South Dakota State University Open PRAIRIE: Open Public Research Access Institutional Repository and Information Exchange

Theses and Dissertations

1994 Raptor Use of the Idaho National Engineering Laboratory Richard Wayne Hansen

Follow this and additional works at: https://openprairie.sdstate.edu/etd Part of the Natural Resources and Conservation Commons

Recommended Citation Hansen, Richard Wayne, "Raptor Use of the Idaho National Engineering Laboratory" (1994). Theses and Dissertations. 457. https://openprairie.sdstate.edu/etd/457

This Thesis - Open Access is brought to you for free and open access by Open PRAIRIE: Open Public Research Access Institutional Repository and Information Exchange. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of Open PRAIRIE: Open Public Research Access Institutional Repository and Information Exchange. For more information, please contact [email protected]. Erratum: Richard Hansen reported eight owl species from his owl surveys on the INEL based on recorded calls and direct observation. All species Hansen recorded are known to occur in Idaho. I can confirm visual observations on short-eared owls, long-eared owls, burrowing owls, and great horned owls from my own visits to the INEL. Most (or all) of the flammulated owl calls that Richard identified were in error in my opinion and instead represent calls of long-eared owls, an abundant species on the INEL. Richard Hansen agrees with this conclusion at least to some degree. Hansen did record three direct observations of flammulated owls at less than 15 meters although no photos were taken. I have no comment on the direct observations of flammulated owls except that one observation in February seems especially unlikely. Lester D. Flake, Thesis Advisor RAPTOR USE OF THE IDAHO NATIONAL ENGINEERING LABORATORY

BY

RICHARD WAYNE HANSEN

A thesis submitted in part ial fulfillment of the requirements for the degree Master of Science , Major in Wildlife and Fisheries Sciences , South Dakota State University 1994 Raptor use of the Idaho National Engineering Laboratory

This thesis is approved as a creditable and

independent investigation by a candidate for the degree

Master of Science , and is acceptable for meeting the thesis

requirements for this degree. Acceptance of this thesis does not imply that the conclusions reached by the candidate

are necessarily the conclusions of the major department .

DO Lester o. Flake Date Thesis Advisor

Charles G. Scalet Date Head , Wildlife and Fisheries Sciences

i Raptor use of the Idaho National Engineering Laboratory

ABSTRACT

Richard Wayne Hansen

Raptors are high trophic-level predators , and thus sensitive to environmental change . I conducted a basic ecological study of raptors using the Idaho National

Environmental Laboratory (INEL) , in southeastern Idaho , between 1991 and 1993 to assess effects of human activity on the site . Results were compared to previous raptor studies conducted on the INEL from 1974-1976 and in 1982 , as well as with studies of rough-legged hawks , long-eared owl s, and burrowing owls conducted during the late 1970 's and early

1980's.

Road-side raptor surveys were conducted from January through May in 1992 and 1993 . Principle species recorded were rough-legged hawks , red-tailed hawks , ferruginous hawks , golden eagles, prairie falcons , and great horned owl s. Most raptors were perched on power poles when I

observed them . Site facilities did not appear to affect wintering raptor distribution . A high occurrence of raptors within 10 km of site facilities was probably a result of power pole distribution , as power poles were usually close to facilities . Raptor populations were comparable to those

ii noted during the 1970's, but lower than raptor numbers in

1982.

I conducted short-eared owl counts during the spring

1992 and 1993 . Counts were accompl ished by walking 5 transects (2 km long) and counting all owls flushed . There were no owls flushed , however owls were observed during other phases of this study . Ten kilometers of transect is probably insufficient to monitor short-eared owl populations on the INEL.

To establish species occurrence and distribution of owls on the INEL, I conducted nocturnal call ing surveys . At regular stops along 5 routes , I played owl calls and recorded all responses . Eight species were recorded during these surveys : great horned owl , long-eared owl , short­ eared , burrowing owl , boreal owl , northern saw-whet owl , western screech owl , and flammulated owl . Most owls were detected in April and May in the juniper forests around the

Lemhi foothills and Twin Buttes .

I monitored raptor nesting from March through

August , concentrating on medium- to large-sized species .

Long-eared owls and great horned owls nested in limited numbers during this study . Owl nesting success was comparable to other studies in the Great Basin region . Red­ tailed hawks , ferruginous hawks , and Swainson 's hawks were common nesters on the INEL. Red-tailed hawk numbers have

iii increased since the 1970's, while Swainson 's and ferruginous hawk numbers have remained relatively stable. Reproductive success was comparable to earlier studies . Nest distribution of ferruginous and Swainson 's hawks was fairly random, with some avoidance of human development in the case of ferruginous hawks . Ferruginous hawks , a Category 2 species under consideration to Threatened and Endangered

Species status , experienced increased nest failure when exposed to increased human activity on site . Red-tailed hawk nests were clustered along the Big Lost River . Food habits comparisons show dietary overlap between Swainson 's hawks , red-tailed hawks , and great horned owls. Ferruginous hawks and long-eared owls had little overlap with other species . Continued monitoring of raptors on the INEL through prey fluctuations would provide insight into relationships between raptors , as we ll as with their prey base and habitats .

iv ACKNOWLEDGEMENTS

Several people provided me with help, gu idance , and

support , without which this project would be wanting . I

would like to thank Les Flake , my advisor , for granting me

the privilege of chasing birds through the desert , and

making sure I glean something useful out of it . I would

like to thank the faculty and grad students in the Wildlife

and Fisheries department for their time and patience . I am

particularly grateful to Bob , Carmen , and Karen for getting

me used to life in the flatlands . Thanks also to Dr . W. L.

Tucker, Agricultural Experiment Station statistician, for

his useful statistical advise .

Thanks to all the pe rsonnel at RESL, particularly

Doyle Markham for coordinating this project in Idaho , Randy

Morris and Tim Reynolds for helpful suggestions, and

especially Russ Mitchell for providing indispensable

assistance in all phases of this proj ect . I would also like

to thank the summer interns of Associated Western

Universities for providing field help: W. Clark, S. Cooper,

A. Croley , L. Maddison , J. Shupe , and A. Yost .

Finally I would like to thank my parents without whom I would never have come this far.

This research was funded by a contract with the

U. s. Department of Energy, Idaho Field Office through the Environmental Science and Research Foundation , Idaho Falls.

v TABLE OF CONTENTS

ABSTRACT ii

ACKNOWLEDGEMENTS v

LIST OF TABLES • viii

LIST OF FIGURES . x

LIST OP APPENDICES. • xii

INTRODUCTION 1

STUDY AREA 3

ROAD-SIDE SURVEYS . 7 METHODS 8 RESULTS 12 DISCUSSION . 24

OWL SURVEYS . 38 METHODS 39 Foot surveys 39 Calling surveys 39 RESULTS 43 Foot surveys 43 Calling surveys . 43 DISCUSSION . 48 Foot surveys 48 Calling surveys . 48

NESTING SURVEYS . 56 METHODS 57 RESULTS 62 Productivity 62 Nest Sites 67 Habitat 67 Human-related development 80 Reuse 80 Nearest neighbors 84 Nest De fense . 84 Food Habits . . 90 DISCUSSION • • 96 Productivity 96 Nest Sites 101 Habitat • • • 101 Human-related development 105 vi Reuse ...... 107 Nearest neighbors 108

Nest Defense • . . 109

Food Habits • . • • • • 112

CONCLUSIONS . • • . 115

LITERATURE CITED 118

APPENDICES .... 128

vii LIST OF TABLES

Table Page

1 Habitat categories used in study of raptors 13 using the INEL, 1991-1993.

2 Results of road-side raptor surveys conducted 14 on the INEL January through May , 1992 and 1993.

3 Results of raptor surveys conducted 5 times 17 from early February through mid April along a 187-km route , on the INEL.

4 Habitat use of 4 raptor species during the 20 nonbreeding season on the INEL expressed as a percentage of total observations during a road-side survey (1992 and 1993).

5 Activities of raptors surveyed during road­ 21 side surveys of the INEL (January-May 1992 and 1993) .

6 Perch types of raptors observed on road-side 22 surveys on the INEL (January-May 1992 and 1993) .

7 Distance to site facilities of raptors observed 23 during road-side surveys of the INEL, 1992 and 1993.

8 Owl species detected during calling surveys 44 on the INEL, 1992-1993 .

9 Detection rate of owls on call ing surveys of the 45 INEL, 1992-1993, expressed as number detected/ survey .

10 Productivity of hawks nesting on the INEL, 1991- 63 1993.

11 Productivity of owls nest ing on the INEL, 1991- 65 1993.

12 Nesting substrates of raptors on the INEL, 1991- 71 1993.

13 Vegetation communities within 100 m of raptor 78 nest sites on the INEL, 1991-1993.

viii 14 Five principle components derived from analysis 79 of vegetation communities surrounding Buteo hawk nest sites and 100 random points on the INEL, 1991-1993 .

15 Distances between human development and hawk 82 nests on the INEL, 1991-1993 .

16 Reuse of nest sites by raptors on the INEL, 83 1991-1993 .

17 Nearest neighbor matrix for hawks nesting on the 87 INEL, 1991-1993 .

18 Nearest neighbor occurrence of hawks nesting on 88 the INEL, 1991-1993 .

19 Nest defense by raptors nesting on the INEL, 89 1991-1993 .

20 Diet diversity and overlap among raptors nesting 91 on the INEL, 1991-1993 .

21 Food of ferruginous hawks nest ing on the INEL, 92 1991-1993.

22 Food of red-tailed hawks nesting on the INEL, 93 1991-1993 .

23 Food of Swainson 's hawks nesting on the INEL, 94 1991-1993 .

24 Food of great horned owls nesting on the INEL, 95 1991-1992 .

25 Food of long-eared owl s nesting on the INEL, 97 1991-1992 .

ix LIST OF FIGURES

Figure Page

1 Map of the Idaho National Engineering Laboratory 4 showing maj or landmarks .

2 Routes used to conduct road-side raptor surveys 9 on the INEL, 1992 and 1993.

3 Chronology of migrating raptors on the INEL as 15 determined by first or last observation dates in 1992 and 1993.

4 Community structure of migratory raptors surveyed 18 on the INEL, January-May 1992 and 19 93.

5 Locations of transects used in short-eared owl 40 survey on the INEL , 19 9 2-1993.

6 Routes used for owl calling surveys on the INEL, 42 1992-1993.

7 Months owl species were detected during a calling 46 survey on the INEL , 19 92 and 1993.

8 Times owl species were detected during a calling 47 survey on the INEL, 1992 and 1993.

9 Vegetation type map of the INEL, adapted from 59 McBride et al . 19 78 .

10 Nesting chronology of hawks on the INEL, 19 91- 64 19 93.

11 Nesting chronology of owls on the INEL, 1991- 66 1993.

12 Nest locations of raptors using the Idaho 68 National Engineering Laboratory , 1991.

13 Nest locations of raptors using the Idaho 69 National Engineering Laboratory , 1992.

14 Nest locations of raptors using the Idaho 70 National Engineering Laboratory , 1993.

15 Height of nests and nesting substrate for hawks 73 on the INEL, 1991-1993.

x 16 Compass aspect of hawks nests on the INEL, 74 1991-1993.

17 Outside diameters of hawk nests on the INEL, 75 1991-19 93.

18 Tree stand characteristics for hawks nesting on 76 the INEL, 1991-1993.

19 Plot of eigenvalues for nesting Buteo habitat 81 use as defined by principle components analysis.

20 Reused nest locations of raptors on the INEL , 85 1991-19 93.

21 Locations of nests used by different raptor 86 species in successive years on the INEL, 1991- 1993.

xi LIST OF APPENDICES

Appendix Page

A Bimonthly weather data from the CFA NOAA weather 128 station (1991-1993) and historical weather data adapted from Clawson et al . (1989) .

B Status and monitoring of sensitive raptor species 129 on the INEL. c Narrowleaf cottonwood size and condition along 132 the Big Lost River, Idaho , in July 1992 .

D Locations of 1-km transects used to assess tree 136 condition along the Big Lost River , 1992. E Narrowleaf cottonwood size and condition along 137 Birch Creek, on the INEL, in July 1992 .

xii INTRODUCTION

Because of their role as principle predators ,

raptors (i.e. eagles, hawks , falcons , and owls) are

sensitive to slight changes in the ecosystem . In addition ,

factors influencing raptor populations may have

repercussions at lower trophic levels (Kennedy 1980) . Both

of these factors make raptors useful as an index to the

effects of human development on the environment . To use

raptor populations as an index , baseline data from which conclusions regarding population fluctuations can be made must be available. There is a lack of long-term data regarding raptor abundance in the intermountain west; thus ,

it is difficult to determine the effects of environmental changes (natural and human caused ) on raptor numbers in this geographical region (Harlow and Bloom 1989, Marti and Marks

1989) . Useful land management decisions depend on our understanding of environmental processes , thus long-term baseline data on raptors opens a window on ecological processes and provides a useful tool for documenting the effects of human development (Kennedy 1980) .

Several raptor studies have been conducted on the

Idaho National Engineering Laboratory (INEL) beginning in the mid-1970's. A baseline study of raptors using the site was conducted from 1974 through 1976 (Craig 1977, Cra ig

1979) . Watson (1984) studied wintering rough-legged hawks 2

(Buteo lagopus) on the INEL. Raptor abundance was reassessed in 1981-1982 using roadside surveys and a nest inventory (Craig et al . 1984) . Two studies specifically targeted burrowing (Athene cunicularia) and long-eared owls

(Asio otus) using the site (Gleason 197 8, Cra ig et al .

1988) . Except for mid-winter raptor counts and a cursory nest inventory in 1987 , raptors have not been studied on the

INEL since the early 1980's.

My study was conducted to update the data base of raptor status on the INEL and to assess the env ironmental factors affecting the community . Specific obj ectives of the study were to: (1) determine raptor abundance and distribution, (2) evaluate raptor production and determine priority nesting areas , (3) determine the influences human use of the site has on raptor distribution , and (4) assess the effects of environmental factors (e.g. prey availability , weather , vegetation, and competition) on raptor productivity and distribution. 3

STUDY AREA

National Environmental Research Parks (NERPs) were created in the 1970's in association with United States

Department of Energy (DOE) research facilities to study the environmental effects of energy development (U.S. Dept .

Energy 1985) . NERPs provide outdoor laboratories with restricted access that minimize uncontrolled human influences on environmental research . The INEL was declared a NERP in 1975. The INEL provides an ideal place to study raptors and their environment in the sage-steppe desert .

The 231,600 ha INEL site is located on the western edge of the upper Snake River Plain in southeastern Idaho .

The Bitterroot , Lemhi , and Lost River mountain ranges adjoin the site to the north and west (Figure 1) . The Big Lost

River, Little Lost River, and Birch Creek drainages empty on the site from these ranges . These water courses have been diverted for irrigation and power production , so water rarely reaches the sinks located on the north-central part of the site . In 1993 the Big Lost River flowed on the INEL

during June for the first time since 1986 (Bennett 1990) •

The only permanent water on the site is associated with waste water ponds near several facilities . The facilities on site are devoted to researching nuclear energy production and are connected with a series of paved roads . 4

Idaho

� INEL i

'

' ,, "

TAN

Clreular0 Butte

NRF

ANL-W TRA •

20 "' ""'"",... --

' -�-- ' ... ' ..,, '- -- ,! . l.09fRiver Sig Key:

M#dd�lltB utte • 1aclllties rlversand Sinks -- roads

SigSouthern Butte 4 8 12 I I I I I I I I I 0 kllometers

Figure 1 . Map of the Idaho National Engineering Laboratory showing major landmarks. 5

The topography of the INEL is flat to rolling , and

is dominated by underlying or protruding Pleistocene basalt

flows . Two maj or (East and Middle, also known as the Twin

Buttes ) and several minor buttes break up the average

terrain . Elevations range from 1463 m to 1829 m (x = 1485 m) above sea level . Soils are composed primarily of silicic

volcanic sands and clays accumulated through alluvial or

aeolian deposition (McBride et al . 1978) . Basalt outcrops

are interspersed through the southern 2/3 of the site and

sand dunes are common in the north . The INEL is located in

a cold, semi-arid desert (Clawson et al . 1989) . The climate

is moderated by the influences of the Pacific Ocean and

surrounding mountain ranges producing cooler summers and warmer winters than locations with similar latitudes in the

continental United States . Winds generally flow from a

southwesterly direction . Temperatures range between -42°C

and 39°C, and precipitation averages 19 .1 cm .

McBride et al . (1978) documented the vegetation communities found on the INEL. Big sagebrush (Artemisia tridentata) , green rabbitbrush (Chrysothamnus viscidiflorus) , and a variety of grasses are the primary plant species. Sagebrush dominated communities are the most common vegetation communities on site. Utah juniper

(Juniperus osteosperma) forests are located around the Twin

Buttes (East and Middle) and the Lemhi foothills. Isolated and clustered juniper trees are scattered though the 6

southern 2/3 of the site . Small groups of narrowleaf cottonwood trees (Populus angustifolia) and willows (Salix spp.) are located along the Big Lost River channel . Birch Creek is bordered by small groups of narrowleaf cottonwoods

and western water birch (Betula occidentalis) . Many of the

trees along these watercourses are dead or dying , probably due to lack of water. Crested wheatgrass (Agropyron

cristatum) , an exotic species , and saltbush (Atriplex spp .) communities also occur on the site . The INEL is surrounded by public rangeland and private cropland. 7

ROAD-SIDE SURVEYS

Craig {19 78) conducted road-side surveys on the INEL

during the winters of 1974-75 and 1975-76 to determine the

relative densities of raptors using the site outside the

nesting season . Rough-legged hawks were the most abundant

species both winters , followed by golden eagles and prairie

falcons . Another wintering raptor survey was conducted

during a time of high black-tailed jackrabbit {Lepus

californ icus) abundance from 1981-82 (Craig et al . 1984) .

Ferruginous hawks , golden eagles , and bald eagles were more

abundant in 1982 than during the previous surveys . Watson

(1984) noted an increase in rough-legged hawk numbers on the

INEL when carrion , in the form of road-killed jackrabbits

(primarily black-tailed) , was abundant during periods of deep snow cover .

I conducted road-side surveys of raptors on the INEL

from January-May during 1992 and 1993 to assess population

fluctuations and distributions . Data from this study were used to examine population trends since the 1970's and

1980's. My study was conducted during a time of moderate to low black-tailed jackrabbit abundance and extremes in winter weather conditions (Appendix A) (J. Anderson, N. Huntly, A.

Porth, unpubl . data , NOAA , unpubl . data) . Obj ectives of these surveys were to determine raptor abundance and distribution and compare it to earlier studies (1974-75, 8

1981) , determine the influence of habitat on raptor distribution, determine the effects of human developments on raptor distribution, and determine the adequacy of current raptor monitoring efforts on the INEL.

METHODS

I conducted surveys along 5 routes (Figure 2) .

These routes were : (1) T-4 from U.S. 26 to U.S. 20 and highway 20 to its intersection with 26 (TWIN) (58 km); (2) Big Lost River from the Union Pacific Railroad tracks to its northern most crossing of Lincoln Boulevard , detouring around the firing range (LOST) (45 km); (3) T-25 from the

Argonne junction on U.S. 20 to Circular Butte , then T-17 from Circular Butte to Idaho 28 (EAST) (63 km); (4) the unimproved road west of Idaho 22 , beginning in T 6 N, R 31

E, Sec . 6, to its intersection with Birch Creek, then Birch

Creek southeast to its intersection with Idaho 22 (BIRCH)

(37 km); and (5) Lincoln Boulevard from Central Facilities to Idaho 33, Idaho 33 to 600E , 600E to Idaho 28, Idaho 28 to

Idaho 22, Idaho 22 to Idaho 33, Idaho 33-22 to U.S. 20, U.S.

20 to the main gate, and back to Central Facilities

(CIRCUIT) (187 km) . The CIRCU IT route was the same one used in previous raptor surveys on the INEL (Craig 197 8, Craig et al . 1984 , Watson 1986a) .

I conducted all surveys in a 4-wheel drive vehicle , except for the first 3 months of 1993 when EAST, BIRCH, 9

Route Key

------· CIRCUIT Birch"­Cfftk '. ----- EAST -·-· · -·- - LOST -··-··-··-··- TWIN ...... BIRCH

L.lttJe \ lost " Riwr l�I I I I I I . . . I . . . I , ' , I ' , ,, I T-25 I I J1 I I ' ' ANL-W TRA • l��pp _...- . ...,...,, ,.' ·, ' . 20 --·- / / ...... n, � / � �/ T-4 ......

Figure 2. Routes used to conduct road-side raptor surveys on the INEL, 1992 and 1993 (see text for route descriptions). 10

LOST , and TWIN where surveyed by snowmobile . The CIRCUIT route was travelled at an average speed of 45 kph , while the other routes were travelled at 24 kph . I init iated surveys after 0800 MST and completed them by 1500 hours . An effort was made to conduct surveys only on calm days , but 9 of 79 surveys were conducted with the wind exceeding 20 kph because of time constraints . I did not conduct surveys during , or immediately after, an episode of heavy precipitation . During 1992 , I surveyed each route twice a month . In 1993 I surveyed the CIRCUIT route twice a month , but weather and road conditions restricted me to surveying along the other routes once a month from January through

April . I traveled each route in alternating directions between successive surveys .

I recorded all raptors seen along the survey routes , and identified them to species with 10x50 binoculars and a

15-60x spotting scope . In addition , I recorded the birds activity (i.e . perching , soaring , stooping, powered flight}, perch site (i.e. power pole, out of service power pole, ground , rock, sign , fence , other), and location . Weather data was obtained from the National Oceanic and Atmospheric

(NOAA ) reporting station at Central Facilities (CFA ) .

A potential problem with this survey method is recounting individuals. The length and speed travelled on the CIRCUIT route decreased the potential for recounting birds , as they were unlikely to move far enough during a 11 survey to be recounted (Craig 1978) . The low number of birds counted along the other routes , and the configuration of the routes (often a straight line) , made it easy to keep track of individuals, thus reducing the probability of recounts .

I used nonparametric statistical tests because they allow comparisons between years and survey routes despite differing survey efforts . I compared the number of birds counted on the wintering surveys between 1992 and 1993 using

2 x contingency tables . I used the Kruskal-Wallis test to compare the February-April counts of this survey with those of the Craigs ' studies (1978 , 1984) , and tested for population trends among years using the Mann-Wh itney U test .

To see how observations along the CIRCUIT route represented the composition of the wintering raptor community on the

2 INEL, I used x 2-row contingency tables to compare the count results of the CIRCUIT surveys with data from the other 4 routes combined , for both 1992 and 1993 . To determine the adequacy of INEL monitoring of wintering raptors , I compared the unpublished results of Radiological and Ecological Sciences Laboratory (RESL) personnel mid­ winter raptor counts (conducted along all the paved roads on the INEL, 211 km) in 1992 and 1993 with my January and

January through March counts (choosing the largest number of each species observed for each time period) using the Mann­

Whi tney U test . I considered results significant at £ < 12

0.05.

To assess the effects of habitat on distribution , I compared the vegetation communities raptors were occupying with total available habitat within 400 m of the survey routes (measured from McBride et al. (1978)]. Habitat categories are summarized in Table 1. I also measured the distance birds were from site facilities , grouping them into

5 categories : 0-1 km , 1-3 km , 3-5 km , 5-10 km , and >10 km .

The amount of transect within each distance-range was also measured to determine availability.

I used x2 contingency tables to compare habitat use and distance from facilities between years and among species. I analyzed raptor activities and perch preferences with the Kruskal-Wallis rank test . Individual species perching preferences were measured using x2 comparisons .

Results were significant at E < 0.05.

RESULTS

Fourteen species of raptors were identified during the road-side surveys (Table 2) . Rough-legged hawks , red­ tailed hawks , ferruginous hawks , American kestrels, and golden eagles were most often sighted on surveys. Prairie falcons, Swainson 's hawks , northern harriers , and great horned owls were less likely to be observed . Chronology of sightings are summarized in Figure 3.

Raptor community structure differed between 1992 and 13

Table 1. Vegetation communities used in study of raptors using the INEL, 1991-1993 .

Catagory Vegatation Community8

Sagebrush-grassland Artemesia tridentata-Agropyron spp . A· tridentata-oryzops is hymenoides Sagebrush-rabbitbrush Artemesia tridentata- Chrysothamnus viscidiflorus

Sagebrush-winterfat Artemesia tridentata-Eurotia lanata

Sagebrush-saltbush Artemesia spp . -Atriplex spp .

Mixed sage Artemesia spp .

Grasslandb Agropyron christatum Agropyron smithii-Iva axillaris

Grassland-sagebrush Agropyron spp . -Artemesia tridentata

Grassland-rabbitbrush Oryzops is hymenoides- Chrysothamnus viscidiflorus Elymus cinereus-Q . viscidifl orus

Rabbitbrush-sagebrush Chrysothamnus viscidiflorus­ Artemesia tridentata

Juniper f orestc Juniperus osteosperma- Artemesia tridentata

Other communities Tetradymia canescens- Chrysothamnus viscidiflorus Atriplex nuttalii-Eurotia lanata

8Adapted from McBride et al . (1978) . b Any grassy area >1 ha was considered grassland for observations . c Any area with >10 junipers within 1 ha was considered a juniper community for observations . Table 2. Results of road-side raptor surveys conducted on the INEL January through May , 1992 and 1993, expressed as number of sightings .

Speciesb

Route8 Year RL RT FH SH GE BE PF PE ME AK NH GH SE TV UK

EAST 1992 35 6 12 0 17 0 3 0 0 2 3 0 0 0 1 1993 2 13 10 0 2 1 2 0 0 6 5 0 1 1 2 TWIN 1992 7 2 4 0 0 0 1 0 0 2 1 0 0 0 0 1993 0 6 1 1 0 0 0 1 0 2 0 0 0 0 1 BIRCH 1992 20 2 7 5 9 1 1 0 0 6 1 0 0 0 1 1993 1 2 2 9 8 0 1 0 0 6 0 0 0 0 0 LOST 1992 2 13 2 4 0 0 3 0 0 18 0 32 2 0 2 1993 2 24 4 11 3 0 0 0 0 31 0 6 0 0 0

Combinedc 1992 64df 23e 25 9df 26 1 8 0 0 2sdf 5 32 2 0 4 1993 5f 451 17 21 13 1 3 1 0 45 5 6 1 1 3 d CIRCUIT 1992 95 16e 18 od 16e 0 12 0 1 ad 5 0 0 0 5 1993 47 26 34 10 41 1 9 1 0 29 3 0 0 0 2

Total 1992 159d 39 e 43 9 d 42e 1 20 0 1 36d 10 32 2 0 9 1993 52 71 51 31 54 2 12 2 0 74 8 6 1 1 5

8 See Figure 2. b RL=rough-legged hawk, RT=red-tailed hawk, FH=ferruginous hawk, SH=Swainson's hawk, GE=golden eagle, BE= bald eagle, PF=prairie falcon, PE=peregrine falcon, ME=merlin, AK=American kestrel, NH=northern harrier , GH=great horned owl , SE=short-earred owl , TV=turkey vulture , and UK=unidentified raptor . c The combined results of the EAST, TWIN, BIRCH, and LOST routes. d Significant difference between years (P < 0. 001; x2 analysis) . e Significant difference between years (P < 0.025; x2 analysis) . f 2 Significant difference between routes (P < 0.05; x analysis) ...... "'" Prairie falcon �--·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·---·�

Golden eagle �--·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·�

Rough-legged hawk �--·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·---��---- () Red-tailed hawk 1()"·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·---� Ferruginous hawk �·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·� 2 Swainson's hawk ::::. f)-·-·-·-·-·-·-·---·..12�---·-·-·-·-·-·-·-·-·-·-·-·-·- Northern harrier � 9 �--·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·---� Great horned owl �--·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·-·---·�

January February March April May

Figure 3. Chronology of migrating raptors on the INEL as determined by first or last observation dates in 1992 (-)and 1993 (-·-·-·-)(continued observations outside the survey period indicated by arrowheads, numbers indicate dates of first or last observation).

...... 01 16

1993 (Table 2). Particularly notable was the much higher rough-legged hawk population in 1992 and the prevalence of red-tailed hawks , Swainson 's hawks , golden eagles , and

American kestrels in 1993. In addition , despite fewer survey hours in 1993 , more raptors were counted that year than in 1992 .

Although there was no significant variation among

1975, 1976, and 1992 population sizes and structure , differences between those counts and those of 1982 and 1993 were noted , based on a 5 count period from February into

April (£ = 0.001, Kruskal-Wallis test) (Table 3) . The raptor community in 1982 was similar in structure to the community in 1993 (£ = 0.20, Kruskal-Wallis test), but was significantly larger (E = 0.003 , Mann-Whitney U test) for each species noted .

Rough-legged hawks composed a higher proportion of the raptors seen on the CIRCUIT route than those surveyed on the other routes (EAST , TWIN, LOST , and BIRCH) (£ = 0.05)

(Figure 4) . In 1992, fewer Swainson's hawks and American kestrels were counted than in 1993 along the CIRCUIT surveys

(£ = 0.001) , and fewer red-tailed hawks were counted in 1993 than in 1992 (E = 0.025) . There was no significant difference between my count data along the CIRCUIT route,

January-March , and the RESL midwinter raptor count data .

Rough-legged hawks , red-tailed hawks , ferruginous hawks , and golden eagles displayed different habitat 17

Table 3. Results of raptor surveys conducted 5 times from early February through mid April along a 187-km route , on the INEL.

Species 19758 19768 1992 1993

Rough-legged hawk 50 59 103 30 24 Ferruginous hawk 5 7 30 9 12 Red-tailed hawk 2 2 8 6 15 Golden eagle 9 4 96 7 29 Bald eagle 0 1 30 0 0 Unidentified raptor 1 1 18 3 1

Total 67 74 285 55 81

8 Craig (1978) b Craig et al . (1984) � Rough-legged hawk - Red-tailed hawk l2:2l Ferruginous hawk 1992 - Swainson's hawk 1993 I : d Golden eagle Prairie falcon CIRCUIT route &Si D American kestrel CIRCUIT route LJOwls c::J Other raptors

Figure 4. Community structure of migratory raptors surveyed on the INEL , January-May 1992 and 1993 (Owls include great horned and short-eared owls, other raptors include northern harriers, bald eagles, peregrine falcons, merlin, turkey vulture, and unidentified; combined routes include every route except CIRCUIT)...... 00 19 preferences in 1992 and 1993 (Table 4) . Rough-legged hawks selected agricultural lands more in 1993 than in 1992. Red­ tailed hawks shifted from sagebrush-grassland use in 1992 to sagebrush-rabbitbrush use in 1993 . Ferruginous hawks used agricultural lands and juniper associations more often in

1993 than in 1992 . Golden eagles usually were seen in sagebrush-grassland communities in both 1992 and 1993 , but were sighted in a wider variety of vegetation communities in

1993 .

Except for rough-legged hawks , I found no difference in raptor activities between survey years (Table 5) . Rough­ legged hawks were more likely to be seen while soaring in

1992 than in 1993 .

Overall perch preference did not differ significantly by year or species (Table 6) ; however, a couple of minor differences were noted . Red-tailed hawks perched in trees more often than the other raptor species .

Rough-legged hawks perched on signs and fenceposts more often than other raptors .

Red-tailed hawks showed a change in distribution with respect to distance from facilities between 1992 and

1993, in contrast to ferruginous hawks , rough-legged hawks , golden eagles , and prairie falcons (Table 7) . All species displayed a similar distribution pattern with respect to distance to site facilities . In both years, red-tailed , rough-legged , and ferruginous hawks , golden eagles, and Table 4. Habitat use of 4 raptor species during the nonbreeding season on the INEL expressed as a percentage of total observations during a road-side survey (1992 and

1993) •

Vegetation Community8 Species Year S-G S-R G-S R-S G G-R J AG Other

Rough-legged hawk 1992 57 . 4cd 6. 8d 20.4 0.0 3.1 1.2 3.1 6.8 c 1.2 1993 26.3 5.3 d 19.3 0.0 0.0 1. 8 7.0 40. 4d o. o

Golden eagle 1992 65. 9d 14. 6cd 12. 2c o.o 7.3 0.0 0. oc o.o 0.0 1993 61. 3d 4.8 d 22.6 o.o 3.2 o.o 6.5 1.6 o.o

Red-tailed hawk 1992 59 . ocd 20. 5c 7.7 0.0 0.0 o.o 10.3 2.6 0.0 1993 36.1 40.3 8.3 0.0 0.0 o.o 8.3 6.9 o.o

Ferruginous hawk 1992 34.1 17 . 1 cd 22. ocd o.o 9.8 c 4.9 12.2 0.o c 0.0 1993 51. 3 2.6 d 0.0 10.3 o.o o.o 23. ld 12.8 o.o

Ava ilabilityb 35.6 32.0 10.0 5.8 3.2 4.0 4.4 2.4 2.6

a S-G=sagebrush-grassland, S-R=sagebrush-rabbitbrush, G-S=grassland-sagebrush, R- S=rabbitbrush-sagebrush, G=grassland , G-R=grassland-rabbitbrush, J-all juniper communities , AG=agriculural land , Other=all other vegetat ion types on the site. b Measured from vegetation map of the INEL (McBride et al. 1978) . c Significant difference between years (P < 0.005: analysis) . d x2 Significant difference from availability (P < 0.05: x2 analysis) .

(\.) 0 21

Table 5. Activities of raptors surveyed during road-side surveys of the INEL (January-May 1992 and 1993) , expressed as number of sightings .

Species Year Perching Flying Soaring Other8

Rough-legged hawk 1992 131 14 5 1993 47 2 2

Red-tailed hawk 1992 33 2 3 1 1993 68 1 3 0

Ferruginous hawk 1992 29 5 3 4 1993 39 3 9c 0

Golden eagle 1992 36 3 2 0 1993 54 5 3 0

8 Includes fighting , courting , and stooping on prey . b Significant difference between years (P < 0.001; x2 analys is). c Significant difference amoung species (P < 0.005; Kruskal­ Wallis analysis) . Table 6. Perch types of raptors observed on road-side surveys on the INEL (January-May 1992 and 1993).

Species PPole8 OPole Pl ine Ground Tree Fence/Sign Sprink

Rough-legged hawk 112 12 2 9 10 4

Red-tailed hawk 55 3 0 3 2 0

Ferruginous hawk 49 2 0 5 6 1 1

Golden eagle 65 4 0 11 10 0 0

8 PPole=power or telephone pole, OPole=power pole converted to platform , Pl ine=power line , Ground=ground , rock , or sagebrush , Tree=tree or nest, Fence/sign=fence posts , road signs , and all miscellaneous posts of similar size, Sprink=center-pivot irrigation structures . b Significant difference between species (P < 0.001; x2 analysis) . Table 7. Distance to site facilities of raptors observed during road-side surveys of the INEL, 1992 and 1993 .

Range8 Species Year 0-1 km 1-3 km 3-5 km 5-10 km >lOkm

Rough-legged hawk 1992 2 3 6 65c 83 1993 0 4 2 14c 33

Red-tail hawk 1992 1 5 10 6bc 17 1993 0 2 23c 27c 19

Ferruginous hawk 1992 1 0 3 12c 25 1993 0 1 6 21c 23

Golden eagle 1992 0 1 2 14c 24 1993 0 1 14c 22c 25

Prairie falcon 1992 0 0 1 12c 7 1993 0 0 0 ac 4

Available8 2.4 13 .6 20.3 37.6 316.1 a Measured from INEL site maps . 2 b significant difference between years (p < 0.0025) x analysis. c Signi ficantly different than expected based on km of transect with in each 2 distance catagory (P < 0.001 ; x analysis) . 24 prairie falcons were observed within 5-10 km of facilities more often than if their distributions were random .

In 1992 snow depth averaged 1 cm (range 0-3 cm,

January-March) and in 1993 it averaged 56 cm (range 0-76 cm,

January-March) (NOAA unpubl . data) (Appendix A) . Snow drifted to greater depths in sagebrush dominated communities , especially in 1993 .

DISCUSSION

My survey results concur with previous descriptions of raptor chronology on the INEL. Great horned owls , golden eagles, and prairie falcons may be seen on site at any time of year (Craig 1979) . Craig (1979) also noted that short­ eared owls , ferruginous hawks , red-tailed hawks , and northern harriers arrived on the INEL in March and left in

November . Swainson 's hawks usually arrived a month after the aforementioned species , and left in September. Rough­ legged hawks winter on site , arriving in October and leaving in May .

The black-tailed jackrabbit population during this study was similar to that of the mid 1970's, but lower than that of the early 1980's. Anderson et al. (unpubl . data ) counted 0-10 jackrabbits along a 45 km spotlight census route in the 1970's, 1000 jackrabbits in 1981 , 700 in 1982 ,

50 in 1992, and 6 in 1993 ; all surveys were along the same route in June . Small mammal densities were lower than during both of the previous survey periods (a catch per unit 25 effort of 0-1 mammal caught per trap night versus a CPUE

>10) (Stoddart 1983 , T. D. Reynolds , unpubl . data) . Watson

(1984) found jackrabbits (in the form of carrion ) and montane voles (Microtus montanus) to be major prey items for rough-legged hawks on the INEL. I observed fewer rough­ legged hawks than were counted in surveys conducted in the

1970's and 1982 (Craig 1978, Craig et al . 1984) . Lower rough-legged hawk abundance over the course of this study may be a result of lower prey availability . Wintering hawk abundance has been associated with prey abundance in other studies (Craighead and Craighead 1969 , Baker and Brooks

1981, Watson 1986a) .

Snow cover apparently affected the number and distribution of rough-legged hawks on the INEL. Previous raptor surveys on the INEL showed rough-legged hawks preferred wintering on the agricultural lands bordering the site in the mouths of the Little Lost River and Birch Creek valleys to the sagebrush communities on the site (Craig

1978, Craig et al . 1984) . Snow depth during those surveys was average for the site (x = 8 cm , range 0-10 cm , January­ March) (NOAA unpubl . data.) . In southwestern Idaho , voles

(Microtus spp.) are most numerous in riparian and agricultural areas (Marks 1984) . In my study area , rough­ legged hawks may be drawn to the agricultural lands around the INEL to exploit rodents . In 1992 , rough-legged hawks ranged widely across the INEL, but in 1993 they were 26 distributed similarly to previous survey observations . Snow depth has been shown to affect rough-leg distribution, probably due to prey availability (Schnell 1968 , Watson

1986a, Bosakowski and Smith 1992) . The lack snow in 1992 may have opened more of the INEL to rough-legged hawk foraging because small mammals were more accessible. The deep snow cover in 1993 probably restricted rough-legged hawks to areas where prey were concentrated and likely to be on the surface of the snow , such as where hay bales were present on agricultural land . Restricted availability of prey in 1993 is probably responsible for their low wintering numbers that year.

I observed more red-tailed hawks during my study than were reported on earlier road-side surveys (Craig 1978,

Craig et al . 1984) . As in Craig's studies, red-tails did not overwinter on the INEL during this study . Craig (1979) indicates winter weather conditions on the INEL may prohibit red-tails from staying year around . However, I saw no difference in the arrival dates of red-tailed hawks , despite the extreme variation in winter conditions between 1992 and

1993. Therefore , weather alone may not be the governing factor with respect to red-tailed hawks wintering off of the

INEL. Prey availability may be another factor responsible for the red-tailed hawks ' winter exodus from the INEL.

Red-tailed hawks were distributed over the entire survey area , but they were most common along the Big Lost 27

River. They were also abundant along U.S. 20 from the

intersection with state highway 22-33 to the river crossing,

and north of ANL-W . The shift of red-tailed hawks from predominantly using sagebrush-grasslands in 1992 to a more

even use of available habitat in 1993 was probably due to the increase in red-tailed hawk numbers from 1992 to 1993 .

Spring snow cover was not a major factor affecting prey availability during later survey months as it was shallow, and small mammals were no longer concealed under the snow .

Increased numbers of red-tailed hawks using the INEL since the 1970's may be a reflection of the increased nesting on site since earlier raptor studies . One nest was recorded in 1976, 4 in 1982 , and 5 in 1987 (Craig et al .

1984 , J. Kirkley , unpubl . data) , compared to 8, 13 , and 12 nests I found in 1991 , 1992 , and 1993 respectively .

Migratory red-tailed hawks in southwestern Idaho tend to return to with in 100 km of their natal area (Steenhof et al .

1984) .

Ferruginous hawk numbers noted during this study were similar to observations in the 1970's (Craig et al .

1984) . Again, jackrabbit numbers were low both in the

1970's and during this study . Ferruginous hawk population size has been linked to population fluctuations in their primary prey species (Smith et al . 1981, Gilmer and Stewart

1984 , Woffinden and Murphy 1989) . Indeed , Craig {1979) suggested the low black-tailed jackrabbit population of the 28

1970's limited ferruginous hawk nesting on the INEL. During the winter of 1981-82, ferruginous hawks were sighted often during raptor surveys and black-tailed jackrabbit numbers were high (Craig et al. 1984) . My data appear to corroborate the relationship between the ferruginous hawk and jackrabbit populations.

The delayed appearance of ferruginous hawks in 1993 may be attributed to prey availability and snow cover .

Black-tailed jackrabbit numbers were decimated by severe winter weather and predation in 1993 (A. Porth , pers . comm. ,

Biology Dept ., Idaho State Univ. , Pocatello, ID 83201) .

Snow depth apparently forced jackrabbits into restricted areas of favorable habitat , increasing their densities .

Bald and golden eagles , as well as coyotes (Canis latrans), were attracted to these jackrabbit concentrations and preyed heavily upon them (pers . obs.). By spring , jackrabbit numbers on the INEL were very low, 6 counted on a 45 km spotlight survey in 1993 versus 50 counted in 1992

(Anderson , Huntly , and Porth unpubl . data) . Ferruginous hawks often hunt from low perches or the ground (Wakeley

1978) , and the >50 cm of snow on the ground in early March would have made this hunting technique difficult by covering perches and small mammals alike .

The observed change in habitat use by ferruginous hawks between 1992 and 1993 may be due to their late arrival on site in 1993 . The ferruginous hawks began nesting at the 29 same time both years (early to mid April), and sightings in

1993 were more concentrated around nesting areas than in

1992 . Ferruginous hawks on the INEL primarily nest in juniper trees (Craig 1979) . Most of the juniper trees on site are in the juniper and sagebrush-grassland communities, hence my increased observation of ferruginous hawks in these areas . The increased use of agricultural land in 1993 may be a result of higher prey availability on farm lands when snow was still drifted in the sagebrush communities in

March .

With the exception of 4 Swainson 's hawks sighted in the fall of 1974 , there is no mention of swainson 's hawks observed during previous wintering raptor surveys of the

INEL (Craig and Trost 1976, Craig 1978 , Craig et al . 1984) .

In 1992, I observed no swainson 's hawks along the CIRCUIT route , but in 1993 I saw 10. Eight of my observations of

Swainson 's hawks were in the mouths of the Little Lost River and Birch Creek valleys , natural north-south migration corridors from the Snake River Plain. A potential explanation for the presence of Swainson 's hawks in the mouths of the valleys is a delay in their migration due to snow conditions north of the INEL. The spring of 1993 was unusually cool and wet (NOAA , CFA reporting station , unpubl . data) . In addition, snow remained on the ground on the INEL until mid April , and later at higher elevations around the site. swainson 's hawks nest on the INEL, but not in the 30 areas traversed by the CIRCUIT route (Craig 1979, pers . obs.). Therefore , hawks would not have been common along the survey route because they would be near their nesting grounds . Swainson 's hawks were most common along the Big

Lost River and Birch Creek , which are historic nesting areas

(Craig 1979) .

I observed small numbers of northern harriers during both years of this study . Earlier surveys yielded slightly higher harrier numbers in the springs of 1975 and 1982 than were present in my study (Craig 1978, Craig et al . 1983) .

Craig (1978) attributes the maj ority of the harriers he saw to migration across the INEL . At least some of the harriers

I observed were nesting on or near the site. Adults and fledgl ings were regularly sighted in several areas on site after road-side surveys were terminated each year. As in earlier studies , northern harriers were common near agricultural fields and open grassland (Craig 1978) . In

1993 harriers arrived on site much later than in 1992. The delay may be attributed to the snow cover of 1993; northern harriers feed primarily on small rodents , particularly

Microtus spp . (Craighead and Craighead 1969) , thus snow may have deterred hunting on site until the end of March .

Golden eagle abundance near the mouth of the Birch

Creek Valley in 1992 resembles Craig 's (1978, 1979) findings of the 1970's. Eagles were also common around Circular

Butte in 1992. Jackrabbits are an important prey species 31 for golden eagles over most of their North American range

(McAtee 1935, Mills 1937, Eakle and Grubb 1986) . Craig

(1979) cites low black-tailed jackrabbit numbers in the mid

1970's as a factor limiting wint ering eagle populations on the INEL. I believe this was also true in 1992 . As much as

60% of golden eagles' winter diets may be carrion (Snow

1973) . Sheep have been reported in the diets of golden eagles in Montana , Colorado , and Texas (Mills 1935, Kalmbach et al . 1964, Mollhagen et al . 1972) . Sheep were grazed near

Antelope and circular Buttes during the survey period in

1992 . The eagles near circular Butte may have been attracted to sheep herds grazing in the vicinity, to take advantage of sheep carrion and stray lambs .

I counted more golden eagles in 1993 than were counted in any other survey year, except for the winter of

1981-82 (Craig 1978 , Craig et al . 1984) . The high concentration of wintering eagles in 1981-82 was attributed to a large black-tailed jackrabbit population (Craig and

Craig 1984) . The jackrabbit population in 1993 was low, thus it is unl ikely eagles concentrated on the INEL to exploit rabbits . Eagles in 1993 were usually seen along paved roads , particularly Lincoln Boulevard and Idaho 22.

Wintering eagles have historically been observed along highway 22 (Craig 1979, Craig and Craig 1984) , but have not been common along Lincoln Boulevard since 1971, a time of jackrabbit abundance (Craig 1979) . Antelope carrion may 32 have served to attract wintering eagles to the paved roads on the study site . Pronghorn were uncommon on the INEL during the winter of 1993 , except along Lincoln Boulevard north of the Naval Reactors Facility (NRF) and along Birch

Creek. Antelope mortality was high in 1993 due to winter stress and vehicle collisions , thus carrion was readily available to wintering eagles.

During both survey years , golden eagles were most likely to be seen in sagebrush-grassland communities . Craig et al. (1986) observed the same phenomenon during the winter of 1981-82. They ascribed the habitat selection of wintering eagles on the INEL to the distribution of jackrabbits on site . Black-tailed jackrabbits on site are common in sagebrush-grassland communities during winter

(MacCracken and Hansen 1984) . In Montana , wintering pronghorn were also found in sagebrush-grassland associations (Bayless 1969) . To take advantage of these food resources , by hunting weakened animals or feeding on carrion, golden eagles would have to hunt the sagebrush­ grassland communities of the INEL. Golden eagles' reduced use of sagebrush-rabbitbrush and increased use of grassland­ sagebrush may be a reflection of their feeding on pronghorn carrion in 1993.

Prairie falcon numbers were comparable to earlier road-side surveys of the INEL (Craig 1978, Craig et al .

1983) . Prairie falcons typically feed on ground squirrels 33

and medium sized passerines (Enderson 1964 , Squires et al .

1989) . In eastern Colorado, wintering prairie falcons were

attracted to large flocks of horned larks (Eremophila

alpestris) (Beauvais et al . 1992) . Of 3 stoops I observed during this study , 2 were on horned larks and 1 was on an unidentified passerine . Horned larks are common winter residents on the INEL (Cieminski 1993) , and snow buntings

(Plectrophenax nivalis) were also abundant in 1993 (pers . obs.). The abundance of passerine prey on the INEL may

attract wintering falcons to the site from surrounding mountain ranges . As opposed to earlier surveys (Craig

1978) , falcons were well dispersed on site. I usually saw prairie falcons in areas with power poles. The distribut ion of power poles , serving as hunting perches , has been cited as a factor influencing prairie falcon habitat use (Beauvais et al . 1992) .

Craig (1978) observed more American kestrels (111) than I did; however in his survey he used 2 observers , whereas I had only 1. We both observed differing numbers of kestrels between years . Craig offers no explanation for this phenomenon . I feel a potential attraction for kestrels on the INEL during my study was an emergence of cicadas

(Cicadidae) in May 1993 .

All great horned owls seen during my surveys were along the Big Lost River . Great horned owl winter ranges are limited and often identical to their nesting territories 34

(Craighead and Craighead 1969, Peterson 1979 , Gatz and

Hegdal 1986) . My findings concur , as most of the owls I saw were near known nesting locations . Fewer owls were seen in

1993 for 2 reasons : inclement weather prevented me from conducting as many surveys of the river, thereby reducing my potential for counting owls in 1993, and only 1 pair of owls attempted to nest along the river (sightings dropped from an average of 3.2 per RIVER survey in 1992 to 0.8 in 1993) .

Great horned owls were not recorded during earlier road-side surveys of the INEL (Craig 1978 , Craig et al . 1984) . The route used for previous surveys (CIRCUIT) does not pass through areas where I found owls during this study .

The 15 raptor species I encountered on this survey was comparable to the 12 species Craig (1978) noted in the

1970's; differences between the studies include observations of Cooper's hawks in the 1970's and my seeing a turkey vulture , great horned owls, a bald eagle, and peregrine falcons (Appendix B) . The merlin I observed was probably incidental , but the species has nested near the northern portion of the site (Craig and Renn 1977) . I saw the merlin perched along Idaho 22 in the vicinity of the nesting observations of the 1970's. Wintering bald eagle numbers on and around the INEL fluctuate greatly, probably due to carrion availability (Craig et al . 1984) . In January of 1993 , a large concentration of eagles was located in the northwestern portion of the site , near Howe , where 35

jackrabbits also were concentrated (0. D. Markham , pers . comm.). Turkey vultures are not common on the INEL (Craig

1979) . The vulture I saw in 1993 was in the northeastern

part of the site where many pronghorn died during winter.

Peregrine falcons have been seen occasionally on site since

the 1970's (Craig 1979 , o. D. Markham , pers . comm.) . However, they have not stayed on site for extended periods .

My observations support the transient nature of peregrine

use of the INEL.

Activities I observed in the raptors were probably

related to preferred hunting techniques of each species . As

in other surveys in the western United States , northern

harriers were usually flying when spotted (Marion and Ryder

1975, Craig 1978, Shupe and Collins 1983) . The ground­ hugging flight of this species is its main method of hunting

(Cl ark and Wheeler 1987) . Buteoine hawks , golden eagles ,

and prairie falcons tend to hunt from tall perches . Utility poles composed a minority of available perches (30% of the

surveyed routes were within 500 m of power poles) , but the maj ority of perching raptors used them . Craig (1978) also describes high use of power poles and attributes it to observer bias towards looking for birds in those locations .

Marion and Ryder (1975) noted raptors on the plains of

Colorado were usually flying when seen. The perching birds in their study preferred ut ility poles to other available perches . Winter ranges of rough-legged hawks on the INEL 36 are influenced by the location of power poles on site

(Watson l986b) . The abundance of raptors within 10 km of

INEL facilities was probably because the majority of power poles on site are near facilities . The birds appear to be attracted to the utility poles and not the facilities themselves .

Schnell (1968) found that rough-legged and red­ tailed hawks wintering in Illinois perch in different places : rough-legs near the ground and red-tailed hawks in trees . I also found that rough-legged hawks used low perches (e.g. , fenceposts, signs , and center-pivot sprinklers) . Golden eagles were also commonly seen on or near the ground , particularly in 1993 when they were feeding on pronghorn carcasses . Red-tailed hawks tended to parch in trees more than the other raptors .

The amount of soaring I saw in rough-legged hawks in

1992 may be due to their wider distribution that year, and hence occupation of areas with fewer favorable perches .

Wakeley (1978) suggests soaring in ferruginous hawks is used as much for marking territories as for hunting . The prevalence of soaring in ferruginous hawks in 1993 may be attributed to their late arrival on the INEL. They had less time to stake territories in 1993 than in 1992 , thus they may have had to soar more in 1993 to mark their territories . In addition, ferruginous hawks nested near the survey routes more often in 1993 than in 1992 . 37

While the CIRCUIT route allows comparisons with previous wintering studies on the INEL (Craig 1978, Craig et al. 1984) , it provides a slightly different portrayal of the raptor community structure on site than a more widespread

survey effort . Rough-legged hawk numbers are exaggerated along the CIRCUIT route , probably due to their affinity for agricultural lands bordering the site . Common spring residents on site , part icularly red-tailed and Swainson's hawks , are missed as the CIRCUIT route does not traverse areas these species prefer to inhabit (i.e. trees along the

Big Lost River and Birch Creek) . Great horned owls were also missed due to their propensity for roosting along the

Big Lost River during the day .

Observed composition of the raptor community overwintering on the INEL by the RESL midwinter survey is biased by route location in the same way as the CIRCUIT route . However, general trends of winter raptor use of the

INEL can be estimated with the midwinter count . To gain insight into transitional raptor communities in the spring and fall additional counts should be conducted in late

October-early November and in April. 38

OWL STUDIES

A study of owl species occurrence on the INEL has not been conducted . As part of his raptor study in the

1970's, Cra ig (1977 ) studied long-eared owl nesting along

the Big Lost River . A food habits analys is of long-eared

owls along Birch Creek and an assessment of nesting owl

activity patterns along the Big Lost River were conducted in

1982 (Craig et al . 19 85, Cra ig et al . 19 88) . Al l long-eared

owl nests were located along the Big Lost River and Birch

Creek , but occas ionally owls were spotted in the western

reg ions of the INEL . Gleason (197 8) studied burrowing owls

in the mid- 197 0's. In addit ion , great horned , short-eared ,

and northern saw-whet owls have been observed on the INEL

(Cra ig 197 9) . I surveyed the INEL in 1992 and 1993 for owl presence us ing foot and call ing surveys . Objectives of this phase of the study were to gather basel ine data regarding species occurrence and distribution of owls on the INEL.

Erratum: Richard Hansen reported eight owl species from his owl surveys on the INEL based on recorded calls and direct observation.

All species Hansen recorded are known to occur in Idaho. I can confirm visual observations on short-eared owls, long-eared owls, burrowing owls, and great horned owls from my own visits to the INEL. Most (or all) of the flammulated owl calls that Richard identified were in error in my opinion and instead represent calls of long-eared owls, an abundant species on the INEL. Richard Hansen agrees with this conclusion at least to some degree. Hansen did record three direct observations of flammulated owls at less than 15 meters although no photos were taken. I have no comment on the direct observations of flammulated owls except that one observation in February seems especially unlikely. Lester D.

Flake, Thesis Advisor 39

METHODS

Flushing surveys--During the weeks of 3 May 1992 and

9 May 1993 , I conducted foot surveys along 2 km transects to

flush short-eared owls. Routes were traversed at a normal

walking pace, on calm days between 0930 and 1400 MST. I

chose 5 transects that were representative of the open ,

grassy habitats preferred by this species (Bent 1938, Clark

1975) . Routes were in the following areas : T6N, R30E, Sec .

24--from Idaho 22, at the north edge of the juniper forest ,

proceeding ESE ; T4N , R28E, Sec . 19--from Idaho 22-33 at mile

marker 8, proceeding NE; T2N , R28E, Sec . 11--Big Lost River

diversion dike intersection with T-12 , proceeding S; T2N,

R30E, Sec . 13--from the intersection of T-6 and the fire

trail south of the Axillary Reactor Area (ARA) , proceeding SE; and T4N , R34E , Sec . 8--from T-8 at the east edge of the

rye grass (Eylmus flavescens) basin, proceeding NW (Figure

5) •

Calling surveys--! conducted owl call ing surveys

intermittently from February through July in 1992 and 1993 .

Surveys consisted of making stops at specified locations along the survey route and playing recorded owl calls

(Kellogg 1962) . I began surveys 1 hour after sunset and ended them 1 hour before sunrise , or when the route was completed, whichever came first . I only conducted surveys when wind was <15 kph and precipitation was light or nonexistent . At each stop I recorded the time , number of 40

� ' ' Birch \ Creek"· �--:....,j..�

G c1rcui.1 sun.

ANL-W • TRA - • ICPP

20 lo ,.,.,,,,,... - ' .,,,· - '·' - '' Big��:, •

Momlc Clly � Big Southern Sam. Q 8

Figure 5. Locations of transects used in short-eared owl survey on the INEL, 1992-1 993 <• ). 41 responses , and the species of responding owls. Stops were

0.8 km apart on the Lost River, Lemhi, and Twin Buttes routes and 1.6 km apart on the T-4 and T-14 routes (Figure

6) . Distances between stops were determined based on the habitat characteristics--the former routes were in wooded areas with more potential for high owl densities and low sound conductance than the later routes . There were 10-13 stops along each route , depending on the amount of time available. Shorter surveys were conducted in the summer because nights were shorter. Calls broadcast on each route were determined by preferred habitats of owl species based on my observations and the literature . I used flammulated, northern saw-whet , western screech , long-eared , and great horned owl calls for the Lost River, Lemhi , and Twin Buttes routes and saw-whet , burrowing , short-eared , and great horned owl calls on the T-4 and T-14 routes .

The 5 routes were located in the following areas :

T-4 from its junction with Twin Buttes Road (east of ANL-W) to its intersection with T-9 ; T-14 from its intersection with T-3 to the INEL border; along the Big Lost River from the railroad tracks northeast of ICPP to the demolition bunker east of NRF ; unnamed fire trails between the Twin

Buttes ; and unnamed fire trails at the foot of the Lemhi

Mountains . Routes were chosen to reflect the range of habitats available to owls using the INEL.

From February through April, 1992, I conducted 42

Route key Lemhi Birch ', Twin c,..1r'., T- 14 T-4

River -, ' '

' ,, ' •.. ,

TAN Little " Lost ' Riwr Clrcullfr0 Butt9

I i i i i i j NRF i ,, •',, ,, i '• ' ; -· ANL-W ,,.,. TRA • ,, -'• ICPP

20 lo,.,,.. ,.,,. --

' ... ,,.. --,,#.. ... ,. ..._ ..� � � .

AtomicCity Butt9 � BigSoutMm Q 8

Figure 6. Routes used for owl calling surveys on the INEL, 1992-1993. 43

territorial calling surveys for great horned and long-eared

owls along all the fire trails in the juniper forests near the Lemhi foothills and Twin Buttes . I only played the

calls of those 2 species , but I recorded the responses of all owl species . The broadcast and recording procedures were similar to those previously described .

RESULTS

Flushing surveys--! conducted these surveys 9 times, walking 5 transects in 1992 and 4 transects in 1993 . Sheep were grazed on the transect site south of ARA in 1993 , thus I was unable to conduct a survey there concurrent with the others . I did not flush any short-eared owls.

Calling surveys--! conducted 11 calling surveys in

1992 and 14 in 1993 . Seven surveys each were conducted along the Lemhi and Twin routes , 4 each along the River and

T-4 routes , and 3 along the T-14 route . Eight species of owls responded to the broadcast calls (Table 8) .

Great horned owl s were the most commonly detected owl species (n = 36) , foll owed by flammulated (n = 29) and burrowing owl s (n = 19) (Table 9) . Great horned owls were present the entire survey period, flammulated owls were most common in early spring, and burrowing owls were first detected in late spring (Figure 7) . Owls usually responded between 2100 and 0100 hours MST (Figure 8) . Great horned 44

Table a. Owl species detected during call ing surveys on the INEL, 1992-1993.

Common name Scientific name

Northern saw-whet owl Aegolius acadicus

Boreal owl Aegolius funereus

Short-eared owl Asio flammeus

Long-eared owl

Burrowing owl Athene cunicularia

Great horned owl Bubo virginianus

Flammulated owl otus flammeolus

Western screech owl Otus kennicottii Table 9. Detection rate of owls on call ing surveys of the INEL, 1992-1993 , expressed as number of responses/survey .

Speciesb

Route8 GHO LEO SEO wso FO swo BO BUO

Lemhi 3.7 0.14 0.0 0.0 2.4 0.43 0.0 0.0

Twin 0.57 0.57 o. o 0.14 1. 7 0.43 o. o o. o

T-14 0.66 o. o 0.33 0.0 o. o 0.33 0.33 2.0

T-4 0.25 0.0 0.0 0.0 o. o o. o o. o 3.0

River 0.75 o. o 0.25 o.o 0.0 o. o o. o 0.25

All routes 1.4 0.20 0.08 0.04 1. 2 0.28 0.04 0.76

a See Figure 6. for route locations . b GHO=great horned owl , LEO=long-eared owl , SEO=short-eared owl , WSO=western screech owl , FO=flammulated owl , SWO=northern saw-whet owl , BO=boreal owl , and BUO=burrowing owl . 5

4

1

0 February April May June July Month

O Greathomed owl � Flammulated owl • Burrowing owl 0 Otherspecies

Figure 7. Months owl species responded to calls during a survey on the INEL, 1992and 1993(Other species indude: long-eared owl, short-eared owl, boreal owl, saw-whet owl and western screech owl). 30

25

U) c 0 20 � � CD U) .a 0 - 15 0 � CD .a E :J 10 z

5

0 2000-2059 2100-2159 2200-2259 2300-2359 0000-0059 0100-0159 0200-0259 Time

• Great homed owl [SJ Rammulated owl • Burrowing owl � Other species

Figure 8. Times owl species were detected during a calling survey on the INEL, 1992and 1993(Other species include: long-eared owl, short-eared owl, boreal owl, saw-whet owl, and western screech owl). 48 owls called later in the evening , while flammulated and burrowing owls responded earlier.

DISCUSSION

Flushing surveys--! made every effort to survey habitats favored by short-eared owls (i.e. open sagebrush , grassland, or marsh) (Bent 1938, Craighead and Craighead

1969 , Call 1978) , but most of the INEL site qualified as

suitab le habitat . Therefore, as foot surveys are inadequate to census short-eared owls on an area as large as the INEL,

an alternate survey method must be adopted, if quantitative data on short-eared population trends is desired.

Conducting intensive ground searches in areas short­ eared owls are commonly observed, such as the north eastern corner of the site , may yield higher flush counts than I observed . Including a rope drag or a field dog can increase the effectiveness of ground searches (Clark 1989) . Ground searches are both labor and time intensive , and therefore are impracticable for merely tracking population trends .

A more efficient means of surveying short-eared owls on the INEL would involve roadside surveys . I frequently observed short-eared owls on the site between 1/2 hour before and 1 hour after sunset. Owls were most active in

May and major activity centers appear to be south of ARA, northeast of NRF , along the northeastern boundaries of the site , and the Big Lost River sinks . Twilight counts along highways 33, 28, and 20, as well as along Lincoln Boulevard 49 in late April and May would be the most practical means of tracing short-eared trends on the INEL.

Calling surveys--Five of the 8 species I encountered during call ing surveys have been reported previously on the

INEL: great horned owl , long-eared owl , short-eared owl , burrowing owl , and northern saw-whet owl (Craig 1979) .

Before my study, the occurrence of western screech owls on the INEL was suspected (Arthur et al . 1984) , and the presence of flammulated and boreal owls was unknown . A northern pygmy owl (Glaucidium gnoma) was detected near the

INEL in 1976 (Craig 1979) , but I encountered none during my surveys .

Great horned owl s have been reported around the Big

Lost River , East Butte , Kyle Canyon , and several facilities

(Cra ig 1979) . I found great horned owl s to be the most widely distributed owl species on the INEL. Th eir greater abundance along the Lemhi route may be due to juniper forest covering most of the foothills and several old ferruginous hawk nests in the vicinity . Great horned owls do not build their own nests , thus they are dependent on those left by other large birds (Bent 1938 , Johnsgard 1988) . Great horned owls also nest in rock cavities and man-made structures , both of which are scattered throughout the INEL site. Their versatile nesting habits are probably responsible for the great horned owl 's wide distribution on site .

As in studies elsewh ere (Craighead and Craighead 50

1969 , Petersen 1979, Gatz and Hegdal 1986) , great horned owls on the INEL appear to maintain territories year round .

Great horned owls were detected in all seasons surveyed , but

I noted a slight decrease in their call ing activity in late spring and summer. Reduced calling activity over the course of the spring was also noted in studies in Virginia and

Pennsylvania (McGarigal and Fraser 1984 , Morrell et al .

1991) .

Long-eared owls have been common along the Big Lost

River (16 nesting pairs in 1976) (Craig 1979) . During this survey, long-eared owls were not commonly encountered (n =

5) , and none of my encounters were along the Big Lost River route. Perhaps low densities of small mammals (T. D.

Reynolds , unpubl . data ) on the INEL during this study were respons ible for the apparent decline of this species . Long­ eared owls are stenophagus and are sensitive to declines in a small number of prey species , particularly voles (Microtus spp.) (Johnsgard 1988) .

The earliest I detected long-eared owls was March .

Craig and Trost (1979) found long-eared owls do not winter on the INEL, but return in mid April to initiate clutches .

I heard no long-eared owls after May .

Long-eared owls are most vocal when staking territories and their activity decreases as the nesting season progresses (Craig et al. 1988, Johnsgard 198 8) . On the INEL, long-ears are most active between 2300 and 0200 51

hours (Craig et al . 1988) , and all of my detections were

within this range .

Short-eared owls nest in the open habitats of the

INEL (Craig 1979) . They are typically crepuscular and are

seldom vocal (Bent 1938, Johnsgard 1988) . I detected 2

short-eared owls during the calling surveys . Short-eared

owl activity patterns make it difficult to detect during

nocturnal calling surveys , thus my methods were inadequate

to sample this species .

The only previously reported northern saw-whet owls

on the INEL have been dead , and they were considered

accidental migrants (Craig 1979, Arthur et al . 1984) . I

detected 7 saw-whets during the calling surveys , one

visually, from February though May . Saw-whet owls may

winter on the INEL. Saw-whets winter in a number of habitat

types and have a tendency to roost in small , dense trees

(Mumford and Zusi 1958 , Johnsgard 1988) . All but 1 of the

saw-whet owls I encountered were in the juniper forests

around the Lemhi foothills and Twin Buttes, apparently

suitable wintering areas for this species .

In Colorado , northern saw-whets were most vocal

February through April , the same period courtship and

nesting take place (Cannings 1987, Palmer 1987) . Saw-whets were vocal on the INEL into the nesting season , but I could not determine if they nested on site . Potential nesting cavities are available to them in the form of common flicker 52

(Colaptes auratus) cavities .

In 1993 I encountered 1 boreal owl during calling surveys . In Idaho , boreal owls inhabit montane , coniferous forests all year long (Hayward et . al . 1993) , thus my observation in the sage-steppe is unusual . These owls are not migratory , but they wander extensively during winter in response to prey density and snow cover (Hayward et al .

1987, Johnsgard 1988) . In 1993 , snowfall was exceptionally heavy in southern Idaho {NOAA , unpubl . data) , therefore the owl I heard was probably an accidental visitor from the mountains surrounding the upper Snake River Plain .

I observed a western screech owl along the Twin

Buttes route in 1993 . This species typically inhabits deciduous riparian areas {Johnsgard 1988) . In Idaho , western screech owls are nonmigratory (Hayward 1983) . The owl I observed is probably a vagrant displaced by the severe winter weather of 1993 .

While conducting calling surveys , I heard and observed the first flammulated owls reported on the INEL.

Flammulated owls nest in montane coniferous forests and probably winter in Mexico {Johnsgard 1988) . In the spring ,

f lammulated owls migrate north at lower elevations than they nest. All my encounters with flammulated owls were in juniper forests along the Twin Buttes and Lemhi routes .

Three times along the Lemhi route I observed f lammulated owls from within 15 m, once in February 1992, and twice in 53

April 1993 . The juniper tracts on the INEL may serve as

staging areas for flammulated owls during spring migration .

In British Columbia flammulated owls reach their

nesting grounds in April, and in Idaho and Utah nest dates

range from 25 April to 1 July (Howie and Ritcey 1987 ,

Johnsgard 1988) . Flammulated owl use of the INEL appears to

peak in April, an appropriate time for them to reach nesting

grounds in the mountains surrounding the upper Snake River

Plain .

The presence of f lammulated owls on the INEL in

February was unexpected . They are not believed to enter the

continental United States before March (Phillips 1942,

Blanda et al. 1975, Howle and Ritcey 1987) . All of my

February detections of this species occurred in 1992 , a year

with an exceptionally mild winter . Temperatures on the INEL

did not fall below -s0c after 15 February , and snow cover was nonexistent by the same date (Appendix A) (NOAA ,

unpubl . data) . The mild weather may have served either to

restrain flammulated owls from migrating south or to promote

their early return. Gleason (1978) reported burrowing owls to be common nesters on the INEL. The burrowing owl population on the INEL appears to be limited by the availability of nesting dens , badger (Taxidae taxis) burrows , and a paucity of open grassy habitat . Gleason

found burrowing owl activity centered around the town of

Howe , and in the east-central portion of the INEL site . I 54

observed burrowing owls along the T-4 and T-14 routes

(Figure 6) . My observations were south of the areas in which Gleason (1978) found most of these owls. Most of the

owls I located were near open grassy habitats , particularly

stands of seeded crested wheatgrass. In south-central Idaho

burrowing owls avoid continuous stands of dense sagebrush

(Rich 1986) . The open areas I found owls in may be preferred activity centers in the sagebrush dominated INEL

site .

Burrowing owls do not overwinter on the INEL. They usually arrive in April (Craig 1979) . I first detected burrowing owls in May , corresponding with the beginning of

nesting season on the site . On the INEL burrowing owls begin to disperse from nest sites in early July (Gleason

1978) ; my increased detections of them in July may be due to

this movement . Most burrowing owls I encountered called within 2 hours of sunset . As this species is primarily

diurnal and crepuscular, such behavior is typical (Collins

1979) .

My calling surveys were not designed for measuring

owl populations on the INEL, but they were adequate for

detecting species presence and as an index to relative

abundance for most owls using the INEL. Continued calling

surveys should be conducted March through June , from 1 hour

after sunset to 0100 hours to develop a database of owl use

of the INEL. 55

Additional owl surveys on the INEL may be justified .

A more thorough calling survey would provide population estimates for vocal nocturnal species (great horned , flammulated , northern saw-whet) not studied previously on the site . A search of the juniper habitats around the Lemhi foothills and Twin Buttes areas for cavity nesting birds may expand the INEL vertebrate data base , and may yield nesting saw-whet owls. Knowledge of the nocturnal avifauna using the INEL would contribute to future management decisions on site and contribute to the overall ecological understanding of the INEL NERP . 56

NESTING SURVEYS

Five species of medium- to large-sized raptors commonly nest on the INEL. These species include: great horned owl , long-eared owl , ferruginous hawk , swainson's hawk, and red-tailed hawk (Craig 1979) . All 5 species nest primarily in cottonwood and juniper trees . Red-tailed hawks , prairie falcons , and golden eagles occasionally nest on cliffs associated with the Twin Buttes area . Northern harriers and short-eared owls, ground nesting species , have also been observed nesting on the INEL. Raptor productivity data are available from Craig's study conducted in the late

1970's, and limited nest count data are available from Craig et al . 1984 for the early 1980's and from an unpublished data set for 1987 . From 1991 through 1993 I monitored nesting raptors on the INEL to assess the effects of habitat composition , human activity , species interactions, and prey availab ility on nesting numbers and reproductive success . 57

METHODS

I initiated nest searches in late May 1991, and late

February 1992 and 1993 . The search effort spanned the

entire area of the INEL site , as well as areas adjacent to

site boundaries . Times devoted to nest searches were 274 hours in 1991, 234 hours in 1992 , and 247 hours in 1993 .

Most searching involved driving the numerous fire trails within the site and examining all areas apparently capable

of supporting a raptor nest . I conducted foot searches of the juniper forests near the Twin Buttes and Lemhi Range , as well as along the Big Lost River and Birch Creek . I ascended the Twin Buttes to look for cliff nesting birds .

An aerial search was conducted over the eastern 1/3 of the

site in 1991. I made no specific effort to locate ground or cavity nesting species , but when located , such nests were noted .

All my observations of nesting raptors followed the cautionary guidelines proposed by Fyfe and Olendorff (1976) .

Visits to nest sites were brief, especially early in the nesting cycle . Observations were made from as great a distance as possible. A mirror-pole apparatus was used when

I had to look inside a nest . Every effort was made to avoid surprising incubating/brooding adults and nestlings nearing fledging to prevent accidental injury to the young . I followed the development of the nestlings closely , visiting 58 nests at least biweekly. On each nest visit I recorded number of young , approximate age , and defens ive behavior of the adults (observer distance when flushed, intensity of defense , and number defending) . I used the Kruskal-Wallis test to assess differences in nest defense due to species and nest stage (significant at £$ 0.05) (Conover 1980) . I also tested for correlation of nest defense and nest stage using Spearman's rho . If a nest failed , I attempted to determine the cause .

For the nests of all medium- and large-sized raptors found , I recorded several habitat characteristics. At the nest site, following fledging , I noted the nesting substrate; heights of the substrate and nest (m) ; outside diameter of the nest (cm) ; species , condition, and diameter at breast height (dbh) (cm) of nesting trees; predominant vegetation community at the nest site (within 100 m) ; compass aspect of the nest (nearest 10°} ; the number of and average height (m) of trees within 30 m; the height of the highest trees within 30 and 100 m (m) ; and the distance to

2 the nearest tree (m) (minimum dbh of 13 cm) • I used x analysis to compare categorical nest site measurements

(e.g. , nesting substrate , tree species , condition of nest tree , and vegetation community) among Buteo species $ (£.,.19.. 0.05) . In addition, I tested nest site vegetation community

2 versus availability using a x goodness-of-fit test with data obtained from an INEL vegetation map (Figure 9) (McBride 59

- Sage-grass E2J Sage-rabbitbrush EJ Grass-rabbitbrush m Rabbitbrush-grass - Juniper-sage fZ3 Saltbush-winterfat � Rabbitbrush-sage D Horsebrush-rabbitbrush

. D Mixed sage [ZJ Sage-saltbush � Seeded wheatgrass - Native grass - Mixed shrubs CJ Sage-winterfat

Figure 9. Vegetation type map of the INEL site, adapted from McBride et. al 1978. 60 et al . 1978) . I compared tree condition and size to availability based on cottonwood surveys I conducted along the Big Lost River and Birch Creek and juniper surveys conducted near the Twin Buttes and Lemhi foothills (Appendix c-E) . With the remaining nest site characteristics , I tested for interspecific differences with Kruskal-Wallis and median tests (.fsig. $ o. 05) . Using United States Geological Survey (U.S.G.S.) 7.5 minute topographic maps I determined nest locations

(Universal Transverse Mercator (UTM ] coord inates), distance to primary and secondary roads (km) , and amount of primary and secondary road (km) within a 3 km radius . From INEL grazing area maps , I measured the distances of nests from facilities . I used logistic stepwise regression (.fs;9• $ 0.05) to determine the relative influence of human development on nest locations for Buteo species .

From grazing maps of the INEL, I measured nearest neighbor distance for Buteo hawks and conducted nearest neighbor analysis (Clark and Evans 1954) . If the observed ratio of the mean observed distance between nests and the expected distance (R) $ 0.9, distribution is clumped , if R �

1.1 distribution is regular, and if 0.9 < R < 1.1, distribution is random. The significance of R was tested with: where c is the test statistic (compared with the Pearson type III distribution, due to small sample size) C.fs;9• $ 60 et al . 1978) . I compared tree condition and size to

availability based on cottonwood surveys I conducted along

the Big Lost River and Birch Creek and juniper surveys

conducted near the Twin Buttes and Lemhi foothills (Appendix

C-E) . With the remaining nest site characteristics , I

tested for interspecific differences with Kruskal-Wallis and

median tests CJ;.;9• :S: O. 05) • Using United States Geological Survey (U. S.G.S.) 7.5

minute topographic maps I determined nest locations

{Universal Transverse Mercator [UTM ] coord inates), distance

to primary and secondary roads (km) , and amount of primary

and secondary road (km) within a 3 km radius . From INEL

grazing area maps , I measured the distances of nests from

facilities . I used logistic stepwise regression (l;.;g. :S: 0.05) to determine the relat ive influence of human development on nest locations for Buteo species .

From grazing maps of the INEL, I measured nearest neighbor distance for Buteo hawks and conducted nearest neighbor analysis (Clark and Evans 1954) . If the observed ratio of the mean observed distance between nests and the expected distance (R) :S: 0.9, distribution is clumped, if R � 1.1 distribution is regular, and if 0.9 < R < 1.1, distribution is random . The significance of R was tested with : where c is the test statistic (compared with the Pearson type III distribution , due to small sample size) CEs;g. :S: 61

0.05) , rA is the mean of the observed distances , rE is the expected mean distance in an infinitely large random distribution of the same density, and are is the standard error of the expected distance .

I used a vegetation map of the INEL to determine relative percentages of various vegetation communities within 1 and 3 km of the nest . Vegetation communities are named by the 2 most common plant species in that area , with the predominating species first . Habitat composition was assessed with principle components analysis (PCA) . I plotted the resulting eigenvectors and interpreted scatter patterns visually between species , and with 100 random points .

On my last visit to the nest site , following fledging , I coll ected castings and prey remains. I used skeletal remains thus obtained to determine the maj or feeding habits of nesting raptors . I quantified dietary diversity by using Shannon's index (Shannon and Weaver

194 9) :

where pi represents the proportion of each prey species in the sample. I also used Pieliou 's (1969) diversity index : 62

J1=H1/H max', where H' is Shannon 's index value and H max ' is the

logarithm of total species richness . For dietary overlap between species I used Pianka 's (1973) index:

where Pu and pik are proportions of prey species in the diets of the raptor species being contrasted . I obtained prey base data from independent small mammal studies (T.D.

Reynolds , unpubl . data) , jackrabbit spotlighting counts (J.

Anderson , N. Huntly, A. Porth , unpubl . data) , and my personal observations .

RESULTS

Productivity

I found 33 red-tailed , 40 ferruginous , and 28

Swainson 's hawk nests (Table 10) during this study . The timing of nesting activities of all 3 species overlapped , but Swainson 's hawks init iated nesting later than both red­ tailed and ferruginous hawks (Figure 10) . In 1992 I found 6 great horned owl and 4 long-eared owl nests, but in 1993 I only found 1 great horned owl nest and no long-eared owl nests (Table 11) . Nesting surveys started too late in 1991 to provide comparable owl nesting numbers , however I did find 2 nests of each species . Great horned owls nested earlier than all other raptors on the INEL (Figure 11) . 63

Table 10. Productivity of hawks nesting on the INEL, 1991- 1993.

RED-TAILED HAWK :

Species 1991 1992 1993

Red-tailed hawk Territorial pairs8 8 13 12 Nesting pairs 8 13 12 Nests with clutches 8 13 12 Number of successful nests 8 11 4 Nesting success 100% 84% 33%

Number of nestl ings 14 28 27 Number of fledglings 12 19 6

Hatched/nest 1.8 2.2 2.3 Fledged/nest 1. 5 1. 4 0.5 Fledged/successful nest 1. 5 1.7 1.5

Ferruginous hawk Territorial pairs8 13 14 17 Nesting pairs 11 13 16 Nests with clutches 9 10 15 Number of successful nests 5 9 7 Nesting success 55% 90% 46%

Number of nestlings 21 27 29 Number of fledglings 16 20 8

Hatched/nest 2.3 3.0 1.9 Fledged/nest 1. 7 1. 5 0.5 Fledged/successful nest 3.2 2.2 1.6

Swainson 's hawk Territorial pairs8 9 11 11 Nesting pairs 8 10 10 Nests with clutches 8 9 10 Number of successful nests 7 7 7 Nesting success 88% 77% 70%

Number of nestlings 14 18 18 Number of fledglings 9 11 9

Hatched/nest 1.8 2.0 1.8 Fledged/nest 1.1 1.1 0.9 Fledged/successful nest 1. 3 1. 5 1. 3

8 Includes nesting pairs . Fledging

Red-tailed hawk (N = 33)

Fledging

Ferruginous hawk (N = 40)

Fladglng

Swainson's hawk (N = 28)

March April May June July August

Figure 10. Nestingchronology of hawks on the INEL., 1991-1993. 65

Table 11. Productivity of owls nesting on the INEL, 1992- 1993.

Great horned Long- eared

Territorial pairs8 14 7 Nesting pairs 7 4 Nests with clutches 5 4 Number of successful nests 3 3 Nesting success 60% 75%

Number of nestlings 5 13 Number of fledglings 4 12

Hatched/nest 1. 0 3.3 Fledged/nest 0.7 3.0 Fledged/successful nest 1. 3 4.0

8 Includes nesting pairs . Great homed owl Layrg (N = 7)

Layrg long-eared owl (N = 6)

March April May June July August

Figure 11. Nesting chronology of owls on the IN EL, 1991-1993. 67 Causes of nest failure included eggs or nestlings falling from the nest , unsuccessful incubation , cracked eggs , predation on young or incubating adult, and broken or fallen nests . In 1993 the majority of nest failures were from exposure to cold, wet weather (Appendix A) .

Additional nesting raptors on the INEL included : burrowing owls (5 nests found) , short-eared owls (2) ,

American kestrels (16) , and northern harriers (1) . These species were not the focus of this study , thus nest numbers should be considered minimums . I located 2 prairie falcon aeries on the Twin Buttes , but their inaccessibility precluded my collection of adequate reproductive data from them .

Nest Sites

Raptors nested over most of the INEL (Figures 12-

14 ) . Ferruginous hawk nests were scattered throughout the site, Swainson's hawks were slightly less widespread , and red-tailed hawks nested near the Big Lost River and Twin

Buttes . Great horned or long-eared owl nests were primarily along the Big Lost River and near the Lemhi foothills.

Habitat--Nesting substrate used by raptors varied

(Table 12) . Red-tailed hawks nested predominantly in narrow-leaf cottonwoods, ferruginous hawks in Utah junipers and on artificial nesting platforms , and Swa inson 's hawks did not display a preference for nesting substrate . Great 68

(\\l �. \ ' Birch ',8 c,..1r'" ' ��--��f--�

TAN

5

.l

NRF ; F

_, ANL-W F TRA • �'G -'• ICPP s

, : 20 --·- R-' , , --- -- �-o·- - -R -_: ' 8/g l.o.tRiver

Figure 12. Nest locations of raptors nesting on or near the Idaho National Engineering Laboratory, 1991 (R =red-tailed hawk, S=Swainson's hawk, F= ferruginous hawk, G=great horned owl , L=long-eared owl). 69

Birch\s er..1r ·,

Clrculfir� Butte

s

:s

NRF

F F ANL-W

s : '-- --"'' }\�)ci-"' 1\-.R ' ,.' °F Sig LostRiver

..

Sig Southern 8""9 Q

Figure 13. Nest locations of raptora nesting on or near the IdahoNational Engineering Laboratory, 1992 (R=red--tailed hawk, S=Swainson's hawk, F= ferruginous hawk, G=great horned owl, L=long-eared own . 70

Birch .. et.elr \

,s

·�

Clrcui.r� Bun. ',

F

NRF ;, A

,'R - ' s ANL-W

: 20 ·-·-

s � sun. A s

Figure 14. Nest locations of raptors nesting on or near the Idaho National Engineering Laboratory, 1993 (A=red-tailed hawk, S=Swainson's hawk, F= ferruginous hawk, G=great horned owl), Table 12 . Nesting substrates of raptors on the INEL, 1991-1993.

RT8 FH SW GHO �o Substrate N % N % N % N % N %

Trees Narrow-leaf cottonwood 26 78.8 2 5.0 14 50.0 6 66.7 3 50.0 Utah juniper 6 18.2 31 77 .5 11 39.3 2 22.2 3 50.0 Western water birch 0 0 3 10.7 0 0

Platform 0 7 17 .5 0 1 11.1 0

Cliff 1 3.0 0 0 0 0

Totals 33 100.0 40 100.0 28 100.0 9 100.0 6 100.0

8 RT=red-tailed hawk , FH=ferruginous hawk , SW=Swainson's hawk , GHO=great horned owl , LEO= long-eared owl . 72 horned owl s usually nested in cottonwoods (the platform used was located in a cottonwood along the Big Lost River) .

Nest trees along the Big Lost River were similar to available trees with respect to both condition and size.

Seventy-three percent of cottonwoods along the river were dead ; 7.9 % of the cottonwoods showed leaves on at least half of their branches . Mean height of trees along the Big

Lost River was 8.9 m (SD=2 .7, N=l53). Juniper and birch trees on site were smaller than cottonwoods .

Red-tailed hawks chose higher nest ing substrates and placed their nests higher above the ground than other nesting hawks (Figure 15) . Four of 6 long-eared owl nests were in trees < 5 m tall, and all nests were $ 4.5 m above the ground . Nesting substrate for great horned owl s ranged from 5.8 to 9.6 m (median = 7.8 m) . Five of the horned owl nests were $ 4.1 m above ground and the highest was 7.3 m.

Long-eared owls nested in old magpie (Pica pica) and ferruginous hawk nests , while great horned owls used ferruginous and red-tailed hawk nests .

Ferruginous hawks nests were centered on their nesting substrate more often than the other Buteos (Figure

16) , and their nests tended to be larger (Figure 17) . I found no other significant interspecific variations in nest aspect or size.

Some differences occurred in tree stand characteristics among nesting hawks (Figure 18) . Red-tailed 15

14 A c 13

12

11 a B b 10

- b 9 -E +J 8 .c. O> 7 Q) I 6

5

4

3

2

1

0 S N S N S N Red-tailed hawk Ferruginous hawk Swainson'shawk (33) (40) (28) Figure 15. Height of nests (N) and nesting substrate (S) for hawks on the INEL, 1991-1993(N in parentheses, boxes are interquartile range, lines are extremes, dashes are medians, and asterisks are means: statistically similar medians indicated by letters [Brown-Mood median test. p < 0.002)). ...J w Nmh Nmh

East

Red-tailed hawk Swainson 's hawk

Welt

Ferruginous hawk

Fore 16 Car1l8B8t.Mt< eepectc:l n01ts lhe on IM:L, 1991- 1 900(irwu croe np-eeents anered m IU>ltnte). 150 B

140

130

120 A 110

- 100 E A 90 -0

...... 80 ...... Q) Q) 70 E ca 60 0 50

40

30

20

10

0 Red-tailed hawk Fe rruginous hawk Swainson's hawk (15) (�) (20)

Figure 17. Outside diameters of hawknests on the INEL. 1991-1993(N in parentheses, boxes are interquartile ranges, lines are extremes, dashes are medians, and asterisks are means; statitistically similar medians indicated by letters [Brown-Mood median test, P <0.001)) -..J \J1 A 15 30 A c A A (36) 14 28 A c 13 26

12 24

A 11 22 B (/) 10 c 20 Q) - ._Q) 9 18 E f;- - B ..__ ...... 8 16 0 .c. B c ._ C> 7 14 Q) Q) ..c I 6 12 E B :::J 5 B 10 z

4 8

3 6

2 4

1 2

0 RT FH SW RT FH SW RT FH SW RT FH SW RT FH SW 0 Nest tree Highest w/in 30m Average w/in 30 m Highest w/in 1 00 m Number of trees w/in 30m

Figure 18.Tree stand characteristics for hawks nesting on the INEL 1991-1993( RT =Red-tailed hawk (33nests], FH=ferruginous hawk (40 nests) SW =Swainson's hawk (28 nests); boxes indicate interquartile range, lines indicate extremes, dashes indicate medians, asterisks indicate mean, statisticallysimilar medians for eachvariable indicated by letters (Brown-Mood median test P < 0.042)). 77 hawk nests were found in taller trees than were f erruginous hawk nests . Swainson's hawks nested in stands with more trees than the other species, and ferruginous hawks often nested in isolated trees . More than 25% of ferruginous hawk nests were > 1000 m from another tree , while most red-tailed and Swainson's hawk nests were within 40 m of an adj acent tree . Owl nests were within 35 m of another tree , except 1 long-eared owl nest in an isolated cottonwood along the lower Big Lost River in 1991.

Raptor nests were more common in juniper habitats than availability would dictate (Table 13 ) . In addition, open habitats (sagebrush-grassland and grassland-sagebrush) supported many nest sites . Six of 15 great horned and long­ eared owl nests were in juniper habitat and another 7 were in sagebrush and grass communities .

Four vegetation communities within 1 km of nest sites, along with 5 communities within 3 km , were sufficient to separate habitat characteristics near nesting hawks using principle components analysis (Table 14) . Three principle components explained 65 .2% of the variation and 4 components explained 76.6%. Principle component 1 is positively related to grassy habitats in the northern INEL and negatively related to sagebrush-grassland communities on site . Principle component 2 is also positively related to northern vegetation communities, but negatively related to sagebrush-rabbitbrush communities . Habitat composition 78

Table 13. Vegetation communities within 100 m of raptor nest sites on the INEL, 1991-1993 .

Vegetation Community8 SG SR GS GR JS RS Other

Red-tailed hawk 14 9 2b 0 5b 4 1

c 5 b Ferruginous hawk 1 o 12 b 1 1 o 2c oc

swainson 's hawk 14 le 2b 0 9b oc 2

8 SG=sagebrush-grassland ; SR=sagebrush-rabbitbrush ; GS=grassland-sagebrush ; GR=grassland-rabbitbrush ; JS=juniper-sagebrush ; RS=rabbitbrush-sagebrush ; Other=all other available habitat types as delineated by McBride et al . 1978 . 2 b Significantly greater than availability (P $ 0.017, x goodness-of-fit , df=6) . 2 c Significantly less than availability (P � o.ooo , x goodness-of-fit , df=6) . 79

Table 14. Five pr inciple c�nents der ived from analysis of vegetation c011111Jnities surrounding Buteo hawk nest sites and 100 random points on the INEL, 1991 - 1993.

Principle C�ant Vegetation Comiuni ty 2 3 4 5

Within 1 km Sagebrush-grassland ·0.512 Sagebrush·rabbi tbrush ·0.643 Sagebrush-winterfat 0.467 0.355 Grassland·rabbi tbrush ·0.625 0.614 0.354

Wi thin 3 km Sagebrush-grass land ·0.510 Sagebrush ·winterfat 0.454 0.361 Horsebrush· rabbi tbrush ·0.546 ·0.643 Rabbitbrush ·sagebrush ·0.442 0.336 Grass l and-sagebrush ·0.394 0.856

Eigenva lue 3.03 1.73 1.10 1.02 0.99 X of var iation explained 33 .7 19.3 12.2 11.4 10.9 Cc.nm.Jlative X explained 33 .7 53.0 65 .2 76 .6 87.5 80 surrounding ferruginous hawk nests approximated that surrounding random points .

Plotting the first 2 principle components revealed some trends (Figure 19) . Red-tailed hawks were highly clumped in the left-central section of the plot indicating an affinity for grassy habitats in the southern regions of the INEL, ferruginous hawks were scattered indicating a random distribution , and Swainson 's hawks displayed an intermediate distribution .

Human-related development--No one measure of development was found to explain nest site selection among nesting hawks on the INEL (Table 15) . I found all hawks nested closer to secondary roads than they were distributed randomly. Ferruginous hawk nests were farther from facilities than were red-tailed hawk nests and random points . Swainson 's hawk nests were also farther from facilities than red-tail nests and random points , but there were more kilometers of road within a 3 km radius of swainson 's nests than ferruginous hawk nests . Red-tailed and Swainson 's hawk nests had more kilometers of road

(primary and secondary) near their nests than random points did.

Reuse--All 3 hawk species nesting on the INEL showed an inclination to reuse nest sites (Table 16) . Judging by size and structure , many of the reused nests were originally constructed by ferruginous hawks . Ferruginous hawks were ------4 ------�

Red-tail Ferruginous Swainson's Random 3 0

2 . ..

1 6

. O•,.,.. .., 9' .. e •"' • •o . 6 N "' . 6 0 .. . . :::> • 0 ...... � . .. .. 0 -1 :

. • D

. . -2 . .,......

-3

-4 -4 -2 0 2 4 6 U1

F1gan19 . PkJIeigenvectors of fornestingButeo habitat 1.198 esdefined by prlnclplecomponants anatysls. Table 15. Distances (km) between human development and hawk nests on the INEL, 1991- 1993 .

�ry Stetfatics:

Distance to faci litx Distance to er imarx road Distance to secondarx road im2 Amount of road within 3 km

Species N Median Mean( SD ) Range Median Mean( SO ) Range Med ian Mean( SO) Range Median Mean(SD ) Range

Red-tai led hawk 31 4.5 5.6(4.4) 0.9-18.2 3.3 3.4(2.2) 0.2-6.7 200 214( 157) 10-560 12.3 14.6(7.3) 4.5-29.3 Ferruginous hawk 39 9.2 9.0(4.0) 0.4-17.0 5.5 5.6(4.3) 0.03-14.0 324 356(31 1) 2-1500 8.9 12.6(7.6) 3.0-30.6 Swa inson' s hawk 25 8.5 8.9(5.5) 1.4-21 .3 1.9 3.4(3.1) 0.3-10.2 100 281 ( 393 ) 10-1500 21 .3 20 .6(7.6) 3.3-46.0 Random points 40 5.2 5.6(3.2) 0.0-11.0 1.4 2.6(2.9) 0.0- 12.2 485 619(483 ) 31-1756 19.6 20 .8(9.0) 6.7-44.9

Standardized Esti•tes:

Red-tai led hawk Ferruginous hawk Swainson's hawk

a b b Random Points OF 0.23 -0.53 - o .5o D1R -0.49 -0.19 -0 .02 b b b D2R 1.29 0.65 0.65 b b RD3 0.50 0.53 0.14

b Swainson's hawk OF - 1.09 -0.02 b D1R 0.88 -0.26 D2R -0.24 0.01 b RD3 -0. 14 0.38

b Ferruginous hawk OF -1.20 D1R 0.55 D2R -0.30 RD3 0.07

a DF=distance to a facil ity (km> ; D1R=distance to a pr imary road Ckm> D2R=distance to a secondary �oads (m); RD3=the amount of road wi th in 3 km of a nest . Assoc iation (P � 0.05) was found us ing stepwise logistic regression; signs before va lues indicate direct ion of relationsh ip (i.e. , -0.53 indicates ferruginous hawk nests were farther from faci lities than random points were ). 83

Table 16. Reuse of nest sites by raptors on the INEL, 1991- 1993.

Species Once Twice Thrice

Red-tailed hawk 6 8 3 Ferruginous hawk 10 5 6 Swainson 's hawk 16 4 0

Total 32 17 9 84

the only species to use the same nest site 3 years in

succession with any regularity during this study .

Swainson's hawks did not use the same site more than 2 years

running , and most nest sites were used once . There was no

correlation between nest location and reuse (Figure 20) .

Some nest sites were used by different species in successive

years (Figure 21) . Twice during this study 2 species nested

in the same nest concurrently (long-eared owl and American

kestrel north of ANL-W ; Swainson's hawk and American kestrel

northwest of Middle Butte ), in both cases the nest structure

was an old ferruginous hawk nest . Reuse of nest sites was

not associated with the success of that nest the previous

year.

Nearest neighbors--Red-tailed hawk nests were

clustered (Table 17) , whereas ferruginous hawk and

swainson 's hawk nests were randomly distributed . Red-tailed hawks were usually found closer to other raptor species than

in a random distribution , but ferruginous hawks were rarely their nearest neighbor (Table 18 ) . Raptor nest dispersion was similar between years .

Nest Defense

Great horned owls were the least defensive nesters over the course of this study (Table 19) . In both great horned and long eared owls nest defense was rarely initiated before I was close to the nest . Swainson 's hawks were the 85

" ', 8lrch CtMlr '.

, ..--���""----+--��---.

' ,_ '

TAN Utt¥ " LMt" River

F F

.'s

NRF F

, '� ANL-W TRA • F -'. ICPP

20 ...... , ... __.

.. le8ulf9 �r

Figure 20. Reused nest locations of raptors on the INEL, 1991 -1993 (R=red-tailed hawk, F=ferruginous hawk, S=Swainson's hawk, capital letters indicate use for 3 years and lower case letters indicate use for 2 years). 86

BirchCl'9fllc\ ',

* NRF

ANL-W TRA • _ ,-� ICPP

...

' ' ------.. - , :·- . - -- Big LOiiRiver

Figure 21. Locations of nests used by different raptor species in successive years on the INEL, 1991-1993 (.& =used by different species indif ferent years; * =used by different species the same year). Table 17. Nearest neighbor matrix for hawks nesting on the INEL, 1991-1993.

Red-tail Ferruginous Swainson's Year Species N r a r R r r R e a a R a Red-tailed hawk 8 8.51 4.34 0. 51b 4.16 0.49 b 5.09 0. 60b 1991 Ferruginous hawk 11 7.25 18 .70 10. 02b 6.44 o.a9 7.93 1.09 Swainson 's hawk 8 8.51 13.20 1. 55b 6.61 0.78 7.20 0.8 4

Red-tailed hawk 12 6.94 2.72 0. 39b 4.70 o. 67b 3.62 0.52 b 1992 Ferruginous hawk 13 6.67 13 .48 2.02 b 7.34 1.10 7.65 1.14 swainson's hawk 10 7.61 5.51 0. 72c 6.85 0.90 6.43 0.85

Red-tailed hawk 12 7.25 3.13 o. 43b 4.18 0.60 b 6.21 0.89 1993 Ferruginous hawk 15 6.21 14 .80 2.39 b 7.92 1. 27c 10.79 1. 74b Swainson 's hawk 11 7.25 7.40 1. 02 5.64 0.77 10.56 1. 45b

8re=expected distance between nearest neighbors, r =mean observed distance between nearest neighbors , R=nearest neighbor ratio (R = 18 in random population , R < 1 in clustered population , R > 1 in diffuse population) . b significantly different than random population (P � 0.01, Pearson Type III distribution) . c significantly different than random population (P � 0.05, Pearson Type III distribution) •

00 -.J 88

Table 18 . Nearest neighbor occurance of hawks nesting on the INEL, 1991-1993.

Neighbor Nester Red-tail Ferruginous swainson's

Red-tail 16 3 14 Ferruginous 9 17 13 Swainson's 10 10 9 Table 19 . Nest defense by raptors nesting on the INEL, 1991-1993 .

a Defense intensi t� Initiationdi stance Nl.Jlt>er defend i OS Species Nb Median Mode Maxin.n Median Range Median Mode Red-tailed hawk 121 1 1 4 100 0-400 1 1 Ferruginous hawk 123 1 0 4 100 0-500 1 1 Swainson's hawk 121 2 0 5 70 0-600 1 1

Great horned owl 32 0 0 3 30 5-150 1 1 Long-eared owl 12 1. 5 0 4 5 0-100 1 1

8Intensity ratings ranged from 0 (no nest defense) to 5 (defenders coming within 5 m of observer). b N=total number of nest vis its .

()) l.O 90 most defensive nesters on the INEL CE = 0.029, Kruskal­

Wallis) . In contrast to owls, Buteos often began nest defense when I was about 100 m from the nest . In all species, nests were usually defended by 1 adult .

I found no significant correlation between nest stage (i.e. , incubation , early nestling, fledging) and defensive behavior, but ferruginous and red-tailed hawks were most defensive from hatch ing unt il nestl ings were about

20 days old (E = 0.025, Kruskal-Wallis) . Swainson 's hawks were most defensive when nestlings were more than 30 days old and 2 adults participated in nest defense during the later nestling stage CE = 0.019 , Kruskal-Wallis) .

Ferruginous hawks were most likely to be flushed with no defense during incubation CE = 0.001 , Kruskal-Wallis) .

Food Habits

Except for long-eared owls, raptors nest ing on the

INEL have broad diet selections (Table 20) . Ferruginous hawk diets overlapped little with those of other species , despite comparable diet breadth . They fed mainly on black­ tailed jackrabbits (Table 21) . Red-tailed hawks also preyed on jackrabbits, in addition to cottontails (Sylvilagus spp.) and rodents (Table 22) . Swainson 's hawks had similar feeding habits to red-tailed hawks (Table 23) . Great horned owls fed primarily on cottontails and bushy-tailed woodrats

(Neotoma cinerea ) (Table 24) . Horned owl diets overlapped 91

Table 20. Diet diversity and overlap amoung raptors nesting on the INEL, 1991-1993 .

DIVERSITY :

Species Na H' anti log H' J'

Red-tailed hawk 26 0.987 9.71 0.861 Ferruginous hawk 23 0.971 9.36 0.826 swainson 's hawk 16 1. 073 11.86 0.892 Great horned owl 5 0.953 8.98 0.883 Long-eared owl 5 0.743 5.53 0.688

8 N=number of nests sampled , H'=Shannon 's diversity index, J'=Pieliou's diversity index ; antilog H' presented to facilitate interspecific comparisons .

OVERLAP :

LE08 GHO SW FH

RT 0. 616b 0.778 0.833 0.623 FH 0.274 0.351 0.557 SW 0.756 0.845 GHO 0.774

8 LEO=long-eared owl , GHO=great horned owl , SW=Swainson's hawk , FH=ferruginous hawk , RT=red-tailed hawk . b Numbers obtained from Pianka 's overlap index . 92

Table 21 . Food of ferruginous hawks nest ing on the INEL, 1991- 1993 (23 nests).

Nl.JN)er Percent of Percent of Prey Spec ies Sc ientific Name of Items Total Items Total Biomass8

Rodent ia Northern Pocket Gopher CThomomys talpoides) 13 14.6 5.2 Townsend's ground squ irrel CSpermophi lus townsendi i> 10 11.2 3.5 Bushy- tai ledwood rat C� cinerea) 5 5.6 2.8 Least ch ipnuik CEutamias mininus) 4 4.5 0.2 Montane vo le CMicrotus montanus ) 3 3.4 0.2 Deer mouse CPeromvscus maniculatus) 2 2.3 tr Western harvest mouse (Rei throdontomys megalotis) 1 1.1 tr Great Basin pocket mouse (Perosnathus earvus) 1 1.1 tr Northern grasshopper mouse (Onochomys leucogaster) 1 1.1 tr

Microtus spp. 5 5.6 0.3 Unident ified Cricetidae 4 4.5 0.2

Lagomorpha Black·tailed jackrabbi t (� cal i fornicus) 22 24 .7 67 .6 M01X1tain cottonta il (Sylvi lagus nuttal lii) 1 1.1 1.3 Pygmy rabbi t (Sylvi lagus idahoensis) 1 1.1 0.7

�spp. 5 5.6 10.9 Sylvi lagus spp. 1 1. 1 1.0

Carnivore Long-tai led wease l (Mustela frenata) 3 3.4 1.0

Aves Black-bi lled magpie CPi ca pi ca) 2 2.3 0.7 Nest ling (� rega l is) 1 1.1 1.2 Sage grouse CCentrocercus urophas ianus ) 1 1.1 2.7

Unident ified bird 2 2.3 0.2

Repti lia Un ident ified lizard 1. 1 tr

Invertebrates b b Unident ified insect 60

8 Adapted from Steenhof (1983) b Invertebrates not included in calculat ion of frequency and biomass . 93

Table 22. Food of red-tai led hawks nest ing on the INEL, 1991- 1993 C26 nests) .

Nl.llt>er Percent of Percent of Prey Spec ies Scientific Name of Items Total Items Total Biomass8

Rodent ia Montane vo le CMicrotus montanus) 8 9.3 0.1 Northern Pocket Gopher CThomomys talpoi des) 7 8. 1 5.0 Least chip!U'lk kangaroo rat CDipodomvs ordii) 1 1.2 0.2 Meadow vo le CMicrotus pennsyl vanicus) 1 1.2 0.1 Sagebrush vo le (Lasurus curtatus) 1 1.2 0. 1

Microtus spp. 13 15.1 1.6 Unident ifiedCric etidae 3 3.5 0.2

Lagomorpha Black-tailedja ckrabbi t C� ca lifornicus) 7 8. 1 38.4 Mountain cottontai l (Sylvi lasus nuttal lii) 7 8. 1 16.2 Pygmy rabbi t CSylvilagus idahoensis) 1 1.2 1.2

Sylvilagus spp. 7 8.1 12.5 �spp. 3 3.5 , 1.6

Aves Nesti ing (� jamaicensis) 1.2 2.,

Unidentifiedbi rd 2 2.3 0.4

Reptilia Gopher snake (Pituopbis melano leucus) 3 3.5 2.2 Western rattlesnake (Crotalus vi ridus) , 1.2 1.4

Unidentified lizard 2 2.3 o. 1

Invertebrates b b Homoptera 77 b b Coleoptera 6

b b Unidentified insect 34

a Adapted from Steenhof (1983) . b Invertebrates not included in ca lculat ion of frequency and biomass 94

Table 23 . Food of Swainson's hawks nest ing on the INEL , 1991-1993 (16 nests).

NU!Oer Percent of Percent of a Prey Spec ies Sc ientific Name of Items Total Items Total Bior.-s

Rodent ia Deer mouse (Peromvscus maniculatus) 6 6.0 1.2 Great Basin pocket mouse CPeroanathus parvus) 6 6.0 1.1 Northern pocket gopher CThomomys taleoides ) 4 4.0 8.6 Montane vole CMicrotus montanus ) 3 3.0 1.1 Western harvest mouse CRei throdontomys megalotis) 2 2.0 0.2 Northern grasshopper mouse (Onochomys leucogaster) 2 2.0 0.6 Least chipnuik (Eutamias !!!.i!l.i!!!!!> 1 1.0 0.3 Townsend's ground squirrel (Spermophi lus townsendi i) 1 1.0 1.9 Ord's kangaroo rat CDipodomys ordi i) 1 1.0 0.6 Bushy· tai led woodrat C� cinerea) 1 1.0 3.0 House mouse (� nusculus) 1 1.0 0.2

Microtys spp. 8 8.0 3.0

Lagomorpha Black-tai led jackrabbit (� ca lifornicus) 2 2.0 21 .5 Mountain cottontai l CSylvi lagus nuttal lii) 1 1.0 7.0 Pygmy rabbi t CSylvi lagus idahoens is) 1 1.0 3.7

Syl vi lagus spp . 5 5.0 26 .9 �spp. 2 2.0 10.8

Carnivore b Unident ified carni vore 1.0

Artiodactyl ia b Unidentifiedung ulate 1.0

Aves Western meadow lark CSturnel la neglects> 4 4.0 4.1

Unidentifiedbi rd 5 5.0 3.0

Reptilia Unident ified lizard 1.0 0.2

Invertebrates c c Coleoptera 37 c c Orthoptera 35

c c Unident ified insect 9

8 Adapted from Steenhof (1983 ). b Not included in calculationof biomass . c Invertebrates not included in calculation of freqency and biomass. 95

Table 24 . Food of great horned ow ls nest ing on the INEL, 1991-1992 CS nests).

Nl.llt>er Percent of Percent of Prey Spec ies Sc ientific Name of Items Total Items Total Biomass8

Rodentia Deer mouse (Peromys cus maniculatus) 9 16.1 1.9 Montane vo le C"'icrotus montanus ) 8 14.3 3.1 Ord's kangaroo rat (Dipidomys ordi i) 6 10.7 3.5 Bushy- tailedwoodrat (� �) 6 10.7 18.4 Great Basin pocket mouse (Perosnathus parvus) 4 7.1 0.8 Western harvest mouse CReithroclontomys megalotis) 3 5.4 0.3 Northern grasshopper mouse (C>nochomys leucogaster) 1 1.8 0.3 Meadow vo le (Microtus pennsylvanicus) 1 1.8 0.3 Northern pocket gopher (Thomomys talpoides> 1 1.8 2.2

Microtus spp. 3 5.4 1.2

Lagomorpha Mountain cottontai l CSylvi lagus nuttal lii) 4 7. , 28.7 Pygmy rabbi t (Sylvi lagus idahoens is) 3 5.4 11.3

Sxlvi lagus spp. 1.8 5.5 �spp. 1.8 12.0

Carnivore Long-tai led weasel 1.8 2.0

Aves Red-tai led hawk C� jamaicensis) 1.8 6.6

Un ident ified bird 3 5.4 1.9

Invertebrates b b Coleoptera 4

8 Adapted from Steenhof (1983 ). b Invertebrates not included in calculation of frequency and biomass . 96 with both red-tailed and Swainson 's hawk diets . Long-eared owls preyed mainly on 3 rodent species (Table 25) . Their diets overlapped most with those of great horned owls.

DISCUSSION

Productivity

Nesting numbers of red-tailed , ferruginous , and swainson 's hawks on the INEL were higher during this study than those noted in the 1970's. One red-tailed hawk nest was reported in 1976 (Craig 1979) . In 1975 and 1976 a total of 3 ferruginous hawk nests were found . Twelve Swainson's hawk nests were located between 1974 and 1976 .

Hawk nesting populations from 1991-1993 were similar to the 1982 population . In 1982 4 red-tailed hawk nests were found (Craig et al . 1984) , whereas I found 8-13 nests .

The 7 Swainson 's hawk and 16 ferruginous hawk nests noted in

1982 are comparable to the nesting densities I encountered .

Nesting success of red-tailed hawks wa s high in 1991 and

1992, but fell dramat ically in 1993 . Cold, wet weather appeared to be the cause of most of the nest failures in

1993.

The range of fledging rates (0.5 to 1.5) I found were comparable to other red-tailed hawk populations . Red­ tailed hawks in southwestern Montana fledged 1.4 young per nest (Johnson 1975) , 1.6 in southern California (Wiley

1975a) , and 1.6-1.9 in south-central Washington (Fitzner et 97

Table 25 . food of long·eared owls nes ting on the INEL, 1991- 1992 (5 nests ).

N\llt>er Percent of Percent of Prey Species Scientific Name of Items Total Items Total Biomass8

Insectivore Merriam's shrew

Rodentia Great Basin pocket mouse (Perognathus parvus) 21 25 .9 9.6 Montane vo le (Mi crotus montanus) 21 25.9 19.9 Deer mouse (Peromys cus maniculatus) 18 22.2 9.3 Ord's kangaroo rat (Oipodomys ordii) 5 6.2 7.1 Meadow vole (Microtus peoosylvanicus) 2 2.5 1.6 Sagebrush vo le (Lagurus curtatus) 2 2.5 1.6 Northern grasshopper mouse (Onochomys leucogaster) 2 2.5 1.4 Bushy- tai led woodrat (� �) 1 1.2 7.5 Western harvest mouse (Rei throdontomys megalotis) 1 1.2 0.3 Northern pocket gopher (Thomomys taleoides ) 1 1.2 5.4 Townsend's ground squi rrel CSpermophi lus townsendi i) 1 1.2 4.8

Microtus spp . 1.2 0.9

Lagomorpha Black-tai led jackrabbi t (� californicus) 2 2.5 27. 1

Aves Unidentified bi rd 2 2.5 3.0

Invertebrates b b Unident ified insect 4

a Adapted from Steenhof (1983). b Invertebrates not included in calculat ionof frequency and biomass . 98

al . 1981) . Lower fledging rates were noted in northern Utah

(1.1 ) and southeastern Wisconsin (1.3) (Platt 1971, Petersen

1979) .

Nesting success of ferruginous hawks was highly

variable from 1991-1993 . Success was lowest in 1993 . Other

studies have shown inconsistent ferruginous hawk nesting

success . In northwestern New Mexico nest success varied

between 60 and 100% (Ramakka and Woyewodzic 1993) , 33 and

50% in Washington (Fitzner et al . 1977) , and 48 and 72% on

the Utah-Idaho border (Howard and Wolfe 1976) .

Fledging rates of ferruginous hawks on the INEL

(0.5-1.7 young/nest} were comparable with observations from

other parts of their range . Long-term studies (8-20 years)

in Utah and New Mexico showed ferruginous hawks had mean

fledging rates of 1.8 and 1.7 young per nest, respectively

(Woffinden and Murphy 1989 , Ramakka and Woyewodzic 1993) .

Fledging rates in Washington and South Dakota were 2.2 and

2.1, respect ively (Fitzner et al . 1977 , Blair and Schitoskey

1982) .

Swainson 's hawk nesting success remained high (70-

88%) throughout this study , and is comparable to the 71%

success rate noted by Craig (1979) in 1975-1976. Nesting success in other parts of their range varied from 64% to

100% (Platt 1971, Fitzner et al. 1981, Gilmer and Stewart

1984) .

Fledging rates were stable, but low for swainson 's 99 hawks on the INEL from 1991-1993 (0.9-1.1 fledglings/nest).

Swainson 's hawks fledged 1.6 young per nest in the 1970's

(Craig 1979) . Fledging rates in other locales ranged from

1.3 to 1.9 in other parts of their range (Platt 1971,

Fitzner et al . 1981, Gilmer and Stewart 1984) . Craighead and Craighead (1967) noted low fledging among hawks nesting in western Wyoming (0.6 fledgl ings/nest).

The winter and spring of 1993 were unusually cold and wet in southeastern Idaho (NOAA unpubl. data) (Appendix

A) . Red-tailed and ferruginous hawks nested earlier than swainson 's hawks on the INEL, and experienced higher nest failure rates in 1993 . Nest destruction, due to wind and water saturation , and nestling exposure were the most common causes of nest failure that year . During 1993 , nest failures were complete , fledging rate was stable for successful nests , and later nesting hawks (i.e. , swainson's hawks) experienced normal nesting success . Therefore , I believe weather was the primary factor influencing poor nest success of early nesting raptors in 1993.

Great horned owl nesting numbers during this study were comparable to those noted on the INEL in the 1970's

(Craig 1979) , however it was apparent only a portion of owl pairs on site actually nested. I found numerous territorial pairs , but only 5 active nests . Long-eared owl nests were uncommon during my study , but constituted a large portion of the nesting raptor community in 1976 (Craig 1979) . The 100 retiring habits of this species may have affected my ability to find their nests . However, a search of all suitable nesting substrates along the Big Lost River produced only 4 nests , whereas in 1976, 16 nests were located along the river.

Great horned owls had 43% nesting success in the

1970's (Craig 1979) , and their success was not much higher during the early 1990's (60%) . In 1975 and 1976 combined ,

0.9 young fledged per nest (2 .0 from successful nests) , a rate similar to my observations . In other parts of their range , great horned owls had fledging rates ranging from 1.1 to 2.7 fledglings per nest (Craighead and Craighead 1969,

Petersen 1979, Fitzner et al . 1981) .

Nesting success of long-eared owls during this study , 75% success fledging 3.0 per nest , was similar to nesting success in previous studies . Nesting success of long-eared owls on the INEL was 84% in the 1970's with 1.7 young fledging per nest in 1975 and 4.1 fledglings in 1976

(Craig 1977) . In western Wyoming and south-central

Washington , fledging rates ranged from 3.3 to 4.3 fledgl ings per nest (Craighead and Craighead 1969, Fitzner et al .

1981) .

Other raptor species I observed nesting on the INEL have been noted previously (Craig 1979) . American kestrels commonly nested throughout the site during my study . I found fewer nesting prairie falcons than noted by Craig in 101 the 1970's. Burrowing owls were present on the study area, but I did not attempt to locate their nests . As with earlier studies of raptors on the INEL, I had insufficient time to search for ground nesting species (i.e. , northern harrier and short-eared owl) .

Nest Sites

Habitat--The selection of nesting substrate by hawks on the INEL was similar to that noted in other open environments . Red-tailed hawks in western Wash ington nested primarily in black cottonwoods (Popu lus trichocarpa )

(Speiser 1990) , and in south-central Washington , Lombardy poplars (Populus nigra) and black cottonwoods were common substrates (Fitzner et al . 1981) . swainson's hawks are less selective of nest trees , several species are used by nesting

Swainson 's hawks in Wyoming , Washington , and North Dakota

(Dunkle 1977 , Fitzner et al . 1981, Gilmer and Stewart 1984) .

In the 1970's Swainson's hawks on the INEL nested along the

Big Lost River and in shelterbelts , usually in cottonwoods

{Craig 1979) . The increase in use of junipers as nesting substrate may be a reflection of hawks nesting away from the river, perhaps due to the deteriorating condition of cottonwoods along the Big Lost River. Ferruginous hawks nested only in trees and on plat forms during this study, as they did in the 1970's (Craig 1979) . This species is usually very adaptive with respect to nest substrate choice, 102

and in the Little Lost River valley north of the INEL, they

typically nest on the edges of alluvial bluffs (Call 1976,

Powers and Craig 1976) .

Nest placement by hawks on the INEL is related to

their preferred nesting substrates . Red-tailed hawks had

the highest nests , in the tallest trees . Cottonwoods are

the tallest trees on the site . Ferruginous hawks nested in

junipers and platforms , and their nests were the lowest .

Swainson's hawks nested at an intermediate nest height .

Similar characteristics were noted in Buteo guilds in Utah ,

Alberta , and Washington (Platt 1971, Schmutz et al . 1980,

Bechard et al . 1990) . Ferruginous hawks were also more likely to nest in the tops of junipers and to center their nests on the substrate . These placement tendencies were probably due to the physical requirements of supporting their large nests . Red-tailed and Swainson 's hawk nests that were centered on the nest substrate were all vacant ferruginous construct ions .

In all cases , great horned and long-eared owls used nests built by other large birds in previous years . Great horned owls nested in vacant red-tailed and ferruginous hawks nests, as well as a platform along the Big Lost River.

In North Dakota , great horned owls used nests of other species in proportion to their availability (Gi lmer et al .

1983) . Two long-eared owl nests were in old ferruginous hawk nests , the remainder were in black-billed magpie nests . 103

In the 1970's 88% of long-eared owl nests on the INEL were

in magpie nests (Craig 1977) . Corvid nests were also commonly used in southwestern Idaho (Marks 1986) .

Tree nesting hawks on the INEL are faced with 3 choices : small stands of dying cottonwoods along the Big

Lost River and Birch Creek, medium to large stands of junipers near the Twin Buttes , Lemhi foothills, and lava margins , or isolated junipers scattered throughout the

southern 2/3 of the site . Nest site characteristics characterize this choice . Red-tailed hawks nested in small cottonwood stands , near tall trees . The few trees within

100 m of ferruginous hawk nests were < 4 m tal l. Swainson 's hawks nested in medium sized stands of varying tree height .

Similar stand characteristics were found around hawk nests

in Alberta , Washington , and Montana (Schmutz et al. 1981,

Bechard et al. 1990, Restani 1991) .

Habitat composition at hawk nest sites on the INEL was probably related to their use of trees . Trees are most common in juniper-sagebrush and sagebrush-grassland

associations on site .

Vegetational composition in the areas surrounding

ferruginous hawks nests resembled a random distribution.

Juniper distribution on the INEL was also random , with the

exception of the Twin Buttes and Lemhi foothill areas , thus habitat composition around nest sights may have been a

reflection of juniper availability. Black-tailed 104

jackrabbits, a major prey species during this study,

typically inhab it open , grass dominated hab itats on the INEL

(Anderson and Johnson 1983) . If hawks were nesting near

jackrabbit concentrations , it should have shown up in

habitat composition around nest sites , but it did not . More

intensive observation of ferruginous hawk habitat use would

clarify the relationship of nest location and habitat

composition .

Habitat composition around red-tailed hawk nests was

similar among individuals. Th is similarity was a reflection

of their use of cottonwoods as nesting substrate . Red­

tailed hawks nested primarily along the Big Lost River , and

vegetational composition did not vary much along the entire

course of the river within the INEL. Availability of

suitable nesting substrate has been documented as the key

requirement of open country red-tailed hawks in Alberta and

Washington (Schmutz et al . 1980, Fitzner et al . 1981) .

Swainson 's hawk habitat selection was intermediate

between the random distribution of f erruginous hawks and the

similarity of red-tailed hawk nesting areas . Craig (1979)

found Swainson 's hawks nested in areas now occupied by red­

tailed hawks . Vegetation characteristics along the Big Lost

River during the late 1970's may have been more favorable

for Swainson 's hawk foraging than areas they now occupy .

Swainson 's hawks in Washington preferentially foraged in areas with little or no cover (Bechard 1982) . Because 105 swainson 's hawks nested later than red-tailed hawks , they may have been pushed into less than optimal nesting habitat

(e.g. , junipers).

Human-related development--Red-tailed , ferruginous, and swainson 's hawks on the INEL occupied nest sites situated closer to secondary roads than were random points .

My survey methods probably biased observations in this regard , as most nests were found by driving along fire trails. I was therefore unable to use distance to secondary roads as a suitable index to tolerance of human activity .

Red-tailed hawks did not display any aversion to development on the INEL. Human activities , such as planting trees and erecting transmission towers , have been shown to benefit red-tailed hawks occupying areas with few natural nesting substrates (Fitzner et al . 1981, Knight and

Kawashima 1993) . In California, human-caused nest failures in red-tailed hawks were usually due to direct intervention

(e.g. , removing nestlings) (Wiley 1975a) . Restricted access on the INEL shields nest ing birds from such activities, and

I found no evidence of direct human-caused nest failure during this study . Because most facilities on the site are near the Big Lost River, and because most red-tailed hawks nested along the river, it was not surprising to find this species closer to facilities than the other hawk species .

Ferruginous hawks exhibited the least tolerance to human development of all hawk species nesting on the INEL. 106

The paucity of roads near their nests and distance to site facilities support this association . In Washington , f erruginous hawks tended to nest farther from human structures than either red-tailed or Swainson 's hawks , and nested more than 3 km from primary roads (Bechard et al .

1990) . Ferruginous hawks on the INEL were also susceptible to nest failure associated with increased human activity in the same vicinity . Two nests failed along Lincoln

Boulevard , 2 failed near gravel pits west of RWMC soon after digging began in the spring, 1 failed on the NPR site during increased human activity associated with site development , and 2 failed in the eastern part of the INEL when sheep were grazing nearby . All failures due to disturbance occurred during nest initiation and incubation . White and Thurow

(1985) found ferruginous hawks exposed to regular human disturbance suffered higher nest failure rates than undisturbed nests , usually due to nest abandonment .

Swainson 's hawks also nested farther from human development than red-tailed hawks , however they had more roads near their nests then ferruginous hawks . This species has been shown to be fairly tolerant of human activity.

They have nested close to human habitation and roads

(Bechard et al. 1990) , and have been shown to be tolerant of agricultural development (Schmutz 1984) . It does not appear human devel opment on the INEL affected nest site selection by swainson 's hawks . 107

Reuse--During this study ferruginous hawks built new nests 3 times, 2 of them on newly constructed artificial nesting platforms . Ferruginous hawks along the Idaho-Utah state border regularly reused old nests (Platt 1971, Howard and Wolfe 1976) . Reuse of nests on the INEL may be due to the durability of the large structures and a scarcity of trees. Red-tailed hawks also reused old nests. The red­ tailed hawks ' preference for tall nesting substrate , a rarity on the INEL, probably contributed to this reuse . Low nest tree availability was proposed as a reason for reoccupation of nests in North Dakota (Gi lmer et al . 1983) .

Swainson's hawks did not regularly reuse old nests on the

INEL, perhaps due to the flimsy nature of their nests .

Their nests rarely survived more than 1 nesting season ; only

2 nests built by swainson 's hawks during this survey were still intact the following year . Swainson's hawks in

Wyoming and North Dakota were found to reuse nests of the previous year (Dunkle 1977 , Gilmer and Stewart 1984) .

Use of nests by different species in successive years was not uncommon during this study . All tree nesting owls on the site occupied nests constructed by another species . Nests along the Big Lost River may have been reused because of the limited availability of cottonwoods .

Reused nests were usually built by ferruginous hawks , based on size. As in North Dakota , I found reuse of nest sites to be mainly intraspecific (Gilmer et al . 1983) . 108

I also found 2 unique examples of sympatric nesting .

In both cases, American kestrels fledged young from a nest

they occupied concurrently with another nesting raptor. All

4 broods fledged successfully . Both nests were originally built by ferruginous hawks . The kestrels nested in cavities

in the bulky understucture below the nests of their

cohabitators . Long-eared owls nested in another understructure cavity . In the second case of cohabitation ,

Swainson's hawks nested on top of the structure . I only witnessed 1 instance of interspecfic defense: when I

flushed a brooding long-eared owl from its nest it was harassed by a female kestrel nesting in the same structure .

Raptors have been reported nesting near each other, but the concurrent use of a nest structure is unusual . Great horned owl nests have been noted in close proximity of Harris' hawk

(Parabuteo unicinctus) , red-shouldered hawk (Buteo lineatus ), and red-tailed hawk nests (Freemeyer and

Freemeyer 1970, Wiley 1975b, Houston 1975) . High occurrence of nest reuse and simultaneous use of nest structures may indicate that suitable nesting substrate is a limiting factor for raptors on the INEL.

Nearest neighbors--The clustered distribution I noted of nesting red-tailed hawks was probably related to the availability of nesting substrate . Both red-tailed and

Swainson 's hawks nested along the Big Lost River and near the Twin Buttes , thus the apparent affinity of red-tailed 109 hawks for Swainson 's hawks was also a reflection of nest tree availability . A similar relationsh ip was found for hawks nesting in Washington (Bechard et al. 1990) .

swainson's hawks on the INEL appear to nest randomly with respect to the locations of other hawks on site, whereas in south-central Idaho , they were closely associated with ferruginous hawk nest sites (Thurow and White 1983) .

This association was attributed to mutual nest defense benefits . The difference I noted may be due to ferruginous hawks on the INEL nesting in isolated junipers , wh ile in the

Raft River valley , they nested on the edge of a juniper forest.

Ferruginous hawk nests were distant from red-tail nesting sites , but again this was probably a reflection of nesting in isolated junipers while red-tailed hawks nested primarily along the river. Ferruginous hawks in south­ central Idaho also selected nest sites randomly (Thurow and

White 1983) . Buteoine hawk nest spacing in Alberta and Utah was attributed to tree distribution (Schmutz et al. 1980,

Woffinden and Murphy 1983) , and this appears to hold true on the INEL as well.

Nest Defense

Potential differences in Strigiform and Falconiform defensive behaviors are not well documented . Despite being of comparable size, great horned owls were passive nest 110

defenders compared to the hawks on site . Perhaps immobility

and cryptic coloration were the great horned owls' best

defenses against diurnal predators on the INEL. Great

horned owls are known to be very defensive when around other

raptors , occasionally killing young in the nests of other

species within their territories (Craighead and Craighead

1969, Petersen 1979) . Twice during my study I observed

evidence of this behavior ; an incubating swainson's hawk was

killed by a great horned owl in the Lemhi foothills and the

remains of red-tailed hawk nestlings were collected from a horned owl nest along Big Lost River. The swainson 's hawk was found below its nest with talon marks in its back and

its breast eaten ; a great horned owl was observed leaving the nest tree .

Long-eared owls were also fairly passive nest defenders . They resorted to physical displays more often than great horned owls, but allowed observers closer to their nests before reacting . Again, it appears concealment was this species preferred mode of nest defense . Vorenetsky

(1987) described a wide range of territorial behaviors in long-eared owls, but I found the most common to be a wing­ spreading defensive posture on the nest , or aerial circling of the observer while calling .

The hawks on site actively defended their nests .

Behavior was initiated from as far away as 600 m and ranged from call ing to stooping on the observer. Swainson 's hawks 111 were most defensive , as noted by Schmutz et al . (1980) and

Thurow and White (1983) . Ferruginous hawks were the least

defensive hawks on the INEL, as they are in other Buteo

communities (Angell 1969, Thurow and White 1985) . Red-tail

defensive behavior was intermediate .

Females were usually the defenders during the early

stages of nesting . In later nest stages I was not able to

determine the sex of defending parents accurately. Males

engaging in the bulk of nest defence has been documented in both Strigiformes and Falconi formes (Andersson and Wiklund

1987 , Wiklund 1990, Sproat and Ritchison 1993) . Andersen

(1990) , however , observed female red-tailed hawks were more

aggressive nest defenders than males.

The lack of a significant correlation between nest

stage and the defense intensity that I observed supports

Knight and Temple's (1986) contention that increases in defense are more related to habituation to humans than

intrinsic cues . A lack of fear of large intruders may

contribute to increased defensive behavior . My nest visits were infrequent and short, thus there was little opportunity

for birds to adjust to my presence .

I did notice some change in defensive behavior

coinciding with different nest stages, despite the lack of a

significant correlation. Red-tailed and ferruginous hawks were most defensive soon after hatching . Perhaps the adults

stayed closer to the nest during the early nestling stages , 112

allowing them to see intruders they may have missed later in

the nesting season .

Swainson 's hawks and great horned owls were most

defensive just before fledging . This occurrence agrees with

the theory that nest defense increases with parental

investment in their offspring (Andersson et al . 1980) .

Andersen (1990) also noted an increase in defense in the

later stages of nestling development in red-tailed hawks .

I approached nests by various modes , and the distance I observed a nest from varied with the behavior and

size of the nestlings . Interpretations of adult defensive behaviors should consider this fact . For example, perched raptors are more likely to flush at a greater distance when approached by a person on foot than when approached by a vehicle (Holmes et al . 1993) . As I did not record my mode of transport , or closest approach to nests , biases may have been introduced into my data .

Food Habits

The preponderance of black-tailed jackrabbits in f erruginous hawk diets during this study is in contrast to

Craig's (1979) findings in the 1970's. During the 1970's f erruginous hawks preyed mainly on pocket gophers (Thomomys talpoides) and ground squirrels (Spermoph ilus townsendii) , which Craig attributed to a low jackrabbit population .

Black-tailed jackrabbit numbers were low, but increasing 113 during this study (Anderson et al. unpubl . data) . Elsewhere in Idaho and Utah , jackrabbits were a key component in ferruginous hawk diets (Howard and Wolfe 1976, Woffinden and

Murphy 1989) . It appears ferruginous hawks were finding enough prey on the INEL to sustain their nesting efforts , but whether jackrabbit densities are limiting nesting densities can only be ascertained whe� rabbit numbers increase.

Red-tailed hawks had a more varied diet than ferruginous hawks , but also preyed on lagomorphs more than rodents . Red-tail diets can range from broad to fairly specific, depending on prey availabilities (Craighead and

Craighead 1969) , but usually lagomorphs and rodents predominate (Platt 1971, Fitzner et al. 1981, Janes 1984) .

Red-tailed hawk diets on the INEL were similar to other populations in sage-steppe habitats .

Voles and cottontails composed the bulk of

Swainson 's hawk diets , as in the 1970's (Craig 1979) . In both cases their diets were fairly wide ranging . Hawks on the INEL had similar diets to those in Wyoming , North

Dakota, and Montana (Dunkle 1977 , Gilmer and Stewart 1984 ,

Restani 1991) . swainson 's hawks on the Hanford Site, in south-central Washington, preyed primarily on snakes

(Fitzner et al. 1981) .

Great horned owls had a diet similar to those of diurnal raptors on the INEL. Cottontails were common prey 114

items during this study and in the 1970's (Craig 1979) . A

variety of rodents composed most of the remaining prey.

Rodents were the major prey items in Utah and Washington

(Platt 1971, Fitzner et al. 1981, Knight and Erickson 1977) .

Long-eared owl s appeared to be more stenophagous

during my study than in the 1970's (Craig and Trost 1979) .

More prey species were taken during this study, but 3 prey

species composed the maj ority of their diet . Long-eared owl

diets on the INEL in 1982 were similar to my observations ,

with pocket mice , deer mice (Peromyscus maniculatus) , and

voles predominating (Craig et al . 1985) . Unlike previous

studies, I did not find many pocket gophers in prey remains .

Gophers were commonly taken by other raptors on site , so the

differences I observed may have been related more to nest

location than prey preferences . Throughout their range ,

long-eared owls tend to be prey specialists , feeding predominately Microtus and Peromyscus species in North

America (Marti 1976) .

Ferruginous hawks and long-eared owls had the least

dietary overlap with other raptors on the INEL. The lack of

overlap may have been related to their size ; ferruginous hawks were the largest raptors nesting on the INEL and took the largest prey, while long-eared owls were the smallest species I studied , and were the only raptors concentrating their foraging efforts on rodents .

Dietary overlap between red-tailed hawks , swainson 's 115 hawks , and great horned owls was noteworthy . All 3 species were of similar size and nested in similar habitats . Gilmer et al. (1983) suggested that great horned owl diet adaptability reduces their competition with red-tailed hawks . I found great horned owls on the INEL preyed on fewer species than hawks , but concentrated on nocturnal and crepuscular species while hawks concentrated more on dinrnal species. Swainson's and red-tailed hawks may have part itioned food resources through nest spacing . Swainson 's hawks were widely dispersed on the site while red-tailed hawks were concentrated along the Big Lost River.

Swainson's hawks also took slightly smaller prey than red­ tailed hawks .

CONCLUSIONS

Two factors having the greatest influence on nesting raptors on the INEL are nest substrate and prey availability . Frequent reuse of nest sites and stable raptor distributions seem to indicate nesting substrate scarcity . Red-tailed hawks have supplanted Swainson's hawks from traditional nesting areas along the Big Lost River. swainson 's hawk production seems to have suffered as a result. Cottonwood mortality along the Big Lost River may serve to limit or halt red-tailed hawk nesting on the INEL and must be considered in future years . An increasing prey base may have contributed to higher nesting densities in the

1990's than the 1970's, but a crash in black-tailed 116

jackrabbit numbers in 1993 did not affect nesting

concentrations . Decreased production in 1993 was probably

more a result of cold, wet weather conditions than low

jackrabbit availability, as birds nesting later in the

season were not affected .

Human activities on the INEL did not appear to

affect the maj ority of nesting raptors on site. Ferruginous

hawks exhibited sensitivity to human disturbance , especially

near roads and in areas of increased human activity (e.g. ,

gravel pits and the NPR site) . A buffer zone of at least

500 m should be maintained near their nest sites (as this was the closest distance a successful ferruginous hawk nest

was from a disturbance} , and historical nesting patterns

should be considered when new developments are proposed ,

especially if th is species is listed as threatened .

Several opportunities for research still exist on

the INEL. Nesting trends should be tracked through

jackrabbit population fluctuations because adequate

reproductive data has not been obtained during peak

jackrabbit numbers . suitable nesting platforms should be placed in remote locations on site to determine the effect of nest site availability on ferruginous hawk nesting densities. A telemetry study of Swainson's hawk and red­ tailed hawk interactions and habitat use would help determine the effects of the recent increase in nest ing red­ tailed hawk densities on site . Nest site fidelity of hawks 117 on the INEL could be determined with a color-banding study .

Finally, a concerted effort to find and monitor nesting northern harriers , short-eared owls, and small owl species would augment the exist ing database concerning raptor use of the INEL. 118

LITERATURE CITED

Andersen, D. E. 1990. Nest-defense behavior of red-tailed hawks . Condor 92: 991-997.

Anderson , J. E. and R. D. Johnson . 1983 . Impacts of jack rabbit herbivory on sagebrush steppe vegetation . in o. D. Markham ed . Idaho National Engineering Laboratory radioecology and ecology programs 1983 progress report . DOE/ID 12098 Nat . Tech . Info. Serv. , Springfield, Virginia . 434pp .

Andersson, M, c. G. Widlund , and H. Rundgren. 1980. Parental defense of offspring : a model and an example . Anim . Behav . 28: 536-542 .

Andersson, s. and c. G. Wiklund . 1987 . sex role partitioning during offspring protection in the rough-legged buzzard Buteo lagopus . Ibis 192 :103- 107 .

Angell, T. 1969 . A study of the ferruginous hawk : adult and brood behavior. Living Bird 8:225-241.

Arthur , W. H. , J. W. Connelly, D. R. Halford , and T. D. Reynolds . 1984 . Vertebrates of the Idaho National Engineering Laboratory . RESL, DOE/ID-12099 , Idaho Falls, ID. 40pp .

Baker , J. A. and R. J. Brooks . 1981. Distribution patterns of raptors in relation to density of meadow voles. Condor 83 : 42-47 .

Balda , R. P. , B. C. McKnight , and c. D. Johnson . 1975. Flammulated owl migration in the southwestern United States . Wilson Bull. 87: 520-533.

Bayless , S. R. 1969 . Winter food habits , range use , and home range of antelope in Montana . J. Wildl . Manage . 33: 538-5 51.

Beauvais, G. , J. H. Enderson, and A. J. Magro . 1992 . Home range , habitat use and behavior of prairie falcons wintering in east-central Colorado. J. Raptor Res . 26:13-18.

Bechard, M. J. 1982 . Effect of vegetative cover on foraging site selection by Swainson 's hawk . Condor 84 : 153-159 . 119

Bechard, M. J. , R. L. Knight , D. G. Smith, and R. E. Fitzner. 1990. Nest sites and habitats of sympatric hawks (Buteo spp. ) in Wash ington. J. Field Ornithol. 61: 159-170.

Bennett, c. M. 1990. Streamflow losses and ground-water level changes along the Big Lost River at the Idaho National Engineering Laboratory , Idaho. U.S.G.S. Water-Resources Invest. Rep . 90-4067 . Idaho Falls, ID. 49pp.

Bent , A. c. 1938. Life histories of North American birds of prey (part 2) . U. s. Govt. Printing Office, Wash ington , D. C. 482pp.

Blair, c. L. , and F. Schitoskey . 1982. Breeding biology and diet of the ferruginous hawk in South Dakota . Wilson Bull. 94: 46-54 .

Bosakowski , T. and D. G. Smith . 1992. Demography of wintering rough-legged hawks in New Jersey . J. Raptor Res . 26:61-65.

Call, M. W. 1978 . Nesting habitats and surveying techniques for common western raptors . U.S. Dept . Int. , BLM Tech . Note 316 . 115pp .

Cannings , R. J. 1987 . The breeding biology of northern saw-whet owls in southern British Columbia. in R. w. Nero, R. J. Clark, R. J. Knapton , and R. H. Hamre eds . Biology and conservation of northern forest owls. USDA Forest Serv . Gen. Tech . Rep . RM-142. 309pp .

Cieminski , K. L. 1993. Wildlife use of wastewater ponds at the Idaho National Engineering Laboratory . M.S. Thesis, South Dakota State Univ. , Brookings , South Dakota . 3llpp .

Clark, P. J. and F. c. Evans . 1954 . Distance to nearest neighbor as a measure of spatial relationships in populations . Ecology 35:445-453.

Clark, R. J. 1975. A field study of the short-eared owl , Asio flammeus , (Pontappidan) in North America . Wildl . Monogr. 47. 67pp .

Clark, R. J. 1989. Survey techniques for owl species in the northeast . pp . 318-327 in Proc . Northeast Raptor Manage. Sympos. and Workshop . Natl. Wildl . Fed. , Washington , D.C. 120

Clark , w. s. and B. K. Wheeler. 1987 . A field guide to the hawks of North America . Houghton Mifflin co ., Boston, Massachusetts . 198pp .

Clawson , K. L. , G. E. Start , and N. R. Ricks . 1989. Climatography of the Idaho National Engineering Laboratory , 2nd edition . DOE/ID-12118. U.S.D.C. , N.O.A.A. , Environ. Res . Lab. , Air Resources Lab . Field Res . Div, Idaho Falls, ID. 155pp.

Collins, c. T. 1979 . The ecology and conservation of burrowing owls. in P. P. Schaeffer and s. M. Ehlers eds . Proc . Nat . Audubon Soc. Symp . Owls of the west : their ecology and conservation . West . Educ . Cent. , Tiburon , California. 97pp.

Conover, J. 1980. Practical nonparametric statistics , �w. 2 edition . John Wiley & Sons , New York . 493pp. Craig, E. H. , T. H. Craig, and L. R. Powers . 1986. Habitat use by wintering golden eagles and rough-legged hawks in southeastern Idaho . J. Raptor Res . 20:40- 45.

Craig, E. H. , T. H. Craig, and L. R. Powers . 1988. Activity patterns and home-range use of nesting long-eared owls. Wilson Bull . 100: 204-213 .

Craig, T. H. 1977. Raptors of the Idaho National Engineering Laboratory . M.S. Thesis, Idaho State Univ. , Pocatello. 102 pp .

Craig, T. H. 1978. A car survey of raptors in southeastern Idaho 1974-76. Raptor Res . 12 : 40-45.

Craig, T. H. 1979 . The raptors of the Idaho National Engineering Laboratory site . ID0-12089. U.S. Dept . Energy , Idaho Falls, Idaho . 28pp .

Craig, T. H. and E. H. Craig . 1984 . A large concentration of roosting golden eagles in southeastern Idaho . Auk 101: 610-613 .

Craig, T. H. , E. H. Craig, and L. R. Powers . 1984 . Recent changes in buteo abundance in southeastern Idaho. Murrelet 65 : 91-93 .

Craig, T. H. , E. H. Craig, and L. R. Powers . 1985. Food habits of long-eared owl s (Asio otus) at a communal roost site during the nesting season . Auk 102 :193- 195. 121

Craig, T. and F. Renn . 1977 . Recent nestings of the merlin in Idaho . Condor 79: 392.

Craig, T. H. and c. H. Trost . 1976. A study of the raptorial species on the INEL site. in o. D. Markham ed. 1975 progress report , Idaho National Engineering Laboratory site radioecology--ecology programs , ID0-12080, Nat . Tech , Info . Serv. , Springfield, Virginia. 205pp .

Craig , T. H. and c. H. Trost . 1979. The biology and nesting density of breeding American kestrels and long-eared owls on the Big Lost River , southeastern Idaho. Wilson Bull. 91: 50-60.

Craighead , J. J. and F. C. Craighead . 1969. Hawks , owls and wildlife . Dover Publ. Inc. , New York . 433pp .

Dunkle, s. W. 1977 . swainson 's hawks on the Laramie Plains , Wyoming . Auk 94 : 65-71.

Eakle, w. L. and T. G. Grubb . 1986. Prey remains from golden eagle nests in central Arizona . West . Birds 17 : 87-89 .

Enderson, J. H. 1964 . A study of the prairie falcon in the centra l Rocky Mountain region . Auk 81:332-352 .

Fitzner, R. E. , D. Berry , L. L. Boyd, and c. A. Rieck. 1977 . Nesting of the ferruginous hawk (Buteo regalis) in Washington 1974-75. Condor 79: 245-249.

Fitzner, R. E. , W. H. Rickard , L. L. Cadwell, and L. E. Rodgers . 1981. Raptors of the Hanford Site and nearby areas of southcentral Washington. PNL-3212 , Pacific Northwest Laboratory, Richland, WA . 6lpp .

Freemeyer, H. and s. Freemeyer. 1970. Proximal nesting of Harris' hawk and great horned owl . Auk 87 : 170.

Fyfe, R. w. and R. R. Olendorff. 1976. Minimizing the dangers of nesting studies to raptors and other sensitive species . Can. Wildl . Serv . Occas . Paper , No . 23. 17pp .

Gatz , T. A. and P. L. Hegdal . 1986. Local winter movements of four raptor species in central Colorado . West . Birds 17 : 107-114 . 122

Gilmer, D. s., P. M. Konrad , and R. E. Stewart . 1983 . Nesting ecology of red-tailed hawks and great horned owls in central North Dakota and their interactions with other large raptors . Prairie Nat . 15: 133-143.

Gilmer, D. s. and R. E. Stewart . 1984 . swa inson's hawk nesting ecology in North Dakota . Condor 86:12-18 .

Gleason, R. s. 1978 . Aspects of the breeding biology of burrowing owls in southeastern Idaho . M.S. Thesis, Univ . of Idaho , Moscow . 47pp.

Harlow, D. L. and P. H. Bloom . 1989. Buteos and the golden eagle. .in Proc . western raptor management symposium and workshop . Natl . Wildl . Fed. , Washington , D.C. pp . 102-110.

Hayward , G. D. 1983 . Resource partitioning among six forest owls in the River of No Return Wilderness , Idaho . M.S. Thesis, Univ . of Idaho , Moscow .

Hayward , G. D. , P. H. Hayward , and E. o. Garton . 1987 . Movements and home range use by boreal owls in central Idaho . in R. W. Nero , R. J. Clark , R. J. Knapton , and R. H. Hamre (eds.). Biology and conservation of northern forest owls. u.s.D.A. Forest Serv. Gen . Tech . Rep . RM-142 . 309pp .

Hayward , G. D. , P. H. Hayward , and E. o. Garton . 1993 . Ecology of boreal owls in the northern Rocky Mountains , U.S.A. Wildl . Monogr. No . 124. 59pp .

Houston , c. s. 1975 . Close proximity of red-tailed hawk and great horned owl nests . Auk 92 : 612-614 .

Holmes , T. L. , R. L. Knight , L. Stegall, and G. R. Craig. 1993. Responses of wintering grassland raptors to human disturbance . Wildl . Soc . Bull . 21:461-468 .

Howard , R. P. and M. L. Wolfe . 1976. Range improvement practices and ferruginous hawks . J. Range . Manage . 29: 33-37.

Howie , R. R. and R. Ritcey . 1987. Distribution , habitat selection , and densities of flammulated owls in British Columbia. .in R. W. Nero , R. J. Clark , R. J. Knapton, and R. H. Hamre (eds.). Biology and conservation of northern forest owls. u.s.D.A. Forest Serv . Gen. Tech . Rep . RM-142. 309pp. 123

Janes , s. W. 1984 . Influences of territory composition and interspecif ic competition on red-tailed hawk reproduction success . Ecology 65: 862-870.

Johnsgard , P. A. 1988. North American owls biology and natural history . Smithsonian Inst . Press, Washington D.C. 295pp .

Johnson, s. J. 1975. Productivity of the red-tailed hawk in southwestern Montana . Auk 92: 732-736.

Kalmbach , E. R. , R. H. Imler, and L. W. Arnold. 1964 . The American eagles and their economic status . USDI FWS , Bur . Sport Fish . and Wildl ., Washington , D. C. 86pp .

Kellogg , P. 1962 . A field guide to western bird songs (cassettes). Houghton Mifflin Co. , Boston Massachusetts .

Kennedy , P. L. 1980. Raptor baseline studies in energy development . Wildl . Soc . Bull. 8:129-135.

Knight , R. L. and A. w. Erickson . 1977 . Ecological notes on long-eared and great horned owls along the Columbia River. Murrelet 58: 2-6.

Knight , R. L. and J. Y. Kawashima . 1993. Responses of raven and red-tailed hawk populations to linear right-of-ways . J. Wildl . Manage . 57:26 6-271.

Knight , R. L. and s. A. Temple. 1986. Why does intensity of avian nest defense increase during the nesting cycle. Auk 103 :318-327 .

Maccracken , J. G. and R. M. Hansen . 1984. Seasonal foods of black-tailed jackrabbits and Nuttall cottontails in southeastern Idaho . J. Range Manage . 37: 256-259.

Marks , J. S. 1984 . Feeding ecology of breeding long-eared owls in southwestern Idaho . Can . J. Zool. 62: 1528- 1533 .

Marks J. s. 1986. Nest-site characteristics and reproductive success of long-eared owls in southwestern Idaho . Wilson Bull. 98: 547-550. 124

Marion , W. R. and R. A. Ryder . 1975 . Perch-site preferences of four diurnal raptors in northeastern Colorado . Condor 77: 350-352 .

Marti , c. D. 1976. A review of prey selection by the long­ eared owl . Condor 78 : 331-336.

Marti , c. D. and J. s. Marks . 1989 . Medium-sized owls. in Proc . western raptor management symposium and workshop . Natl . Wildl . Fed. , Washington , o.c. pp . 124-133.

McAtee , W. L. 1935. Food habits of common hawks . USDA Cir. No . 370. U.S. Govt . Printing Office , Washington , D. c. 36pp . McBride , R. , N. R. French , A. H. Dahl , and J. E. Detmer. 1978 . Vegetation types and surface soils of the Idaho National Engineering Site . ID0-12084 Idaho Operations Office , U.S.D.E. , Idaho Falls, ID. 28 pp .

McGarigal , K. and J. D. Fraser. 1984 . The effect of forest stand age on owl distribution in southwestern Virginia. J. Wildl . Manage . 48: 1393-1398 .

Mills, H. B. 1937 . Some Montana birds : their relationship to insects and rodents . Montana State Coll ., Ag . Exper . Sta . Cir. 151, Bozeman , Montana . 48pp.

Mollhagen , T. R. , R. W. Wiley , and R. L. Packard . 1972 . Prey remains in golden eagle nests : Texas and New Mexico. J. Wildl . Manage . 36:784-792 .

Morrell, T. E. , R. H. Yahner, and w. L. Harkness . 1991. Factors affecting detection of great horned owl s by using broadcast vocalizations . Wildl . Soc . Bull. 19 : 481-488.

Mumford , R. E. and R. L. Zusi. 1958 . Notes on movements , territory , and habitat of wintering saw-whet owls. Wilson Bull. 70: 188-191.

Palmer, D. A. 1987. Annual, seasonal, and nightly variat ion in calling activity of boreal and northern saw-whet owls. in R. W. Nero , R. J. Clark, R. J. Knapton , and R. H. Hamre eds . Biology and conservation of northern forest owls. U.S.D.A. Forest Serv . Gen. Tech . Rep . RM-142. 309pp. 125

Petersen , L. 1979 . Ecology of great horned owls and red­ tailed hawks in southeastern Wisconsin. DNR Tech . Bul l. No . 111, Madison , Wisconsin. 63pp.

Phillips , A. R. 1942. Notes on the migrations of the elf and flammulated screech owls. Wilson Bul l. 54 : 132- 137 .

Pianka , E. R. 1973. The structure of lizard communities . Ann . Rev . Ecol . Syst . 4:53-74 .

Pielou , E. C. 1969 . An introduction to mathematical ecology . Wiley Interscience , New York . 286 pp .

Platt, J. B. 1971. A survey of nesting hawks , eagles , falcons , and owls in curlew Valley , Utah . Great Basin Nat . 31:51-65.

Powers , L. R. and T. H. Craig. 1976. Status of nesting ferruginous hawks in the Little Lost River valley and vicinity , southeastern Idaho . Murrelet 57 : 26- 31.

Ramakka , J. M. and R. T. Woyewodzic. 1993. Nesting ecology of ferruginous hawk in northwestern New Mexico . J. Raptor Res . 27 : 97-101.

Restani, M. 1991. Resource partitioning among three Buteo species in the Centennial Valley , Montana . Condor 93 : 1007-1010 .

Rich , T. 1986. Habitat and nest-site selection by burrowing owls in the sagebrush steppe of Idaho . J. Wildl . Manage . 50: 548-555.

Schmutz, J. K. 1984. Ferruginous and Swainson's hawk abundance and distribution in relation to land use in southeastern Alberta . J. Wildl . Manage . 48:1180-1187 .

Schmutz, J. K. , R. W. Fyfe, o. A. Moore , and A. R. Smith . 1984 . Artificial nests for ferruginous and Swainson's hawks . J. Wildl . Manage. 48: 1009-1013 .

Schmutz, J. K. , s. M. Schmutz, and D. A. Boag. 1980. Coexistence of three species of hawks (Buteo spp.) in the prairie-parkland ecotone . Can. J. Zool. 58: 1075-1089. 126

Schnell, G. D. 1968 . Differential habitat utilization by wintering rough-legged and red-tailed hawks . Condor 70: 373-377.

Schupe , s. and R. Collins . 1983 . A winter roadside survey of hawks in eastern Nemaha County , Nebraska . Nebr. Bird Rev . 51: 19-22 .

Shannon, c. E. and w. Weaver. 1949 . The mathematical theory of communication. Univ. of Illinois. Press, Urbana . 125 pp .

Smith , D. G. , J. R. Murphy, and N. D. Woffinden . 1981. Relationships between jackrabbit abundance and ferruginous hawk reproduction. Condor 83: 52-56.

Snow , c. 1973 . Golden eagle, Aguila chrysaetos . Hab . Manage . Ser. for Unique or Endang . Spp. , Rep . No . 7. USDI BLM Tech . Note 239. 52pp.

Speiser, R. 1990. Nest site characteristics of red-tailed hawks in western Washington . Northwest Nat . 71: 95- 97.

Sproat , T. K. and G. Ritchison . 1993 . The nest defense behavior of eastern screech owls: effects of nest stage , sex , nest type , and predator location . Condor 95: 288-296 .

Squires , J. R. , s. H. Anderson , and R. Oakleaf. 1989 . Food habits of nesting prairie falcons in Cambell county , Wyoming . J. Raptor Res . 23: 157-161 .

steenhof, K. 1983 . Prey weights for computing percent biomass in raptor diets . Raptor Res . 17: 15-27.

Steenhof, K. , M. N. Kochert , and M. Q. Mortisch. 1984. Dispersal and migration of southwestern Idaho raptors. J. Field Ornithol . 55: 357-3 68 .

Stoddart, L. c. 1983. Relative abundance of coyotes, lagomorphs, and rodents on the Idaho National Engineering Laboratory . in o. D. Markham ed . Idaho National Engineering Laboratory radioecology and ecology programs 1983 progress report . DOE/ID 12098 Nat . Tech . Info. Serv. , Springfield , Virginia . 434 pp .

Thurow, T. L. and c. M. White . 1983 . Nest site relationship between the f erruginous hawk and swainson 's hawk . J. Field. Ornithol . 54 :40 1-406 . 127

U. s. Dept . of Energy . 1985. Department of Energy National Environmental Research Parks . U.S.D .E. , Office of Health and Environ . Res. , Washington , D.C. 48pp.

U. s. Fish and Wildlife Service . 1992 . Endangered and threatened wildl ife and plants . 50 CFR 17 .11 & 17 .12. U.S. Government Printing Office , Washington . 38pp .

Voronetsky , V. I. 1987 . Some features of long-eared owl ecology and behavior : mechanisms maintaining territoriality . in R. w. Nero, R. J. Clark, R. J. Knapton , and R. H. Hamre eds . Biology and conservation of northern forest owls. U.S.D.A. Forest Serv . Gen. Tech . Rep . RM-142. 309pp .

Wakeley, J. s. 1978. Hunting methods and factors affect ing their use by ferruginous hawks . Condor 80:327-333.

Watson , J. w. 1984. Rough-legged hawk winter ecology in southeastern Idaho . M.S. Thesis, Montana State Univ. , Bozeman . 101 pp .

Watson , J. W. 1986a . Temporal fluctuations of rough-legged hawks in southeastern Idaho . Raptor Res . 20:42-43.

Watson, J. W. 1986b. Range use by wintering rough-legged hawks in southeastern Idaho. Condor 88: 256-258 .

Wh ite, c. M. and T. L. Thurow . 1985. Reproduction of ferruginous hawks exposed to controlled disturbance . Condor 87 : 14-22 .

Wiklund, c. G. 1990. The adaptive significance of nest defense by merlin, Falco columbarius, males. Anim. Behav . 40:244-253 .

Wiley, J. W. 1975a. The nesting and reproductive success of red-tailed hawks and red-shouldered hawks in Orange County , California. Condor 77: 133-1 39.

Wiley, J. w. 1975b . Relationships of nesting hawks with the great horned owl . Auk 92: 157-159 .

Woffinden, N. D. and J. R. 1983. Ferruginous hawk nest site selection . J. Wildl. Manage . 47: 216-219.

Woffinden, N. D. and J. R. Murphy . 1989 . Decline of a ferruginous hawk populat ion : a 20-year summary . J. Wildl . Manage . 53 : 1127-1132. Appendix A. Bimonthly weather data from the CFA NOAA weather station (1991-1993) and historical weather data adapted from Clawson et al . (1989) .

1992 1993 Normal8 b .1221 Month High Low Prec ip. High Low Precip. Snow D. High Low Prec ip. Snow D. Ave. T�. Precip. Snow D.

January 1-15 -3.9 -13.6 tr 0 -7.2 -20.3 1.1 47 -10.0 1.8 10.4 16-31 2.5 -15. 1 0.0 0 -4. 1 -17 .8 1.8 56 -10.5 February 1-14 6.6 -8.8 0.9 0 -3.6 -15.7 1.1 55 -8.5 1.6 10.4 15-28 10.6 -2.2 0.3 0 -6.4 -28.3 2.5 73 -4.6 March 1-15 14.3 -2.6 0.6 0 -1 .6 -12.3 0.0 65 -3.0 1.5 5.6 16-31 13.3 -3.8 0.1 0 7.7 -2.4 1.4 22 -0.1 Apri l 1-15 13.1 -2.3 0.1 15.7 -0.5 0.0 0 10.4 -3.4 1.9 tr 4.2 1.9 0.2 16-30 13.3 -1. 1 1.2 19.3 -0.1 0.2 0 14.0 -0.9 1.2 0 8.3 May 1-15 13.9 3.0 4.3 24 .2 1.4 0.8 17.8 2.3 4.4 10.9 3.0 16-31 21 .0 5.8 0.8 26 .5 6.7 tr 24 .1 6.8 0.7 12. 1 June 1-15 23 .9 6.0 2.7 26 .5 6.5 2.5 17.9 4.2 6.4 13.9 3.0 16-30 25 .4 7.3 0.2 27.9 11.3 1.4 23 .8 5.1 0.5 17.2 July 1-15 33 .2 9.5 0.0 21 .4 8.9 0.6 24 .3 4.4 0.2 20 .3 1.3 16-31 31 .8 9.8 1.3 31 .8 9.3 0.0 24 .7 6.7 0.0 21 .1 August 1-15 32.2 10.9 0.1 35 .0 10.9 0.0 28.4 7.9 0.9 20.2 1.4 16-31 32 .2 10.0 1.9 27.9 6.3 0.2 24 .8 5.1 0.9 18. 1

a 0 Based on weather records col lected on the INEL since 1950; Ave. T�. = average daily t�rature c c), Precip.= average total precipi tation for month (cm) , Snow D.= average snow depth at CFA (cm) .

b 0 0 High= average high t�rature for 2-week per iod c c), Low= average low t�rature for 2-week period c c), Prec ip.% total precipitation for 2-week period (cm), Snow D.= average snow depth at CFA for 2-week per iod (cm) . 129

Appendix B. Status and monitoring of sensitive raptor species on the INEL.

Two federally endangered species were sighted on the

INEL during this study (U. S. Fish and Wildlife Service

1992) . I observed bald eagles during the winters of 1992 and 1993 in small numbers (1-3) . In early January 1993 , a large concentration of bald eagles was seen southeast of

Howe , Idaho along the INEL border (0. D. Markham , pers . comm.), but it dispersed by the end of the month . I saw 2 adult peregrine falcons during the winter of 1993 , once near

Middle Butte and once near NRF . Neither species was observed during the breeding season .

Ferruginous hawks , a Category 2 species under consideration for Category 3 status (C. Harris , Natural

Heritage Program , Idaho Fish and Game , pers . comm. ), nested

in stable numbers during all 3 years of this study (Table

11) . Woffinden and Murphy (1989) estimated that ferrug inous hawks must fledge 1.5 young/pair/year to maintain a stable population. Nesting ferruginous hawks on the INEL appear to meet this requirement . Low nest success in 1993 was unusual

for this populat ion , based on my data and that of the 1970's

(Craig 1979) . Human activity on the INEL appears to be an

important factor influencing nest site selection and

fledging success . Ferruginous hawks selected nest sites that minimized their contact with human development (Table 130

15) , and they displayed a tendency to desert their nests when exposed to increased human activity early in the nesting cycle. Seven of 13 nest failures over the course of this study were directly attributable to nest abandonment following increased human activity . This behavior must be considered when making decisions concerning increased development on the INEL, especially if the ferruginous hawk receives Threatened Species status . The addition of nesting platforms in areas isolated from human development may serve to increase the nesting population . Schmutz et al . (1984) found it possible to increase ferruginous hawk populations in suitable habitat with the addition of platforms when natural nest platforms were limiting .

Other species of special concern by the Bureau of

Land Management and/or Idaho Fish and Game noted during this study included : prairie falcon , merlin , and burrowing owl

(C. Harris, pers . comm. ). Prairie falcons were present all year on the INEL. They nested around the Twin Buttes , and were among the most common wintering raptors on site (Table

2) . They were also common during previous wintering raptor surveys (Craig 1978 , Craig et al . 1983) . The prairie falcon population appears to be stable, and the INEL may serve as an important wintering area for this species . Merlins are probably accidental on site . One merlin nest was reported near the INEL (Craig and Renn 1977) , and sightings were 131 infrequent . Burrowing owls appeared to be common nesters on site, but my survey methods were inadequate to accurately census the nesting populat ion . I cannot make a definitive statement regarding the status of burrowing owls on the

INEL.

The midwinter raptor count on the INEL appears to give an adequate index for monitoring long-term population trends , but by increasing the number of surveys conducted during the winter months (November-March) trends in rarer species (e.g. bald eagles and peregrine falcons) may be noted . Nest monitoring on the INEL is inadequate . Long­ term data regarding ferruginous hawk nesting populations is required to aid in assessing population stability and their potential for Threatened Species status . By monitoring known nest sites annually for activity , an index to f erruginous hawk nesting numbers may be developed and the populations stability determined . 132

Appendix c. Narrowleaf cottonwood size and condition along the Big Lost River, Idaho , in July 1992 .

Trees > 10 cm dbh Transect8 Height (m) dbh (cm) Cond . 6 Sapl . c Deadf . d 2e 8 12 .5 0 10 14 0 12 14 0 11.5 20.3 0 9 15.5 0 9.5 16 .3 0 8 14 .1 0 7.5 13 .4 1 6.5 14 .3 0 5 13 .7 0 8 28 .9 0 7 31.6 0 12 41 0 12 .4 39 0 6.5 19.1 0 7.9 46.1 2 6.5 44 2 3 27.3 1 8.3 28.8 0 7.5 17 0 9.2 24.2 0 6 25 0 10.8 56 2 6.2 21. 5 4 17 62 .3 2 74 16

5 13 .3 55.6 1 5.8 13.5 1 7.1 20.7 2 33 9

12 8.5 25.9 1 22 3

16 10.2 23.l 0 4.5 14 .5 0 6.5 21.1 0 14 .4 27.9 0 6 18 .7 0 5.3 19 0 13 25.3 1 10.7 35.1 0 6.1 28.1 0 8.5 32 .4 2 10.8 58.8 0 133

Appendix c. Continued . Trees > 10 cm dbh 15 Transect8 Height (m) dbh (cm) cond . Sapl. c Deadf. d

10 33 0 5.6 32.9 0 7.5 14 .1 0 8 15.5 2 12 .1 43.9 0 9 36.8 0 8 22 .5 2 5.5 13 0 10 33 1 4.9 20.8 1 9.1 25.2 0 6 21. 1 0 8.5 27 .5 0 8.5 24 .3 1 10 18 .1 1 10 33.4 0 10.2 30.9 0 4 21.4 0 6.2 18 .2 0 5 21 0 177 32 7.5 31.1 0 8.6 29 .2 0 10.1 38.6 0 8.2 31.2 0 8.8 28 .5 0 9.6 35.9 0 4 10.8 0 5.8 11 . 5 0 11. 2 32.2 0 6 16.9 0 5 11.8 0 5.1 10 .9 0 4 13.4 0 5.7 22 .9 0 5.8 17 .8 0 10.2 33.6 0 10.5 34.9 0 8.3 28 .6 0 8 34.4 1 9.2 30.9 0 6.1 19 0 9.5 30.3 0 6.8 20.7 0 9.7 34.7 0 134

Appendix c. Continued . Trees > 10 cm dbh 6 c Transect8 Height (m) dbh (cm) Cond . Sapl . Deadf . d

7.8 24.6 0 8 33.9 0 8.8 29.2 0 10.3 23 .2 1

18 17 .2 55.7 4 17 36.4 4 9.5 24.7 1 6.4 14 .7 0 11 .1 27.4 1 7.8 14 0 10.8 73 .8 0 14 .5 60 3 8.2 18 .2 0 8 18 .2 0 8.5 15.3 0 8.5 14 .5 0 6.3 10 .5 0 10.4 36.6 4 7 17 .5 0 7.7 18 4 13 42.1 3 11. 6 46 2 7.8 42 .5 0 8.3 26.8 1 101 16

22 14 .2 46.1 3 5.6 26.7 0 35 31

24 8.4 26.9 0 6.5 22 .1 0 10.3 36.3 0 9.9 49.2 0 12 .5 37.5 3 11. 6 46.2 0 8.8 42.7 0 9.4 46.8 2 14 39 0 11.5 33.8 0 7.9 14 .7 0 13 .1 32.9 2 7.4 12 .5 0 11 .4 22 .6 0 135

Appendix c. Continued . Trees > 10 cm dhb 6 d Transect8 Height (m) dbh (cm) Cond . Sapl . c Deadf.

11. 7 19.7 0 9.6 22.3 1 8.5 18 .4 0 10.2 17 .7 0 10 19 .5 0 12 .1 33.7 4 9.3 23.5 0 9 19 .4 0 7.5 24.4 0 9.8 28 0 11.2 34 .5 4 7.9 14 .8 0 9.5 19 .2 0 10.3 17 .5 0 24 6 12 .6 0 72 46 9.2 21.9 0 12 .7 46.5 3 5.4 13 0 12 29 .3 1 7.1 17 .2 0 8 20.2 0 9.8 23.5 0 11. 6 44 0 8.4 29 0

26 0 26

29 13.7 58 .2 0 11. 5 45.9 0 10.8 49.5 0 14 .2 64 0 10.2 36.6 0 0 15

8Randomly selected 1 km transects along the Big Lost River. b o=dead, 1=25% foliated, 2=50% foliated , 3=75% foliated , 4=100% foliated . c Number of trees in transect < 10 cm dbh . d Number of deadfalls > 10 cm dbh . eTransects were 1-km sections of river, numbered for number of km above northern Lincoln Blvd . crossing . Appendix . Locations of 1-km transects used to assess tree condition along the Big Lost River, 1992.

NRF �

+ :' 5

TRA • \_ _ :----. ICPP

.\--� .. 1tl PBF CFA • 22 _, _\--Y -\- ,_ - - 18 • 2tJ ,\" >,- ;... Big Lost River /, _ RWMC :zg 24 •

.... w O'I 137

Appendix E. Narrowleaf cottonwood size and condition along Birch Creek, on the INEL, in July 1992.

dbh (cm) cond . 8 Height (m)

27.7 1 5.5 34.7 0 6 52 .2 2 10 54 .6 1 10 .5 55 .4 3 10 .5 52 .6 0 5.5 23.3 0 9 23.7 0 9.5 54 .9 3 11 46.1 2 10.5 10.6 0 3 14 .5 0 4 24.9 1 9 11. 8 0 3.5 17 .8 0 4.5 18 .3 0 3.5 25.8 1 10.5 28.1 1 10.5 20.8 0 10.5 28.9 1 10 57 .2 0 11 29.9 0 9 27 0 8.5 42.3 2 11 26.3 2 10.5 35.8 2 11 . 5 20.1 0 5 13 .5 0 4.5 31. 6 1 9.5 54 .1 4 12 40 4 8.5 36.5 2 10 51. 5 1 10 23.6 0 4 13 .2 0 5 21. 2 0 6.5 17 .3 0 5 14 .9 3 5.5 17 .2 3 6.5 10.4 4 5.5 18.6 1 6 16.9 0 4.5 23.9 2 7.5 15.4 1 6.5 138

Appendix E. Continued .

dbh (cm) cond . 8 Height (m)

13 .6 0 4 20.5 3 8 21.2 2 6.5 20 1 7 23.2 2 8 10.1 0 4 17 .3 0 6 18 .5 0 6.5 19 .6 0 8 22 .2 3 8.5 13 0 7.5 18.5 0 7 28.5 1 9 28 3 9 11. 5 0 4.5 11.2 0 4 15.6 0 5.5 25.6 0 5 21.1 0 4 22.8 0 6 20.1 1 6 47.9 0 12 62 .4 0 12 29.1 0 8 30.3 0 9 24.2 0 6 13.9 0 4 35.8 0 6 59 .4 0 7.5 40.9 0 4 28 0 6.5 29 0 8 68 .5 0 7.5 38.4 0 8.5 15.8 4 6.5 34.8 0 9.5 21.2 0 4 21.7 2 5.5 53 .8 1 10 15.6 0 4 23 0 4 21.2 0 4 63 .9 3 11 36 3 8 41 1 8 139

Appendix E. Continued .

dbh (cm) cond. 8 Height (m)

19 .1 0 7 31.8 2 8 95 3 10 23 0 9 16.9 0 7.5 18.9 0 8.5 22.3 0 8.5 30.2 1 9 12 .5 0 3 21. 5 0 8.5 24.2 1 9 18 .1 1 6.5 15.7 0 8 25.2 1 8.5 22.2 0 6.5 15.3 0 7 25.1 0 8.5 18 .4 0 7.5 26.9 1 7.5 21. 2 0 6 14 .8 0 4 31. 2 0 7 42 .7 2 7 41.2 0 8.5 43.9 3 9.5 11. 2 0 3.5 17 .3 0 5 34.1 3 10.5 27 .8 4 9.5 29.7 4 9 33.4 3 10 39.3 2 10 28 3 8 19 .4 4 4.5 28.5 4 10 27 .8 4 10 28.1 4 10 19 4 4.5 26.7 3 5.5 24.2 2 5 16.4 1 3.5 17 3 5 20.3 3 5 25.8 2 6 19 .8 0 2 140

Appendix E. Continued .

dbh (cm) cond . 8 Height (m)

12.9 3 2 18 0 4 21. 9 0 4 19 .9 1 5.5 61. 3 2 10.5 18 .4 3 2 18 .1 1 2.5 34 .1 2 8 18 .5 0 2 20.1 3 6 11.4 0 2 35.1 0 10 29.2 0 9.5 19 .9 3 9 21. 6 2 9 15 3 4.5 22.4 3 9.5 16.2 3 6 28.3 3 10 32 4 10 15.8 4 4 28.4 4 10.5 21. 5 3 10 16.2 0 5 21. 9 3 8.5 21.4 2 9 21. 2 2 6 56.6 1 11.5 15.7 2 10.5 23 1 9 27.1 1 11 24.4 2 10.5 15.7 0 7 28.3 2 10 17 .4 2 9 11 0 8.5 15.7 0 9.5 14.3 0 9 13 .1 0 9 24.4 2 10.5 26.2 3 10 22 .4 1 10 16.9 1 8 23 3 9.5 14.3 1 9 141

Appendix E. Continued .

dbh (cm) cond . 8 Height (m)

12 0 7 11. 4 0 7 20.8 2 10 20.7 3 9 22.8 3 9.5 19 .7 1 9 18 .5 0 8 20.6 2 9.5 19 .1 4 8 16 .5 0 6 19 .5 3 7 18 .2 1 5.5 16 2 7 14 .3 2 6 15 0 6.5 12 .5 2 5.5 11 . 8 0 5 22.7 3 7.5 14 .3 2 5.5 22.1 1 6 17 .5 0 5.5 24 .4 1 6 22.7 1 6 13 .4 3 4 33 0 5.5 17 .9 0 5 20.7 0 3.5 25.9 3 7

12 .9 0 4 . 5 20.1 1 4.5 16.8 0 4.5 15.5 0 5 16.2 0 4.5 13 .5 0 5 16.3 0 4.5 21 0 9 15.5 0 4.5 18.5 1 4.5

14 0 4 . 5

8cond.= condition of tree based on foliage cover; O=dead , 1=25% foliated, 2=50% foliated , 3=75% foliated, 4=100% foliated .