Cooper’s Hawk and Long-Eared Owl

Nest Occupancy and Productivity

in Piceance Basin, Colorado

Technical Note 442  May 2013
Suggested citation:

Smithers, B.L. 2013. Cooper’s Hawk and Long-Eared Owl Nest Occupancy and Productivity in Piceance 

 Basin, Colorado. Tech Note 442. Bureau of Land Management, White River Field Office, Meeker, CO.

Production services provided by:

Bureau of Land Management

National Operations Center

Information and Publishing Services Section

P.O. Box 25047

Denver, CO 80225

BLM/OC/ST-13/003+6600
Cooper’s Hawk and Long-Eared Owl

Nest Occupancy and Productivity

in Piceance Basin, Colorado

Technical Note 442  May 2013

Brett Smithers

Wildlife Biologist

Bureau of Land Management

White River Field Office

Meeker, Colorado
ii  Technical Note 442  Cooper’s Hawk and Long-Eared Owl Nest Occupancy and Productivity in Piceance Basin, Colorado

Acknowledgments

The author expresses appreciation to Brady Dunne for assisting with data 

collection and for providing helpful comments regarding the development 

of this report. Without his ability to work with people and adapt to 

unique and often stressful situations, the successful outcome of this 

project would be uncertain. Brady’s contribution to this project included 

many hard-worked hours searching for raptor nests, fighting off biting 

insects, and providing valuable insight into the daily nesting activities of 

Accipiter. The author also thanks Ed Hollowed, with the Bureau of Land 

Management’s White River Field Office, for his continued interest and 

support. Lastly, the author extends thanks Dr. Clint Boal, with the Texas 

Cooperative Fish and Wildlife Research Unit at Texas Tech University, for 

reviewing various drafts of this document and for providing inspiration 

to publish this information. Funding for this project was provided by the 

Bureau of Land Management.
Technical Note 442  Cooper’s Hawk and Long-Eared Owl Nest Occupancy and Productivity in Piceance Basin, Colorado  iii

Table of Contents

Table of Contents

Abstract . . . . . . . . . . . . . . . . . . . . . v

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

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

Methods . . . . . . . . . . . . . . . . . . . . . 5

Statistical Analysis . . . . . . . . . . . . . . . . 9

Results . . . . . . . . . . . . . . . . . . . . .11

Discussion . . . . . . . . . . . . . . . . . . . 15

Literature Cited . . . . . . . . . . . . . . . . .21
iv  Technical Note 442  Cooper’s Hawk and Long-Eared Owl Nest Occupancy and Productivity in Piceance Basin, Colorado

The purpose of this 

publication . . . is to expand 

the current understanding

of Cooper’s hawk

(Accipiter cooperii) and 

long-eared owl

(Asio otus) nesting density, 

nest occupancy,

and productivity information 

in areas affected by

oil and gas development.
Technical Note 442  Cooper’s Hawk and Long-Eared Owl Nest Occupancy and Productivity in Piceance Basin, Colorado  v

Abstract

Abstract

The purpose of this publication is to convey research 

findings to the Bureau of Land Management and 

other federal and state agencies to expand the 

current understanding of Cooper’s hawk (Accipiter 

cooperii) and long-eared owl (Asio otus) nesting 

density, nest occupancy, and productivity information 

in areas affected by oil and gas development. The 

Piceance Basin in northwestern Colorado was 

chosen as the study area because it has a high 

density of nesting Cooper’s hawk and oil and gas 

exploration, extraction, and production activities. 

Of the 304 nest structures visited during the 2011 

breeding season, 34% were classified as occupied, and 

65% were classified as unoccupied. Of the occupied 

nests in which the outcome of the nesting attempt 

(i.e., failed, successful, or unknown) was recorded, an 

observed success rate of 50% and a nest failure rate 

of 34% was recorded. Consequently, the outcome 

of the nesting attempt was not recorded at 16% of 

the monitored nests. When considering all 2011 

Cooper’s hawk and long-eared owl nesting attempts, 

fledging rates were 1.3 (± 0.23) and 1.4 (± 0.36) 

fledglings produced per nesting attempt, respectively. 

The number of fledglings produced per nesting 

attempt among Cooper’s hawk and long-eared owl 

significantly declined from 2010 to 2011; however, 

there was no statistical difference among occupied 

Cooper’s hawk nests and long-eared owl nests when 

examining differences in fledging rates at successful 

nests only. When pooling occupancy data involving 

several other raptor species, nesting area reoccupancy 

during the 2011 breeding season was low, with 

only 29% of occupied areas in 2010 reoccupied in 

2011; however, nesting area reoccupancy was high 

for Cooper’s hawk. The results provide additional 

support of the stochastic nature of nest occupancy 

and annual reproduction of Cooper’s hawk and 

long-eared owl in northwestern Colorado. To benefit 

management planning and conservation efforts, 

these findings suggest additional research is needed 

to assess the importance of habitat heterogeneity 

and the ability of Cooper’s hawk to maintain overall 

production of young at a relatively high rate at the 

site-specific scale, while overall production might be 

low at the landscape scale.
vi  Technical Note 442  Cooper’s Hawk and Long-Eared Owl Nest Occupancy and Productivity in Piceance Basin, Colorado

The objective of this 

publication is to provide a 

descriptive summary of

nest occupancy and 

productivity information

on a distinct population

of Cooper’s hawk

and long-eared owl . . .
Technical Note 442  Cooper’s Hawk and Long-Eared Owl Nest Occupancy and Productivity in Piceance Basin, Colorado  1

Introduction

Introduction

Only a small amount of published information relates 

oil and gas exploration, extraction, and production 

activities to nest occupancy and productivity of 

Cooper’s hawk (Accipiter cooperii) and long-eared owl 

(Asio otus) (Kennedy 1980). Moreover, very little 

published literature describes Cooper’s hawk nesting 

densities in pinyon-juniper woodlands (Slater and 

Smith 2010).

In April 2008, the Bureau of Land Management 

(BLM) White River Field Office received funding 

via the BLM’s competitive Budget Planning System 

for a project designed to collect breeding season 

information from woodland raptors in the Piceance 

Basin, Colorado. The purpose of this project was 

to collect information that would allow for an 

assessment of annual reproduction and territory 

occupancy over time in areas heavily influenced by 

natural gas exploration and extraction activities. For 

purposes of statistical comparison, the target species 

were the Cooper’s hawk and long-eared owl.

The objective of this publication is to provide 

a descriptive summary of nest occupancy and 

productivity information on a distinct population 

of Cooper’s hawk and long-eared owl that occupy 

an area of northwestern Colorado where oil and gas 

development activities have resulted in a patchwork 

of discrete disturbance features.
2  Technical Note 442  Cooper’s Hawk and Long-Eared Owl Nest Occupancy and Productivity in Piceance Basin, Colorado

The Piceance Basin

in Rio Blanco County 

(northwest Colorado)

was identified as an area

that supports exceptionally 

high densities of nesting 

Cooper’s hawk . . .
Technical Note 442  Cooper’s Hawk and Long-Eared Owl Nest Occupancy and Productivity in Piceance Basin, Colorado  3

Study Area

The Piceance Basin in Rio Blanco County 

(northwest Colorado) was identified as an area that 

supports exceptionally high densities of nesting 

Cooper’s hawk (Smithers 2009). The images in 

Figure 1 represent typical Cooper’s hawk and longeared owl nesting habitat common throughout the 

Piceance Basin.

Figure 1. Typical Cooper’s hawk and long-eared owl nesting 

habitat in Piceance Basin

This region of Colorado is also rich in mineral 

resources, primarily natural gas and oil. Natural gas 

extraction infrastructure, such as well pads, pipeline 

and road corridors, compressors, and water treatment 

facilities, are the dominant anthropogenic disturbance 

features throughout the study area. See Figure 2. 

The construction and installation of these facilities, 

combined with the reclamation and revegetation 

of these sites through both natural and humaninfluenced processes, have created a mosaic of mixedaged vegetative cover classes.

Figure 2. Dominant anthropogenic disturbance features found 

throughout the study area (beginning with the top photo): 

treatment facility, vehicle traffic, pipeline installation and 

vegetation removal, and natural gas well pad

Study Area
4  Technical Note 442  Cooper’s Hawk and Long-Eared Owl Nest Occupancy and Productivity in Piceance Basin, Colorado

The study area is located in northwestern Colorado, 

Sixth Principal Meridian, Township 2 N. - 4 S., 

Range 95 - 100 W., an area ranging from 1,737 

to 2,590 m in elevation (Sedgwick 1987). Figure 

3 illustrates the geographic extent of the study 

area (symbolized as a black box) where raptor 

nest occupancy and productivity information was 

collected during the 2010 and 2011 breeding seasons 

in Piceance Basin.

The dominant overstory vegetation in the study 

area is pinyon pine (Pinus edulis) and Utah juniper 

(Juniperus osteosperma). Low-elevation woodlands on 

shales are dominated by juniper with an understory 

of scattered prairie Junegrass (Koeleria macrantha), 

bluebunch wheatgrass (Pseudoroegneria spicata), 

needle and thread (Hesperostipa comata), bottlebrush 

squirreltail (Elymus elymoides), Indian ricegrass 

(Achnatherum hymenoides), and sometimes stunted 

antelope bitterbrush (Purshia tridentata) and mountain 

mahogany (Cercocarpus montanus). Common forbs 

include groundsel (Senecio spp.), scarlet gilia (Gilia 

aggregata), penstemon (Penstemon spp.), phlox (Phlox

spp.), and rayless tansyaster (Haplopappus nuttallii). 

Pinyon pine, big sagebrush (Artemisia tridentata), 

and western wheatgrass (Pascopyrum smithii) join on 

sandstone to form a more diverse plant community.

Above 2,100 m, pinyon pine is the predominant 

tree species, and the shrub layer is composed of big 

sagebrush, rabbitbrush (Chrysothamnus spp.), antelope 

bitterbrush, and occasionally mountain mahogany, 

chokecherry (Prunus virginiana), and Saskatoon 

serviceberry (Amelanchier alnifolia). Gambel oak 

(Quercus gambelii) is prominent on steep slopes and 

frequently occurs in shady ravines. The grass-forb 

community above 2,100 m includes most species 

found at lower elevations, but percentage ground 

cover is higher; arrowleaf balsamroot (Balsamorhiza 

sagittata) and lupine (Lupinus spp.) are also frequently 

present. 

Figure 3. Geographic extent of the study area in Piceance Basin
Technical Note 442  Cooper’s Hawk and Long-Eared Owl Nest Occupancy and Productivity in Piceance Basin, Colorado  5

Table 1. Summary of recorded raptor species and their number 

of occupied nests in Piceance Basin, Colorado, during the 2011 

breeding season

Species No. Occupied Nests % 

Cooper’s hawk 42 40.8

Common raven 18 17.5

Long-eared owl 17 16.5

Red-tailed hawk 13 12.6

Golden eagle 4 3.9

Northern goshawk 3 2.9

Prairie falcon 2 1.9

American kestrel 1 1.0

Bald eagle 1 1.0

Great horned owl 1 1.0

Osprey 1 1.0

Total 103

Known nest structures were relocated using a global 

positioning system (GPS), specifically a Garmin 

GPSMAP 76CSx. A Trimble GeoXT GPS unit, 

with submeter accuracy, was used to further refine 

position information for all nests in this project. To 

help navigate to each nest, the Universal Transverse 

Mercator (UTM) coordinates of each nest were 

uploaded into the GPS unit using the DNR Garmin 

version 5.4.1 software. Once at the nest, and to 

help alleviate any discrepancies between the nest 

identification number, UTM coordinates, and the 

actual physical location of the nest, a photo was 

taken of the GPS screen, in which both the nest 

ID number and UTM coordinates were displayed. 

Next, a photo of the nest tree and nest were taken, 

followed by a closeup photo of the nest and a 

representative photo of the nest stand. Each series 

of photos was grouped by the nest ID number and 

stored in separate folders using the nest ID number 

as the folder name.

In some cases, the datum was not recorded for a 

known nest or was unknown. For these nests, a 

procedure was developed that included converting 

UTM coordinates from the North American Datum 

of 1927 (NAD27) to NAD83 or vice versa, while 

in the field using the Garmin GPSMAP 76CSx 

unit. For a detailed description of this process, see 

Smithers 2009. While conducting spring presence/

absence surveys, information regarding raptor 

Methods

Methods

Nest Inventory and Monitoring 

During the 2008, 2009, 2010, and 2011 accipiter 

breeding seasons, potential nesting habitat was 

identified manually using 1-meter resolution 

National Agriculture Imagery Program (NAIP) 

imagery and terrain information (e.g., Digital 

Elevation Model data). Figure 4 illustrates areas that 

were selected for nest surveys using the 2011 NAIP 

imagery. The survey areas were identified based on 

canopy closure, dominant cover type, and tree stem 

density.

Figure 4. Areas selected for accipiter nest surveys

Monitoring tasks included visiting known nest 

areas and assessing the breeding season status using 

procedures outlined in Smithers 2012. Occupancy 

information was collected opportunistically 

throughout the study area for all species and pooled 

together. However, because Cooper’s hawk and 

long-eared owl were the dominant species where 

these data were collected, the pooled results should 

be interpreted with caution. Table 1 provides a list of 

species whose occupancy information was collected.
6  Technical Note 442  Cooper’s Hawk and Long-Eared Owl Nest Occupancy and Productivity in Piceance Basin, Colorado

detections was recorded on the “Nest Monitoring 

and Raptor Detection Data Form,” and ongoing 

monitoring information collected throughout the 

breeding season was recorded on the “White River 

Field Office Nest Monitoring Form” (Smithers 

2012).

Known nesting territories from the 2011 accipiter 

breeding season were monitored from April 24 - 

September 18, 2011. The start date was the day when 

full-time nest inventory and monitoring tasks began. 

The end date was the day when all accipiter juveniles 

of the nests being monitored had dispersed from the 

nest stands.

Determining Nest Occupancy 

For spring surveys, emphasis was placed on 

documenting whether or not a nest stand was 

occupied, rather than evaluating whether the nesting 

territory was occupied. As such, an “occupied” nest 

stand included a nest in which either an adult was 

observed incubating eggs, as suggested by an adult 

being in an incubating posture on the nest, or by 

direct observation of eggs in the nest. A “successful” 

nest was defined as a nest that produced at least one 

fledgling. Nests that were determined to be occupied 

during the spring surveys and that did not produce 

at least one fledgling, for either known or unknown 

causes, were defined as “failed” nests. For purposes 

of this research, we define a fledgling as young of 

the year capable of flying either short distances 

or capable of sustained flight to and from the nest 

structure or within the nest stand or post-fledgling 

area, prior to dispersal from the post-fledgling area.

Nest occupancy was determined based on methods 

from Steenhof and Newton (2007). Evidence 

that suggested nests were used during the 2011 

breeding season included white streaks of excrement 

(whitewash) under the nest tree or at the roost site, 

prey remains in the nest stand, down present on the 

perimeter of the nest, castings under the nest tree, 

or fresh nesting material on the nest. According to 

Smithers (2009, 2010, and 2011), the breeding season 

status of the nest (i.e., occupied or unoccupied) 

could be confirmed mid-September due to the 

amount of residual whitewash that was present under 

occupied nests. This was true for nests that did not 

fail during the incubation or early nestling phase, 

regardless of the amount of young present in the 

nest during the nestling or fledgling phase. Figure 5 

shows whitewash under an occupied Cooper’s hawk 

nest in the study area. 

Figure 5. Whitewash under an occupied Cooper’s hawk nest in 

the study area

The condition of individual nests was used as a 

general guide to assess the status of the nest prior 

to incubation. Occupied nests most often had 

fresh material (e.g., branches) and tended to appear 

less compressed or compacted than unoccupied 

nests. Unoccupied nests tended to have a flattened 

or compressed appearance, presumably from the 

effects of snow compacting nest material during the 

previous winter (Smithers 2010). Figure 6 shows a 

typical occupied Cooper’s hawk nest in the study 

area. 

Figure 6. Typical occupied Cooper’s hawk nest in the study 

area

All Cooper’s hawk and long-eared owl nests that 

were identified as being occupied during the 

2011 spring nest monitoring surveys were visited 

approximately 17 times throughout the breeding 
Technical Note 442  Cooper’s Hawk and Long-Eared Owl Nest Occupancy and Productivity in Piceance Basin, Colorado  7

season to assess nest occupancy and predation events, 

record presence/absence information of adults, and 

record fledgling dispersal information. Figure 7 

provides examples of eggs from the study area that 

most likely experienced predation events. 

Figure 7. Eggs from the study area that most likely 

experienced predation events (from top to bottom): an egg that 

was most likely depredated by an avian predator, a membrane 

still attached to an egg shell, and the jagged edges of an egg 

shell that was most likely depredated by a mammalian predator

All nest areas that were identified as being occupied 

during the spring surveys were visited to verify 

nesting attempt outcomes from June 15 - August 28, 

2011. For those nest areas that remained occupied 

throughout the breeding season, information 

pertaining to fledging rates and fledging and dispersal 

dates were recorded for each successful nest.

Nest Reoccupancy Analysis

To complete this analysis, we first selected all 

occupied 2009 Cooper’s hawk nests and exported 

the nest data to a shapefile. We repeated this process 

for 2010 and 2011. We then used the near tool to 

calculate the distance from occupied 2009 Cooper’s 

hawk nests to the nearest occupied 2010 Cooper’s 

hawk nest, using a 500-meter search radius. Nest 

structures that were reoccupied in 2010 (using the 

2009 occupied nests as the near feature) and nest 

structures that were reoccupied in 2011 (using the 

2010 occupied nests as the near feature) resulted in 

a distance value of zero. The resultant distance values 

for the 2009/2010 nests were stored in the attribute 

table of the input file (i.e., the 2010 occupied 

Cooper’s hawk shapefile). We then exported this 

table to Excel and added the distance values to the 

data table used for this analysis. We repeated these 

steps to generate distance values for 2010/2011 nests. 

Moreover, this process was used to derive distance 

estimates for reoccupied 2009/2010 and 2010/2011 

long-eared owl nests.
8  Technical Note 442  Cooper’s Hawk and Long-Eared Owl Nest Occupancy and Productivity in Piceance Basin, Colorado

The sampling units of this 

project consisted of nests that 

were opportunistically selected 

from a sample of all occupied 

nests based on accessibility.
Technical Note 442  Cooper’s Hawk and Long-Eared Owl Nest Occupancy and Productivity in Piceance Basin, Colorado  9

Statistical Analysis

Statistical Analysis

The sampling units of this project consisted of nests 

that were opportunistically selected from a sample of 

all occupied nests based on accessibility. Thus, nests 

used in this study were not randomly selected from 

the population of nests within the study area. All 

statistical tests were completed using the R statistical 

software package (R Development Core Team 2005). 

An alpha of 0.05 was used for all statistical tests, 

and results are reported as the mean ± SE (standard 

error).

Because the reoccupancy data were not normally 

distributed, and because efforts to normalize these 

data failed, a two-sample, nonparametric Wilcoxan 

rank sum test (W) was used to examine differences in 

productivity rates, nest reoccupancy distance values, 

and differences in total precipitation and minimum 

daily temperatures of the study area in 2010 and 

2011. 
10  Technical Note 442  Cooper’s Hawk and Long-Eared Owl Nest Occupancy and Productivity in Piceance Basin, Colorado

A total of 304 known

raptor nesting territories

were visited during the

2011 field season. 
Technical Note 442  Cooper’s Hawk and Long-Eared Owl Nest Occupancy and Productivity in Piceance Basin, Colorado  11

Table 2. Summary of 2010 and 2011 success rates of occupied raptor nests in Piceance Basin, Colorado*

2010 2011

N Successful Failed Unknown N Successful Failed Unknown

Occupied 162 (89%) 109 (67%) 15 (9%) 38 (23%) 103 (34%) 52 (50%) 35 (34%) 16 (16%)

Unoccupied 21 (11%) 198 (65%)

Unknown 0 3 (1%)

Total 183 304

* All percentages are rounded to the nearest percent.

Table 3. Summary of 2010 and 2011 raptor species productivity information in Piceance Basin, Colorado (parentheses indicate 2010 

values)

Species Failed Successful Unknown

Total

Occupied 

Nests

No. 

Fledged 

(NF)

Mean NF/

Successful 

Nest

Mean NF/

Nesting

Attempt

American kestrel 1 (2) 1 (2)

Bald eagle 1 (NA) NA (1) NA (NA) 1 (1) NA (1) NA (1.0) NA (1.0)

Cooper’s hawk 22 (8) 20 (49) NA (5) 42 (62) 53 (147) 2.7 (3.0) 1.3 (2.4)

Common raven 5 (1) 6 (2) 7 (7) 18 (10) 15 (7) 2.5 (3.5) 0.8 (0.7)

Great horned owl NA (1) 1 (2) 1 (3) NA (1) NA (1) NA (0.3)

Golden eagle 2 (NA) 2 (1) 4 (1) 2 (1) 1 (1) 0.5 (1)

Long-eared owl 4 (6) 10 (43) 3 (10) 17 (59) 23 (126) 2.3 (2.9) 1.4 (2.1)

Northern goshawk 3 (3) 3 (3) 3 (7) 1.3 (2.3) 1.0 (2.3)

Prairie falcon 2 (2) NA (1) 2 (3) 3 (2) 1.5 (1) 1.5 (0.7)

Red-tailed hawk 1 (NA) 8 (6) 4 (10) 13 (16) 18 (15) 2.3 (2.5) 1.4 (0.9)

Sharp-shinned hawk NA (1) NA (1) NA (3) NA (3.0) NA (3.0)

Saw-whet owl NA (1) NA (1)

Osprey 1 (NA) 1 (NA) 2 (NA) 2.0 (NA) 2.0 (NA)

Total 35 (15) 52 (109) 16 (38) 103 (162) 119 (310)

during the 2011 breeding season. Of the occupied 

nests, the majority were occupied by Cooper’s 

hawk (41%, N = 42). For species composition, nest 

occupancy, and productivity information, see Tables 

1, 2, and 3.

Results

Results

Nest Monitoring

A total of 304 known raptor nesting territories were 

visited during the 2011 field season. Of these nests, 

34% (N = 103) were classified as being occupied 

during the spring surveys, and 65% (N = 198) of 

these nests were confirmed as being unoccupied 
12  Technical Note 442  Cooper’s Hawk and Long-Eared Owl Nest Occupancy and Productivity in Piceance Basin, Colorado

Productivity was similar among Cooper’s hawk 

and long-eared owl in the study area during the 

2011 breeding season. When considering only 

successful (e.g., excluding failed nesting attempts) 

Cooper’s hawk nests (20 of 42 occupied nests in 

2011), produced on average 2.7 (± 0.20) fledglings 

per successful nest (see Table 3). This finding was 

statistically similar to what was recorded in 2010, 

with 3.0 (± 0.14) fledglings produced per successful 

Cooper’s hawk nest (W = 594.5, p-value (P) = 

0.194). When considering only successful longeared owl nests, (10 of 17 occupied nests in 2011), 

an average of 2.3 (± 0.30) fledglings were produced 

per breeding pair. And similar to Cooper’s hawk, 

the number of long-eared owl young produced per 

successful nest in 2010 and 2011 did not significantly 

differ (W = 280, P = 0.097). During the 2010 

breeding season, long-eared owl produced an average 

2.9 fledglings per successful nest.

When considering all Cooper’s hawk and long-eared 

owl nesting attempts (i.e., including both successful 

Of the occupied nests in which the outcome of 

the nesting attempt (i.e., failed or successful) was 

recorded (N = 87 nests), we observed a success rate 

of 60% (N = 52), and a nest failure rate of 40% 

(N = 35) (see Table 2). Compared to 2010 findings, 

nest failure rates in 2011 represented a 233% increase 

(15 of 124 nests failed in 2010 vs. 35 of 87 nests 

failed in 2011). The outcome of 16 nesting attempts 

(N = 16 nests, or 16% of all occupied nests) was not 

recorded during the 2011 breeding season.

Fledging rate information was collected at all nests 

that successfully fledged young (N = 52 successful 

nests). These nests produced a total of 119 fledglings 

(an average of 2.3 ± 0.14 fledglings produced per 

successful nest). This represents an 18% decline in the 

number of fledglings produced per successful nest 

in 2010 (N = 310). When considering all occupied 

nests of all species in 2011, fledging rates within the 

study area were 1.2 fledglings produced per nesting 

attempt (N = 103 occupied nests). Compared to 

2010 findings, only 38% of the total number of 

fledglings produced in 2010 were produced in 2011.
Technical Note 442  Cooper’s Hawk and Long-Eared Owl Nest Occupancy and Productivity in Piceance Basin, Colorado  13

and failed nesting attempts), fledging rates were 

1.3 (± 0.23) and 1.4 (± 0.36) fledglings produced 

per nesting attempt, respectively, in the study area 

(see Table 3). When assessing productivity on the 

basis of nesting attempts, not just successful nests, 

we documented a significant decline in the number 

of fledglings produced per nesting attempt among 

Cooper’s hawk (W = 1782.5, P = 0.00005) and 

long-eared owl (W = 460, P = 0.034) from the 2010 

to the 2011 breeding season.

Nest Stand Reoccupancy Analysis

During the 2011 breeding season, occupancy 

information was collected at an additional 114 nest 

territories compared to 2010, representing a 67% 

increase in the number of known nesting territories 

in the study area from 2010 to 2011 where 

monitoring information was collected.

Of the nest structures that were occupied in 2010, 

136 out of 162 (84%) were visited in 2011. Nest 

structure reoccupancy during the 2011 breeding 

season was low, with only 29% (N = 40) of nest 

structures occupied in 2010, and consequently 

visited in 2011 (N = 136), also being reoccupied 

in 2011. Of the nesting areas that were occupied 

in 2010, 59% (N = 96) were determined to be 

unoccupied in 2011. Fourteen nests that failed in 

2010 (N = 15) were visited in 2011. All of the 14 

nests were occupied in 2011; however, only 14% 

(N = 2 nests) successfully produced young during 

the 2011 breeding season. Twenty-six nests (16%) 

that were occupied in 2010 (N = 162) were not 

revisited in 2011.

Nesting area reoccupancy was high for Cooper’s 

hawk, with 11 pairs (26% of all known 2011 

Cooper’s hawk territories, N = 42) returning to the 

nest structure that was used in 2010. Additionally, 

20 pairs returned to the same nest stand to either 

construct a new nest or occupy an alternate nest. 

This calculates to a total of 31 pairs (or 74% of all 

known 2011 Cooper’s hawk territories) reoccupying 

known nesting territories (e.g., nesting areas) during 

the 2011 breeding season.

The mean distance between 2010 and 2011 

occupied Cooper’s hawk nests was 177 m (± 18.9 

m, N = 20 nest pairs, range = 33.8 to 340.2 m) (see 

Table 4). The mean distance between 2009 and 2010 

occupied Cooper’s hawk nests was 186 m (± 43.2 

m, N = 7, range = 115.4 to 436.2 m). The mean 

distance between 2009 and 2010 Cooper’s hawk 

nests and 2010 and 2011 Cooper’s hawk nests did 

not significantly differ (W = 66.5, P = 0.8681).

Table 4 shows the data that was used to compare 

nest reoccupancy information for Cooper’s hawk 

and long-eared owl in the study area. Cooper’s 

hawk tended to reoccupy or construct nests during 

the 2011 breeding season that were closer to 2010 

occupied nests when compared to those nests that 

were reoccupied or constructed in 2010 from the 

2009 occupied nests, though this relationship was 

not statistically significant (W = 66.5, P = 0.8681). 

Long-eared owl tended to reoccupy or construct 

nests during the 2011 breeding season that were 

farther from occupied 2010 nests, when compared to 

the distance between occupied 2009 and 2010 nests 

(W = 31.5, P = 0.02807).
14  Technical Note 442  Cooper’s Hawk and Long-Eared Owl Nest Occupancy and Productivity in Piceance Basin, Colorado

Table 4. Summary of distances (in meters) of occupied Cooper’s hawk and long-eared owl nests from 2009 to 2010 and from 2010 

to 2011 

Distances Between 

2009 and 2010

Occupied

Cooper’s Hawk Nests

Distances Between 

2010 and 2011

Occupied

Cooper’s Hawk Nests

Distances Between 

2009 and 2010

Occupied

Long-Eared Owl Nests

Distances Between 

2010 and 2011

Occupied

Long-Eared Owl Nests

436.22 113.07 23.71 240.35

115.43 185.65 61.72 262.55

115.68 317.41 101.24 290.11

117.59 252.87 181.39 291.53

150.22 223.45 291.53 333.67

181.60 38.47 433.28

181.95 230.42 436.22

33.84

129.56

178.94

108.19

138.96

93.56

340.21

124.33

234.13

224.72

150.22

276.15

145.17

185.53 176.97 131.92 326.82

114.40 84.68 106.66 79.11

43.24 18.94 47.70 29.90

0.62 0.48 0.81 0.24

7 20 5 7

Mean =

Standard Deviation (SD) =

Standard Error (SE) =

Coefficient of Variation (CV) =

Population Size (N) =
Technical Note 442  Cooper’s Hawk and Long-Eared Owl Nest Occupancy and Productivity in Piceance Basin, Colorado  15

Discussion

Discussion

Although a causal relationship was not discovered for 

increased nest failure rates during the 2011 breeding 

season, one factor that could have influenced nest 

failure rates from 2010 to 2011 is stochastic spring 

weather events (e.g., rain and snow showers) and cold 

temperatures (see Figures 8, 9, and 10). Estimated 

nest failure dates showed that most failures occurred 

during the months of April and May, which also 

coincided with early spring rain, snow showers, 

and cold temperatures. Data used to develop these 

figures was obtained from the Pinto Ridge Remote 

Automated Weather Station, which is located in 

northwest Colorado.

 
16  Technical Note 442  Cooper’s Hawk and Long-Eared Owl Nest Occupancy and Productivity in Piceance Basin, Colorado

Figure 8. Comparison of April 2010 and April 2011 mean, maximum, and minimum temperatures and precipitation events in the 

study area 

60

50

40

30

20

1 4 7 10 13 16

days of April

days of April

days of April

degrees Fahrenheit degrees Fahrenheit degrees Fahrenheit

days of April

inches

19 22 25 28

1 1 7 10 13 16 19 22 25 28

1 4 7 10 13 16 19 22 25 28

1 4 7 10 13 16 19 22 25 28

10

0

0.30

0.25

0.20

0.15

0.10

0.05

0.00

80

60

40

20

0

50

40

20

30

10

0

Maximum April 2010 Temperatures Maximum April 2011 Temperatures

Minimum April 2010 Temperatures Minimum April 2011 Temperatures

Total April 2010 Percipitation Total April 2011 Percipitation

Mean April 2010 Temperatures Mean April 2011 Temperatures
Technical Note 442  Cooper’s Hawk and Long-Eared Owl Nest Occupancy and Productivity in Piceance Basin, Colorado  17

Figure 9. Comparison of May 2010 and May 2011 mean, maximum, and minimum temperatures and precipitation events in the 

study area

60

70

80

50

40

30

20

1 4 7 10 13 16

days of May

days of May

days of May

degrees Fahrenheit degrees Fahrenheit degrees Fahrenheit

days of May

inches

19 22 25 28 31

1 4 7 10 13 16 19 22 25 28 31

1 4 7 10 13 16 19 22 25 28 31

1 4 7 10 13 16 19 22 25 28 31

10

0

0.30

0.35

0.25

0.20

0.15

0.10

0.05

0.00

80

100

60

40

20

0

50

60

40

20

30

10

0

Maximum May 2010 Temperatures Maximum May 2011 Temperatures

Minimum May 2010 Temperatures Minimum May 2011 Temperatures

Total May 2010 Percipitation Total May 2011 Percipitation

Mean May 2010 Temperatures Mean May 2011 Temperatures
18  Technical Note 442  Cooper’s Hawk and Long-Eared Owl Nest Occupancy and Productivity in Piceance Basin, Colorado

Figure 10. Comparison of June 2010 and June 2011 mean, maximum, and minimum temperatures and precipitation events in the 

study area 

60

70

80

50

40

30

20

1 4 7 10 13 16

days of June

days of June

days of June

degrees Fahrenheit degrees Fahrenheit degrees Fahrenheit

days of June

inches

19 22 25 28

1 1 7 10 13 16 19 22 25 28

1 4 7 10 13 16 19 22 25 28

1 4 7 10 13 16 19 22 25 28

10

0

0.60

0.70

0.50

0.40

0.30

0.10

0.10

0.00

80

100

60

40

20

0

50

60

70

40

20

30

10

0

Maximum June 2010 Temperatures Maximum June 2011 Temperatures

Minimum June 2010 Temperatures Minimum June 2011 Temperatures

Total June 2010 Percipitation Total June 2011 Percipitation

Mean June 2010 Temperatures Mean June 2011 Temperatures
Technical Note 442  Cooper’s Hawk and Long-Eared Owl Nest Occupancy and Productivity in Piceance Basin, Colorado  19

Mean, maximum, and minimum temperatures during 

April 2011 were generally lower compared to April 

2010 temperatures. Moreover, precipitation events in 

April 2011 were more numerous and more equitably 

distributed (i.e., precipitation events were distributed 

equally throughout the month) compared to April 

2010 (see Figure 8).

After reviewing the temperature and precipitation 

information, the only item that stood out was the 

number of days in which a measureable amount 

of precipitation was recorded during the month of 

April 2011. In April 2010, there were a total of 

8 days in which a measurable amount of 

precipitation was recorded, compared to 17 days 

in April 2011; however, total precipitation did not 

significantly differ between April 2010 and April 

2011 (W = 93, P = 0.15), with 0.87 and 1.13 

in, respectively. Also, mean values for minimum 

temperatures did not significantly differ between 

April 2010 and April 2011 (W = 490.5, P = 0.55), 

with 32 and 30° F, respectively. Although there was 

no statistical difference in mean April precipitation 

estimates between years, findings suggest that the 

distribution of precipitation events may influence 

Cooper’s hawk nest productivity more than the total 

volume of precipitation produced per event.

Unfortunately, prey delivery rates were not assessed 

during the 2011 breeding season. However, based 

on anecdotal evidence, and using the same observer 

in 2011 as 2010, there was a noticed decline in the 

overall presence of either adult in the nest stand 

during routine monitoring visits. In addition, unlike 

observations recorded during the 2010 breeding 

season at occupied nests, during the 2011 breeding 

season, a measurable decline was noticed in the 

number of occurrences in which prey items were 

observed on the rim of the nest, in the nesting area, 

or on the ground directly below the nest. See Figure 

10.

Moreover, when compared to 2010 observations, 

fledglings tended to disperse from the nest stand, 

and presumably from the post fledging area, sooner 

in 2011 than in 2010. These anecdotal observations 

suggest that prey abundance and possibly prey 

availability may have been reduced during the 2011 

breeding season, which may have also influenced nest 

occupancy and productivity results. 

In 2011, fledging rate information was collected at 

all successful nests (N = 52), though these data are 

heavily skewed towards Cooper’s hawk (N = 20) 

and long-eared owl (N = 10). Of those nests that 

were successful, a total of 119 fledglings (an average 

of 2.3 ± 0.14 fledglings produced per successful 

nest), represents an 18% decline in the number of 

fledglings produced per successful nest in 2010 

(N = 310). When considering all occupied nests 

for all species in 2011, fledging rates within the 

study area were 1.2 fledglings produced per nesting 

attempt (N = 103 occupied nests). Compared to 

2010 findings, only 38% of the total number of 

fledglings produced in 2010 (N = 310 fledglings) 

were produced in 2011 (see Table 3).

Because the same observer was used in both 2010 

and 2011, and because the amount of survey effort 

was similar during the 2010 and 2011 breeding 

seasons, we are led to believe that there was both a 

numerical and biological difference (i.e., decline) 

in the number of fledglings produced per successful 

nest in 2011 compared to 2010. Also, as mentioned 

previously, both seasonal weather patterns and prey 

abundance and prey availability most likely led to the 

observed pattern. 

Figure 10. Uneaten prey found below two occupied Cooper’s 

hawk nests in 2010 in the study area 
20  Technical Note 442  Cooper’s Hawk and Long-Eared Owl Nest Occupancy and Productivity in Piceance Basin, Colorado

Compared to 2010 findings, productivity was 

similar among Cooper’s hawk and long-eared owl 

for successful nests in the study area during the 

2011 breeding season. When considering 2011 

successful Cooper’s hawk nests (N = 20), Cooper’s 

hawk produced an average 2.7 (± 0.20) fledglings, 

which is statistically similar to what was recorded 

in 2010, with 3.0 (± 0.14) fledglings produced per 

successful nest (W = 594.5, P = 0.194). This finding 

contradicts our supposition that prey abundance and 

availability might have been a key factor influencing 

nest occupancy and productivity results. If prey 

availability and abundance was a primary factor 

influencing nest occupancy and productivity results, 

then one would expect a noticeable decline in the 

number of fledglings produced per successful nest 

when looking at all successful Cooper’s hawk nests. 

However, this finding provides support for the 

clustered distribution of occupied Cooper’s hawk 

nests as it relates to prey abundance and availability, 

which is most likely also clustered and patchy during 

years when prey density is low. Cooper’s hawk nest 

occupancy and productivity may be more heavily 

influenced by the distribution, abundance, and 

availability of small mammals in the study area.

Based on nest occupancy and productivity 

information, findings recorded in 2010 most likely 

represent an exceptional year for Cooper’s hawk nest 

occupancy and productivity, and findings recorded 

in 2011 most likely represent a low year for Cooper’s 

hawk nest occupancy and productivity. When 

comparing 2010 and 2011 results, 2011 findings 

provide support for the theory that even though nest 

occupancy may be low in years when prey densities 

are low, because of the patchy distribution of prey 

(both avian and mammalian) within a territory, 

nest productivity can still be high at the local scale, 

though nest occupancy and productivity estimates 

based on the number of fledglings produced per 

nesting attempt will be low at the regional scale. 

Additional support of this concept includes the fact 

that a significant decline was documented in the 

number of fledglings produced per nesting attempt 

among occupied Cooper’s hawk nests (W = 1782.5, 

P = 0.00005) and long-eared owl nests (W = 460, 

P = 0.034) from the 2010 to the 2011 breeding 

season.

During the 2011 breeding season, occupancy 

information was collected at an additional 114 nest 

territories compared to 2010, representing a 67% 

increase in the number of known nesting territories 

in the study area from 2010 to 2011 where 

monitoring information was collected. This finding 

can best be explained by the fact that the individual 

responsible for monitoring nests had more time to 

visit additional known nest locations and confirm 

occupancy status because of low nest occupancy 

and high nest failure rates in the study area. It would 

have required more time to conduct routine visits to 

confirm occupancy status if the latter had been true 

(i.e., higher occupancy rates and lower nest failure 

rates). As such, these findings do not represent a 

statistical increase in the number of new territories 

in the study area. Rather, these findings illustrate 

a decreased level of effort required to confirm 

occupancy status when overall nest occupancy is low 

and nest failure rates are high in the study area.
Technical Note 442  Cooper’s Hawk and Long-Eared Owl Nest Occupancy and Productivity in Piceance Basin, Colorado  21

Literature Cited

Literature Cited

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129-135.

R Development Core Team. 2010. R: A language 

and environment for statistical computing. 

ISBN 3-900051-07-0. R Foundation for 

Statistical Computing, Vienna, Austria. 

http://www.lsw.uni-heidelberg.de/users/

christlieb/teaching/UKStaSS10/R-refman.

pdf.

Sedgwick, J.A. 1987. Avian habitat relationships 

in pinyon-juniper woodland. The Wilson 

Bulletin 99 (3): 413-431.

Slater, S.J., and J.P. Smith. 2010. Accipiter use of 

pinyon-juniper habitats for nesting in 

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HawkWatch International, Salt Lake City, 

UT. 

Smithers, B.L. 2009. 2009 White River Field Office 

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Bureau of Land Management, White River 

Field Office, Meeker, CO.

Smithers, B.L. 2010. 2010 White River Field Office 

Raptor Nesting Productivity and Nest 

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White River Field Office, Meeker, CO.

Smithers, B.L. 2012. White River Field Office 

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01/25/12. Bureau of Land Management, 

White River Field Office, Meeker, CO. 

http://www.blm.gov/co/st/en/fo/wrfo/

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Smithers, B.L. In progress. An assessment of Cooper’s 

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equipment. Bureau of Land Management, 

White River Field Office, Meeker, CO.

Steenhof, K., and I. Newton. 2007. Assessing Nesting 

Success and Productivity. p. 181-191. In: 

Bird, D.M., and K.L. Bildstein (eds). Raptor 

Research and Management Techniques, 

Hancock House, Blaine, WA. 
22  Technical Note 442  Cooper’s Hawk and Long-Eared Owl Nest Occupancy and Productivity in Piceance Basin, Colorado

Based on nest occupancy 

and productivity information, 

findings recorded in 2010 

most likely represent an 

exceptional year . . . and 

findings recorded in 2011 

most likely represent a low 

year for Cooper’s hawk nest 

occupancy and productivity.
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