BLM Technical Note 435

Accipiter Use of Pinyon–Juniper Habitats

for Nesting in Northwestern Colorado

BLM
BLM/WY/ST-10/010+1110
Steven J. Slater1

 and Jeff P. Smith

HawkWatch International, Inc.

2240 South 900 East

Salt Lake City, Utah 84106

1 Phone: (801) 484-6808 x108 

 Email: sslater@hawkwatch.org

Prepared for:

U.S. Department of Interior, 

Bureau of Land Management

Utah State Office, Salt Lake City,

Wyoming State Office, Cheyenne, and

Colorado State Office, Lakewood

BLM Technical Note 435

Accipiter Use of Pinyon–Juniper Habitats

for Nesting in Northwestern Colorado

iii Accipiter Nesting in Pinyon-Juniper • BLM Technical Note 435

List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . xi

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Study Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Accipiter Broadcast Surveys . . . . . . . . . . . . . . . . . . . 3

Nest Searches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Vegetation and GIS-based Metrics . . . . . . . . . . . . . . 4

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Appendix A. Example survey 

data sheets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Appendix B. Example nest 

area photographs. . . . . . . . . . . . . . . . . . . . . . . . 19

Table of Contents

v Accipiter Nesting in Pinyon-Juniper • BLM Technical Note 435

Table 1. Vegetation and GIS-based metrics at Sharp-shinned 

Hawk (n = 3), Coopers’ Hawk (n = 10), and Northern 

Goshawk (n = 10) nests in pinyon–juniper habitats of 

the Piceance Basin, Colorado study area. . . . . . . . . 8

List of Tables

Table 2. Bureau of Land Management (BLM) and U.S. Forest 

Service (USFS) field and district offices with greater 

than 50,000 ha of pinyon–juniper (P–J) habitat on 

public lands, ranked by total coverage area (compiled 

from Pugh [2004]). . . . . . . . . . . . . . . . . . . . . . . 11

vii Accipiter Nesting in Pinyon-Juniper • BLM Technical Note 435

Figure 1. Piceance Basin study area in northwestern Colorado, 

with areas of pinyon–juniper vegetation and locations of 

2007 broadcast surveys shown. . . . . . . . . . . . . . . . 3

Figure 2. Diagram of a typical artificial nest structure (ANS) used 

for Ferruginous Hawks in the BLM Rawlins Field Office 

in south-central Wyoming (originally represented in 

Tigner and Call 1996). . . . . . . . . . . . . . . . . . . . . . 7

Figure 3. Aspect of Sharp-shinned Hawk (red; n = 3), Cooper’s 

Hawk (blue; n = 10), and Northern Goshawk 

(green; n = 10) nest sites in the Piceance Basin 

study area. . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Figure A1. Example accipiter broadcast survey data sheet. See 

the methods section for a detailed overview of the 

survey protocol. . . . . . . . . . . . . . . . . . . . . . . . . 17

List of Figures

Figure A2. Example vegetation survey data sheet used at 0.04-ha 

plots centered on accipiter nests. See the methods 

section for a detailed overview of the 

survey protocol . . . . . . . . . . . . . . . . . . . . . . . . . 18

Figure B1. Photograph of typical Sharp-shinned Hawk nesting 

area in the Piceance Basin, Colorado. The nest is 

indicated with an arrow . . . . . . . . . . . . . . . . . . . 19

Figure B2. Photograph of typical Cooper’s Hawk nesting area in 

the Piceance Basin, Colorado. The nest is 

indicated with an arrow . . . . . . . . . . . . . . . . . . . 20

Figure B3. Photograph of typical Northern Goshawk nesting 

area in the Piceance Basin, Colorado. The nest is 

indicated with an arrow . . . . . . . . . . . . . . . . . . . 21

ix Accipiter Nesting in Pinyon-Juniper • BLM Technical Note 435

other regions and vegetation types. Vegetation surveys 

also suggested that Sharp-shinned Hawk (n = 3) nest sites 

contained smaller trees and a greater density of small 

trees compared to the other two accipiters. Cooper’s 

Hawk (n = 10) and Northern Goshawk (n = 10) nest and 

nest-site characteristics overlapped considerably. All 

of the surveyed accipiter nests that we found were on 

northwestern through eastern aspects and none was in 

pinyon–juniper patches less than 4.8 ha in size. Based 

on our results and other available records, we do not 

believe that accipiter nesting use of pinyon–juniper 

vegetation is restricted to northwestern Colorado. Federal 

lands contain and estimated 65% of current pinyonjuniper vegetation, and we recommend that federal land 

managers consider the potential of these areas to support 

nesting accipiters during land management planning. We 

identified the BLM and U.S. Forest Service field and 

district offices with the greatest pinyon–juniper coverage, 

and provide some general recommendations for accipiter 

nest surveys in this vegetation type.

Although substantial research has been directed at 

understanding accipiter nesting-habitat relationships, 

little attention has been paid to pinyon–juniper (Pinus 

edulis or monophylla – Juniperus spp.) vegetation of the 

western United States. Previous surveys conducted by 

the Bureau of Land Management (BLM) in northwestern 

Colorado suggested that accipiter nesting in these habitats 

might be more common than previously acknowledged, 

leading to a BLM-sponsored study of this phenomenon 

by HawkWatch International in the Piceance Basin of 

Colorado. Broadcast surveys and nest searches suggested 

that in 2007 there were a minimum of 0–2 Sharp-shinned 

Hawk (Accipiter striatus), 13–29 Cooper’s Hawk (A. 

cooperii), and 3–6 Northern Goshawk (A. gentilis) nests 

active in the study area. Although we did not design our 

surveys to produce robust estimates of nesting density, 

spacing of active nests located between 2005 and 2007 

suggested that all three accipiters probably occur at 

relatively low densities in pinyon-juniper habitat in the 

study area compared to estimates derived from studies in 

Abstract

xi Accipiter Nesting in Pinyon-Juniper • BLM Technical Note 435

Office for providing additional accipiter nesting data 

used for comparative purposes. This technical report 

also benefited from input provided by the project’s BLM 

Steering Committee, which included Dave Mills (BLM 

Utah, Richfield Field Office), Steve Madsen (BLM 

Utah State Office), Dave Roberts (BLM Wyoming 

State Office), and Wes Anderson (BLM Colorado State 

Office), as well as independent reviews by Bill Mannan 

(University of Arizona) and Richard Reynolds (USDA 

Forest Service, Rocky Mountain Research Station, CO).

This project was undertaken with funding provided by 

BLM Assistance Agreement JSA065003. We thank our 

seasonal field technicians, James Cederstrom, Corey 

Kanuckel, and Robert Spaul, for their dedication to the 

field effort, as well as HawkWatch International staff 

biologist Mike Neal for his assistance in the field. We 

thank Ed Hollowed and Brett Smithers of the WRFO 

for providing raptor survey data, their insight into 

accipiter nesting use of pinyon–juniper in the Piceance 

Basin and northwestern Colorado, and reviews of this 

report. We thank the BLM Rawlins, Wyoming Field 

Acknowledgements

1 Accipiter Nesting in Pinyon-Juniper • BLM Technical Note 435

species to make use of pinyon–juniper habitats for 

nesting. Indeed, an extensive review of the literature 

revealed only a single documented case of a Northern 

Goshawk nesting in pinyon–juniper habitat (Johansson 

et al. 1994), no published reports of Cooper’s Hawks 

nesting in pinyon–juniper, and only two studies that 

recorded Sharp-shinned Hawks nesting in pinyon–juniper 

and juniper habitats (Platt 1976, Hennessy 1978). Note 

that few of the studies reviewed actually searched 

pinyon–juniper habitats for accipiter nests, and therefore, 

the previously mentioned bias may partially account 

for the few available published records. Unpublished 

raptor survey records from the White River Field Office 

(WRFO) of the Bureau of Land Management (BLM) 

suggest that accipiters, especially Cooper’s Hawks, may 

use pinyon–juniper for nesting more commonly than 

previously believed (Ed Hollowed and Brett Smithers, 

BLM-WRFO, personal communication).

Pinyon–juniper habitats are found scattered throughout 

the western United States and occupy and estimated 20.9 

million hectares of western land (Smith et al. 2004). 

It is widely believed that pinyon–juniper vegetation 

has expanded in distribution and density over the last 

150 years, driven by a complex of human-caused 

environmental changes (Tausch et al. 1981). In the 

United States, the majority (83%) of pinyon–juniper 

vegetation occurs in New Mexico, Arizona, Utah, 

Nevada, and Colorado (in order of total land area). 

Federal lands contain 65% of all pinyon–juniper habitats 

(Smith et al. 2004). The BLM manages the largest 

share of these lands (56%), followed by the U.S. Forest 

Service (USFS; 40%) and other federal agencies (4%; 

Pugh 2004). Pinyon–juniper habitats on public lands are 

subjected to various human-associated activities that may 

compromise potential nesting habitat. These activities 

include prescribed burns, mechanical treatments (i.e., 

chaining, plowing, disking, or related tillage treatments), 

chemical treatments, timber harvest, energy extraction, 

livestock grazing, and recreation.

If accipiter nesting use of pinyon–juniper woodlands is 

not an isolated phenomenon, federal land-management 

agencies need to be informed of this and be provided 

with survey guidance for the protection of accipiter 

nests in these habitats. All three accipiter species are 

protected by the Migratory Bird Treaty Act, which 

prohibits the “taking” of these species, their young, or 

nests. Additionally, the U.S. Fish and Wildlife Service 

(USFWS) has been petitioned to list the Northern 

Goshawk under the Endangered Species Act on numerous 

occasions and, although these petitions have been denied, 

Three accipiter species breed in North America, the 

Sharp-shinned Hawk (Accipiter striatus), Cooper’s 

Hawk (A. cooperii), and Northern Goshawk (A. gentilis). 

Accipiters are known to use a variety of habitats for 

nesting throughout the United States, but in the West, 

attention has focused primarily on conifer-dominated 

habitats (Rosenfield and Bielefeldt 1993, Squires and 

Reynolds 1997, Bildstein and Meyer 2000). Very little 

published information is available on accipiter nesting 

use of pinyon–juniper habitats (in most of western U.S., 

typically Pinus edulis or monophylla – Juniperus spp.), 

although use is known outside the nesting season (e.g., 

Northern Goshawk use of pinyon–juniper habitats during 

post-fledgling dispersal and winter periods; Drennan and 

Beier 2003, Kennedy and Ward 2003, Wiens et al. 2006). 

The scarcity of information on nesting use of pinyon–

juniper habitats in part may be due to a bias toward 

searching for nests in habitats assumed a priori to be 

suitable (Siders and Kennedy 1996, Squires and Reynolds 

1997). It is also possible, however, that pinyon–juniper 

habitats do not regularly provide the individual tree, 

habitat, or landscape-structure characteristics sought 

by nesting accipiters, as pinyon–juniper woodlands are 

often more shrub-like and open relative to other conifer 

woodlands.

Nesting habitat characteristics vary for all three North 

American accipiters. Historically, it was believed that 

habitat characteristics varied in relation to accipiter body 

size. For example, it has been hypothesized that the 

smaller Sharp-shinned Hawk selects smaller trees and 

denser woodlands, the Cooper’s Hawk medium-sized 

trees and medium-density woodlands, and the larger 

Northern Goshawk the largest trees and most open 

woodlands. While some studies have found support 

for this body-size hypothesis (Reynolds et al. 1982, 

Moore and Henny 1983, Trexel et al. 1999), others have 

found that few nesting habitat characteristics follow 

the predicted trends and that much variability and 

overlap exists for each species (Hennessy 1978, Siders 

and Kennedy 1994, Siders and Kennedy 1996). Not 

surprisingly, it does appear that overlap is most likely to 

occur between species closer to one another in size; i.e., 

Sharp-shinned Hawk and Northern Goshawk nesting 

habitat characteristics are most often distinct from each 

other (e.g., Hennessy 1978, Siders and Kennedy 1996). 

Additionally, the preference of Northern Goshawks for 

more mature forest stands is well-documented (Squires 

and Reynolds 1997).

Given these considerations, one might expect Northern 

Goshawks to be the least likely of the three accipiter 

Introduction
2 BLM Technical Note 435 • Accipiter Nesting in Pinyon-Juniper

the USFWS and USFS consider goshawks a species of 

concern, and the BLM lists it as sensitive in six states 

(Squires and Kennedy 2006).

The specific objectives of our research were to:

1) Assess accipiter use of pinyon–juniper habitats 

for nesting in the Piceance Basin of Colorado.

2) Describe basic vegetation and landscape 

characteristics of accipiter nest sites found in 

pinyon–juniper vegetation in the study area.

3) Provide federal land managers with accipiter 

survey recommendations relating to pinyon–

juniper habitats.
3 Accipiter Nesting in Pinyon-Juniper • BLM Technical Note 435

Study Area

The Piceance Basin study area was located in Rio Blanco 

County in northwestern Colorado, approximately 12 km 

west of the town of Meeker (Figure 1). The study area 

encompassed 2,112 km2, 86% of which is managed by 

the BLM WRFO. The remainder is privately owned 

or managed by the state of Colorado. Elevations in 

the study area ranged from 1,671–2,554 m, and annual 

precipitation in Meeker averaged 42 cm from 1900–2007 

(Western Regional Climate Center, Reno, NV). The study 

area is best characterized as a series of predominantly 

pinyon–juniper ridges, intermixed with small Wyoming 

and mountain sagebrush (Artemisia tridentata 

wyomingensis and A. t. vaseyana) openings, and separated 

by narrow, ephemeral drainages dominated by basin big 

sagebrush (A. t. tridentata). Pinyon–juniper vegetation 

was most common and reached its greatest stature along 

numerous ridges within the area. The Piceance Basin was 

undergoing extensive natural-gas development during the 

study, and gas wells, associated roads, and human activity 

were common in the study area. The WRFO manages 

a database of raptor nest locations documented within 

the field office, gathered by field-office personnel and 

gas-industry consultants. Between 1987 and 2003, only 

five active (eggs laid) accipiter nests were located in the 

Piceance Basin study area; however, 11 active nests were 

found in 2005 and six active nests were found in 2006.

Accipiter Broadcast Surveys

To detect accipiters, we used broadcast surveys from 

unpaved roads transecting larger patches of pinyon–

juniper within the study area. We acknowledge that 

constraining our surveys to roads potentially biased our 

effort with regard to extrapolating results to the larger 

landscape; however, our intent was not to develop 

a rigorous assessment of nesting density or overall 

distribution. Instead, we sought simply to confirm 

presence of the three species in pinyon-juniper habitats 

of the area and then locate sufficient numbers of active 

or recently used nesting territories to enable reasonable, 

initial quantification of the site and nest characteristics 

involved. Moreover, in this particular landscape, pinyonjuniper habitat is distributed primarily along relatively 

narrow side ridgelines interspersed with finger canyons 

and drainages, and most such ridges have dirt roads that 

lead up them from the basin below to the main ridgetop 

above. Therefore, the available road network comprised 

a nearly systematic and highly representative series 

of transects from which to conduct the surveys. That 

said, the one component of pinyon-juniper habitat that 

surveying only along these roads may not have sampled 

well is isolated patches of particularly steep, rugged, and 

largely inaccessible habitat located near the tops of some 

of the ravines. In many cases, however, we were able to 

survey at least partially even these habitats by virtue of 

broadcasting from roads that ran along the main ridgetop 

above. We also did not estimate the probability of 

detection during our surveys, which imparts an unknown 

bias with respect to using our survey results to estimate 

overall abundances and nesting densities across the 

broader landscape.

Previous research suggested that accipiter broadcast 

surveys may be most effective during courtship and 

nestling periods (Rosenfield et al. 1988, Kennedy and 

Stahlecker 1993, Joy et al. 1994); however, surveys 

during the post-fledging period also may be effective 

due to the increased number of potential responders 

(i.e., adults plus young; R. Reynolds, personal 

communication). To increase our chances of detecting 

all three accipiter species, which generally vary in their 

nesting chronology, we conducted two rounds of surveys 

Methods

Figure 1. Piceance Basin study area in northwestern Colorado, with 

areas of pinyon–juniper vegetation and locations of 2007 broadcast 

surveys shown.
4 BLM Technical Note 435 • Accipiter Nesting in Pinyon-Juniper

between 30 April and 1 June and between 4 June and 7 

July. We based our broadcast survey methods on those 

outlined by Rosenfield et al. (1988) and Kennedy and 

Stahlecker (1993), with modifications to suit our specific 

needs. We established broadcast stations every 300 m 

along sections of accessible road traversing pinyon–

juniper habitat, with the first station in a particular 

patch established 150 m from the patch edge. We based 

our choice of spacing between stations on evaluating 

literature concerning typical spacing of accipiter nesting 

areas and rough estimates of the broadcast range of our 

caller, striving for an interval we felt would maximize 

our chances of detecting any existing territories. We 

also limited our surveys to road sections that transected 

(or in very few cases paralleled along one side of and 

immediately adjacent to) a minimum of 200 m of 

continuous pinyon-juniper habitat, again based on review 

of the literature concerning typical minimum forest-patch 

sizes occupied by nesting accipiters. Surveyors used 

portable GPS units (Garmin GPSMAP 60CSx) to record 

all survey routes and station points.

At each station, the observer broadcast alarm calls in four 

directions, at 45° angles to the road. Surveyors reduced 

the number of broadcast directions to two if a patch 

was located on only one side of the road. Each 10-sec 

broadcast in a specific direction was followed by 30 sec 

of scanning and listening for responses. At each station, 

the observer broadcast Sharp-shinned Hawk, Cooper’s 

Hawk, and Northern Goshawk alarm calls, in that order, 

to avoid potential size-related inhibitory effects. When 

a focal species was detected, the observer recorded the 

species, age, and sex, when possible; an assessment of 

the observer’s confidence in identifying the responding 

species (i.e., confident or not confident, as supported by 

a description of what was heard and/or seen); time of 

response; time elapsed since first call broadcast; species 

of call broadcasted immediately preceding the detection; 

response type (i.e., call, call and approach, call and flyby, silent approach, silent fly-by); estimated distance 

and bearing to response; station number and location; 

and general vegetation characteristics surrounding the 

detection point (i.e., maturity and stature of pinyon–

juniper vegetation). Appendix A contains an example of 

the data sheet used during the broadcast surveys.

While time of day may or may not influence response 

rates of accipiters to broadcast calls (e.g., see Kimmel and 

Yahner [1990] and Roberson et al. [2005] for differing 

perspectives), lighting undoubtedly affects an observer’s 

ability to visually detect non-vocal responses. To reduce 

variation in conditions affecting response detection, all 

surveys occurred between 30 min after sunrise and 30 

min before sunset, and only during periods of fair weather 

(i.e., no rain, snow, or winds exceeding 20 kph) and no 

heavy vehicle passage. The volume of all broadcast calls 

was approximately 110 dB at 1 m from the source (Fuller 

and Mosher 1987), which we achieved by connecting a 

portable MP3 player to a hand-held megaphone.

Nest Searches

We used broadcast-survey detection locations to guide 

on-foot nest searches later in July (to correspond to 

likely post-hatch periods and reduce the likelihood of 

nest desertion). To augment nest sample sizes, we also 

attempted to revisit nests found between 2005 and 2007 

by BLM and industry personnel. As noted in the study 

area description, only a few, scattered accipiter nest 

records were available from before this period. “Active” 

nests were identified as those for which we obtained 

positive confirmation of incubation, adult behavior 

indicative of nestling attendance (e.g., feeding, mantling, 

etc.), fresh prey remains or extensive whitewash 

indicative of extended nestling presence, or actual eggs or 

nestlings.

Vegetation and GIS-based Metrics

We collected detailed vegetation data around identified, 

active accipiter nests after young had fledged (i.e., during 

mid- and late July). Vegetation sampling followed 

methods developed by James and Shugart (1970) and 

Noon (1981), with minor modifications to suit our 

specific needs. Sampling occurred within 0.04-ha 

circular plots centered on nests, with each plot divided 

into quadrats by two perpendicular, 11.3-m long, crossed 

transects aligned with the cardinal directions. Within 

each quadrant, the number of live and dead trees in 

10 diameter-at-breast-height (DBH) size classes was 

recorded: <3, 3–8, 8–15, 15–23, 23–38, 38–53, 53–69, 

69–84, 84–102, and >102 cm (James and Shugart 1970). 

Surveyors also recorded the number of fallen logs ≥1.5 

m long and ≥8 cm diameter in each quadrant (Noon 

1981). Surveyors estimated shrub density by walking 

the perpendicular transects and counting the number of 

live and dead shrubs occurring within the span of the 

observer’s outstretched arms. We multiplied the number 

of shrubs encountered within a subplot by 125 to produce 

an estimate of shrub density per hectare (Noon 1981). 

Surveyors estimated canopy and ground cover by walking 

the two transects, stopping on each alternate step, and 

sighting up and down through an ocular tube with crossed 

sights. Thus, they recorded the number of canopy and 

ground vegetation “hits” at 20 stops (James and Shugart 

1970). Nest-specific measures included the nest-tree 

species, overall height, lower canopy height, and DBH; 
5 Accipiter Nesting in Pinyon-Juniper • BLM Technical Note 435

the height of the nest above the ground; and the nest 

dimensions. Total-plot (i.e., nest site) measures included 

species-specific tree densities and average DBHs; density 

of species-specific saplings (i.e., trees <3 cm DBH); 

density of downed logs ≥1.5 m long; percent canopy and 

ground cover; and shrub density. Appendix A contains an 

example of the vegetation data sheet used.

We used a GIS incorporating a 30-m resolution digital 

elevation model (DEM) and 1-m resolution aerial 

photography (captured in 2006) to provide additional 

insight into nest-site and overall patch characteristics 

at active nests. From the DEM, we determined the 

elevation, slope, and aspect of each nest site. From the 

aerial photos, we determined the distance from each nest 

site to the nearest habitat edge (i.e., a discernable road, 

pipeline or other gas and oil infrastructure, powerline 

right-of-way, or sagebrush or grass opening) and the 

minimum width of the patch associated with each nest 

site. We assessed the latter by identifying and measuring 

the shortest axes through the nest-site habitat patches to 

the nearest two patch breaks, with relevant breaks defined 

as those corresponding to adjacent habitat patches of at 

least 30 m by 30 m in size (i.e., excluding small roads 

or other narrow linear features). We used all known 

2005–2007 active nests to provide initial, rough (due 

to possible survey biases) estimates of average nearestneighbor distances for each species, with the distances 

calculated for each nest on an annual basis and averaged 

across years.

Due to small sample sizes for each accipiter species, 

we limited our analysis to species-by-species univariate 

t-tests (alpha = 0.05, separate variances assumed). We 

also present a full complement of descriptive statistics for 

each metric to provide land managers with a basic sketch 

of potential accipiter nesting habitat.

Because our nest samples cannot be considered either 

exhaustive or completely random, we caution that the 

results of our habitat and inter-specific analyses should be 

considered preliminary indicators that may be confounded 

by unknown sampling biases. For example, the 

characteristics we describe may be more representative of 

birds nesting near roads than the population as whole. In 

this particular case, however, with regard to the primary 

motivation behind our research—expanding knowledge 

of nesting accipiters in a habitat that is broadly subject to 

oil and gas development—this bias may in fact be entirely 

appropriate given the close relationship between roads 

and development activities.

7 Accipiter Nesting in Pinyon-Juniper • BLM Technical Note 435

We detected probable accipiter responses during 66 of 

541 (12%) broadcast surveys conducted during the 2007 

nesting season. After consolidating proximate (i.e., <1 

km apart) responses from the same species (either within 

or between survey periods) and removing responses 

with low observer confidence, we produced distinct, 

confident response locations for 2 Sharp-shinned Hawks, 

22 Cooper’s Hawks, and 4 Northern Goshawks. We 

documented responses during both survey rounds at 

7 Cooper’s Hawk and 1 Northern Goshawk response 

locations.

Our 2007 surveyors found no active Sharp-shinned 

Hawk nests, 13 active Cooper’s Hawk nests, and 3 active 

Northern Goshawk nests. They found seven of the active 

Cooper’s Hawk nests and one of the active Northern 

Goshawk nests during nest searches conducted from 

broadcast response locations. Other active nests were 

located by BLM personnel, industry consultants, and 

incidentally by our surveyors. We illustrate examples of 

typical nesting situations for each species in Appendix B. 

Vegetation surveys occurred at 3 Sharp-shinned Hawk, 

10 Cooper’s Hawk, and 10 Northern Goshawk nests. 

We used nests located in 2005 and 2006 to augment 

vegetation survey locations for all three species. We 

did not conduct vegetation surveys at four Cooper’s 

Hawk nests discovered in 2007 to avoid disturbance 

of nests with young still in them. We were unable to 

obtain a larger sample of Sharp-shinned Hawk nests due 

to difficulty relocating nests from previous years and 

an inability to assign a confident species association to 

located nests. We included in our vegetation surveys 

only nests with confirmed species associations, based on 

documented activity or nest size.

Vegetation data and GIS-metrics suggested that nest-site 

characteristics of Sharp-shinned Hawks, Cooper’s Hawks, 

and Northern Goshawks commonly overlapped (Table 1). 

Nests of all three species were located in both pinyon and 

juniper trees, but most Northern Goshawk nests (90%) 

were in pinyon pines. Although limited by small sample 

sizes, our results suggest that Sharp-shinned Hawk 

nest sites contained significantly smaller DBH pinyon 

pines, smaller DBH live trees, and a greater density of 

15–23 cm DBH trees than those of the other two species 

(Table 1). In addition, Sharp-shinned Hawks nests were 

lower in the nest tree relative to Cooper’s Hawk nests, 

and their nest sites contained a greater density of live 

trees and 8–15 cm DBH trees compared to Northern 

Goshawks. Cooper’s Hawk and Northern Goshawk nest 

and nest-site characteristics only differed in regards to 

distance to edge, with Northern Goshawk nests found 

closer to edges. All three species used nest sites with very 

similar average canopy cover (64–65%) and densities of 

live trees between 23–38 cm DBH (168–175 trees/ha). 

All surveyed accipiter nests were on aspects between 

205° and 81° (Figure 2). Average elevations and slopes 

at nest sites were similar for all three species, whereas 

average distance to road and edge were lower at Northern 

Goshawk nest sites (Table 1). The shortest minimum 

patch axis of all three species was very similar and had a 

lower bound of 221–229 m. This equates to a minimum 

pinyon–juniper patch size of 4.8–5.2 ha surrounding 

surveyed accipiter nests. Average nearest-neighbor 

nesting distance was 9.0 ± SE of 3.3 km for Sharpshinned Hawks (n = 5 [note: n is the number of distances 

calculated between active nests, not the total number of 

nests]), 6.0 ± 1.1 km for Cooper’s Hawks (n = 19), and 

10.2 ± 1.8 km for Northern Goshawks (n = 3).

During the 2007 nesting season, a minimum of 13 

Results

Figure 2. Aspect of Sharp-shinned Hawk (red; n = 3), Cooper’s Hawk 

(blue; n = 10), and Northern Goshawk (green; n = 10) nest sites in the 

Piceance Basin study area.
8 BLM Technical Note 435 • Accipiter Nesting in Pinyon-Juniper

Table 1. Vegetation and GIS-based metrics at Sharp-shinned Hawk (n = 3), Coopers’ Hawk (n = 10), and Northern Goshawk (n = 10) nests in 

pinyon–juniper habitats of the Piceance Basin, Colorado study area.

Sharp-shinned Hawk Cooper’s Hawk Northern Goshawk

Vegetation or GIS metric Mean (SE)1 Range Mean (SE) Range Mean (SE) Range

Nest tree DBH (cm) 32 (8) 19–46 45 (6) 31–93 43 (5) 31–77

Nest height (m) 16 (1)A 15–18 22 (2)B 17–32 18 (1)AB 12–26

Tree height (m) 23 (2) 21–26 27 (2) 20–40 26 (1) 20–34

Lower canopy height (m) 6 (1) 4–7 6 (1) 3–9 4 (0) 3–6

Ground cover (%) 5 (0) 5–5 23 (4) 5–40 21 (3) 5–40

Canopy cover (%) 65 (8) 55–80 64 (4) 45–85 65 (3) 55–90

Live shrubs per ha 45 (22) 5–80 100 (23) 10–245 74 (24) 25–265

Dead shrubs per ha 12 (12) 0–35 7 (3) 0–20 4 (2) 0–15

Avg. pinyon DBH (cm) 17 (3)A 14–23 36 (3)B 19–53 29 (2)B 21–41

Avg. juniper DBH (cm) 28 (3) 23–34 29 (3) 18–44 31 (3) 18–44

Avg. live tree DBH (cm) 21 (2)A 17–25 30 (3)B 16–43 29 (2)B 19–42

Avg. dead tree DBH (cm) 17 (4) 11–25 22 (5) 10–61 20 (5) 12–61

Pinyons per ha 450 (95) 275–600 345 (102) 75–1175 278 (33) 100–450

Junipers per ha 300 (38) 250–375 198 (30) 100–400 245 (34) 75–425

Live trees per ha 750 (58)A 650–850 543 (100)A 275–1375 523 (47)B 275–725

Dead trees per ha 133 (8) 125–150 133 (25) 25–275 115 (34) 25–325

Saplings per ha 267 (169) 50–600 315 (94) 0–900 430 (72) 0–725

Downed logs per ha 283 (17) 250–300 458 (80) 125–900 318 (66) 0–650

3–8 cm DBH trees per ha 100 (52) 0–175 70 (40) 0–400 50 (16) 0s125

8–15 cm DBH trees per ha 167 (46)A 75–225 108 (50)A 25–550 73 (18)B 0–150

15–23 cm DBH trees per ha 242 (22)A 200–275 73 (21)B 0–200 88 (21)B 0–200

23–38 cm DBH trees per ha 175 (29) 125–225 175 (38) 0–350 168 (27) 25–300

38–53 cm DBH trees per ha 58 (17) 25–75 83 (14) 25–150 108 (24) 25–250

53–61 cm DBH trees per ha 0 (0) 0 25 (10) 0–100 23 (9) 0–75

61–84 cm DBH trees per ha 8 (8) 0–25 5 (3) 0–25 15 (8) 0–75

84–102 cm DBH trees per ha 0 (0) 0 3 (3) 0–25 0 (0) 0

>102 cm DBH trees per ha 0 (0) 0 3 (3) 0–25 0 (0) 0

Aspect (30-m cell) – 228–68 – 294–78 – 205–81

Elevation (30-m cell) 2061 (63) 1996–2187 2039 (21) 1896–2108 2075 (20) 1970–2144

Degree slope (30-m cell) 10 (2) 8–13 7 (1) 2–11 7 (1) 2–14

Distance to road (m) 212 (64) 85–285 222 (59) 74–611 131 (37) 34–425

Distance to edge (m) 130 (80)AB 21–285 146 (23)A 56–283 78 (16)B 34–196

Shortest patch axis (m) 658 (224) 227–978 523 (78) 229–980 627 (109) 221–1048

1

 Where significant differences among means for different species occurred, within rows means that do not share 

superscript letters indicate significant differences (univariate t-test, P ≤ 0.05).
9 Accipiter Nesting in Pinyon-Juniper • BLM Technical Note 435

Cooper’s Hawk and 3 Northern Goshawk nests were 

active in the Piceance Basin study area. Additional 

consideration of confident broadcast responses in 

conjunction with active nest locations suggested possible 

totals of 2 Sharp-shinned Hawk, 29 Cooper’s Hawk, and 

6 Northern Goshawk nests in surveyed areas. The WRFO 

raptor database contained records of five active Sharpshinned Hawk and seven active Cooper’s Hawk nests in 

2005, and two active nests of each of the three accipiter 

species in 2006. It is important to note that these nests 

were located through clearance surveys in association 

with proposed gas development, and therefore were not 

intended to provide a comprehensive inventory of the 

area. As a result, even taken all together, these figures 

probably under-represent the potential total number 

of accipiter nests active each year in pinyon-juniper 

habitat within the Piceance Basin. Because we did not 

quantify the probability of detection for our surveys, and 

therefore cannot confidently establish accurate estimates 

of spatial representation, we cannot provide more 

confident assessments of overall abundance or density by 

extrapolating across the broader landscape.

Our results clearly demonstrate that all three accipiter 

species will utilize pinyon–juniper for nesting, with 

the Cooper’s Hawk apparently the most likely to do so. 

Our work suggests, however, that the density of nesting 

accipiters in pinyon–juniper is likely low compared to 

other North American vegetation types. Average nest 

spacing of accipiters reported by other studies ranges 

from 1.2–5.6 km (Rosenfield and Bielefeldt 1993, 

Squires and Reynolds 1997, Bildstein and Meyer 2000, 

Reynolds, et al. 2005). Due to limitations of our survey 

effort, we cannot confidently claim to have generated 

truly representative samples and, if anything, our samples 

likely under-represent the overall abundance of nesting 

accipiters in the pinyon-juniper habitats sampled. This 

may be particularly true for Northern Goshawks, which 

other research has shown may require 3–4 broadcast 

surveys per season to yield robust assessments of 

presence/absence (Boyce et al. 2005). Nevertheless, 

based on what we did document, only the Cooper’s 

Hawk even approached the upper range of previously 

documented nearest-neighbor distances, averaging 6.0 km 

between nests.

Similar to results previously reported in studies conducted 

in other western conifer habitats (Siders and Kennedy 

1994, Siders and Kennedy 1996), we found considerable 

overlap between the nest-site characteristics of the three 

species. Although compromised by small samples sizes 

and possibly non-random sampling, our results suggest, 

however, that Sharp-shinned Hawks used nest sites 

featuring primarily smaller trees and a greater overall 

density of small trees compared to their larger congeners. 

This result appears to agree with accipiter body-size 

predictions of nesting habitat use; i.e., that the smaller 

Sharp-shinned Hawk will utilize young, dense patches 

containing smaller trees relative to the larger accipiters. 

In contrast, the Cooper’s Hawk and Northern Goshawk 

overlapped in most nest-tree and nest-site characteristics. 

Reciprocal use of nests has been documented for these 

two species on the Kaibab Plateau (R. Reynolds, personal 

communication) and may have contributed to the overlap 

in nest site characteristics if this phenomenon is also 

occurring in the Piceance Basin. Compared to the Sharpshinned Hawk, both Cooper’s Hawks and Northern 

Goshawks apparently used nest sites with larger diameter 

trees and lower densities of small trees. In addition, 

Northern Goshawk nest sites contained a lower density 

of all trees relative to the Sharp-shinned Hawk, likely 

reflecting their preference from more mature, open 

forest stands (Squires and Reynolds 1997). Overall, the 

characteristics of accipiter nest sites in pinyon–juniper 

woodlands appear to fall within the range of variation of 

basic nest-site characteristics (e.g., nest tree height and 

DBH, and average site tree DBH and density) recorded 

in other western forests (e.g., Reynolds et al. 1982, 

Siders and Kennedy 1994, Siders and Kennedy 1996). 

We found accipiter nests most commonly on slopes 

with northwestern through eastern aspects, and none 

on southeastern exposures (Figure 2). Other western 

accipiter studies have reported similar results (Hennessy 

1978, Reynolds et al. 1978, Moore and Henny 1983, 

Kennedy 1988). Our results also suggested a minimum 

patch size of 4.8-ha for accipiters nesting within pinyon–

juniper patches, which is similar to the finding that 

Northern Goshawks nesting in California occupied only 

stands >4.1 ha in size (Woodbridge and Detrich 1994).

Although we did not detect a high density of nesting 

accipiters in the Piceance Basin study area, we do not 

believe their use of pinyon–juniper for nesting is unique 

to northwestern Colorado. Few published studies have 

documented accipiter nesting use of these habitats, 

but this may be due to nest-search biases (Siders and 

Kennedy 1996, Squires and Reynolds 1997). That 

said, the only previously published account of Northern 

Goshawk nesting use of pinyon–juniper indicated that a 

fairly high 13% (n = 30) of known nests were located in 

this vegetation type (Johansson et al. 1994). Platt (1976) 

reported that 44% (n = 61) of Sharp-shinned Hawk nests 

Discussion
10 BLM Technical Note 435 • Accipiter Nesting in Pinyon-Juniper

examined in Utah were located in conifer trees, including 

pinyon and juniper, but the author did not provide any 

additional detail. In the Wasatch–Cache National Forest 

of northeastern Utah, 20% (n = 20) of Sharp-shinned 

Hawk nests were found in juniper (Hennessy 1978). 

While this is the extent of the formally published record, 

additional records do exist. For example, Smith and 

Hutchins (2006, 2007) opportunistically documented two 

Cooper’s Hawk nests in junipers and a third in a pinyon 

pine in northeastern Nevada and northwestern Utah. 

Similarly, although not located in pinyon–juniper habitat, 

five Cooper’s Hawk nests in south-central Wyoming 

were located in juniper trees (H. Cline, BLM Rawlins 

Field Office, personal communication). It is quite likely 

that additional unpublished records exist in databases 

maintained by various state and federal agencies.
11 Accipiter Nesting in Pinyon-Juniper • BLM Technical Note 435

The previous discussion suggests that accipiters nesting 

in pinyon–juniper habitats is not a phenomenon restricted 

to northwestern Colorado. With federal agencies 

managing 65% of all pinyon–juniper habitats (Smith et al. 

2004), it is critical that they are aware of the heretofore 

largely overlooked potential of these habitats to support 

nesting accipiters. We recommend that federal agencies 

responsible for managing pinyon–juniper habitats make 

an effort to incorporate this knowledge during land-use 

planning, and conduct surveys for nesting accipiters 

when management actions that may affect pinyon–juniper 

habitats are proposed (i.e., prescribed burns, mechanical/

chemical treatments, timber harvest, energy extraction, 

etc.). We have identified BLM field offices and USFS 

district offices with substantial coverage (>50,000 ha) 

of pinyon–juniper woodlands on public lands (Table 2). 

Recommendations for Pinyon–Juniper

Accipiter Nest Surveys

Table 2. Bureau of Land Management (BLM) and U.S. Forest Service (USFS) field and district offices with greater than 50,000 ha of pinyon–

juniper (P–J) habitat on public lands, ranked by total coverage area (compiled from Pugh [2004]).

BLM Field Office States P–J cover

(1,000s of ha) USFS District Office States P–J cover(1,000s of ha)

Ely NV 1313 Humboldt-Toiyabe NV, CA 973

Battle Mountain NV 527 Gila NM 599

Grand StaircaseEscalante NM FO UT 399 Cibola NM 312

Tonto AZ 301

Cedar City UT 383 Kaibab AZ 286

Arizona Strip AZ 324 Apache-Sitgreaves AZ 280

Elko NV 299 Lincoln NM 249

Monticello UT 297 Coconino AZ 249

Price UT 267 Dixie UT 217

White River CO 267 Prescott AZ 207

Richfield UT 230 Modoc CA 202

Grand Junction CO 226 Fishlake UT 179

Carson City NV 221 Santa Fe NM 179

Moab UT 220 Carson NM 145

Dolores CO 211 Coronado AZ, NM 119

Farmington NM 174 Inyo CA 108

Uncompahgre CO 173 Manti-LaSal UT 102

Vernal UT 172 Grand Mesa-Uncomp-Gunnison CO 66

Fillmore UT 159 Ochoco OR 62

Royal Gorge CO 140 Los Padres CA 56

Salt Lake UT 137 San Juan CO 50

Little Snake CO 117

Kanab UT 116

Rio Puerco NM 107

Deschutes OR 103

Glenwood Springs CO 94

Alturas CA 90

Saint George UT 77

Winnemucca NV 75

Taos NM 73

Kingman AZ 66

Soccoro NM 59

Three Rivers OR 53

Owyhee ID 53

Rock Springs WY 52
12 BLM Technical Note 435 • Accipiter Nesting in Pinyon-Juniper

Managers of these federal lands should be particularly 

cognizant of the potential for accipiters to nest in pinyon–

juniper habitats.

Numerous documents outlining raptor nest-survey 

protocols exist; hence, we do not provide a detailed 

overview of survey methodologies here. Instead, it is 

our intention to provide guidance in the identification 

of potential pinyon–juniper habitats for survey, suggest 

specific survey techniques based on our experiences 

during this study, and highlight attributes of accipiter 

nesting ecology that may aid nest surveyors.

Pinyon–juniper woodlands are highly variable in terms 

of relative pinyon and juniper composition, tree age 

and stature, and associated vegetation composition (i.e., 

pinyon–juniper does not represent a single natural land 

type; Tausch 1999). As a result, the value of pinyon–

juniper to nesting accipiters likely varies widely across 

the West and among local landscapes. To assess fully the 

potential value of pinyon–juniper habitats to accipiters at 

a specific locale will require the identification of patches 

meeting the nest-site characteristics sought by these 

species. Although Table 1 and example photographs 

of typical nest-site vegetation provided in Appendix B 

provide some initial insight into these characteristics, our 

results are based on limited sample sizes, and additional 

work is needed to refine further our understanding of 

accipiter nesting use of this vegetation type. Additionally, 

we recognize that obtaining a detailed characterization of 

pinyon–juniper habitats is often prohibitive. Therefore, 

we recommend that land managers initially use coarsescale approaches to identify potential pinyon–juniper 

accipiter nesting habitat.

We suggest that land managers begin with the 

identification of pinyon–juniper habitats on their 

lands, then isolate patches meeting minimum patchsize requirements for nesting accipiters (i.e., ≥4.8 

ha), and finally focus in on patches likely to contain 

higher densities of 23–38 cm DBH trees. Again, we 

stress that further refinement of an accipiter nest-site 

“search image” will require additional research in this 

vegetation type, and it is possible that accipiters will be 

found nesting in a wider range of conditions in different 

localities (e.g., smaller patches or wider range of canopy 

covers). Vegetation maps depicting the distribution of 

pinyon–juniper habitats are available through a variety 

of sources (e.g., see Smith et al. 2004, USGS National 

Gap Analysis Program 2004) and many federal land 

managers possess their own highly detailed vegetation 

maps. High-resolution (1 m) aerial photography is 

available for much of the United States from the U.S. 

Department of Agriculture (http://datagateway.nrcs.usda.

gov/). Aerial photography can be used to refine existing 

vegetation maps, as well as to assess patch size and tree 

density. Identification of patches with larger trees can 

be accomplished by using GIS software combined with 

field-personnel knowledge of where such patches occur. 

While some research suggests that there is no strong 

association between pinyon–juniper woodlands and any 

particular aspect (Tausch et al. 1981), in some areas, 

larger trees appear to occur on north facing slopes due to 

the relatively moist and cool conditions found there (Lei 

1999). Although the establishment and stature of pinyon–

juniper woodlands are influenced by topography, soil, 

elevation, latitude, disturbance, and other environmental 

conditions in complex ways (Tausch 1999), we believe 

it should be possible to identify where pinyon–juniper 

patches containing larger trees are most likely to occur in 

a given area with the aid of GIS software, DEMs (from 

which elevation, slope, and aspect can be derived), aerial 

photography, and limited ground-truthing.

If broadcast surveys are used to locate nests, we suggest 

playing Sharp-shinned Hawk calls first and Northern 

Goshawk calls last, to avoid potential interspecific 

inhibitory effects. Alternatively, broadcasting Great 

Horned Owl calls also is effective in eliciting responses 

from a variety of raptor species (Mosher and Fuller 

1996). Personnel conducting broadcast surveys should 

be aware that jays often mimic broadcasts. For example, 

Steller’s jays (Cyanocitta stelleri) accounted for 17% 

of all “positive” broadcast survey detections in New 

Mexico (Kennedy and Stahlecker 1993). When they 

detect a response, we suggest that personnel record their 

confidence level in the assessment of the responding 

species and provide detailed notes based on that 

confidence level. Additionally, we recommend surveyors 

employ a minimum of two broadcast survey rounds 

during the nesting season, but as many as three or four 

intensive surveys may be required to accurately assess 

their presence/absence or generate robust estimates of 

goshawk abundance (Boyce et al. 2005). More generally, 

if the goal is to generate accurate assessments of nesting 

density or more thoroughly evaluate presence/absence 

of nesting birds in a specific area, more extensive, 

systematic surveys than we did in this study, as well as 

rigorous assessments of the probability of detection in 

the particular habitat(s) and study area of interest, will be 

required.

We caution that it can be very difficult to locate active 

accipiter nests in pinyon–juniper habitats from response 

locations after the birds have left the nest. We recorded 

responses associated with active nests as much as 540 

m from the actual nest. Additionally, accipiters will 

often fly toward broadcast locations prior to responding. 
13 Accipiter Nesting in Pinyon-Juniper • BLM Technical Note 435

Therefore, when attempting to locate actual nests, it is 

critical to visit potential vegetation during the breeding 

season (i.e., pair formation through post-fledging 

period). Although one can effectively locate nests 

using comprehensive foot surveys outside the breeding 

season, such efforts will require follow-up breeding 

season visits to determine nest status and species 

associations. Territories of all three accipiter species 

may contain alternate nests and individual nests may 

not be used in consecutive years. Northern Goshawks 

are the most likely to reuse nests, while Sharp-shinned 

Hawks and Coopers’ Hawks may build a new nest each 

year, but often close (~100 m) to the previous year’s nest 

(Reynolds and Wight 1978). When attempting to locate 

nests in pinyon–juniper, prior experience (Ed Hollowed, 

BLM WRFO, personal observation) and our efforts 

suggested that searching for stick piles on the ground 

(i.e., fallen nesting material; see Figure 3) was often more 

effective than searching within potential nest trees, as 

nests were often hidden by canopy vegetation.

In conclusion, we reiterate that it was not our intent here 

to outline thoroughly a robust survey protocol for nesting 

accipiters, the details of which should be designed around 

specific objectives and in relation to the particular habitats 

and study area being investigated. Other resources 

currently available to help guide such endeavors 

include, for example, U.S. Forest Service protocols for 

goshawk surveys (Woodbridge and Hargis 2006), a basic 

woodland raptor “clearance” survey protocol developed 

by the BLM White River Field Office (Smithers 2007), 

general guidelines for developing raptor nest-monitoring 

programs recently compiled for the BLM by Smith et al. 

(2009), and more generally several chapters contained in 

Bird and Bildstein (2007).

Figure 3. Typical stick pile observed below an accipiter nest in the Piceance Basin study area.

15 Accipiter Nesting in Pinyon-Juniper • BLM Technical Note 435

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Service, Washington, DC, U.S.A.
17 Accipiter Nesting in Pinyon-Juniper • BLM Technical Note 435

Appendix A.

Example survey data sheets.

Figure A1. Example accipiter broadcast survey data sheet. See the methods section for a detailed overview of the survey protocol.
18 BLM Technical Note 435 • Accipiter Nesting in Pinyon-Juniper

Figure A2. Example vegetation survey data sheet used at 0.04-ha plots centered on accipiter nests. See the methods section for a detailed 

overview of the survey protocol.
19 Accipiter Nesting in Pinyon-Juniper • BLM Technical Note 435

Appendix B.

Example nest-area photographs.

Figure B1. Photograph of typical Sharp-shinned Hawk nesting area in the Piceance Basin, Colorado. The nest is indicated with an arrow.
20 BLM Technical Note 435 • Accipiter Nesting in Pinyon-Juniper

Figure B2. Photograph of typical Cooper’s Hawk nesting area in the Piceance Basin, Colorado. The nest is indicated with an arrow.
21 Accipiter Nesting in Pinyon-Juniper • BLM Technical Note 435

Figure B3. Photograph of typical Northern Goshawk nesting area in the Piceance Basin, Colorado. The nest is indicated with an arrow.



The mention of trade names or commercial products does not constitute endorsement

or recommendation for use by the Federal Government.
BLM Technical Note 432 Raptor Nesting Near Oil and Gas Development:

An Overview of Key Findings and Implications

for Management Based on Four Reports by

Hawk Watch International 

BLM Technical Note 433 An Assessment of the Effects of Oil and Gas Field 

Activities on Nesting Raptors in the Rawlins, 

Wyoming and Price, Utah Field Offices of the 

Bureau of Land Management

BLM Technical Note 434 Artificial Nest Structures as Mitigation for NaturalGas Development Impacts to Ferruginous Hawks 

(Buteo regalis) in South-Central Wyoming

BLM Technical Note 435 Accipiter Use of Pinyon–Juniper Habitats for 

Nesting in Northwestern Colorado

BLM Technical Note 436 Recommendations for Improved Raptor Nest 

Monitoring in Association with Oil and Gas 

Development Activities

These five documents are an integrated series.