunting low
Hawk (2min.) at SE corner
of site
“ “ American 1241-1243 10 Flying along
Kestrel (2min.) N edge of site
“ “ American 1259-1301 30 Flies through
Kestrel (2min.) structures on
eastern
portion of site
“ “ Red-tailed 1313-1321 80 Perched on
Hawk (8min.) M-2 tower
“ “ American 1317(1min.) 15 Hunting NW
Kestrel corner of site
“ “ Red-tailed 1331-1334 80 Perched on
Hawk (3min.) M-2 tower
4/29/10 Red-tailed 1354-1356 80 Soars over

19 
 
 Hawk (2min.) guard house
4/30/10 American 1005-1007 15 Hover hunting
Kestrel (2min.) S of M-2
tower
“ “ American 1340-1346 20 Hover hunting
Kestrel (6min.) 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.

20 
 
 Figure 5.2 Histogram of NWTC April 2010 Resident Raptor Visit
Durations
35

30

25
Number of Visits

20

15

10

0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 20 25 27 30 32 33 48
Length of Visit (Minutes)

60
Figure 5.3 Histogram of Raptor Flight Height in Relation to
Structure Height and Number
50
Number of Resident Raptor Flights 

40
and Structures 

30
Raptors
Towers and Guywires
20

10

0
0‐40 41‐80 >80
Height Classes (m)

21 
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.  

22 
 Table 5.4   Comparison of Migrant Raptor Species Observed at NWTC 
    April 1995 versus April 2010 
Species April 1995 Number April 2010 Number
Observed 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) 

23 
 
3000
2500
2000
Spring Total
1500
April Total
1000

500
0
2004 2005 2006 2008 2009 2010
Source: Hawk Migration Association of America, 2010
Figure 5.4 DRRMS Total April and Spring Raptor Count 2004 -2010

24 
 
10

8
Spring Observed
6
Raptors /Hour
April Observed
4
Raptors/Hour

0
2004 2005 2006 2008 2009 2010
Source: Hawk Migration Association of America, 2010
Figure 5.5 DRRMS Recent Spring and April Migrating Raptors Per Observation
Hour
 

25 
 
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:
 
 
 
 
26 
 
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]).

27 
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 NWTC 2010 DRRMS

1995 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.
nd
2 most abundant observed Am. kestrel Not avail. Red-tailed Not avail.
hawk
3rd most abundant observed Red-tailed Not avail. Unid. raptor Not avail.
hawk

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 large-
bodied 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

28 
 
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 mid-
March 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.  

 
 

29 
 
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 Migrant Resident Shortest Range Tallest Shortest Highest

flight flight flight turbine of rotor turbine met met
heights heights heights heights diameter heights tower tower (ft)
when when (ft) (ft) above (ft) (ft)
hunting flying the
(ft) through ground
the area
shortest 0 197 0 30 30
ht.
Highest 65 820 1050 295 443
ht.
shortest 24 131
blade ht.
ags
Highest 36 459
blade ht.
ags
Levels 6
of
guywires
No. of 3 6
anchors

30 
 
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).

31 
 
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

32 
 
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,

33 
 
 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.

34 
 
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Berry, M.E.; Bock, C.E. (1998). “Effects of Habitat and Landscape Characteristics on Avian
Breeding Distributions in Colorado Foothills Shrub.” Southwestern Naturalist (43): pp. 453–461.

Birek, Jeff. Personal Communication. Telephone conversation between Brenda Beatty and Jeff
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Chesser, R. Terry, R. C. Banks, F. K. Barker, C. Cicero, J. L. Dunn, A. W. Kratter, I. J. Lovette,

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Department of Energy (DOE) and U.S. Fish and Wildlife Service (FWS), 2006). Memorandum
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%20Focus%20Species%20Profiles/EcoRiskProfile_bald_eagle.pdf. Accessed May 3, 2013.

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35 
 
Goodrich, L.J. and J.P. Smith. 2008. Raptor Migration in North America. Pp. 37-150 in K.L.
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Hallock, Dave and Stephen Jones, 2011. Boulder County Avian Species of Special Concern.
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Hawk Migration Association of North America (HMANA), 2010. Dinosaur Ridge Count Site
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