Brigham Young University Science Bulletin, Biological Series

Volume 10 Number 4 Article 1

10-1969

Nesting ecology of raptorial birds in central Utah

Joseph R. Murphy Department of Zoology and Entomology, Brigham Young University

Franz J. Camenzind

Dwight G. Smith

J. Bradford Weston

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Recommended Citation Murphy, Joseph R.; Camenzind, Franz J.; Smith, Dwight G.; and Weston, J. Bradford (1969) "Nesting ecology of raptorial birds in central Utah," Brigham Young University Science Bulletin, Biological Series: Vol. 10 : No. 4 , Article 1. Available at: https://scholarsarchive.byu.edu/byuscib/vol10/iss4/1

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NESTING ECOLOGY OF RAPTORIAL BIRDS IN CENTRAL UTAH

by

Joseph R. Murphy, Franz J. Camenzind/ Dwight G. Smith, and J. Bradford Weston'

BIOLOGICAL SERIES — VOLUME X, NUMBER 4 OCTOBER, 1969 BRIGHAM YOUNG UNIVERSITY SCIENCE BULLETIN BIOLOGICAL SERIES

Editor: Wblmer W. Tanner, Department of Zoology and Entomology Brigham Young University, Provo, Utah

Associate Editor: Stanley L. Welsh, Department of Botany, Brigham Young University, Provo, Utah

Members of the Editorial Board: Tipton, Zoology Vernon J.

Ferron L. Anderson, Zoology

Beck, Microbiology J. V. Joseph R. Muedock, Botany

Wilmer W. Tanner, Chairman of the Board

Ex officio Members:

A. Les'Lester Allen, Dean, College of Biological and Agricultural SciencesSci

Ernest L. Olson, Chairman, University Publications

The Brigham Young University Science Bulletin, Biological Series, pubhshes acceptable papers, particularly large manuscripts, on all phases of biology.

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Frontispiece. Thirty-four-day-okl Ferruginous Hawks, Thorpe Hills, Utah County, Utah, June 21, 1968. The Weston. brood consists of 3 dark-phase individuals and one light ("normal") phase (see p. 34). Photo by J. B. Brigham Young University

Science Bulletin

NESTING ECOLOGY OF RAPTORIAL BIRDS IN CENTRAL UTAH

by

Joseph R. Murphy, Franz J. Camenzind,

Dwight G. Smith, and J. Bradford Weston

BIOLOGICAL SERIES — VOLUME X, NUMBER 4 OCTOBER, 1969 TABLE OF CONTENTS

Page

INTRODUCTION 1

Description of the study area 1

Geology I

Climate I Biotic communities 2 Human utilization 2

Advantages of the study area 3 The individual studies 4 Acknowledgements 4

NESTING ECOLOGY AND BEHAVIOR OF THE GOLDEN EAGLE 4 Introduction 4 Literature review 5 Location and description of the study areas 5 Methods and procedures 5 Results 7 Nest locations 7 Nest density and home range 8 Nesting activity 8 Productivity 10 Behavior 11 Discussion 13 Nest locations 13 Nest density and home range 13 Nesting activity 14 Summary 15

NESTING ECOLOGY OF THE GREAT HORNED OWL 16

Introduction , 16 Methods and procedures 16 Study area 16

Field studies , 16 Results 18 Nesting population and distribution 18 Territoriality 21 Predation 21

Discussion and conclusions 23 Breeding 24 Territory 24 Predation 24 Summary 24

NESTING ECOLOGY OF THE FERRUGINOUS HAWK 25 Introduction 25 The study area 25 Methods and procedures 26 Results 28 Population and distribution 28 Nest locations 28

Nest sites, composition, and size 29 Initial nesting activity 30 Productivity 30 Behavior 30 Food habits 31 Mortality 31

Discussion and conclusions 32

LITERATURE CITED 35

LIST OF ILLUSTRATIONS

Figure Page

1. View of a portion of the study area, Thorpe Hills 2

2. Map of study areas A and B 6 3. Tower nest located at U. S. Army Deseret Depot 7

4. Diagram of home range of Golden Eagles 9

5. Map of Great Homed Owl study area 17 6. Nest locations of the Great Horned Owl 19

7. Territories of three pairs of Great Homed Owls 22

8. Ferruginous Hawk study area locations 26

9. Intensive study area nest site locations 27 10. A typical Ferruginous Hawk nest with paper lining 29 11. Typical Ferruginous Hawk ground nest 33 12. Typical Ferruginous Hawk tree nest 33

LIST OF TABLES

Table Page

1. Golden Eagle nesting activities at various altitudes 7

2. Exposures of active cliff nests 7

3. Clutch size for various Golden Eagle nests 10

4. Productivity records, 1967-1968 11

5. Nest site selection by the Great Homed Owl 18 6. Nest dimensions of the Great Horned Owl 18 7. Nesting success of the Great Homed Owl 20 8. Food of Homed Owls in the study area 23

9. Number and percentage of Ferruginous Hawk nests in various sites 29 10. Exposures of occupied nests, 1967-1968 29 11. Eggs laid, young hatched, and young fledged, 1967-1968 30 12. Observed dates of reproductive activity of the Ferruginous Hawk 30 13. Food of nesting Ferruginous Hawks in the west-central Utah study area 32 AN INTRODUCTION TO NESTING ECOLOGY OF RAPTORIAL BIRDS IN CENTRAL UTAH

by

Joseph R. Murphy'

In contrast to many areas of North America, geology of the area is characterized by synclinal the sparsely-settled Great Basin deserts of Utah folds and a series of anticlines developed in a still support large and diverse raptor popula- thick series of Paleozoic rocks ranging in age tions. For the past several years these predatory from Lower Cambrian to Upper Pennsylvanian. birds have been the subject of intensive studies These strata are folded, faulted, and subsequent- by personnel of the Department of Zoology, ly eroded in such fashion as to expose numerous Brigham Young University. The present report cliff lines of resistant limestones alternating with deals with the nesting ecology of three of the softer shales and sandstones (see Fig. 1). Small largest and most conspicuous species. Other exposures of Tertiary volcanic materials, mainly studies in progress, to be the subjects of future basalts and ignimbrites, are scattered through publications, deal with behavior and ecology of the area. Quaternary alluvial materials, lacus- wintering Bald Eagles, causes and influence of trine and fluvial in origin, form aprons around raptor mortality, economic relations of Golden the bases of the hills and in the stream valleys Eagles, and factors which control total raptor and passes. Pleistocene sediments and fan grav- densities and population dynamics. els of ancient Lake Bonneville comprise the bulk of these deposits. Most of the present stream Description of the Study Area drainage in the study area is intermittent or ephemeral; as a result, only thin mantles of The research has been conducted within a recent alluvium are visible. maximum area of roughly 3000 square miles, involving portions of six counties in central and As is typical of most valleys in the Great western Utah. The majority of the field work, Basin, there is very litde outside drainage of however, has been restricted to a smaller area surface water. For the most part, the surface of about 600 square miles at the eastern edge of water is either absorbed by the soil or flows over the Great Basin, lying approximated between the surface to the lowest part of the valley, North latitudes 40°0()' and 40°20', 'and West where it is slowly absorbed or evaporated. longitudes ni°55' and 112°25'. The area in- Climate cludes, from to east west, the following physio- Climatic features of the study area are char- graphic features (see map. Fig. 2): Lake Moun- acteristic of the northern or cold desert regions tains, Cedar Valley, southern of the Oquirrh end (Fautin, 1946; Shelford, 1963). Mean amiual Mountains and northern portions of the East precipitation is generally less than 15 inches Tintic Range with intervening groups of hills (.'38 cm) for most of the region, with larger (Thorpe Hills and Topliff Hills), a large and amounts at higher elevations. Distribution of part of Rush Valley. Elevations range from precipitation is very uneven in both space and some 4500 feet above sea level at the west mar- time. Moisture graphs for the area indicate that gin of Utah Lake (east of area) to edge study most of the precipitation comes during two nearly 8000 feet in the Oquirrh and Tintic periods: early spring (March through May) Mountains. Local relief varies from 3200 feet and midsummer (July and August). Significant at Lake Mountain to 1600 feet in (maximum) variations in annual and monthly precipitation the Thorpe and Topliff hills. patterns are characteristic. Geology The annual range in temperature may be as The geology of major portions of the study great as 65° C in some parts of western Utah, area has been treated by Bullock ( 1951 ) for the with maxima of 45°C and minima down to Lake Mountain vicinity and by Bissell et al -.30 °C. July is generally the hottest month, with

( 1959 ) for the southern Oquirrh Mountains, a mean temperature of about 2.3°C in an "aver- Thorpe Hills, and adjacent areas. The structural age" year. Wide daily fluctuations in tempera-

'Depaiinient nf Zoology and Enloiiioiopy. Bripliam ^'niinf; UniversitV- .

Bricham Young University Science Bulletin

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Fig. 1. View of a portion of the study area in the Thorpe Hills, UtiUi Co., Utah, showing resistant limestone strata forming numerous cliff lines appropriate for raptor nesting sites. ture are also to be expected, amounting to as saltbush {Atriplex canescens), winterfat {Etiro- much as 30° or more in the summer months. tia lanata), horsebrush {Tetradymia glabrata),

As is true of most arid regions, wind is an and bud sage {Artemesia spinescens) . Original- important cHmatic feature in the Great Basin ly there were significant quantities of perennial deserts, and combined with the prevailing low grasses, especially Onjzopsis hymenoides, inter- relative humidity and dry atmosphere, accounts mixed with the shrubs, but overgrazing and this for tlie observed high rates of evaporation ( Fau- other forms of abuse have drastically altered tin, 1946). The dominant southwesterly winds situation ( see next section ) are particularly strong during the spring months; The principal herbivorous vertebrates include superimposed upon these imd occurring at all the mule deer, jack rabbit, ground squirrels, seasons, are shallow diurnal mountain and vtdley kangaroo rats, wood rats, deer mice, pocket mice, winds. grasshopper mice, a number of herbivorous birds

(mainly passeriforms , and lizards. Vertebrate Biotic Communities ) carnivores, other than raptors, include coyote, A variety of soil types occurs within the area badger, kit fox, bobcat, gopher snake, whipsnake, of study ranging from well-drained, gravelly and Great Basin Rattlesnake. The raptors are soils on higher ground to fine-textured soils of particularly well-represented; principal nesting moderate to high salinity in the lower valleys. species are the Golden Eagle, Ferruginous This diversity in edaphic conditions is reflected Hawk, Red-tailed Hawk, and Great Homed Owl. in the plant and animal commimities present. Represented by smaller nesting populations are In tenns of plant dominants, the high ridges and the Swainson's Hawk, Harrier, Goopcr's and hills support a dwarf conifer community consist- Sharp-shinned Hawks, Prairie Falcon, Kestral, ing primarily of Junijwrus osteosperma witli Burrowing Owl, Screech Owl, and Long-eared smaller amounts of pinyon pine {Finns mono- Owl. Of these, the conspicuous pennanent resi- phijUa) and associated shrubs (e.g., Cercocarpus, dents are the Golden Eagle, H;vrrier, Prairie Fal- Cowania, Purshiu). Big sagebrush (Arte7nesiii con, and Great Homed Owl. Important winter tridentata) is dominant on the well-drained soils residents are the Bald Eagle and Rough-legged of the uplands and lower slopes, while shadscale Hawk. Little infomiation is available regarding (Atriplex confertifolia) and greasewood (Sar- population evcles in prev species and how these cobatus vermicuJatus) communities cover large may in him affect the raptor populations. areas of the valley floors. Other prominent shrubs, becoming dominant under certain Human Utilization edaphic and biotic conditions, include rabbit- Intensive human use of the area extends back brush (Chnjsothamntis nauseosus). four-wing a little over a century, and involves three major Biological Series. Vol. 10, No. 4. Ecology o?- Raptobial Birds

activities: livestock grazing, farming, and mineral evidently not without consequence for the rap- extraction. Although the main line of the Union tors which present a large and tempting target Pacific Railroad between Salt Lake City and for the undisciplined gunner. In fact, our obser- Los Angeles extends the length of Rush Valley, vations indicate that shooting is probably a pri- the four or five towns existing within the study mary cause of post-fledgling raptor mortality in area are all small, with populations of less than the study area (see Ellis et al, 1969) 1000. Important events in western history have ADVANTAGES OF THE STUDY AREA influenced the area. Camp Floyd, which sup- ported the largest troop concentrations of any The area offers a number of substantial ad- United States military post in the LSSO's, was vantages to the raptor populations as well as to located at Fairfield in Cedar Valley. The route students of raptor ecology. These may be sum- of the Pony Express ran through Cedar and marized as follows: Rush Valleys with relay stations in each valley. 1. Human population density is minimal, Later the booming mining towns of Ophir and and the effects of human manipulation or utili- Mercur were established in the nearby Oquirrh zation of the habitat have had no obvious ad- Mountains. Mining activity has drastically verse effects on the raptors except for the shoot- waned in recent years, and the only significant ing noted previously. mineral extraction at present involves quarrying 2. Prey populations appear to be sufficiently of limestone, clay, and calcite; this activity is large and diverse to support the concentration of widespread but pursued only intermittently different raptors present at optimum densities throughout the study area. in all seasons. Although we as yet have no The principal use of the area at the present reliable data on cycles or other population dy- time is for livestock grazing, notably as winter namics in the prey species, indications are that sheep range. According to Bureau of Land the raptors readily resort to alternate prey when Management personnel, upwards of 40,000 sheep a prime prev species is temporarily at low winter in the general study area each year. levels. The diversity in size and behavior of Such intensive use has had detrimental effects potential prey species permits allocation of the upon forage conditions, and much of the range various raptors to specific trophic niches; this has been overgrazed. Most of the desirable will be elaborated upon more fully in the indi- forage species such i\s winterfat, bud sage, four- vidual studies. wing saltbush, big sage, and Indian rice grass are reduced both in number and vigor. Less 3. While the nearly treeless, shrub-covered desirable invading species such as halogeton, valleys provide adequate hunting territories, the nesting Russian thistle, matchweed, and cheat grass are adjacent hills and mountains provide and now common. Range revitalization programs roosting sites in close proximity to the areas of sequence are presently in effect in parts of the area, utiliz- greatest prey density. Geologically, the ing desirable native and introduced species of of strata in the hills is conducive to providing grasses. excellent nesting situations for ledge and hole nesting species. Resistant layers of limestone, Agricultural practice is limited to the val- in- leys, especially Cedar Valley. Dry-fami wheat alternating with softer beds and generally angle, provide the favorable raising is the principal activity; the recent intro- clined at an most duction of portable overhead sprinkling systems sites. Tree-nesting species such as the Ferrugi- accipi- will no doubt extend the acreage involved in nous Hawk, Great Homed Owl, and the wheat production. ters frequentlv utilize the junipers which form A sizable portion of Rush Valley (about 30 an open stand in a characteristic belt through- square mOes) serves as a military reservation out the lower elevations of the mountains. for ordnance storage, the Deseret Depot Activity 4 Since raptors in the area are in contact of the Tooele Amiv Depot. with livestock for much of the year, an excel- An additional hmnan activity of increasing lent opportunity is provided for assessment of

importance in recent years is the utilization of alleged or actual predation on sheep and other the area for sport shooting of rabbits in the domestic stock. Tlie Golden Eagle, which is winter. Cedar and Rush Valleys usually support the species most often indicted by stockmen, is dense jack rabbit populations, and owing to present vear-round in the area; we have been, their proximity to the urban centers of northern therefore, paying particular attention to the Utah, receive surprisingly heavy hunter visita- food habits and economic relations of this

tion, especially on weekends. This activity is species. .

Brigham Young Univehsity Scfence Bulletin

THE INDIVIDUAL STUDIES within the same area, utilize a common pool of prey species, and yet avoid direct confrontation The individual reports are primarily based or competition. upon research conducted by each of the junior authors in the process of obtaining a graduate degree in zoology, under the supervision of the ACKNOWLEDGMENTS senior author. The papers by Camenzind and The authors are grateful for support, facili- Smith deal with species whose biology, although ties, and equipment furnished by the Depart- extensively studied in other habitats, is poorly ment of Zoology and Entomology, Brigham known for desert regions. Weston's study of the Young University. These studies were also ma-

Ferruginous Hawk is evidently the first detailed terially aided by financial support from the treatment of the nesting ecology of this species; National Audubon Society; we express apprecia- it adds a great deal of new information to exist- tion to the society, and particularly to Roland C. ing published accounts of this interesting raptor Clement, Vice-President. Our studies have been in such general works as Bent ( 1937 ) , and encouraged and assisted by personnel of several

Brown and Amadon ( 1968 ) federal and state resource management agencies, One of the more significant conclusions that including the U S. Fish and Wildlife Service, may be drawn from a comparison of the three Forest Service, Bureau of Land Management, papers relates to trophic niche allocation. Al- U. S. Anny (Tooele Army Depot), Utah Fish though there is considerable overlap in the kinds and Game Department, and Utah Woolgrowers of prey items taken, it should be noted that the Association. three species tend to hunt during different parts The junior authors wish to express apprecia- of the diel cycle. Thus the Golden Eagle is pri- tion to Drs. C. Lynn Hayward, Herbert H. Frost, marily diurnal in hunting activity, the Great and J. R. Murdock who served as members of Homed Owl is nocturnal, and the Ferruginous advisory committees and assisted with prepara- Hawk is to a large extent crepuscular, as indicat- tion and editing of the individual theses. They ed by the fact that Kangaroo Rats accounted are also especially grateful to their wives, JoAnn for nearly 45 percent of the prey taken by this Camenzind, Beth Anne Smith, and Linda Wes- species. This utilization of separate hunting ton, who provided moral support, encourage- periods may be an important factor in permitting ment, and assistance with field work and prepa- populations of several large raptors to coexist ration of manuscripts.

NESTING ECOLOGY AND BEHAVIOR OF THE GOLDEN EAGLE. AQUILA CHRYSAETOS L.

by Franz Camenzind J. INTRODUCTION

The Golden Eagle, Aquila chnjsaetos cana- restrictions wherever it is alleged to interfere denis Linn., is facing extinction in much of its with livestock operations. In order to better range. In the Western Hemisphere its distribut- manage this species, its life histoiy and local be- ion is restricted primarily to the mountainous havior patterns must be thoroughly understood. regions west of the 99th meridian, from extreme Although mmierous reports have been pub- northern Alaska to northern Mexico (Hobbie lished from various geographic areas concerning and Cade, 1962) and from timberline in Colo- the Golden Eagle's productivity, nesting ecology, rado to below sea level in Death Valley, Cali- and behavior, little is known of its biology in fornia (Summer, 1929a). Utah. Although a Federal Law was enacted in It was the principal objective of this study

1962 protecting this species, it continues to suf- to determine the productivity, nesting ecology, fer losses through illegal killing and the enact- and behavior of the Golden Eagle in west ment of temporary moratoriums on the hunting central Utah. Biological Series, Vol. 10, No. 4. Ecology of Raptorial Birds

LITERATURE REVIEW State Highway 36 on the west, south by a Une running from Vernon to the extreme north front Gordon ( 1927 ) wrote one of the first thor- of the East Tintic Mountains, and east to Utah ough reviews of the nesting behavior of a pair Lake. As indicated in Fig there are three of Golden Eagles in Scotland. Since that time 2, broad land areas between the two parallel moun- much has been done to qualify and expand his tain ranges. These include ) the lowland be- findings. Watson (1957), Sandeman (1957), ( 1 tween Utah Lake and the Lake Mountains; Lockie (1964), and Lockie and Ratcliffe (1964) (2) Gedar Valley west of the Lake Mountains; and have all added knowledge to the breeding ac- parts of Rush Valley west of the Oquirrh tivities of this species in various areas of Scot- (3) Mountains, Thorpe Hills, and East Tintic Moun- land. tains. Similarly, countless notes and articles have Area B includes several nest locations been published in the United States concerning tliroughout west central Utah. Included are various aspects of the Golden Eagle's ecology. nests on the west front of the Wasatch Moun- Finley (1906) published one of the first of a tains near Provo in Utah Gounty, Utah, as well long series of reports concerning the activities as in several adjacent canyons. Other nests of this species in Slevin Cahfomia. ( 1929 ) pub- under observation are located in and around lished an account concerning seven pairs of Yuba State Park, Gounty, Utah. Golden Eagles over a six-year period in Santa Juab Glara and San Benito Gounties, Galifomia. Dix- METHODS AND PROCEDURES on's ( 1937 ) attempt to map the territories of various pairs of eagles has provided a record The study was conducted from January 1967 that has been substantiated by numerous ac- until July 1968 with the majority of the field counts from several localities in North America work occurring during the nesting seasons (late

With the exception of Gameron's work ( 1905, Februar\' to earlv June) of both years. Field 1908) concerning some Golden Eagles in Mon- observations were made with the aid of a 20- tana, little information was published for the power spotting telescope and a pair of 7x35mm Rocky Mountain Region until the late 1940's. field glasses; notes were taken on a portable It was then realized that this region had one of tape recorder. Approximately 600 hours were the largest concentrations of Golden Eagles re- spent in the field, which includes 150 nest visits. maining in the continental United States. Many Locating nest sites proved to be a demanding of the later reports (Wood, 1946; Williams and physical task which included observing the ac- Matteson, 1947; Brown and Galley, 1950; Gamie, tivities of adult Golden Eagles, thoroughly 1954; Fevold and Graighead, 1958; Strandtmann, searching suspected nesting areas on foot, and 1962) were concerned with eye-witness accounts taking two survey flights over the area. Addi- of various behaviorial patterns or reports on the tional infonnation was obtained by consulting food habits of different populations of Golden U. S Fish and Wildlife Service personnel, Eagles. The latest effort to report life history through personal communication with John data as well as detailed food habit infonnation Hutchings of Lehi, Utah, and by reading the is that of McGahan (1967, 1968) concerning a personal field notes of the late R. G. Bee of population of eagles in southwestern Montana. Provo, Utah; these two individuals are veteran Unfortunately, only scattered reports (Two- field naturalists who compiled long-tenn records mey, 1942; Hardy, 1945; Hayward, 1967) on the dealing with central Utah ornithology. status of the Golden Eagle in Utah have been In order to obtain an accurate timetable of published; it is anticipated that this studv will events, eyries were kept under observation dur- begin to fill the void that exists in our knowl- ing the stages of nest construction, incubation, edge. :ind brooding. Burlap hides were placed near three eyries nt the time of hatching to facilitate LOGATION AND DESGRIPTION more detailed observations of the behavior as- OF THE STUDY AREAS sociated with brooding and fledging. A photo- The nest sites were grouped into two study graphic record of activity was taken with the areas which will be referred to as Area A and aid of a 35mm single-lens reflex camera Area B. The principal study area (Area A) equipped with telephoto lenses. is located west of Utah Lake in Utah and Home range was determined by recording Tooele Gounties, Utah and contains approxi- the direction of flight activity in relation to indi- mately 540 square miles. It is bordered on the vidual nests. The directions were then plotted east by Utah Lake, on the north by a line run- on circle graphs as percentages, and these were ning from Utah Lake to its intersection with pkiced around a map of the studv area. Brigham Young UNrvERsiTv Sctence Bulletin

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Biological Series, Vol. 10, No. 4. Ecology of Raptorial Birds

Unsuccessful attempts were made through- out the winter of 1967-68 to hve trap and mark Golden Eagles with the use of a Bal-chatri trap with both mammalian and avian bait. Victor No. VA and No. 2 spring jump traps with paddetl jaws were also placed on favored roosting posts in Cedar Valley This resulted in the capture of two Great Homed Owls (Bubo virginiamis) one Short-Eared Owl (Asia flammeusj and one Long-Eared Owl (Asia otus), but no eagles were obtained. RESULTS

Nest locations. A total of 31 active nests' were observed during the two-year period. Of these, 11 were recorded in 1967 and 20 in 1968. ~j£as:jrR,i. The increase in 1968 was owing to the addition of the Yuba State Park area nests, as well as the discovery of additional nests in Areas A and B.

Eighty-seven percent of all nesting activity occurred on cliffs (27 activities), 6.5 percent (2 activities) on the ground and 6.5 percent (2 activities ) on an artificial structure. The latter structure, located on the U. S. Anny Deseret

Depot Activity (Nest A-14), is an abandoned gunnery tower extending 11 meters above the ground. The nest is situated at the top and surrounded by a 1 meter high metal railing and Fig. 3. Tower nest (A-14) located at U. S. Army Des- a 2 meter high wooden frame (Fig. 3). eret Depot Activity, Tooele County, Utah (1968).

The ground nest ( A-5 ) is located atop a rock ledge three meters high. The ledge remains TABLE 1 level for about 1.5 meters, and then rises at an Number and percentage of Golden Eagle nesting activi- angle of approximately 13 degrees to the top of ties at various altitudes in Area A and B. the hill. Altitude All except five nests were reached without the aid of ropes. The nests ranged from two meters (A-4) to 13 meters (A-8) above the base of the cliffs. Most of these cliffs are lime- stone, with the exception of one (B-3) which is sandstone, and three others (B-4, B-5a and b, and B-6 ) which are shale.

Altitudinal distribution. Of all active nests in the study area, 61.3 percent were at altitudes between 5,000 and 6,000 feet, 9.7 percent below 5,000 feet, and 29.0 percent above 6,000 feet

( Table 1 ) . The altitudinal extremes are ne^ts A-5 and A-13 at approximately 4,750 feet and 6,800 feet respectively, with the average altitude for all active nests being 5,750 feet.

Nest aspect. For both years, 55.5 percent of all active cliff nests faced west, 22.2 percent nortli, 18.6 percent south and 3.7 percent East

(Table 2). There is no apparent difference either in the number of available cliffs facing various directions or in the distribution between Bbigham Young University Science Bulletin and 1.6 meters high, and had a large amount er than A-4. A third nest active in 1968 (A-5) of nesting materials on the ground immediately completed a triangle whose sides measured: below. This was the only nest with an easterly A-.3b to A-4, 0.7 miles; A-4 to A-5, 1.5 miles; exposure and was located on a small ledge half- and A-5 to A-3b, 1.5 miles. The area of this way up on a 9 meter cliff. The smallest nest triangle was 0.5 square miles. was believed to be A-1, but unfortunately, it Home range- Only in area A was an attempt was destroyed before accurate measurements made to measure home range. The direction of could be taken. With the aid of photographs all eagles going to or from their nests was re- and measuring the debris, it was estimated to corded and only those nests with four or more be one meter long, 0.7 meter from front to back, sightings appear in Fig. 4. Recordings for both and 0.5 meter high. This nest was located on years were included when the same nest or loose rock in a limestone quarry and collapsed, known alternate nests were involved. presumably under its own weight, when the The birds of nest A-15a and b and A-8a and Eaglets were ten days old. b had their activity rather evenly distributed in Nest materials. Nest materials usually re- four directions. Those of nest A-15a and b had flected the surrounding vegetation in both com- 33.3 percent of their activity to both the north position and abundance. Those nests with juni- and south and 16.6 percent to both the east and per or pinyon pine nearby had an abundance of the west; while A-8a and b had 18.2 percent to shredded bark as lining, leafed branches as the east and the west, .36 percent to the south, decoration, and larger branches in the main and 27 percent to the north. structure. Nest A-2a had as its lining both Pairs A-5, A-4, A-3, and A-2a and b had their shredded juniper bark and juniper leaves in activity somewhat restricted to various direc- almost ecjual proportions. This is in contrast tions. The birds of nest A-2 had no sightings to A-6 which had predominately pinyon to the north while 37.5 percent were to both branches, leafed and about 0..3 meter long, not south and east, with 25 percent to the west. shredded into a fine mulch but packed with use. Pairs A-.3, A-4, and A-5 had their activity pri- marily in opposite directions Fifty-four percent Only nest A-3 had foreign material incor- of the activity of A-.3a and b was to the north, porated into the nest. It contained a piece of while 45.5 percent and 31.8 percent of the ac- wire about two meters long placed verticidly on tivity of A-5 was to the east and south respec- the outside of the nest and extending slightly tively. All of the activity of pair A-4 was evenly above the top edge. divided between both the south and west. Similarly, pairs A-9, A-10, and A-11 had the Nest Density and Home Range greater part of their activity in directions op- Pair density. Pair density and all home range posite to each other. Sixty-one percent of the data were taken only from Area A which con- activity of pair A-10 was to the south, while pair tained approximately 540 square miles of land A-11 had .50 percent to the north and .33.3 per- area including villages and cultivated fields. In cent to the east. Pair A-9 had 66.6 percent to 1967, nine nesting pairs were recorded in the the south and .33.3 percent to the cast and no area, for an average density of one pair per reported activity to the north and west. 60 square miles. This is in contrast to 14 nesting Alternate nests. of the pairs of eagles pairs in 1968 averaging 38 square miles per paii". Many These figures express the maximum area per had one or more alternate nests Eleven of the pair, for rarelv during the nesting season were 21 p;iirs considered had known alternate nests. adult birds seen in the townships east of Faust Of these, 8 pairs had three nests and 3 pairs had in Rush Valley or east of Fairfield in Cedar Val- two. Three of the 11 pairs with alternates are ley, and unknown nesting pairs may also have known to have used different nests the same been present year. The distance from active to alternate nests raneed from slightly less than 25 meters at A-4 The two active nests with the greatest un- to 1.3 miles at A-8. occupied distance between them were A- 15 and A-14, 16.1 miles apart. This area was occupied Nesting AcrrviT^' predominantly by sheep and cattle operations and provided virtually no nesting habitat for Nest preparation. Preparation of the nests eagles. The closest active nests, 0.7 miles apart, varied greatly, with several being almost were found in 1968 when A-3b and A-4 were doubled in size prior to nestine, while others lx>th active. The divide between the two nests remained virtually unchanged. Nest A-10 was was 125 feet higher than A-.3b and 225 feet high- increased in height nearly 0.4 meter for the Biological Series, Vol. 10, No. 4. Ecology of Raptorial Birds

o

c

11 S o

bC

\- \/^- 10 BniGH.\M Young Univer.sitv Science Bulletin

1968 .season; in contrast, nest A-5 was enlarged days later, on June 5, the nest was empty, and veiy little during the same period. Of all the what is believed to be the remaining bird was nests studied for the two years, 54.5 percent had seen 1.5 miles east of the nest resting on a low an increase in height of 0.1 meter or more, while ledge. It allowed our vehicle to approach within the remaining 45.5 percent had less than 0.1 100 meters, and then flew poorly for about 0.25 meter of new material added. mile, and landed roughly on a hillside. One young at nest A-15b fledged in approxi- Egg Im/ing. Egg laying dates varied from mately ten weeks in 1968, and two young fledged year to year as well as within a given year. in 9 weeks in 1967. Three other pairs (A-5, The first egg was noted in nest A-3b on Febru- A-12, and A-14) each fledged one bird in nine ary 25, in A-10 on February 26, and in A-14 weeks in 1968, and pair A-4 fledged two birds on March 1, 196S. Two eggs were seen in in this same period in 1968 nests B-5 and B-6 on the second of March, 1968, and in nest A-15 on the fourth of March, 1968. Productivity This is in contrast to 1967 when, based on a 42-day incubation period, the first eggs were In 1967 and 1968, 23 nesting pairs of Golden laid on March 6 at nest A-10 and March 9 at Eagles produced eggs for an average of 1.91 Nest .'\-14. Two eggs were found in nest A-3a eggs per nest. Three pairs produced one egg on March 11, 1967, but unfortunately no laying each, 19 produced two eggs each, and the re- date could be detennined, as the nest was de- maining pair produced three eggs (Table 3). stroyed before hatching. A total of 31 active pairs in 1967 and 1968 The last two eggs laid in 1968 were found hatched 35 birds for an average of 1.13 young on April 11 at nest B-5b, and were abandoned per pair. Twenty-six or 74 percent of these about May 16, with evidence that incubation subsequently fledged for an average of 0.84 had lasted nearly 35 days. This is believed to fledglings per pair for both years (Table 4). be the second nesting effort for this pair, their Unsuccessftd nests- Six of the 13 nesting at- first having ended with two abandoned eggs tempts that failed are believed to have been about March 17, 1968 at nest B-5a. Another interfered with by humans. Nest A-3a and b late nesting date occurred at nest A-7 where one were destroyed during incubation in both years young hatched about May 5. Based on a 42-day of study. In 1967 the female was shot off the incubation period, the egg would have been nest by a small caliber bullet and both eggs laid about March 24. destroyed, and in 1968 all three eggs were Temperature data from the two years indi- found broken in the same area below and to cate no appreciable differences and provide no the side of the nest. Nests A-6, A-10, and A-11 obvious correlation between the laying dates for had the eggs removed, and no remains could be both years found in or around the sites. Four people with Incubation periods. Fairly accurate incuba- a rope were seen near nest A-11 on the last day tion periods were obtained from three nests. the eggs were reported present. Nest A- 15b had an egg present on March 4 and Human interference is also blamed for the the young eaglet was cstiinated to have hatched abandonment of nest A-8a in 1967 before eggs between April 12 and 14. This would indicate were laid. It is known that at least one man a of minimum 39 to 41 days of incubation. attempted to approacli this nest with a rope on However, it is believed that this egg was laid the last day it was reported active. March 2 when the bird was observed for 35 One nest (A-2) was abandoned in 1967 after minutes to be sitting on the nest. This would a fire of unknown origin destroved a wood rat increase the incubation period to a probable 41 den 12 meters to the side of the nest. Although to 43 days. Similarly, nests A-14 and B-6 held the nest and egg were undamaged they were two eggs each on March 1 and 3, respectively. The estimated dates of hatching are April 13 to TABLE 3 15 for the eggs of nest A-14 and April 14 to 16 Clutch size for various Golden Eagle nests for for the eggs of nest B-6. This results in a mini- Areas A and B mum 41- to 43-day incubation period for both CIntrb clutches. Size

Fledging. The length of time required for the young to fledge varied from 8.5 weeks to 10 weeks. Pair A-8b hatched two birds about April 7, shortly after which one died Fifty-nine Biological Series, Vol. 10, No. 4. Ecology of Raptori,\l Birds 11

TABLE 4 Productivitv records, 1967-1968. Areas A and B

Area A

Number of nesting efforts 1967 9 Number of nesting efforts 1968 14 Numlier of nesting efforts with eggs 1967 7 Number of nesting efforts with eggs 1968 13 Number hatched 1967 10 Number hatched 1968 17 Average number hatched per nest 1967 1.11 Average number hatched per nest 1968 1.21 Average number hatched per nest 1967-68 1.17 Number fledged 1967 7 Number fledged 1968 12 Average number fledged per nest 1967 0.78 Average number fledged per nest 1968 0.86 Average number fledged per nest 1967-68 0.83 Percent of hatched that fledged 1967 70.0 Percent of hatched that fledged 1968 70.5 Percent of hatched that fledged 1967-68 70.3 12 Bricham Young University Science Bulletin forced the smaller bird to occupy an exposed wing jumps The following day under similar position. conditions, this bird did manage to fly about 200 meters dowTi the hill with a combination of Nest Defense. Only three of the nesting pairs glides and stiff wing flaps. Once on the level showed any indication of defensive behavior. valley floor it was unable to regain flight and Most of the birds would fly out of sight as soon was then captured and carried to the top of the as they were aware of my approach. One ex- hill and released. ception was the pair at nest A-5. From incuba- The first flight of the eaglet in nest A- 11 tion to fledging, these adults would stay in the was one of little hesitation. When approached area when disturbed and at time call in a rather from above, it walked to the nest edge and im- shrill "cherop" sound. Particularly during incu- mediately jumped off and with a series of very bation the female would remain within 45 stiff wing and tail flaps it glided 250 meters to meters and circle and call while the male, if the valley floor. Once there it also could not present, would be higher and usually silent. regain flight and was captured after attempting This type of behavior continued through the to defend itself by lying on its back and extend- fledging period with only the call being omitted. ing its open talons. This bird was also returned At this nest the young began leaving the nest to the hilltop and released. and walking about on the hillside when 6.5 The eaglets of nest A- 15 in 1967 and A-4 in weeks old, and on at least one occasion before 1968 had very similar initial flights. Only the the young fledged, the female came to within last eaglet to leave nest A- 15 was observed as 50 meters of me, circling and calling. Although it jumped off the nest after making two ap- this bird made no aggressive moves when dis- proaches to the edge. It glided at least 350 turbed, she did remain nearby and defensive. meters with no real wing flapping and only the All defensive activit)' decreased sharply if two most abrupt turning. When about halfway into or more persons visited the nest at the same its flight, it produced two very plaintive time "cherops" and continued to its destination, a The female at nest B-5 would not leave the juniper tree. nest until observers were almost within touch- Both young in nest A-4 left the nest at about ing distance; when she occupied nest B-5a, I 7.5 weeks and were found on another ledge 20 could approach to within 10 meters of her, and meters above and to the northwest of their nest later when at nest B-5b, this same female would These birds apparently walked slowly to this not leave until someone was looking over the point, possibly taking several days, as droppings edge of the nest. Upon leaving she usually could be seen leading from their nest to this circled two or three times in the immediate final roosting area. These birds both jumped vicinity before disappearing. off this ledge and glided, one to the west 300 The third eagle to show any indication of meters and the other southeast 500 meters, be- defensive behavior was at nest A-12 during the fore landing in low sagebrush. 1967 nesting season. On one occasion, I was able The first flights of all these young, with the to approach to within 2.5 meters in a vehicle exception of those of nest A-11, were witnessed and then subsequently climb to the nest while by at least one of the adult birds. Only nest in full view of the bird and touch her wing. A-Sb contained large amounts of food after the She left, however, when the nest was reap- young had left. At none of the other nest sites proached to within two meters from above. At was there anv food found, although all had re- this time there were two one-week old eaglets mains scattered about. No adults were seen near in the nest. the young before their flight, and adults made attempts to to their aid during the Fledging behavior. Prior to leaving the nest, no come flicht. the eaglets appeared to perform wing exercises. This was noted on several occasions at nests Renesting attempts. There were two cases A-5, A- 15, and B-3. The eaglets started stretch- of renesting attempts after the failure of the ing their wings over their backs when five weeks first nest. In 1967 nest A-2 was abandoned after old and proeressed to wing flapping until their a fire destroyed a nearbv wood rat den. Three first flight. The eaglet of nest A-5 left the nest weeks later both eagles were seen carrying when 6.5 weeks old and two weeks later was sticks to an alternate nest site 150 meters south- seen walking on the hillside with a combination west of the original nest. Examination of this of wing flaps and jumps. When approached, site indicated that material had been added to this bird attempted to fly down the liill but was a height of 0.8 meter above the existing struc- unable to get off the ground for more than short ture in the fonn of a pyramid. Later examina- Biological Sefiies, Vol. 10, No. 4. Ecology of Raptorial Birds 13

tion indicated that nest construction had ended all on cliffs, in Alaksa. Bee (unpubhshed field at this stage notes) reported an active eagle nest in a fir Nest B-5a was abandoned with two eggs tree on the eastern slope of the Lake Mountains about March 17, 1968, and six days later on of Area A in 1936. This nest could not be March 23, new decorations were evident on an located in the present study. alternate nest 0.7 miles south of the first. On The altitude of the active nests reflected the April 11 two eggs were reported in this new surrounding topography. The average altitude nest, only to be abandoned between May 12 and for all active nests in the study was 5,750 feet, 16. Examination of these last two eggs revealed with no nesting activity seen above 6,800 feet feathered embryos with well-developed claws Sixty-one percent of all active nests were located and bills. This would indicate that incubation between 5,000 and 6,000 feet with 9.7 percent lasted had at least 30 to 35 days, ;md that the below 5,000 feet and 29 percent above 6,000 eggs were laid about April 10 or 11. This al- feet. A probable explanation is that only limited lowed a period of 24 days from the time of the hunting is possible in areas at altitudes above first nest the abandonment to time the second 6,500 feet. McGahan (1968) has suggested that set of eggs were laid. nest site preference is influenced by the direc- tion of the sun's rays; he reported that 49 per- DISCUSSION cent of the nests in his studv faced south, 14 Nest Locations percent west, and 23 percent east. Twenty-two percent of the active nests in my study faced The distinctive topographic features of Area north, 56 percent faced west, and 19 percent A are the parallel mountain ranges with broad south. These figures fail to provide any evi- open valleys on either side. These mountain dence for or against the effect of the sun as a ranges provide an abundance of cliffs for po- detennining factor in nest selection. tential nest sites, while the valleys with their rodent and lagomorph populations furnish ex- Nest Density and Home Range cellent hunting areas. This undoubtedly ac- Pair density counted for the fact that of the 31 nesting activi- and home range were determined ties reported in the two years, 87 percent for Area A only. The nine nesting activities reported in 1967 occurred on cliffs, 6.5 percent on the ground, averaged 60 square miles per and 6.5 percent on an artificial structure. There pair. With the addition of five more nests in 1968, were few trees large enough for nest sites in the average dropped to 38 square miles area A or around nests of Area B. per pair. This figure indicates a maximum area per pair, The utilization of the gunnery tower at site because few adult eagles were reported in A- 14 had enabled this pair of eagles to use this areas near Faust in Rush Valley and Fairfield otherwise unsuitable portion of Rush Valley as in Cedar Valley. their hunting range The closest natural nest This density seems to coincide with reports

site would be in the Oquirrh Mountains five from other areas. Dixon ( 1937 ) found that the miles to the east or on the ground on several 27 pairs observed in Southern California aver- low hills. This location in the U. S. Army Des- aged 36 square miles per pair. Arnold (1954) eret Depot Activity offered protection from in his studv in Colorado found six pairs occupy- human hunting pressures and an abundant jack- ing six adjacent townships, and Lockie (1964) rabbit population. reported that his Scottish eagles averaged 27.1

The ground nest at A-5 is believed to be an square miles per p;ur Another Scottish eagle abandoned Ferruginous Hawk (Buteo regalis) study (Watson, 1957) indicated that five pairs nest. Both McGahan (196S) and Wellein and of eagles averaged nine square miles of territory.

Ray ( 1964 ) have reported unoccupied eagle McGahan (1968) found the birds in his study nests on the ground. to average one pair per 66.3 square miles. McGahan (1968) reported that about 71 The shortest distance between active nests percent of the active nests in his study in south- was 0.7 miles. A nearby nest provided the third western Montana were on cliffs, with the re- point in a triangle the area of which was 0.5 mainder in trees. Wellein and Ray (1964) re- square miles. McGahan (1968) reported two ported that of the 79 occupied and unoccupied active nests being 1.0 mile apart but left it un- nests found in Texas and New Mexico, 87 percent clear as to the topography and home range allot-

were on cliffs, 11 percent in trees, and one nest ment between the nests. Dixon ( 1937 ) stated was on the ground. Only Murie (1944) had no that nesting pairs are sometimes within two tree nests to report when he observed 2.3 nests. square miles. 14 Brigham Young University Scienc:e Bulletin

On a basis of the information concerning 1968 averaged 3.2 degrees F. higher than for direction of activity of the various eagle pairs, the same period in 1967, and the low tem- their home ranges generally appeared to be perature for February 1968 averaged 4.4 de- opposite each other. Pair A-3a had 54.5 percent grees F. higher than for the same period in of their activity to the nortli and 27.3 percent 1967. It is doubtful that these temperature dif- and 18.2 percent to the east and south, respec- ferences account for the difference in egg-laying tively. By way of contrast, the activity of pair dates. A-4 was evenly distributed between west and The incubation rate for die eagles in this south. Pair A-5 had 45.5 percent and 31.8 per- study was detennined to be 42 days. This cor- cent to the east and south respectively, with responds closely to the 45 days determined by only 18.2 percent to the west. This activity to Hobbie and Cade ( 1962 ) in their work on the west was centered toward the roost located some Alaskan eagles but is somewhat above the 0.25 mile in that direction, and at no time was 33 to 35 days listed by Camie (1954) for his there an indication of any bird of nest A-5 going study in Santa Clara County, California. Walker far beyond this point. These birds were often and Walker (1939) reported that a single nest seen hunting the area between their nest and in Southern California had a 43-day incubation the lake. It appears that these three nests are period. Bent (1938) lists 28 to 35 days as the at the apex of their home ranges and that these incubation time but adds that 35 seems more ranges radiate away from each other. This cor- correct. responds to Dixon's ( 1937 ) statement that The male was seen to assist during incub- eagles have their nests on the margin of their ation by relieving his mate in the afternoons. hunting range, and their activity radiates away Walker and Walker (1939) also found this to from each other. be the case with one pair of eagles in Southern A similar situation appears to exist with California. They stated that the male incubated nests A-9, A-10, A-ll, and A-14. The direction from noon until 4 p.m., while the female rested. of their activities indicates that their proximity The male spent much of his time hunting or to each other is only on the basis of nest site, sitting on nearby roosts. and does not indicate an overlapping of ranges. Summer (1929b) and Camie (1954) both Considering the lack of adecjuate nest sites in the stated that the young remain in the nest from valleys, this is the most effective way to utilize 65 to 70 days. Seventy-five days is the length the available hunting area. of time Hobbie and Cade ( 1962) reported as recjuired for fledgmg. In the present study, fled- The birds of nest A-14 had most of their ging periods varied from 59 to 70 activity directed toward the north away from days. The nest with a 59-day fledging period had only the Deseret Depot Activity. Those of nests A-8a one young for the last seven weeks, and this bird and b and A- 15 had their activity distributed in was always supplied with more food than was all directiom, with slightly more to the north eaten. This would indicate that the young are and south. This corresponds to the isolation of not starved from their nests but leave for other the respective hills and the somewhat north- reasons as yet unknown. south oriented highlands around their nests. The combined data for 1967 and 1968 re- Particularly at nest A-14, the adults were never sulted in 23 nesting pairs producing 44 eggs seen low over the valley floor but always along for an average of 1.91 eggs. This is slightly the open ridge tops extending both north and only lower than the 2. 1 eggs per nesting pair reported south. It appears, therefore, that these nests by McGahan (196

16 Bhicham Young University Science Bulletin NESTING ECOLOGY OF THE GREAT HORNED OWL BUBO VIRGINIANUS

by Dwight G. Smith INTRODUCTION

The Great Homed Owl, Bubo virginianus, the adjacent Topliff Hills which increased the is a common nocturnal raptor of North and study area to approximately 2.5 square miles. South America, occurring in most habitat types. These hills are low-lying extensions of the Tintic Although it presumably exerts considerable pred- Range, with their highest elevations rising to atory influence on certain mammalian and avian about 1000 feet above the surrounding valley populations, relatively little is known of its nat- floors of Cedar and Rush Valleys. ural history and ecology owing to its secretive habits and the inacessability of its nesting sites. Field Studies Further interest is warranted in this raptor be- The field work for this study was conducted cause of its decreasing numbers caused by a for a period of two nesting seasons, the spring combination of factors, including habitat dis- of 1967 and the spring of 1968. Field obser- ruption owing to encroaching civilization, road vations were aided by the use of 7 by 50 power kills, and indiscriminate shooting by himters. binoculars and a 20 power spotting telescope. This study deals with the nesting ecology Nests on the study area were located by a of the Great Homed Owl in the Great Basin systematic search of all potential sites, that is, deserts of central western Utah. Major aspects all cliff lines, rock outcroppings, abandoned include nesting population and distribution, ter- quarries, and wooded areas. Those nests found ritoriality, and predation. were then plotted on a master map to determine There are few comprehensive studies on the the relative nesting population and distribution ecology of this species, ;md there appear to be of the owls. Gaps in the suspected distribution no previous detailed investigations made in arid were then rechecked several times during the or semiarid habitats. Included among the more nesting season for signs of roosting birds or nests noteworthy studies are Baumgartner's ( 19.38 previously missed. It was believed that all nests work on its distribution and territorality in New active during a given season were discovered York forests, Errington's (1932) behavioral and and that an accurate picture of the nesting pop- predation studies in the cottonwoods of Wiscon- ulation and distribution of the Great Homed sin and the midwest, Orian's and Kuhlman's Owl was established during the two nesting ( 1956 population and distribution study, also ) seasons of the study. in the Wisconsin area, and Fitch's (1940, 1947) studies conducted at three population and predation studies in the Calif- Territorial were nests, representative of a different topo- omia chaparral. Utah literature on this species each graphic site. These included a west-facing quar- is limited but includes notes on its nest site of which was selection (Sugden, 1928), breeding records (Bee ry nest and two cliff nests, one located in an east-opening canyon and the other and Hutchings, 1942), some life history and a western exposure. ecology data (Bee, unpublished data), a trap- situated on a plateau with Territorial behavior was detennined by visual ping note reported by Stanford ( 1931 ) , and the hours from sunset until numerous local distributional records as exem- observations during dark and was conducted from constmcted blinds. plified by Haywards (1967) Birds of the Upper Colorado River Basin. Additional infomiation on bird movements was gained from the plotting of sighting occurrences METHODS AND PROCEDURES during the night. Owl movements were plotted, then territory size detemiined by planimeter Study Area measurements of the polygon formed by con- distances The study locale of the first ( 1967 ) nesting nection of points denoting the extreme season was the Thorpe Hills of central westem from the nest site, following the metliod dis- Utah, an area of approximately 12 square miles cussed by Odum and Kuenzler (19.55). of elevated topography located in Tooele and Owl predation was detemiined by ( 1 ) week- Utah counties (see Fig. 5). In the second study ly nest visitations to record prey types and fre- season ( 1968 ) . the area was enlarged to include qucncv. and (2) pellet analysis. Biological Series, Vol. 10, No. 4. Ecology of Raptoiii.\l Birds 17

it

Fig. 5. Study area location and vegetation 18 Brigham Young UNrvERsiTv Science Bulletin

On discovery of a nest site, egg productivity nesting pairs per square mile during the 2 study was checked and the nest placed under weekly years. During this period 5 additional nests were observation to note adult habits and egg-pipping found outside the study area but in similar hab- time. After the young hatched the nest was itat type; unless othei"wise noted, all observations inspected more frequently to ascertain food pro- and tabular information pertain to the study curement items and adult territorial behavior. area nests. At nest abandonment with the development of flight by the young, nest measurements were Nests And Nest Site taken and the remaining food items recorded Nest site and nest dimension data are pre- and pellets removed for analysis. Data taken in- sented in Tables 5 and 6. All four of the 1967 cluded nest size and material composition, its season nest sites were used the following year, height relative to cliff size, and its elevation. thus 10 different sites were located during the Additional data gathered inicluded infomi- two-year study. Birds on the study area utilized ation on interspecific relations as detennined by cliffs, abandoned quarries, caves and junipers a search and location of active predatory bird for nest sites. Favored sites were large sandstone nests in the vicinity of the owl nest sites. or limestone cliff faces or rock outcroppings. Three sites were deep in canyons, one on a plat- RESULTS eau, and the remainder in the hills and foothills. Six of the nests were on western exposures, three Nesting Population And Distribution faced south, and one had an eastern exposure. Fourteen nests were found on the study area Site elevations ranged from 5300 to 5750 feet during the 2 breeding seasons, including 4 dur- and averaged 285 feet above the valley floors. ing the 1967 season and 10 the following year None of the nests were actually built by the owls, on the enlarged study area ( Fig. 6). Homed Owl but eggs were deposited in old Raven, Redtailed population density on the study averaged .35 Hawk, and Ferruginous Hawk nests appropriated

TABLE 5 Nest site selection of the Great Homed Owl

Nest Valley floor number Location Site Elevation elevation Difference

1 Biological Series, Vol. 10, No. 4. Ecology of R.\ptoki.\l Birds 19

Fig. 6. Nest locations of the Great Homed Owl : . )

20 Brigham Young University Science Bulletin by the early nesting owls, or were laid directly Productivity And Nesting Success on unmodified rock or ground. Of the 10 nests, A mid-September visit to the nests of the 4 v/ere old Raven nests located in cUff crevices, previous year (1967 season) disclosed no adult one a RedtaOed Hawk nest in a quarry crevice, birds in the vicinity, but on December 2, ( 1967 one Ferruginous Hawk nest in a juniper tree, 1 found a male occupying its nest site, presum- and the remaining 4 were on quarry and cave ably of the previous year. Owl pairs were in- ledges. The nests outside the study area con- itially observed together on January 5, and again formed to this pattern with the exception that on January 16, of the second study year. Egg- one was located in a large Red-tailed Hawk laying dates were between March 20 and April nest high in a cottonwood tree. 10, in 1967, and between January 20 and Feb- The height of nests ranged from 11 to 55 feet ruary 10 in 1968. above ground level. Nest size varied in propor- In 1967 the 4 nests contained 8 eggs for an tion to crevice size, and as most nests were in average clutch of 2.0 eggs per nest. In 1968 8 poor repair, their external structure spilled nests contained 24 eggs for an average clutch throughout the crevice, confomiing to the size size of 3.0 eggs per nest. Maximum clutch size and shape of the crevice. The larger nests act- for the two-year study was 4 eggs while the ually formed the floors of huge cracks in the minimum clutch size was one egg. In every case, vertical stone faces of quarries, while the smallest repeat nests of the 1968 season contained one nests were one ledge sites. Nests were composed more egg than was contained in the 1967 season of juniper and sagebrush twigs of varying sizes. ( see Table 7 ) All nests had extensive litter accumulation in The fourteen nests of the 2-year study period the form of fecal material and prey and pellet hatched 29 young (2.1 per nest) and of these remains. Toward the end of the nesting season 23 (1.7 per nest) fledged, but yearly success this accumulation often spilled over the nests' varied. In the 1967 season, all nesting efforts edge forming large white streaks on the cliff were successful, but 5 of the 1968 study nests face. Actual nest structure depended on 3 factors failed. Reasons for failure were ascribed to sev- age, exposure, and the original avian builder. The eral causes based on circumstantial evidence. In partially protected Raven nests tended to hold at least 2 cases nest failure was attributed to hu- up best; they may thus provide several years of man interference. Roth nests failed after the service. young had hatched and survived for a period of Nest Distribution 2 weeks. A third failure was attributed to inter- specific action between close nesting Homed Nests were evenly distributed in the study Owls and Red-tailed Hawks. In this case both area, with sites averaging one mile apart. Maxi- species abandoned their nests after eggs had and minimum distances mum between nearest been deposited and were under incubation. neighbors for both nesting seasons were three Causes of the other 2 nest failures are unknown. miles and three-quarters of a mile. The closest sites were on opposite sides of the same mountain Adult Nesting Rehavior range, hence the nesting birds hunted in separate valleys. The extreme distance between succes- The 2 sexes differed in nest habits. During sive sites occured in an area of fewer potential the daylight hours males generally roosted in nest sites, and supported lesser numbers of pred- favorite secretive spots such as trees, unoccupied atory birds for any species. All nests were lo- nests or nesting crevices, and ledges at distances cated in the periphery of the hills on sites over- from 5 to 250 vards from the nest site. looking the deserts. No nests were found in the Females roosted in the nest during the day, study area interior (see Fig. 6) although poten- taking flight only when alarmed. Typical fe- tial sites were available. male diurnal activity after the young hatched

TABLE 7 Nesting success of the Great Homed Owl

Yean ,

Biological Series, Vol. 10, No. 4. Ecology of Raptorial Birds 21 consisted of positioning herself between the Hawk and Prarie Falcon. These birds often nest young and the edge of the nest. In this stance she close to the Homed Owl nests. Examples of nest- would doze, occasionally awakening to visually ing proximity include a Red-tailed Hawk nest sweep the nest site environs. The female would situated on a ledge 25 yards from a Homed usually leave the nest approximately 45 minutes Owl cave nest, a Ferruginous Hawk juniper after sunset regardless of weather conditions, nest approximately 100 yards below a Homed and join her mate at the roost, after which the Owl cliff nest, and a Homed Owl sharing quarry pair would begin their nocturnal activities. nesting sites with Golden Eagles and Prairie Falcons. Nest Defense Interaction was observed in only one instance, Nest defense behavior differed with respect wherein a Red-taQed Hawk attacked a flushed to sexes. Males took flight readily, but females Homed Owl. There was no apparent injury to either bird, remained on the nest until I flushed them. Six although both nests were later a- nesting pairs simply disappeared after being bandoned by the 2 nesting pairs. alarmed, but females of three nests would hoot protests from concealment. The female at nest Predation ten repeatedly carried out determined attacks Hunting activity of Homed Owls was often against me when I was near the nest. observed normally in late evening after sunset. Although there was some hunting in the hills Territorality and canyons, the principal prey procurement act- Territories of 3 nesting pairs were deter- ivity occurred in the deserts. Owls were observed mined during the growth of the young and are to hunt either alone or in pairs, employing one presented in Figure 7. Both hunting activity of two hmiting techniques. One method con- and sighting locations were plotted, thus maxi- sisted of flying slowly, harrier fashion, over the mum territory was established (Odum and ground, diving suddenly on prey spotted; the Kuenzler, 1955). Results indicate territorial sizes other method consisted of perching on suitable of 172, 237 and 376 acres for the nests, or an elevated objects such as telephone poles, rock average territory coverage of 261.6 acres. outcroppings, and road banks to facilitate sight- Territory size varied with topography. The larg- ing of prey which would then be attacked by est territory ( Fig. 7; nest 9) had the nest site on a direct flight to it. a plateau, and the nesting birds hunted in both Food items recorded were either prey re- surrounding valleys. The smallest determined mains found in the nest or items identified from territory (Fig. 7; nest 6) had its site in a narrow pellets removed from the nest sites for analysis. canyon with owls extending their hunting act- A total of 173 food items was recorded; this ivity along a narrow strip into the desert. All de- included 101 prey remains and 72 items based termined territories extended only slightly into on pellet analysis. Mammals recorded from pel- mountains but ranged far into the desert val- lets were counted only on the basis of skulls leys. In no case did the size of the feeding present, thus avoiding possible duplication of territory have a radius exceeding one-fourth mile individuals. Table 8 hsts the prey items found, from the nest site into the hills, but the radius along with percentage calculations based on extending into the deserts ranged from one mile numbers of individuals of each species relative to one and one sixth miles, and spot observations to the total number of prey items, without on a fourth nest indicate a possible territorial regard to volumetric considerations. range of two miles into the valleys. Mammals comprised 156 items, or 90.2 per- cent of the total food intake. The Black-tailed Intra And Interspecific Relations jackrabbit (Lepus californicus) and desert cot- No interactions were observed between nest- tontail (Si/Ivilagus auduboni) accounted for 67 ing pairs of Homed Owls, and no other large items or 38.7 percent of the total of all items. owls occurred in the study area. The medium This was equalled by the kangaroo rat (Dipod- sized Long-eared Owl (Asio otiis) and Short- omtjs ordii), which totaled 68 items or 39.3 eared Owl {Asio flammetts) occur in the sur- percent of the total food items. Other mammals rounding valleys, but neither species was found less frequently recorded included the white- to be nesting in the study area. Several large footed deer mouse (Peromijsctis maniculatus) avian raptors do nest in the study area, however, meadow mouse (Microtus sj).), ground squirrel including the Golden Eagle, Red-tailed Hawk, (CiteUiis toivemendii) , and wood rat (Neotoma Ferruginous Hawk, Swainson's Hawk, Cooper's sp.). 22 Bricham Young UNrvERsiTv Science Bulletin

Key

roads ^=^=

sighting points o

territory boundary

nest site *

scale:

63mm. equals Imi.

Fig. 7. Territories of three pairs of Great Homed Owls Biological Series, Vol. 10, No. 4. Ecology of Raptorial Birds 23

TABLE 8 Food of horned owls in the study area 24 BmcHAM Young University Science Bulletin vided no actual benefits to the owl occupants. radius into or along tlie periphery of the hill This conclusion is reinforced by my observations sites, they do range widely into the deserts, that these desert Homed Owls may use the with activity radii often exceeding one mUe. same nest for several years in succession, where- Both Baumgartner (1938) and Bent (1937) as Orians and Kuhlman ( 1956 ) found that the state that homed owls will not tolerate other owls of their study were unable to use old nests avian predators nesting in proximity, but I found for more than one season, as the nests would several instances of other raptors nesting within become too dilapidated unless repaired by a hundred yards of homed owl nests. This in- hawks. cluded one example of a Red-tailed Hawk nest- Breeding ing only 25 yards from an active homed owl nest. However, in agreement with Baumgartner

Bent ( 1937 ) notes the average homed owl ( 19'38 ) I found no other species of large owls clutch size to three be from two to eggs, with nesting in the proximity of the homed owl nests, one or four occurring uncommonly. In their although both the Long-eared Owl and Short- later study, Orians found and Kuhlman (1956) eared Owl occur in the adjacent valleys. a clutch size of two eggs to be the most com- mon, with one-egg clutches appearing less fre- quently, and three-egg clutches rarely occurring. Predation

Bee (unpublished data) records Utah clutch The results of my predation studies agree sizes ranging from one to four eggs, with two with those of Errington, Hamerstrom and Ham- appearing most commonly. erstrom (1940), Alcom (1942), Fitch (1947),

Bent ( 19.37 ) further noted that egg-laying and Orians and Kuhlman (1956), who found dates were between January and May, depend- that lagomorphs constitute the principle volu- ing on the locality. My observations seem to metric items in the homed owl diet. Bond's indicate that both clutch size and egg-deposi- ( 1940 ) Nevada desert study demonstrated a tion date vary with yearly differences in winter similar utilization of several small species, in- temperature and severity, as there was a signifi- cluding kangaroo rats, white-footed deer mice, cant difference in egg-deposition dates (eggs meadow mice, and wood rats, but failed to show were laid one full month earlier in the second- comparable use of rabbits and c-ottontails. I study year) and clutch size (from an average of found that owls on the study area utilized fewer 2 eggs to an average of 3 eggs per clutch) in different species, but relied heavily on three the two years of study. However, further ob- species: the black-tailed jackrabbit, the desert servations would be necessary to substantiate cottontail, and the kangaroo rat. this hypothesis. Territory SUMMARY

My discovery of a male occupying his terri- This study describes the nesting ecology of tory in December agrees with the findings of a the Great Homed Owl, Bubo vir<^ini(inus in the number of observers, including Ridgway, Baird, desert regions of western central Utah. Study and Brewster (1874), Errington (19.32), Baum- aspects included population and distribution as gartner (19.38), and Orians and Kuhlman detennined by the location of all active nests in (19.56), all of whom observed male Homed a unit area; territoralitv as detemiined by ob- Owls occupying their territories in the late fall servations from constructed blinds and sighting and winter. infonnation; and predation as demonstrated by The actual territory of these desert-dwelling a tabulation of prey remains found in nests and owls seems to be larger than those found in analvsis of pellets removed from the nest site. previous studies. Ridgway, et. al. (1874) noted Data were gathered for a period of two nesting that homed owls rarely go more than a mile seasons, the spring of 1967 and the spring of from their nest sites. Miller's (1930) observa- 1968. in the Thorpe and Topliff Hills of Utah tions indicated that homed owls patrol an ir- and Tooele Counties, Utah. regularly shaped territory with the largest di- Owl nesting densities were found to be .36 ameter of no more than one half mile. Baum- nests per square mile in 1967 and .40 nests per gartner (1938) obtained similar results, noting scjuare mile in 1968. Nest sites included cliff that his study birds also held territories of not lines, abandoned quarries, and junipers. Cliff more than a quarter mile radius in any direction nests occurred most often, with the eggs de- from the nest site. I found that while the desert posited either on bare rock or in old Raven, homed owls generally do not exceed a half mile Red-tailed Hawk, or Ferruginous Hawk nests. Biological Series, Vol. 10, No. 4. Ecology of R.\ptorial Birds 25

Both egg-deposition dates and clutch size The nesting pairs maintained large territories, differed between the two years of the study often ranging over one mUe from the nest site period, with an average clutch of 2 eggs being into the surrounding valleys. These owls ap- laid in late March or early April in the 1967 parently tolerate the close nesting of diurnal season, and an increased average clutch of 3 avian predators, but not other owls. eggs being laid in late January or early February The black-tailed jackrabbit and desert cotton- tail contribute the bulk of the horned owl food, in the 1968 season. In 1967 four nests produced followed by the kangaroo rat. Other mammals, eight young, all of which were successfully birds, and invertebrates are utilized to a lesser fledged. In 1968 ten nests produced 21 young, extent. of which 15 fledged. Nesting failures were fre- Economically these owls are of little impor- in the season, in quent second-study occurring tance to agriculture or game management in five of the ten study-area nests. Reasons for this area, but mav contribute to range manage- failure were attributed to human interference ment through their predation on the lagomorph and avian interspecific competition. and rodent populations.

NESTING ECOLOGY OF THE FERRUGINOUS HAWK BUTEO REGALIS

by John Bradford Weston

INTRODUCTION

The Ferruginous Hawk (Buteo regalis) is adequately covers the nesting ecology of the the largest and most powerful of the North Ferruginous Hawk. American buteos. It is a bird of the western The purpose of this study is to furnish more plains and arid regions, breeding from south- infonnation on the nesting ecology of the Ferru- western Canada to southern Arizona, New ginous Hawk in Utah, with particular attention Mexico, and Kansas, It winters from California being paid to distribution, density, and nest and Montana to Lower California and northern sites. Observations on territorial behavior, feed- Mexico, seldom occurring east of the Mississippi ing mechanics, nest behavior, hunting tech- River. m(|ues, and mortality were also included. The Ferruginous Hawk may live on grassy prairies where it nests in the timber belts along THE STUDY AREA the streams, or in the barren, treeless plains or The study area embraces some three town- badlands where it usually builds its nest on some ships and three ranges of desert and seinidesert convenient cliff, butte, or cutbank. Its chief land immediately south and west of Fairfield, habitat requirement seems to be a good supply Utah County, west central Utah (Fig. 8). The of small rodents on which it characteristically major portion of the area is fonned bv Cedar feeds (Bent, 19.37). Valley and several ranges of low hills imme- Fuertes (1920), May (19.35), Bent (1937), diately adjacent to the valley. Topography is Sprunt (1955), and Grossman (1964) have generally flat, broken only by the low hOls. each described some phases of tlie natural his- Elevation of the desert floor is about 4,900 feet tory of this species. Bowles (1931) described above sea level. Within the eastern edge of the ground and tree nesting of the Ferruginous study area lie the Lake Mountains, and along

Hawk in Washington. Cameron ( 1914 ) de- the area's western edge the Topliff and Thorpe scribed nest sites and food preferences of this Hills are prominent. These mountains vary in species in Montana. Salt (1939) listed migra- altitude from 5,900 feet to 7,690 feet above sea tion routes of hawks banded in Alberta, Canada. level. To the north the Ocjuirrh Mountains rise To date, however, I have found no work that well above 10,500 feet. The southern boundary Brigham Young Uni\'ersity Science Bulletin 26

Fig. 8. Study area location

lecting data. In the spring of 1968, field work of the study area is an arbitrary line drawn again conducted from late February through through the soutliem portion of Cedar Valley was 30. In the course of this two-year project, and the adjoining hills. Agriculturally developed June fewer than 450 man-hoius were devoted to areas fonn portions of the northern and eastern no studies. boundaries. the field Owing to the large size of the study area, a METHODS AND PROCEDURES small portion was chosen in which a more de- intensive study could be conducted ( Fig. during the spring tailed, Field work was conducted repre- 9). The area chosen for intensive study seasons of 1967 and 1968. During the spring of the larger, more Febru- sents a typical portion of 1967, I spent nearly every weekend from general study area. It was chosen because it was ary 20 through June 30 in the study area col- Biological Series, Vol. 10, No. 4. Ecology of RAPToniAL Birds 27

3 28 Bricham Young University Science Bulletin

easily accessible and contained a high density of Valley comprised a large portion of the study nesting Ferruginous Hawks. area, with the hawks nesting in the foothills on During the spring of 1967, a systematic its perimeter. search was conducted throughout the study Seasonal displacement area for Ferruginous Hawk nests. Behavior as- The Ferruginous Hawk population in the sociated with hunting, courtship, and territorial study area was present at a high density only defense of the hawks was often utilized in locat- during the nesting season, which lasted from ing nests and delineating hunting ranges. Aerial early March to late July. By September 1 no surveys were also utilized to locate possible nest Ferruginous Hawks were observed in the study sites. Such surveys were valuable only to the area. Several intensive searches conducted dur- extent of locating suitable nesting terrain. ing the winter of 1967-68 revealed the presence After Ferruginous Hawk nests were located, of only one Ferruginous Hawk in the west desert those thought to be "active" or in use that year area of central Utah. This bird was located some were kept under observation throughout the 25 miles west of the study area. study. The remains of prey were observed and By mid-November a large population of rap- pellets were gathered from all nests in which tors presumed to nest in more northemly areas they occurred. Pellets were also collected from had moved into the study area and surroimding roosting sites of the male hawks. Such sites were terrain. The Rough-legged Hawk Btiteo lago- usually located near the nest. ( pus) and the Bald Eagle (Haliaeetus leucoceplia- Pennanent blinds composed of a wire mesh his) were present in particularly large numbers. frame with a burlap covering were placed upon These raptors remained in the area imtil late hillsides above several nests. From these blinds February and early March, when they departed observations were recorded on a portable tape and were replaced in part by Ferruginous Hawks recorder, and nesting activities were photo- which had presumably wintered farther south. graphed with a 35mm. single-lens reflex camera Also occupying the study area only during the equipped with several telephoto lenses ranging spring and summer months were the Red-tailed up to 600mm. in length. Hawk (Biiteo janiaicensis), Swainson's Hawk Known breeding pairs of hawks were as- (Buteo sicainsoni), and Sparrow Hawks (Falco signed numbers, and each nest of that pair was

sparverius) . The Golden Eagle (Aquihi chnjsae- assigned the corresponding number and an al- tos) and Great Horned Owl {Bubo virginianus) phabetical designation. Therefore, nest la, lb, were pennanent residents of the area through- ... Id all belonged to pair number 1. out the study. Analysis of prey remains and pellets was Fish and Wildlife Service locktite bands were made by the writer. Bones were identified by placed on nesting hawks during the 1968 season. comparison with those of known specimens. No recoveries which might indicate direction of Pellets were analyzed on the basis of bones con- seasonal movements had been made by the end tained therein and the presence of general types of this study. of hair found. Where p)ossible, all prey items were identified to species. Nest Locations

Nest distribution RESULTS Ferruginous Hawk nests, often very old and Population and Distribution in poor condition, were located on most low hills and many in scattered trees throughout During 1967-1968 the general study area sup- the study area. However, occupied nests oc- ported 21 known pairs of Fermginous Hawks curred in groups. Tliat is, one portion of a large for an average density of one pair per 15.4 habitat area contained a series of active nests, square miles. These figures express the maxi- while another nearly identical portion of the mum area per pair, for areas inhabited by hu- same habitat contained only old inactive nests. mans are included, and unknown nesting pairs Active nests occurred in the same areas during may also have been present. The majoritv of both years of study. The Tenmile Pass and the hawks nested in the foothills surrounding Blowhole Hill-Long Point areas exhibited a high Cedar Valley. The valley floor, which comprises concentration of active nesting pairs during the a major portion of the study area, is essentially present study. void of suitable nest sites. The smaller intensive study area supported Proximitt/ of Nests 11 knovyn pairs of hawks for an average density The maximum and minimum distances be- of one pair per 7 square miles. Again, Cedar tween nearest neighbors for 13 occupied nests Biological Series, Vol. 10, No. 4. Ecology of Raptorial Birds 29

TABLE 9

Number ( and percentage ) of Ferruginous Hawk nests in various sites 30 Bkic.iiam Young University Science Bulletin

inches, a pocket diameter of 17 inches, and a the eighth and twelfth days and full juvenile pocket depth of 4 inches. Tlie smallest nest plumage had developed by the seventh week. measured, a ground nest, had a diameter of 29 Observed dates reproductive activity inches, a thickness of 8 inches, a pocket diameter of of 11 inches, and a pocket depth of 2 inches. During both 1967 and 1968 every pair of hawks had selected its nest site by March 10. In 1968 the average laying date was April Initl\l Nesting Activity 8, the average hatching date was May 10, the The earliest date in 1967 that hawks were average brood departure date was June 25, and observed near their nest sites was March 6. The the resulting average number of days in the first nesting pair observed in 1968 was seen nest was 45. Those differences observed in 1967 February 25. In both 1967 and 1968 March was were probably not significant (Table 12). the month in wliich nearly all nests were con- structed, with most of them being essentially Behavior completed by March 25. Territorial behavior Three different pairs of hawks were observed The observations on territorial behavior building their nests. In each instance both seemed to be rather inconclusive. At times each members of each pair were active in nest con- pair was seen to vigorously defend its territory struction. Sticks of various sizes, which made against any intruding raptor. On other occas- up the bulk of every nest, were always carried ions predatory birds flew directly over active to the nest site in the hawks' feet. On one oc- nests without eliciting any response whatsoever casion a strip of bark was seen to be carried to from the occupant pair. On one occasion tvvo the nest in a hawk's bill. The female, distin- Swainson's Hawks were seen to attack a soaring guished by her larger size, did the actual ar- male Ferruginous Hawk whose mate was sitting ranghig of materials in the nest. Once nest with her three young on a nest directly below building had begun, each pair spent several him. The intruding Swainson's Hawk flew high hours every day carrying material to the nest until it was finished. TABLE 11 Eggs laid, voiinj; hatched, and voung fledged, Productivity 1967-1968 Chitch size Years In 1967 thirteen nests produced 20 eggs for 1967 1968 an average of 1.5 eggs per nest. One nest pro- duced 4 eggs, two nests produced 3 eggs, five Number of nesting efforts nests produced 2 eggs, and the remaining five nests were abandoned before any eggs were laid. In 1968 fourteen nests produced 50 eggs, an average of 3.57 eggs per nest. Eleven nests con- tained 4 eggs, two nests contained three eggs, and the remaining nest was abandoned before

any eggs were laid (Table 11 ).

Hatchina^ fledgino success, and description of t/oitng In 1967 a total of thirteen nesting efforts produced S young, an average of .67 young per nest. Of those hatched 8 ( 100 percent ) fledged.

In 1968 fourteen nesting efforts hatched .33 young for an average of 2..36 per nest. Of those hatched 28 (85 percent) fledged, an average of

2 fledged per nest ( Table 11).' Within hours after hatching, the young hawks exhibited a dense covering of white down. Their gapes were noticeably broader than those of other species of hawks of similar size. Their ceres were greenish-yellow and their irises were dark gray. Pin feathers began to appear between Biological Series, Vol. 10, No. 4. Ecology of Raptorial Birds 31 above the resident Ferruginous Hawk, then little osseous material. Prey species can be iden- swooped on hirn at steep angles, barely missing tified readily from most pellets, but it is fre- him several times. Finally, one of the Swain- (juently impossible to determine the number of son's Hawks came too close and was grasped in individuals concerned. In all, 219 samples were the talons of the larger Ferruginous Hawk, who taken from 26 eyries, which included 53 pellets immediately released him. Upon being released, and numerous parts and pieces of rabbits and the Swainson's Hawk tumbled several feet, re- other prey. From these, 283 prey specimens, or gained his balance and quickly flew away, fol- 1.3 per sampling, were identified, indicating the lowed closely by his partner. The Ferruginous minimum number of individuals that the various Hawk returned to his leisurely soaring flight identified items could represent. above the nest. Identified prey On five occa.sions as I visited five separate Of the 283 prey specimens identified, 92 per- active nests, three adult Ferruginous Hawks cent were mammals (Table 13). Kangaroo rats soared above me, screaming, diving, and protest- comprised 48 percent of the mammals and 44 ing my presence. At no time was hostility ex- percent of all prey items. Black-tail jackrabbits hibited by the nesting pair toward the third were the next most plentiful, representing 33 hawk that had joined them in their attack on me. percent of the mammals and 30 percent of the These were the only instances in which more total. It is likely, however, that jackrabbits than two Ferruginous Hawks were observed rep- resent the greatest amount of food material in above any one nest site. the diet of these hawks. Birds composed 5 per- Several times during the study Ravens (Cor- cent of the total number of prey items, with vus corax) were attacked by nesting hawks, al- Homed Larks the most prevalent, representing though no actual contact was ever observed. S6 percent of the birds. The only reptiles tallied On one occasion a Golden Eagle was chased were 3 snakes and 6 homed lizards, representing from a Ferruginous Hawk's nest site, although only 3 percent of the total mmiber of prey eagles were generally tolerated wherever they roamed. individuals. Hunting techniques Nest behavior The most common hunting technique ex- Only female hawks were observed incubating hibited by this species was to fly low over open eggs, although the males may have participated country, never rising more than a few feet above in incubation during periods of my absence. the groimd, with rapid wingbeats propelling it Both members of each pair hunted prey for their swiftly forward. Capture was attempted of any young, but the male always left his catch on the small animal encountered in the flight path. edge of the nest and departed. By contrast, I often saw Ferruginous Hawks soaring in when the female carried prey to the nest, she true Buteo style. On one occasion a soaring remained to feed the young. hawk su'ooped on a black-tail jackrabbit, which Throughout incubation and while the young it missed. Twice during my study, hunting were covered with down, the female remained hawks were observed to hover in one place by at the nest almost constantly. However, as the rapid vibration of their wings, then fly on a few young grew older and more self-sufficient both feet and repeat the procedure until prey was parents spent increasing amounts of time soaring found and captured. and roosting together in areas near the nest. Observation of several nests from concealed As the young reached their sixth week after blinds revealed that intensive hunting was usual- hatching, the female was present at the nest ly initiated at first light of dawn, often being only during the night time and on subsequent completed by sunrise. Evening hunting usually short visits to bring food to the nest during the started at sundown. Food demands by nestling day. hawks necessitated extended hunts as the nest- Food Habits ing season progressed.

Remains of prey were counted, recorded, re- MORTALITi- moved from the nest, and identified. When pos- sible, food was squeezed from the crop of nest- During this study several dead raptors were lings (Errington, 1932). Pellets regurgitated by found. A fellow student conducted a mortality both adults and young were counted, collected, census of raptors along a utility pole line located and stored. However, pellet analysis is a poor in the center of my study area (EllLs, et. al., method of determining quantitatively the food 1969). In 1967-1968 he counted 28 dead raptors of nestling hawks, for such pellets contain very (primarily eagles) along the nine-mile stretch 32 Brigham Young University Science Bulletin

TABLE 13 Food of nesting Ferruginous Hawks in the west-central Utah study area"

1967 1968 Total

Prey species No. Biological Series, Vol. 10, No. 4. Ecology of Raptorial Birds 33

or summits of low hills. If constructed on the summit of a hill, it is usually situated atop a rock, and if on the side of a hill, a jutting ledge

of rocks forming a natural shelf is selected to hold the bulky, loosely-constructed nest in place

( Fig. 11 ). Behle, Woodbury, and Cottam ( 1943) describe ground nests located along the 1846 route of the Donner Party on the Great Salt Lake Desert north of Knolls, Utah. Tliese nests varied in size up to 6 feet in height and 9 feet in diameter at the base and were composed of AUenroIfea roots and stems. The second most common type, the juniper nest, closely resem- bles the ground nest. It is usually constructed five to eight feet above the ground (Fig. 12). Nests similar to those found in the present study area are described by Cameron (1914) in Mon- tana and Bowles ( 1931 ) in Washington. Bent

( 1937 ) describes nests in North Dakota which were situated 40 feet above the ground in tall swamp oaks. Nests I observed were composed of heavy sticks, manure, and other rubbish, and lined with grass, strips of bark, and paper. Wil- liams and Matteson ( 1947b ) recorded a nest lined with paper in Wyoming. They also re- ported that nests located by Wyoming biologists in the year 1S80 were composed almost entirely of dried buffalo bones. During the present study, 17 of 22 active nests were on southern or eastern exposures. Fig. 12. A Ferruginous Hawk Juniper tree nest in the west-central Utah study area, Nlay 30, 1969. These data suggest that nest site preference is influenced by the direction of the sun's rays. Exposure should be important during the earlv Direction of nest exposure seems to be a spring months. Even in the wanner months of major factor in detemiining proximity of nests of pairs. to June and July, nests facing south and east might neighboring Nests exposed the same be preferable. Those with easterly exposures hunting areas were constructed no closer than would receive the warm morning sun and be 1.3 miles, whereas nests facing different hunting shaded in the hot afternoons. McGahan (196S) areas were constructed as close as 0.4 mile apart. found that most of the Golden Eagle nests in his Weather, prey densities, and human inter- study area faced either south or east. ference may each be partially responsible for the dramatic variation in productivity experienced between 1967 and 1968. It may well be that inclement weather factors such as low tempera- ture, high humidity, or a violent stomi may oc- cur at a critical period in the hawk's reproduc- tive cycle, thereby affecting its reproductive capability. Bee (19.35) noted that at times hawks of this species would abandon their nests during inclement weather. Weather data ob- tained from the Brigham Young University Physics Deparhnent revealed little variation in monthly temperature and precipitation averages during this study. However, day-to-day varia- tions obviously occurred and may have had '^r ^ marked influence on hawk productivity. Another influencing factor may have been Fig. 11. Nest .5b, a Ferruginous Hawk ground nest in the west-central Utah study area, June 24, 1968. variable densities in prey populations present 34 BniGHAM Young University Science Bulletin during each of the two years, but no data on total melanistic plumage and one possessing prey densities was gathered during this study. characteristic light-colored plumage ( see Frontis- Human interference may also have been a piece). This Ls especially intriguing in view of factor in productivity variations. In 1967 three the fact that this was the only nesting melanistic nests containing eggs and four nests ready for adult in my study area and that the pair pro- eggs were abandoned early in the nesting season duced the only melanistic young observed during after one visit to each nest by the author. Al- the study. Bent (1937) found that .50 percent though identical nest observation procedures of the hawks of this species observed in North were conducted ag;iin in 1968, no nests were Dakota were melanistic. abandoned. This leads me to believe that the Territorial behavior appears to depend upon human interference factor was negligible both the mood of the hawks. Various raptors were years. often allowed to fly over the nest site through- Ferruginous Hawk pair #3 nested in the cen- out the nesting season, and during my visits ter of Cedar Valley atop the two foot high road- other Ferruginous Hawks were allowed to come bank of a well-traveled, improved gravel road. into the nesting territory to join the resident pair Two days after the initial discovery of the nest in attacking me. On other occasions, however, the female was found shot and killed at the any bird that ventured near the nesting area nest site. Both of her legs and five rectrices had was immediately attacked. been removed (Weston and Ellis, 1968). This Ferruginous Hawk food-habit data indicate is the only known instance of direct human inter- that mammals make up the bulk of the hawks' ference with nesting hawks during the study. diet, with kangaroo rats and black-tailed jack- Both members of each pair participated in rabbits comprising 74 percent of the total prey most nesting activities, exceptions being that consiuned. These findings seem to agree with only female hawks were observed incubating other researchers, for May ( 1935 ) found this eggs and feeding the young. species to eat primarily mammals, but observed Reproductive activity began with territory that in one area it was a conspicuous enemy of selection in early March and ended when the the California Clapper Rail (RaUiis lon^irostris). young were fledged in late June. The average Bent (19'37) indicates that its food consists al- incubation period was 32 days, which is 5 days most exclusively of mammals, ranging in size longer than the 27-day average recorded by from jackrabbits to meadow mice. Cameron Cameron (1914). (1914) reported that in eastern Montana this In 1968 nest *13, which contained a single hawk feeds chiefly upon prairie dogs and fifteen-day-old hawk, became too bulky for the meadow mice. He saw Ferruginous Hawks kill juniper in which it was located and fell from a jackrabbit which they could not carry away. the tree. The adult hawks quickly remedied However, during the present study several adult the situation by constructing a crude ground nest jackrabbits were observed in different nests.

directly under the original tree. The young hawk Hunting is usually conducted in this area be-

remained in this nest until it successfully fledged fore sunrise and after sunset of each day, in-

on June 24. Also of special interest in 1968 was dicating that the Ferruginous Hawk is primarily nest #11, constructed in a juniper by a melan- crepuscular in its hunting activities. This ex- istic male and a light-colored female. Four young plains whv a nocturnal animal such as the kang-

were fledged from the nest, three exhibiting aroo rat is so often captured as prey. LITERATURE CITED

Alcorn, J. R. 1942. Notes on the food of the Great . 1937. The Golden Eagle in San Diego Coun- Homed Owl near Fallon, Nev. Condor 44:284-285. ty, Califomia. Gondor 39:49-56. its Ellis, Arnold, L. W. 1954. The Golden Eagle and eco- D. H., D. G. Smith and J. R. Murphy. 1969. nomic status. U. S. Fish and Wildlife Serv. Gircular Studies on raptor mortahty in Western Utah. Gt.

No. 27:1-35. Basin Nat. 29( 3) : 165-167. Baird, S. F., T. M. Brewer and R. Ridgeway. 1874. Errington, Paul L. 1932a. Techniques of raptor North American birds. Little, Brown, and Go., food habits study. Condor 34:75-86.

3:300-304. . 1932b. Studies on the behavior of the Great Baumcartner, F. M. 1938. Gourtship and nesting of Homed Owl. Wilson Bull. 44:212-220.

the Bull. •, Breckenridge. habits Great Homed Owl. Wilson 50:274-285. and W. J. 1938. Food

. 19.39. Territory and population in the Great of Buteo Hawks in North-Central United States. Homed Owl. Auk 56:274-282. Wils. Bull. 50:113-121.

Bee, R. G. UnpubUshed field notes, 1932-59. Brig- , F. Hamerstrom, and F. H. Hamerstrom, Jr. ham Yoimg University Life Sciences Museum, 1940. The Great Homed Owl and its prey in Provo, Utali. North-central United States. Iowa State College

, and John Hutchincs. 1942. Breeding records Agri. Exp. Sta. Res. Bull. 277:757-850. of Utah birds. The Great Basin Naturalist 3:61-88. Fautin, R. W. 1946. Biotic communities of the north- Behle, W. H., a. M. Woodbury, and W. P. Gottam. ern desert shrub biome in Western Utah. Ecol. 1944. Further light on the fossil bird nests of the Monog. 16:251-310. Desert. Proc. Arts, Sci. 1958. re- Great Salt Lake Ut. Acad. Fevold, H. R. and J. J. Craighead. Food & Letters, 21:2.3. quirements of the Golden Eagle. Auk 75:312-317. Bent, A. C. 19.37. Life histories of Nortli American FiNLEY, W. L. 1906. The G

. 1938. Life histories of North American birds U. S. and their relation to agriculture. U. S. De-

of prey ( pt. 2). U. S. Natl. Mus. Bull. 170:1-482. partment of Agri., Division of Ornithology and Biggs, Agnes. 196.5. Are the roughlegs (B. regahs) Miunmalogy. Bull. 3:1-210. retuming? Cixn. Audubon 27(3):78-79. Fitch, H. S. 1940. Some observations on homed owl RiGBY, P. nests. Condor 42:7.3-75. BissELL, H. ]., J. K. D. Proctor, and R. W. Moyle. 1959. Geology of the southern Oquirrh . 1947. Predation bv owls in the Sierran foot- Califomia. 49:137-151. Mountains and tlie Five Mile Pass—northern Boul- hiUs of Condor ter Moimtain area, Tooele and Utah Gounties, Utah. Fuertes, Louis A. 1920. American birds of prey. Guidebook to the geology of Utah, No. 14:1-2.54. Little, Brown, and Co. pp. 176-178. Utali Geol. Soc. Gordon, S. P. 1927. Days with the Golden Eagle. London, Williams and Norgate. Bond, R. M. 1940. Food habits of homed owls in the Pahranagat Valley, Nev. Gondor 42:164-165. Grossman, Mary L. and John Hamlet. 1964. Birds of prey of the world. Clarkson N. Potter, Inc., Bowles, ]. H. 1931. The Ferruginous rough-leg in York. Washington. Murrelet 12:65-70. New Hamerstrom, Francis, and James D. Wea\'eh. 1967. Brown, D. L. and F. M. Galley. 19.50. Eagle-game Ageing and se.xing Rough-legged Hawks in Wiscon- relationships in Montana. Third Annual N. W. sin and lUinois. Unpublished paper. Wildlife Gonference. Hanna, W. C. 1930. Notes on the Golden Eagle in Brown, L., and D. Amadon. 1968. Eagles, Hawks, Southem Califomia. Condor 32( 2 ) : 121-123. and Falcons of the world. McGraw-Hill Book Go., Hardy, R. 1945. Breeding birds of tlie pigmy conifers New York. in the Book Cliffs of Eastem Utah. Auk 62:523- Bullock, K. G. 1959. Geology of Lake Mountain, 542. Utah. Utah Geol. & Mineral Surv.. Bull. 41:1-48. Hayward, C. L. 1967. Birds of the Upper Colorado Cameron, E. S. 190.5. Nesting of the Golden Eagle River Basin. BYU Sci. Bull., Biol. Series 9(2): in Montana. Auk 25(2) :158-167. 1-64. . 1907. Birds of Custer and Dawson Counties, Hobbie, E. and T. Cade. 1962. Observations on Montana. Auk 24:264. J. the breeding of Golden Eagles at Lake Peters in . 1908. Obser\'ations on the Golden Eagle in Northem Alaska. Condor 64(3) :2.35-237. Montana. Auk 25(3) :251 -268. Imler, Ralph H. and E. R. Kalmbach. 1955. The . 1914. The Ferruginous rough-leg in Montana. its status. Fish and Wild- Auk 31:159-167. Bald Eagle and economic life Service, Circular 30:1-51. Carnie, S. K. 1954, Food habits of Golden Eagles in 1964. breeding density of the the coast ranges of California. Condor 56:3-12. LocKiE, J. D. The Craighead, Golden Eagle and fo.\ in relation to food supply in John J. and Frank C. 1956. Hawks, Wester Ross, Scotland. Scottish Nat. 71:67-77. owls and wildlife. The Stackpole Co., Harrisburg.

, Ratcliffe. 1964. Insecticides and Penn. and D. a. Scottish eagles. Brit. Birds ,57(3) :89-101. Decroot, Dudley S. 1927. The California Clapper Rail: Its nesting habits, enemies and habitat. Gon- Mathiak, H. a. 1938. A key to hair of the miunmals Mgmt. 2:251-268. dor 29:2,59-270. of .southern Michigan. J. Wildl. May, B. 1935. The hawks of North America. Dixon, J. B. 1914. History of a pair of Pacific homed John owls. Condor 16:47-54. Nat. Aud. Soc, New York.

35 36 Brigham Young University Science Bulletin

MoGahan, J. 1967. Quantified estimates of predation Strandtmann, R. W. 1962. Notes on nest building

by a Golden Eagle population. ]. Wildlite Mgmt. and mating in the Golden Eagle. Southwestern 31 (3): 496-501. Nat. 7 (.3/4): 267-268.

. 1968. Ecology of the Golden Eagle Auk Sucden, ]. W. 1928. A nest site of the western 85(1):1-12. Horned Owl in Utah. Condor 30:324-325. Miller, Loye. 1930. The territorial concept in the Summer, E. L. 1929a. Golden Eagle in Death Valley. Great Homed Owl. Condor 32:290-291. Condor 31:127.

MuRiE, A. 1944. The wolves of Mount McKinley. . 1929b. Notes on tlie growth and behavior of Fauna of the Natl. Parks of the U. S. No. 5: 1-238. young Golden Eagles. Auk 46:161-169. P. Odum, Eugene and E. J. Kuenzler. 1955. Meas- TwoMEY, A. C. 1942. The birds of the Uinta Basin, urement of territory and home range size in birds. Utah. Ann. Carnegie Mus. 28:341-490. Auk 72:128-137. Walker, L. and M. Walker. 1939. From egg to

. 1959. Fundamentals of ecology. W. B. Saun- eaglehood. Natural History 43(5) :284-289, 302. ders, Philadelphia. Watson, A. 19.57. The breeding success of Golden Orians, G. and F. Kuhlman. 1956, Red-tailed Hawk Eagles in Northeast Highlands. Scot. Nat. 69:153- and horned owl populations in Wisconsin. Condor 169. 58:371-385. Wellein, E. G. and T. Ray. 1964. Eagle Investiga- RiDGWAY, R., S. F. Baihd, and T. M. Brewster. 1874. tion—New Mexico and Western Texas. MS, Wildl. North American landbirds, 3:66. Research Lab., Denver, Colorado. Salt, W. P. 1939. Notes on recoveries of banded Weston, John B. and David Ellis. 1968. Ground Ferruginous rough-legged Hawks. Bird Banding nesting of the Ferruginous Hawk in west-central 10:80-84. Utah. Gt. Ba,sin Naturalist 28(2): 111. Sandeman, p. W. 1957. The breeding success of Williams, Ralph B. and Clyde P. Matteson. 1947a. Golden Eagles in the Southern Grampians. Scot. Nat. 69:148-1.52. Wyoming hawks. Wyoming WikUife IK 4 ): 1.5-18.

. 1947b. Wyoming hawks. Wyoming Wildlife Shelford, V. E. 1963. The ecology of North America. 11(5):38. Univ. of Illinois Press, Urbana. . 1947c. Wyoming Hawks; Golden Eagle. Wy- Slevin, R. 1929. A contribution to our knowledge J. oming Wildlife 11(7): 16-19. 34-38. of the nesting habits of the Golden Eagle. Proc. CaUfomia Acad. Sci. 18:45-71. Wood, D. T. 1946. Eye witness account of Golden Sprunt, Alexander. 1955. North American birds of Eagle killing calf. Condor 48:143.

prey. Harper and Bros., New York. Woodbury, A. M., C. Cottam, and |. W. Sugden.

Stanford, J. S. 1931. Notes on hawks and owls in Annotated checklist of birds of Utah. Bull, of U. Sevier County. Utah. Auk 48:618-620. of Utah, 39(16): 1-40. INFORMATION FOR CONTRIBUTORS

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