BLM Technical Note 434

Artificial Nest Structures as Mitigation for 

Natural-Gas Development Impacts to 

Ferruginous Hawks (Buteo regalis)

in South-Central Wyoming

BLM
BLM/WY/ST-10/009+1110
Mike C. Neal, Jeff P. Smith, and Steven J. Slater1

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 434

Artificial Nest Structures as Mitigation for 

Natural-Gas Development Impacts to 

Ferruginous Hawks (Buteo regalis)

in South-Central Wyoming

iii Ferruginous Hawk use of Artificial Nest Structures • BLM Technical Note 434

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

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

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

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

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

Study Area and Methodological

Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

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

Survey Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Analytical Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Summary of Relevent Modeling and 

Statistical Results . . . . . . . . . . . . . . . . . . . . . . . 7

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

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

Table of Contents

v Ferruginous Hawk use of Artificial Nest Structures • BLM Technical Note 434

Table 1. Ferruginous Hawk reproductive metrics by nestaccessibility categories (1976–2004). . . . . . . . . . . . 8

List of Tables

vii Ferruginous Hawk use of Artificial Nest Structures • BLM Technical Note 434

Figure 1. BLM Rawlins Field Office boundary (solid line) and 

general study area extent (dashed line) in south-central 

Wyoming. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 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). . . . . . . . . . . . . . . . . . . . . . 4

List of Figures

Figure 3. Kernel density (within a 5-km radius) of active 

Ferruginous Hawk nests in the Rawlins study area 

relative to 105 artificial nest structure (ANS) locations 

before (1970–1987, upper panel) and after (1990–

2006, lower panel) their installation. . . . . . . . . . . . . 7

ix Ferruginous Hawk use of Artificial Nest Structures • BLM Technical Note 434

exhibited opposite relationships with development and 

drought. Productivity was significantly greater at ANSs 

relative to ground or rock-feature nests, and both nest 

success and productivity were greater at inaccessible 

(to mammalian predators) nests compared to accessible 

nests. Ferruginous Hawks utilizing ANSs maintained 

fewer alternate nests. Nest attendance and prey delivery 

were greater at ANSs and other inaccessible nests, likely 

contributing to their greater success and productivity. Our 

research suggests that birds in the study area responded 

rapidly to newly available manmade structures and fared 

well on ANSs. We suggest that the rapid shift to naturalgas structures and ANSs was likely due to their location 

within attractive foraging habitat lacking natural nest 

sites. Despite the apparent success of this ANS program, 

we caution that research on post-fledging survival 

associated with ANSs must be conducted before ANSs 

can be fully endorsed as a viable mitigation tool.

The Ferruginous Hawk (Buteo regalis) may be 

particularly sensitive to human disturbance, but is also a 

highly versatile nester. South-central Wyoming supports 

one of the largest breeding populations of Ferruginous 

Hawks, as well as significant natural-gas development. 

Ferruginous Hawks were first documented nesting on 

natural-gas structures in the area during the 1980’s. 

Because nests on natural-gas structures commonly failed 

due to human activities, the Rawlins Field Office of the 

Bureau of Land Management installed 105 artificial nest 

structures (ANSs) between 1987 and 2004 to provide 

area birds with viable nesting alternatives. Herein 

we draw on previous research to assess the efficacy 

of ANSs as a mitigation tool. Overall, Ferruginous 

Hawks used 85% of all available ANSs for nesting at 

some point. Nesting activity appeared to be positively 

associated with areas of natural-gas development and 

subsequent ANS installation. However, activity patterns 

at nest clusters (see definitions) with and without ANSs 

Abstract

xi Ferruginous Hawk use of Artificial Nest Structures • BLM Technical Note 434

Office), Dave Roberts (BLM Wyoming State Office), 

and Wes Anderson (BLM Colorado State Office) for 

their input and guidance. This manuscript also benefited 

from independent reviews provided by Heath Cline 

(BLM, Rawlins, Wyoming), Dr. Tom Thurow (University 

of Wyoming), and Dr. James Tigner (retired Wildlife 

Biologist, BLM, Wyoming).

Funding for portions of the research summarized in this 

technical note was provided by the U.S. Department 

of Energy through the BLM Utah State Office (BLM 

Assistance Agreement JSA065003) as part of a larger 

undertaking called the “Raptor Radii Research Project.” 

We wish to thank members of the Radii Project Steering 

Committee, which included Dave Mills (BLM Utah, 

Richfield Field Office), Steve Madsen (BLM Utah State 

Acknowledgements

1 Ferruginous Hawk use of Artificial Nest Structures • BLM Technical Note 434

11 artificial nest structures (ANSs). The objective 

was to provide alternative substrates for birds nesting 

on natural-gas infrastructure, and thereby mitigate for 

apparent negative effects of natural-gas development 

on Ferruginous Hawk nesting success. Additional ANS 

installations continued to occur in subsequent years to 

mitigate for attempts by other hawks to nest on naturalgas structures. By 1991, 31 ANSs were available in the 

RFO, and by 2004 the number had increased to 105, with 

100 of those specifically installed for Ferruginous Hawks 

and the rest for Golden Eagles (Aquila chrysaetos) and 

Red-tailed Hawks (B. jamaicensis).

Initial results for the 1988–1991 breeding seasons 

suggested that Ferruginous Hawks using ANSs produced 

an average of 2.7 fledglings per nest, compared to a 

range of “natural” nests that produced an average of 

2.2 fledglings per nest (Tigner and Call 1996). While 

these results were informative, they were based on small 

sample sizes and largely haphazard sampling of natural 

nests. In 1995, inventory and monitoring efforts were 

increased to keep pace with natural-gas development, and 

in 2000 author Mike Neal began coordinating the project 

and further standardized the inventory and monitoring 

efforts throughout the RFO.

These factors culminated in an effort to determine if 

ANSs were more frequently active, successful, and 

productive than natural nests within the study area, and 

the resulting investigations ultimately formed the core 

of Neal’s master’s thesis at the University of Wyoming 

(Neal 2007), as well as a key component of the analyses 

presented in Smith et al. (2010). Here we summarize 

the analytical results presented in these two documents 

and synthesize resulting insight to formally evaluate the 

efficacy of ANSs as a mitigation tool for off-setting the 

effects of natural-gas development in the RFO. Together 

this report and Smith et al. (2010) comprise the primary 

products resulting from Phase I of the “Raptor Radii 

Research Project,” for which HawkWatch International 

served as the Principal Investigator.

The Ferruginous Hawk (Buteo regalis) is commonly 

described as a species of raptor that is particularly 

sensitive to human disturbance (White and Thurow 1985, 

Bechard et al. 1990, Olendorff 1993), but also is one of 

the most versatile in use of nest substrates. Across its 

range, the Ferruginous Hawk is known as a consummate 

open-country specialist, and is known to nest on a diverse 

array of natural substrates, including the ground, piles of 

bison bones, small rock piles, larger rock outcroppings 

and cliffs, stout shrubs, low-growing trees, and a variety 

of erosional formations (Olendorff 1993, Bechard and 

Schmutz 1995, Neal 2007). Ferruginous Hawks have 

also nested on a variety of manmade substrates, including 

chimneys or roofs of abandoned buildings, windmills, 

haystacks, shelterbelts, and power-line towers (Gaines 

1985, Olendorff 1993), and more recently natural gas 

infrastructure (Tigner and Call 1996, Neal 2007).

In the late 1970s, Bureau of Land Management (BLM) 

personnel in the Rawlins Field Office (RFO) in southcentral Wyoming became concerned about the proximity 

of proposed natural-gas developments to historically 

active Ferruginous Hawk nests and prime foraging areas 

(J. Tigner personal communication). South-central 

Wyoming supports one of the largest known breeding 

populations of Ferruginous Hawks (Olendorff 1993). 

The species is currently listed as a state sensitive species 

in Wyoming and in most other states where it occurs, 

and garners similar recognition in several state-level 

and bird conservation region (BCR) plans developed by 

the overarching Partners in Flight avian conservation 

program (Rich et al. 2004). In 1980, at which point 

roughly 1,500 natural gas wells had been installed in the 

RFO, the first pair of Ferruginous Hawks nested on a 6-m 

tall, cylindrical, natural-gas condensation tank. Although 

these structures offer a flat, elevated substrate for nesting, 

they require regular maintenance visits by industry 

personnel. In the 1980s, when a further rapid expansion 

of natural-gas development occurred in the RFO, 

anecdotal evidence suggested that industry operators 

frequently destroyed nests or maintenance activities 

prompted nest abandonment by breeding adults.

In autumn 1987, the BLM and some of the natural-gas 

companies operating in the area cooperated in erecting 

Introduction

3 Ferruginous Hawk use of Artificial Nest Structures • BLM Technical Note 434

Study Area

The BLM – RFO administers an area encompassing 

approximately 1.72 million hectares of public land 

(Figure 1), largely associated with the northern 

sagebrush-steppe ecoregion (Kuchler 1964). The study 

area for analyses included all public land and any tracts 

of accessible private land within the RFO, encompassing 

44,425 km2. The BLM administers 223 Key Raptor 

Areas nationally. Individual field offices identified 

such areas based on biological and administrative 

characteristics. Key Raptor Areas have unusually high 

raptor nesting densities and habitat is a key activity-plan 

issue surrounding commodity production values (oil 

and gas, coal, gravel, etc.; Olendorff 1989). Throughout 

the shrubsteppe ecoregion, 78 such areas have been 

delineated, with 13 (17%) of these located within the 

RFO and specifically recognizing Ferruginous Hawks as a 

primary species of concern (Olendorff and Kochert 1992).

Within the RFO, a variety of natural substrates have 

provided traditional nesting locations for Ferruginous 

Hawks (Tigner and Call 1996), such as erosional 

formations, rock pillars and outcrops, hillsides, sagebrush 

(Artemisia spp.), juniper trees (Juniperus spp.), limber 

pines (Pinus flexilis), and aspen/cottonwood trees 

(Populus spp.). Documented use of manmade structures 

includes power poles, a pump house, an abandoned 

sheepherder’s wagon, a windmill, and, more recently, 

natural-gas infrastructure and ANSs (Neal 2007). Most 

ANSs in the RFO were erected within 1 km of natural gas 

structures (e.g., condensation tanks) where Ferruginous 

Hawks had constructed nests during the same or previous 

nesting season. Any remaining nest material was moved 

from the natural-gas structure to the relevant ANS. ANSs 

were placed such that the spatial relationships to other 

nearby natural-gas infrastructure were similar to those 

associated with the natural-gas structure used for nesting. 

The rationale for such placements assumed that the 

Ferruginous Hawks would return to the area and use the 

newly available ANS containing their nest materials. A 

handful of other ANSs were erected to substitute for nests 

that had been built on other decrepit manmade structures 

unrelated to natural-gas development.

The ANSs used in the RFO consisted of a 3.5-m tall, 

pressure-treated post sunk 1 m into the ground, which 

Study Area and Methodological Background

Figure 1. BLM Rawlins Field Office boundary (solid line) and general study area extent (dashed line) in south-central Wyoming.
4 BLM Technical Note 434 • Ferruginous Hawk use of Artificial Nest Structures

supported a 1-m2 nest platform (Figure 2; from Tigner 

and Call 1996). The nest platform was made of a 2” x 

6” board frame, including one longer board to provide a 

perch to the side of the main nest area, and with hardware 

cloth stapled to the top of the frame to provide a base 

for the beginnings of a sagebrush “nest wreath.” The 

platform was then bolted to the top of the post. Many 

early prototypes were constructed with limber pine 

branches extending above the nest to help stabilize nest 

materials and nestlings, and to deter avian predators. 

While the addition of nest material or the sagebrush 

wreath helped attract breeding hawks to newly installed 

ANSs, the limber pine branches proved to be unnecessary. 

Due to the elevated platform created by ANSs, hawks 

augment nest construction 2.5 m above ground level.

Survey Methods

Detailed survey histories and methods are provided in 

Neal (2007) and Smith et al. (2010). Briefly, somewhat 

haphazard, annual nest monitoring, including gathering 

of limited productivity and mortality data, began in 

the RFO in 1976, although no data were collected for 

the 1977 nesting season. More consistent Ferruginous 

Hawk nest monitoring occurred from 1988 through 

2004, while surveys in 2005 and 2006 were limited to 

priority nests (i.e., ANSs and other nests with documented 

activity in the previous 10 years; Heath Cline, Rawlins 

BLM, personal communication). The 1988–2004 

efforts focused on documenting all active or occupied 

Ferruginous Hawk nests located on accessible public land 

(federal and state) within the RFO, with special emphasis 

on ANSs, high-density nesting areas, and developmentrelated project areas. All raptor nest locations were 

hand-plotted on USGS 7.5’ quad maps, recorded in the 

BLM GIS database, and georeferenced in ArcMap 9.1 

(Environmental Systems Research Institute [ESRI], Inc., 

Redlands, CA).

Surveys generally involved visits during April and May 

to document the initial activity status of nests, second 

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).
5 Ferruginous Hawk use of Artificial Nest Structures • BLM Technical Note 434

visits in late May and early June to all “occupied” nests to 

reassess status and age any chicks present, and final visits 

in late June or early July to enumerate “near-fledglings” 

(i.e., chicks that reached at least 80% of the average 

fledging age for the species: 32–34 d). Most surveys 

occurred on the ground, but occasionally by way of aerial 

transects. “Active” nests were those where reproductive 

activities such as copulation, incubation, brooding, 

and nest attendance were observed. “Occupied” nests 

were those where one or more hawks were observed in 

the immediate vicinity of the nest or signs of presence 

(e.g., recent addition of nest materials, mute, pellets, 

or prey remains) were documented during the breeding 

season. Nests where no physical presence or signs of 

recent use were observed were classified as “inactive.” 

“Alternate” nests were found within “nest clusters” 

(sensu Smith et al. [2010] and explained further below) 

in close proximity to well-established primary nests. In 

this context, designation of an “active alternate” nest 

implied that the nest was recently tended but another nest 

within the cluster was used for actual nesting, whereas 

designation of an “inactive alternate” nest implied a 

similar circumstance but that the nest had not been tended 

recently. “Used” nests received a final classification of 

active, active-failed (depredation specified as cause of 

failure when relevant), or occupied. Successful nests 

were defined as those in which at least one near-fledgling 

was produced, and “productivity” was estimated as the 

actual number of near-fledglings produced (Steenhof 

and Newton 2007). Nestling age was estimated using 

Moritsch (1985).

For purposes of the analyses summarized here, an 

“inaccessible” nest was defined as a nest that was >2.5 m 

above ground or otherwise could not be reached by most 

mammalian predators or an average human without the 

aid of specialized equipment (Neal 2007). The majority 

of nests classified as “accessible” were located on natural 

substrates and included no ANSs, whereas inaccessible 

nests typically were located on manmade substrates 

and especially ANSs. Accordingly, analyses comparing 

accessible and inaccessible nests largely reflected 

contrasts between natural and manmade nesting substrates 

and non-ANS versus ANS nests.

Among most raptor species, individual breeding pairs 

often maintain variable numbers of clustered, alternative 

nest sites, which they may tend each year and use in 

different years for actual nesting (Steenhof and Newton 

2007). This means that in a given year no eggs will be 

laid in many nests in an area only because established 

breeding pairs laid their eggs in other nests within a 

given territory, which includes a cluster of nests. In 

turn, this means that the best definition of a sample unit 

for any multi-year assessments is for it to represent 

identifiable “nest clusters” used by individual breeding 

pairs (analogous to the “nesting territory” definition 

used by Steenhof and Newton [2007] but avoids 

potential confusion related to the ethologists’ concept 

of territoriality). We used a combination of GIS-based 

nearest neighbor analyses, graphical consideration of 

topographic features, and inspection of individual nest 

histories to identify all distinct Ferruginous Hawk nest 

clusters within the RFO (Neal 2007, Smith et al. 2010).

Analytical Methods

We derived all analytical results discussed here from 

Neal (2007) and Smith et al. (2010), where detailed 

descriptions of data-preparation and analytical methods 

can be found. Here we provide a brief review of key 

methodological elements and introduce some important 

terminology, and then follow in the next section with a 

concise synthesis of results germane to evaluating the 

efficacy of ANSs as a mitigation tool for Ferruginous 

Hawks in the RFO. For several reasons, most of the 

analyses presented in Smith et al. (2010) evaluated 

patterns and relationships apparent from 1998 through 

2006, whereas most of the analyses presented in Neal 

(2007) incorporated data from a longer period: 1976–

2004.

In classifying the breeding status of nests and nest clusters 

for analytical purposes, we used three overlapping, 

binomial classification scenarios, and we conducted 

separate logistic regression analyses for each to evaluate 

differences and similarities in the results:

(1) “Used” = evidence of recent nest tending or actual 

breeding attempt (eggs laid) obtained

“Unused” = no such evidence obtained, despite nest 

check

(2) “Active” = breeding attempt confirmed

“Other” = all other cases, including some used (i.e., 

evidence of tending, but not actual breeding) and all 

unused classifications

(3) “Successful” = one or more chicks reared to at least 

80% of the average fledging age for the species (32-

34 d)

“Failed” = breeding attempt confirmed but no chicks 

reared to 80% of fledging age
6 BLM Technical Note 434 • Ferruginous Hawk use of Artificial Nest Structures

We developed and evaluated multiple-regression and 

multiple-logistic-regression models that described 

relationships between Ferruginous Hawk nesting activity 

and potential predictor variables, which included:

1) Oil and Gas (OG) Development Factors derived from 

Principal Components Analysis (PCA) of original 

variables describing well and road densities and 

proximities at two spatial scales: within 0.8-km and 

2.0-km radii of nest clusters.

2) Climate variables, including average current and 

prior-year Palmer Drought Severity Indexes and 

winter (Nov – Feb) precipitation totals (NOAA 2007).

3) Vegetation Factors derived from PCA of original 

variables describing proportional coverage of roughly 

20 landcover/vegetation classifications represented 

in high-quality, 30-m-resolution maps derived from 

classified satellite imagery.

4) Selected two-way interaction terms; i.e., between 

development factors and vegetation/climate variables.

The dependent variables for these analyses included 

several “annual activity” metrics applied to ANS and 

non-ANS nest clusters, as well as a “proportional 

activity” metric applied to only non-ANS clusters. For 

the annual-activity analyses, the sample units were 

individual-year nest-status and productivity records. The 

dependent variables included three binomial response 

variables analyzed with multiple logistic regression: 

“used” versus “unused”, “active” versus “other”, and 

“successful” versus “failed.” The response variable for 

the proportional activity models (analyzed with multiple 

regression) was the proportion of years between 1998 

and 2006 in which a given nest cluster was either used 

or active, with analyzed nest clusters restricted to those 

for which at least five years of consecutive annual 

monitoring data were collected during the relevant period. 

We also analyzed two other suites of annual-activity 

models with multiple regression, where productivity was 

the continuous response variable. We independently 

considered all nest starts in one suite of models and only 

successful nests in the other suite of models. Modeling 

of annual-activity relationships with nesting success 

(successful or failed) and productivity as the dependent 

variables also involved independent analyses for 

accessible and inaccessible nests.

To provide some perspective on samples sizes, the 

original unfiltered 1976–2006 dataset available for 

analyses encompassed 3,745 individual Ferruginous 

Hawk nests, which we divided into 463 distinct nest 

clusters and among which 2,096 documented nest starts 

occurred. However, only 417 nest clusters and 1,234 nest 

starts were associated with adequate survey methods and 

histories to include in the analyses. Of the 1,234 nesting 

attempts, 750 (61%) occurred at 89 different ANSs, 88 

(7%) at 13 other manmade-substrate nests, and 396 (32%) 

at 315 natural-substrate nests (mostly rocky-substrate, 

ground-hillside, and evergreen-tree nests).
7 Ferruginous Hawk use of Artificial Nest Structures • BLM Technical Note 434

Figure 3. Kernel density (within a 5-km radius) of active Ferruginous 

Hawk nests in the Rawlins study area relative to 105 artificial nest 

structure (ANS) locations before (1970–1987, upper panel) and after 

(1990–2006, lower panel) their installation.

Between 1987 and 2004, 89 of 105 (85%) available 

ANSs were used at some point by Ferruginous Hawks, 

with annual occupancy rates ranging from 52–70% (Neal 

2007). The average time between installation and use of 

ANSs (n = 98) by nesting Ferruginous Hawks was 0.68 ± 

SE of 0.15 years (Neal 2007). Moreover, a comparison 

of pre-1988 and post-1990 kernel densities of active 

Ferruginous Hawk nests (Kernel Density Estimator, 

Spatial Analyst Extension, ArcGIS 9.2; ESRI, Inc. 

2007) confirmed a distinct distributional shift related to 

the availability of ANSs. Although pockets of limited 

nesting activity remained in other scattered locations, 

the activity centers clearly shifted to ANS locations once 

they were available (Figure 3). We also examined the 

distributions of 54 well-monitored Ferruginous Hawk 

nest clusters in relation to three energy-development 

“hotspots” in the RFO between 1998 and 2006 (Neal 

2007, Smith et al. 2010). Overall, active nest clusters 

averaged significantly closer to development-hotspot 

centers than inactive clusters. In combination, these 

data illustrate that human provisioning of new substrates 

(natural-gas infrastructure and ANSs) led to a rapid shift 

in Ferruginous Hawk nesting activity to areas favored 

for natural-gas development. In turn, this suggests that 

these core development areas provided prime foraging 

habitat for the species, but previously did not afford 

viable opportunities for nesting due to a lack of suitable 

substrates.

Multivariate modeling of apparent relationships between 

Ferruginous Hawk nesting activity and various OG 

development factors, climate variables, and vegetation 

factors revealed stark contrasts between results for nonANS and ANS nest clusters. For example, both the 

annual-activity and proportional-activity models indicated 

that the probability of nesting at non-ANS clusters 

increased in areas of low overall OG development activity 

and where the relative prevalence and proximity of nonOG roads was high within 0.8 km but low at the 2.0 km 

scale. We detected exactly the opposite relationship 

between annual nest activity and development at ANS 

clusters, suggesting that nest accessibility serves to alter 

substantially the relationship between nesting Ferruginous 

Hawks and development. Similarly, we found that 

activity at non-ANS nest clusters tended to increase as 

annual drought severity decreased, whereas ANS clusters 

appeared to benefit from increasing current-year drought 

severity. We hypothesize that the latter relationship may 

derive from the fact that OG structures and water sources 

uniquely associated with development (e.g., evaporation 

ponds) may concentrate prey during drought periods by 

providing otherwise scarce water and shade (Smith et 

al. 2010). Again, these results suggest that the nesting 

ecology of ANS birds differed markedly from that of 

pairs using primarily accessible natural nests. Finally, we 

found that top models of annual nest activity generally 

performed poorly (i.e., failed relevant goodness-of-fit 

tests) for both ANS and non-ANS nest clusters. We 

suggest this was because nest accessibility was the 

primary driver of Ferruginous Hawk nesting success 

and productivity in this system. As a result, we believe 

additional development, climate, and vegetation variables 

added little to the explanatory ability of top models.

Neal (2007) also found that accessibility was a better 

predictor of nest success and productivity than substrate. 

In his study, all nests were classified into 10 nest-substrate 

classes and 4 “accessibility” categories (i.e., manmade 

accessible and inaccessible, and natural accessible and 

inaccessible). Statistical analyses (independent ANOVA 

and logistic regression analyses) revealed that nest 

substrate was a significant predictor of only productivity 

Summary of Relevant Modeling

and Statistical Results
8 BLM Technical Note 434 • Ferruginous Hawk use of Artificial Nest Structures

per nest start. Post-hoc comparisons further revealed that 

the productivity per nest start of inaccessible ANS nests 

was significantly greater than that of relatively accessible 

nests located on ground/hillside or rock features (e.g., 

rimrock and miscellaneous pillars and outcrops, but 

excluding high buttes and cliffs). In contrast, statistical 

analyses revealed that nest-accessibility was a significant 

predictor of all three modeled success and productivity 

metrics (i.e., nest success, productivity per nest, and 

productivity per nest start). Additionally, simultaneous 

evaluation of all three dependent variables against 

both nest substrate and accessibility as predictors with 

MANOVA (i.e., multivariate analysis of variance) further 

confirmed that accessibility was a much stronger predictor 

of Ferruginous Hawk nesting success and productivity. 

Overall, nest success was significantly lower at accessible 

nests (62.5 ± SE of 2.9%) compared to inaccessible 

nests (93.6 ± 0.8%; t = -10.382, df = 326, P < 0.001). 

Similarly, average productivity was significantly lower 

at accessible nests (1.61 ± SE of 0.08 large nestlings 

produced per nesting attempt) relative to inaccessible 

nests (2.74 ± 0.04 large nestlings per attempt; t = -12.166, 

df =397, P < 0.001). Comparison of summary success 

and productivity statistics for all four accessibility 

categories further revealed a consistent hierarchy, 

with natural-inaccessible nests being most successful 

and productive, followed by manmade-inaccessible, 

manmade-accessible, and finally natural-accessible 

nests (Table 1). Again, pronounced differences between 

success and productivity driven by nest accessibility may 

have swamped any potential influence of development, 

vegetation, or climate variables. We suggest success 

and productivity at accessible nests may have been more 

chance driven, due to the vulnerability of such nests to 

predators. In contrast, inaccessible nests were nearly 

always successful and at least minimally productive.

Neal’s (2007) work evaluating comparative use of 

alternate nests and nest attendance and foraging 

relationships at accessible and inaccessible nests provided 

further insight about how ANSs and nest inaccessibility 

yielded greater success and productivity for Ferruginous 

Hawks using such nests. A significant reduction in the 

number of alternate nests used suggested that birds using 

ANSs were less dependent on maintaining large suites 

of alternate nests. Several factors may be involved in 

this dynamic. For example, increased long-term stability 

of ANS substrates may allow for greater perennial use 

of individual nests. Another possibility, though purely 

speculative at present, is that the wire bases of the 

ANSs may allow for greater annual “flushing” of the 

nest substrate by rains, thereby reducing the need to 

rotate nest use to help control build-up of nest-parasites 

(Philips and Dindal 1977). Perhaps most importantly, 

however, in light of Neal’s findings, reduced sensitivity 

to disturbance from ground based activities and predators 

may be the most likely explanation for why reliance on 

multiple, alternative nests is reduced for breeding pairs 

that utilize ANSs. Mammalian predators such as coyotes 

(Canus latrans), badgers (Taxidea taxus), bobcats (Lynx 

rufus), and foxes (Vulpes or Urocyon spp.) are thought to 

be serious threats to ground nesting Ferruginous Hawks 

and recently fledged young (Bechard and Schmutz 

1995). Human intrusions also likely function directly and 

indirectly as predation events, through direct persecution 

of hawks, nest destruction, and the provision of scent 

trails or roads for other ground predators (L. Apple and 

J. Tigner personal communication; M. Neal personal 

observation). The apparent significant benefit of nesting 

on inaccessible substrates makes perfect sense in light 

of this evidence. Lokemon and Duebbert (1976) also 

suggested that Ferruginous Hawks nesting on relatively 

secure nesting substrates (i.e., trees) might be less 

sensitive to human activity than ground nesters.

Neal’s (2007) nest-attendance and prey-delivery results 

further clarified the fact that the apparent reduction in 

sensitivity to ground-based disturbance and depredation 

potential clearly translated to breeding adults at ANSs 

and other inaccessible nests being able to dedicate 

significantly more time to ensuring efficient provisioning 

and care of their young, which in turn translated to greater 

Table 1. Ferruginous Hawk reproductive metrics by nest-accessibility categories (1976–2004).

Accessibility Fledglings / Nest Start Fledglings / Succ. Nest Nesting Success

Category1 n Mean ± SE n Mean ± SE n Mean ± SE

MMAC 30 2.19 ± 0.23 25 2.76 ± 0.15 30 0.79 ± 0.07

MMIN 112 2.60 ± 0.07 111 2.82 ± 0.05 112 0.92 ± 0.09

NATAC 190 1.37 ± 0.10 112 2.51 ± 0.09 190 0.55 ± 0.04

NATIN 84 3.11 ± 0.10 83 3.19 ± 0.09 84 0.98 ± 0.01

1

 MMAC = manmade accessible; MMIN = manmade inaccessible; NATAC = natural accessible; and NATIN = natural 

inaccessible.
9 Ferruginous Hawk use of Artificial Nest Structures • BLM Technical Note 434

overall success and productivity for such nests compared 

to accessible nests. Schmutz et al. (1984) also found that 

ANSs were more successful than natural nests (95% vs. 

67% of nests reaching near fledging age) for Ferruginous 

Hawks nesting in Alberta, and concluded that ANSs 

can be a useful tool for augmenting Ferruginous Hawk 

nesting in areas where prey are available but natural nest 

sites are sparse. Suter and Joness (1981) also suggested 

that nest security could be an important determinant of 

raptor responses to human disturbance.

A final bit of evidence that further testifies to the relative 

susceptibility of accessible natural nests to mammalian 

predation is that over the years a total of 13 adults and 8 

nestlings were confirmed killed by mammalian predators 

at such nests, whereas only two other mammalian 

depredation events were ever documented and these 

involved adults killed away from their nests (Neal 2007).

11 Ferruginous Hawk use of Artificial Nest Structures • BLM Technical Note 434

Analyses of reproductive metrics clearly demonstrated 

the importance of accessibility to the nesting ecology of 

Ferruginous Hawks. Significant differences in vital rates 

for accessible and inaccessible nests suggested that nest 

accessibility to mammalian predators and subsequent 

depredation might be the primary drivers of Ferruginous 

Hawk nest reoccupancy rates, success, and productivity in 

south-central Wyoming. Trade-offs between foraging and 

predator avoidance may affect survival and reproduction, 

especially if foraging profitability and predation risk 

differ among substrate choices (Sharp and Van Horne 

1998). It appeared that natural-inaccessible substrates 

provided Ferruginous Hawks with the highest quality 

nesting sites, based on nest success and productivity. At 

accessible nests, success and productivity appeared to 

increase with long-term, consistent nest use. In contrast, 

success and productivity were higher on average at 

inaccessible nests, but did not vary markedly with 

consistency of nest use. Documentation of repeated 

nest site use may be one of the strongest indicators of 

overall habitat suitability, as elements in the environment 

such as prey abundance, availability of perch sites, and 

previous experience may be important in influencing 

yearly nest-site selection (Stalmaster 1982). In turn, prior 

nest success may influence whether a historical nesting 

site or territory is reoccupied (Dechant et al. 2003). Our 

research confirms that Ferruginous Hawks were more 

likely to reoccupy nest sites that were inaccessible to 

mammalian predators and that reproductive output was 

greater at such sites. Nesting Ferruginous Hawks may 

benefit from high nest reoccupancy rates, due to repeated 

exposure to localized resources (also see Gaines 1985, 

McDonald and Staats 1996).

The more obvious benefits of inaccessible nests are 

minimal destruction of nests or young by predators and 

reduced nest abandonment (Tigner and Call 1996). Many 

studies have reported significant losses of eggs or young 

to mammalian predators (Lokemoen and Duebbert 1976, 

White and Thurow 1985, Keough 2006), but RFO data 

also suggest that adult mortality may be significantly 

higher at accessible nest sites. Depredation of adults 

occurred throughout the breeding season, even during 

the fledging stage. The presence of extensive naturalgas road networks exacerbates adult depredation at 

accessible nests during incubation, because high traffic 

volume keeps these roads open in spring and they act as 

efficient travel corridors for mammalian predators. While 

enhancement of Ferruginous Hawk vital rates through 

inaccessibility to mammalian predators (Schmutz et al. 

1984) is more readily quantified, increased survival of 

breeding adults may be more critical to the viability of the 

species.

Monitoring data from the RFO indicate that accessible 

nest clusters, comprised primarily of nests located on 

miscellaneous rock features, the ground or hillsides, and 

erosional features, were the norm prior to the period 

of intensive natural-gas development and availability 

of ANSs. Smith and Murphy (1978) reported that 

Ferruginous Hawk pairs constructed or renovated from 

1–5 nests within their territories, with 86% of the nesting 

pairs constructing alternate nests. Other studies have 

noted that occupied nests often occurred in groups, with 

certain habitat areas containing multiple active nests and 

other seemingly identical habitat areas containing only 

old inactive nests (Weston and Ellis 1968), and that such 

distributions may represent significant portions of local 

populations (Bates and Norvick 1992). We suggest that 

historically and in natural settings, Ferruginous Hawks 

typically play a “shell game” centering on suites of 

accessible nests that are proximate to prey resources. 

The shell game is played when hawks variably tend and 

occupy several nests prior to incubation in an attempt to 

confuse roaming mammalian predators (Neal 2007).

There is little doubt that Ferruginous Hawks responded 

to the increased availability of manmade structures in 

the RFO by shifting their activity centers to the areas 

where such substrates were made available. While this 

shift may be indicative of a strategic response to an 

altered landscape, which includes greater mammalian 

depredation risks, it may come at the price of reducing 

nesting options or putting them in closer proximity to 

human disturbances. Many of the breeding territories 

currently exhibiting the highest reoccupancy rates in the 

RFO contain a single, primarily ANS, nest site. If this 

substrate is removed or destroyed, the territory could be 

rendered useless, as Ferruginous Hawks infrequently 

build new nests, apparently preferring to repair and reuse 

old nests instead (White and Thurow 1985, M. Neal 

and J. Smith personal observation). In other instances, 

hawks may return to their previous nesting territory even 

though their previously used nests have been removed or 

destroyed (Stalmaster 1982) and attempt to nest on less 

desirable substrates to take advantage of their experience 

with local prey resources. Considering these facts, it is 

critical that extensive use of ANSs as a mitigation tool 

is accompanied by a long-term commitment to their 

maintenance to avoid the potential scenario in which a 

significant portion of the nesting population is attracted to 

nests sites that are only temporary.

Discussion
12 BLM Technical Note 434 • Ferruginous Hawk use of Artificial Nest Structures

Presumably, Ferruginous Hawks began nesting on 

natural-gas infrastructure due to the elevated platform 

they afford and their proximity to primary prey resources 

(Neal 2007). The location and density of active 

Ferruginous Hawk nest sites may be positively correlated 

with spatial proximity to their prey (Rosamonde et al. 

2003), with high nesting densities often corresponding 

to high abundance of ground squirrels (Spermophilus

spp.; Schmutz and Hungle 1988). Although no data 

were available to evaluate it, we believe the rapid and 

almost wholesale shift of Ferruginous Hawk nesting in 

the RFO to use of natural-gas structures and ultimately 

ANSs was due to these new structures providing nesting 

substrates in the midst of attractive foraging areas 

previously devoid of suitable nesting sites (Smith and 

Murphy 1978). Additionally, one might expect burrowing 

mammals to increase in abundance in OG development 

areas due to increased soil disturbance. However, even if 

ground squirrels are less abundant in these broad desert 

flats, they may be more accessible. Ferruginous Hawks 

avoid foraging in dense vegetation due to reduced prey 

vulnerability (Sharp and Van Horne 1998, Dechant et al. 

2003).

Clearly, under certain circumstances such as those 

that pertained in the RFO, ANSs can be very effective 

in directly mitigating for nests that may be destroyed 

or prone to disturbance from activities such as OG 

development. That said, we submit several caveats 

regarding their utilization:

1) Hawks previously habituated to disturbance or 

the use of manmade substrates are more likely to 

occupy ANSs.

2) Optimal occupancy occurs only when ANSs are 

placed proximate to areas with extensive prey 

bases (Call 1979, Howard 1980, and Olendorff et 

al. 1981).

3) Nestling and egg relocation to ANSs may be most 

effective when the new site is within 1 km and 

line-of-sight (Stalmaster 1982).

4) ANSs may not always prove acceptable to 

Ferruginous Hawks because they sometimes 

attract non-target species and they are typically 

more conspicuous than natural sites (Olendorff 

and Stoddard 1974).

5) Adequate resources should be allocated for the 

long-term monitoring, repair, and replacement of 

ANSs.

6) In dense development areas, installation of ANSs 

may create or aid in perpetuating an “ecological 

trap” (i.e., an inappropriate attraction to 

ultimately poor quality habitat [Gates and Gysel 

1978]).

Very few data exist regarding survival of post-fledgling 

Ferruginous Hawks (Zelenak et al. 1997, Keough 

2006). Survival of young at this stage may be very 

low, primarily due to inexperience in acquiring prey 

and avoiding predators and other agents of mortality 

(Mannan et al. 2004). If post-fledgling survival is 

low, estimates of the number of young fledged per 

nest typically reported in reproductive studies are poor 

estimates of actual reproductive success (Zelenak et al. 

1997). Human activity near nests may raise the mortality 

rate by causing nestlings to fledge prematurely and 

render them more susceptible to predation (White and 

Thurow 1985). Fledglings from ANSs are thrust into a 

landscape containing a matrix of well-used roads that 

likely expand the distribution of mammalian predators 

by providing more efficient travel corridors for them. 

While a distributional shift to manmade-inaccessible nest 

sites that are more proximate to available prey resources 

may offer advantages in the short-term, in the longterm it may restrict nesting options, promote premature 

fledging, and decrease post-fledging survival. As such, 

a shift to nesting on ANSs may represent an ecological 

trap. It is currently unknown whether raptors possess the 

requisite feedback mechanisms to respond to decreased 

post-fledgling survival. Until future research more 

fully addresses issues of post-fledgling survival, overall 

foraging-range habitat use, and long-term population 

growth and recruitment dynamics in similar systems, it 

would be unwise to speculate whether ANS mitigation for 

large-scale natural-gas development actually introduces 

“source” or “sink” dynamics to existing Ferruginous 

Hawk populations (Neal 2007).

Another significant unknown and possible problem 

with regard to large-scale application of ANSs as a 

management tool for Ferruginous Hawks concerns the 

possibility that such could potentially result in a longterm change in the nesting behavior of the species. 

Clearly, at least in this south-central Wyoming ecological 

system, the shift to use of ANSs resulted in a significant 

change in the nest-use, nest-defense, and nestlingprovisioning behaviors of the associated breeding 

birds compared to those nesting on natural substrates. 

Although we can only speculate on what any potential 

alteration in nesting ecology may mean for Ferruginous 

Hawks in the RFO in the future, we do believe the topic 

deserves consideration, given the scale at which ANSs 
13 Ferruginous Hawk use of Artificial Nest Structures • BLM Technical Note 434

are being used in the area. In the worst-case scenario, 

one might envision a population that, across several 

generations and through inter-generational transfer of 

learned behavior, can no longer function effectively in 

otherwise typical Ferruginous Hawk nesting situations. 

While this extreme may be unlikely, given the apparent 

adaptability of Ferruginous Hawks to a variety of both 

natural and manmade nesting substrates (Gaine 1985, 

Olendorff 1993, Tigner and Call 1996, Bechard and 

Schmutz 1995, Neal 2007), land managers should give 

thought to the potential dangers that may be associated 

with drastically altering the natural nesting ecology of 

a species. If there was a good reason to suspect that 

Ferruginous Hawk nesting use of ANSs would produce 

negative effects, it may be preferable to avoid immediate 

mitigation for use of OG structures and thereby “force” 

birds exposed to such developments to learn through 

repeated nest failures that nesting on OG infrastructure is 

not productive, with the hope that after a few failures they 

would return to nesting on natural substrates. However, 

given that a number of birds in the RFO successfully 

nested on OG structures and current legal protection of 

raptor nests under the Migratory Bird Treaty Act, this 

option may not be realistic.

Yet another facet of evaluating the overall merits of 

ANSs as a mitigation tool, which is worthy of attention 

but again impossible to evaluate in the current context, 

concerns possible impacts on the underlying ecosystem 

of changing the distribution and ecology of nesting 

Ferruginous Hawks. For example, significant concern 

has emerged recently over the possible influence 

that installation of power-lines and other manmade 

structures in habitats otherwise devoid of elevated 

hunting perches and nest substrates for raptors may 

be having on sensitive species such the Greater SageGrouse (Centrocerus urophasianus). Growing evidence 

suggests that the availability of manmade hunting perches 

may significantly increase depredation of sage-grouse 

by Golden Eagles and lead to the demise of historic 

breeding-display leks. Other sensitive species that may 

be similarly impacted in relevant habitats include for 

example, Burrowing Owls (Athene cunicularia), which 

typically occupy relatively open habitats compared to 

most other raptor species and are known to be susceptible 

to predation by other raptors (Klute et al. 2003), as well 

as species such as white-tailed prairie dogs (Cynomys 

leucurus). Ripple effects of shifts in the distribution of 

Ferruginous Hawks on populations of other key prey 

species, such as ground squirrels, cottontails (Sylvilagus

spp.), and jackrabbits (Lepus spp.), also may occur with 

unknown consequences for overall ecosystem stability.

Thus, while the evidence gathered in the RFO clearly 

suggests that ANSs served effectively to mitigate for 

use of natural-gas structures as substrates for nesting 

Ferruginous Hawks, at this point we cannot fully evaluate 

the long-term implications for the overall health of the 

population and the ecosystem as a whole.

15 Ferruginous Hawk use of Artificial Nest Structures • BLM Technical Note 434

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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.