(1SSN^0892-I01ft) The Journal OF Raptor Research Volume 33 June 1999 Number 2 Contents Survival and Movements of Immature Bald Eagles Fledged in Northern California. J. Mark Jenkins, Ronald E. Jackman and W. Grainger Hunt 81 Prey of Nesting Bald Eagles in Northern California. Ronald E. Jackman, w. Grainger Hunt, J. Mark Jenkins and Phillip J, Detrich 87 Bald Eagle Response to Boating Activity in Northcentral Florida. Petra BohallWood 97 The Golden Eagle {Aquila chrysaetos) in the Bale Mountains, Ethiopia. Michel Clouet, Claude Barrau and Jean-Louis Goar 102 Selection of Nest Cliffs byBonelli’s Eagle {Hieraaetus fasciatus) in Southeastern Spain. Diego Ontiveros 110 Effectiveness of Conservation Measures on Montagu’s Harriers in Agricultural Areas of Spain. C. Corbacho, J.M. Sanchez and a. Sanchez 117 Cooperative Hunting of Jackdaws by the Fanner Falcon {Falco biarmicus) . Giovanni Leonardi 123 Methods for Gender Determination of Crested Caracaras. Joan l. Morrison and Mary Maltbie 1 28 Why do Grass Owi.s (Tyto capensis) produce clicking calls? d. Crafford, j.w.h. Ferguson and A.C. Kemp 134 Diet Composition and Reproductive Success of Mexican Spotted Owls. Mark E. Seamans and RJ. Gutierrez 143 Philopatry and Nest Site Reuse by Burrowing Owls: Implications for Productivity, r. Scott Lutz and David L. Plumpton 149 Use of Raptor Models to Reduce Avian Collisions with Powerlines. Guyonne F.E. Janss, Alfonso Lazo and Miguel Ferrer 154 Short Communications Winter Diet of the Barn Owl {Tyto alba) and Long-eared Owl {Asiootus) in Northeastern Greece: A Comparison. Haralambos Alivizatos and Vassilis Goutner 160 SiBLICIDE, SpLAYED-TOES-FLIGHT DISPLAY, AND GRAPPLING IN THE Saker Falcon. David H. Ellis, Peter L. Whitlock, P. Tsengeg and R. Wayne Nelson 164 Improving the Success of a Mounted Great Horned Owl Lure for Trapping Northern Goshawks. Jon T. McCloskey and Sarah R. Dewey 168 Prey Size Matters at the Upper Tail of the Distribution: A Case Study in Northcentral Chile. David P. Santibahez and Fabian M. Jaksic 170 Spatial and Temporal Variations in the Diet of the Common Kestrel {Falco tinnunculus) in Urban Rome, Italy. Emanuele Piattella, Luca Salvati, Alberto Manganaro and Simone Fattorini 172 Letters 176 Book Review. Edited by Jeffrey S. Marks 178 The Raptor Research Foundation, Inc. gratefully acknowledges a grant and logistical support provided by Boise State University to assist in the publication of the journal. THE JOURNAL OF RAPTOR RESEARCH A QUARTERLY PUBLICATION OF THE RAPTOR RESEARCH FOUNDATION, INC. VoL. 33 June 1999 No. 2 J. Raptor Res. 33(2):81-86 © 1999 The Raptor Research Foundation, Inc. SURVIVAL AND MOVEMENTS OF IMMATURE BALD EAGLES FLEDGED IN NORTHERN CALIFORNIA J. Mark Jenkins Technical and Ecological Services, Pacific Gas and Electric Company, 3400 Crow Canyon Road, San Ramon, CA 94583 U.S.A. Ronald E. Jackman Garcia & Associates, P.O. Box 776, Pall River Mills, CA 96028 U.S.A. W. Grainger Hunt Predatory Bird Research Group, Long Marine Laboratory, University of California, Santa Cruz, CA 95064 U.S.A. Abstract. — ^We studied survival and movements of 13 radiotagged immature Bald Eagles (Haliaeetus leucocephalus) fledged in 1989 and 1990 from nests at Lake Britton in northcentral California. Initial observations were consistent with a previously-described postfledgling northward migration into Canada and Alaska. First-year eagles returned to northern California between January and May of the following year and moved extensively in the general region of northcentral California. Of the two cohorts, 10 birds were located within a year of fledging for a minimum first-year survivorship of 76.9%. Seven eagles returned to our study area. Five of these birds returned briefly to their natal territories. Three of 10 returning birds were not observed again in our study area but were recorded infrequently at distances of 50-190 km outside the study area. Two different movement patterns emerged within the 10 returning birds: five birds showed a high degree of affinity to the study area and five did not. In their second year of life, radiotagged immatures showed less affinity for our study area during late summer and fall. We could not determine if this disappearance indicated a regular or repeated migration, or merely an increased tendency to wander. Key Words: Bald Eagle, Haliaeetus leucocephalus; movements', survival; mortality; radiotelemetry; California. Sobrevivencia y movimientos de juveniles de aguilas calvas del norte de California Resumen. — Estudiamos la sobrevivencia y movimientos de 13 aguilas calvas juveniles {Haliaeetus leucoce- phalus) dotados de radiotransmisores nacidas en 1989 y 1990 en nidos del lago Britton en el centro-norte de California. Las observaciones iniciales fueron consistentes con la migraci6n norte previamente descrita hacia Canada y Alaska. El primer aho las aguilas regresaron al norte de California entre enero y mayo del ano siguiente y se movilizaron extensivamente en la region centro-norte de California. De las dos cohortes, 10 aves fueron localizadas al ano de haber nacido, para un minimo de sobrevivencia del 76.9%. Siete aguilas regresaron a nuestra ^ea de estudio. Cinco de estas aves regresaron brevemente a sus territorios de natalidad. Tres de 10 aves que regresaron no fueron observadas nuevamente en nuestra area de estudio pero si infrecuentemente a distancias entre 50-190 km por fuera del area de estudio. Dos patrones distintos de movimientos emergieron a partir de las 10 aves que regresaron: cinco aves mostraron un alta afinidad al area de estudio y cinco no. En el segundo afio de vida, los juveniles con radiotransmisores mostraron una afinidad menor al area de estudio durante el verano y el otoho. No pudimos determinar si esta ausencia indico un repetido patrdn de migracion o si era una tendencia a deambular. [Traduccion de Cesar Marquez] 81 82 Jenkins et al. VoL. 33, No. 2 Since Broley’s (1947) pioneering studies of ea- glet movements from Florida, various researchers have studied Bald Eagle {Haliaeetus leucocephalus) movements across North America (e.g., Southern 1963, Gerrard and Bortolotti 1988, McClelland et al. 1994). Studies of Bald Eagle movements have focused on migration of adults between breeding and wintering grounds (Gerrard et al. 1978, Hodg- es et al. 1987, McClelland et al. 1994), movements within breeding or wintering grounds (Buehler et al. 1991a, Gerrard et al. 1992, Garrett et al. 1993, Harmata and Stahlecker 1993) and movements of nestlings from their natal territories (Broley 1947, Gerrard et al. 1974, Harmata et al. 1985, Hunt et al. 1992, McClelland et al. 1996). Eagle movements may be affected by a wide variety of biotic and abi- otic factors, including the age of birds, the distri- bution and behavior of various prey species or the prevailing environmental conditions such as cli- mate, topography, and latitude. Breeding Bald Eagles in North America include resident and migratory populations, or a combi- nation, in which some birds are migratory and oth- ers remain on breeding territories in winter. New- ton (1979) believed that residency is the preferred condition when prevailing environmental condi- tions, principally food supply, allow for year-round occupancy of a nesting territory. Milder winter cli- mates in lower latitudes of North America appear to provide conditions necessary for residency, whereas harsh winters of northern latitudes induce breeding eagles to migrate south in search of de- pendable food supplies. Residency for breeding pairs appears the norm in California (Jenkins and Jackman 1993), southern Oregon, Florida and Chesapeake Bay, Maryland (Buehler et al. 1991a). Migratory breeding populations probably occur in most of the Canadian provinces (Gerrard et al. 1978, Gerrard and Hatch 1983) and Alaska. Sher- rod et al. (1976, Alaska) and Swenson et al. (1986, Greater Yellowstone) provided two examples where some breeding eagles move in winter and others do not. Available information now suggests, in general, that eaglets that hatch in the southern latitudes of North America migrate north, while those that hatch in northern latitudes migrate south. Broley (1947) first discovered a northward migration of Bald Eagle fledglings from their Florida nest sites. Immature Bald Eagles from Saskatchewan migrate south and move throughout the midwestern U.S. (Gerrard et al. 1974). Bald Eagles hatched in Maine similarly moved south down the Atlantic sea- board (McCollough 1986). Five fledglings followed from our study area in northern California in the mid-1980s all migrated northward, and four of five continued to British Columbia or southeast Alaska (Hunt et al. 1992). Mabie et al. (1994) also re- ported a northern postfledging dispersal pattern of fledgling Bald Eagles from nests in Texas. Broley (1947) first suggested that these northward migra- tions allowed eaglets to reach runs of anadromous fish in rivers of the northern portion of the con- tinent in summer and early fall. In 1989 and 1990, we monitored the movements of two cohorts of six and seven nestlings, respec- tively, in our northern California study area. We assumed that these eaglets would undertake the previously discovered northward migration (Hunt et al. 1992) and made no attempt to follow them after their initial migration. Our objectives were to locate these birds following their return from their northern migration, determine first year survivor- ship and monitor movements into their second year of life. Study Area and Methods The Pit River originates in Modoc County, drains much of northeastern California and is a major tributary of the Sacramento River system. The Pit River Study Area (PRSA) consists of 78 km of the Pit River in Shasta Coun- ty. Lake Britton is the system’s largest reservoir; it is ap- proximately 13-km long and less than 1-km wide in most places, and has a surface area of approximately 520 ha. Lake Britton supported six occupied Bald Eagle nesting territories during the study period. Three additional small reservoirs, all less than 50 ha in surface area, are found downstream from Lake Britton; four Bald Eagle nesting territories occurred at these reservoirs. Our study area included an intergradation of habitat types characteristic of Cascade and Sierra Nevada moun- tain regions. The area around Lake Britton is dominated by ponderosa pine {Pinus ponderosa) forest, which oc- curred as open stands :S70 m in height (Holland 1986). Downstream from Lake Britton, the Pit River canyon, in- cluding the three downstream reservoirs, was dominated by Sierran mixed coniferous forest. This habitat was sim- ilar to ponderosa pine forest, but was denser, often slight- ly taller (75 m), and composed of several dominant spe- cies, including ponderosa pine, Douglas-fir {Pseudotsuga menziesii), incense-cedar (Libocedrus decurrens) and sugar pine {Pinus lambertiana) . Nestlings were radiotagged in 1989 and 1990 backpack style with teflon ribbons over and under the wings, se- cured on the breast with one or more stitches of cotton thread. The thread was designed to eventually deterio- rate, allowing the transmitter package to fall off in 3-5 yr. Transmitters weighed 65 g with a battery life expec- tancy of approximately 1000 d. All 13 nestlings were tagged in nests at Lake Britton at 8-10 wk of age. All June 1999 Bald Eagle Movements 83 Table 1. Movements of 10 Bald Eagles radiotagged as nestlings at Lake Britton in 1989 and 1990. Bird Date Radiotagged AS Nestling Date of First Detection"* Total Number of Detections Number of Detections in Study Area Greatest Distance ( km) From Study Area JM25'’ 19 May 89 6 Feb. 90 17 13 105NE JM26 4 June 89 19 April 90 21 20 llONW JF27" 4 June 89 21 Feb. 90 2 1 115N JM28 10 June 89 20 Feb. 90 25 24 SOS JM30 11 June 89 29 March 91 1 0 130NE JF31 24 May 90 8 March 91 11 6 160S JF32 24 May 90 8 Feb. 91 2 0 90SW JM34 1 June 90 5 Jan. 91 2 0 190NE JF36 7 June 90 29 March 91 2 1 135SW JM37 9 June 90 23 May 91 38 25 50SW “ Eollowing initial migration. ^ Male. Female. eaglets were banded with standard USGS aluminum leg bands. Birds were sexed on the basis of morphometric measurements (Bortolotti 1984, Garcelon et al. 1985). Radiotagged eagles were monitored weekly in the study area with a scanning receiver and hand-held two- and three-element Yagi antennae. Two-element antennae were mounted on the wing struts of a fixed-wing aircraft for covering larger geographic areas. We conducted weekly helicopter surveys from March 1983-December 1984 (Jenkins 1992) and again from July 1987-July 1991. The age of each observed eagle was classified as adult, near-adult, subadult or juvenile following age class de- scriptions of McCollough (1989). To assist in data inter- pretation, birds not appearing in adult plumage were grouped in a category called nonadults. In addition to weekly helicopter surveys, fixed-wing aircraft surveys were conducted in 1989-92 at about monthly intervals over northern California and southern Oregon, outside the Pit River study area. Results Of the six fledglings radiotagged in 1989 at Lake Britton, four birds were located the following year and a fifth bird was located in 1991. Seven addi- tional fledglings were radiotagged in 1990 and five of these were located in 1991. The survival rate was 76.9% (10 of 13) for the first year of life. Our sur- vival rate is a minimum estimate, because it ignores possible transmitter loss or failure, the possibility that fledglings were missed on aerial surveys over northern California and southern Oregon or that some eagles never returned to the region. All 13 radiotagged juvenile eagles departed the study area by 1 September of the fledging year. Of the four immature eagles from the 1989 cohort re- located in northern California in 1990, three were first located in February and one in April (Table 1 ) . A fifth bird from this cohort was first located in March 1991. Five of the seven nestlings from the 1990 cohort were subsequently located in 1991. The first of these birds was found in southern Oregon on 5 January 1991. Others from the 1990 cohort were first located in February, March, and May 1991 (Table 1). Returning birds wandered throughout our study area, northern California and southern Oregon during the subsequent mon- itoring period (Fig. 1). Seven of the 10 surviving ieagles actually (both cohorts) returned to parts of the PRSA. Five of these birds returned briefly to their natal territo- ries and other locations on Lake Britton. Two of seven birds that returned to the PRSA also subse- quently wandered distances over 100 km outside the PRSA (Table 1). Three juveniles that were fre- quently recorded in the PRSA after their initial mi- gration disappeared for a time in the summer and fall of their second year of life but were recorded again in the PRSA a few months later. Three sur- viving fledglings were not recorded in the PRSA despite weekly helicopter and periodic ground sur- veys. These three birds were recorded infrequently at distances of 50-190 km from the PRSA. In pre- vious studies, we recorded a marked juvenile Bald Eagle from the PRSA subsequently establishing a nesting territory. This eaglet, originally banded in our study area in 1983 at Lake Britton, later was trapped as a breeding adult on Shasta Lake in 1990, a distance of about 55 km southwest of Lake 84 Jenkins et al. VoL. 33, No. 2 Figure 1. Locations throughout northern California of 10 radiotagged Bald Eagle nestlings fledged from nests at Lake Britton in 1989 and 1990. Britton (Jenkins 1992). This is our only record of future breeding by a Bald Eagle fledged from our study area. Based on weekly helicopter surveys, the total number of Bald Eagles recorded in nonadult plumage in the PRSA declined markedly during the late summer and fall but increased again be- ginning in December (Fig. 2) . Three immature ea- gles radiotagged from PRSA nests visited the Klam- ath Basin in their second winter, about 100 air km from the PRSA. This area is one of the largest win- tering congregations of Bald Eagles in the lower 48 states, supporting hundreds of migrant Bald Eagles which feed on migrating waterfowl. Discussion McCollough (1986) estimated a minimum sur- vival of first-year Bald Eagles in Maine of 54%, and a 73% survival for first-year birds when artificial feeding was provided. Gerrard et al. (1978) re- ported 37% first-year survival for 43 Bald Eagles wing-marked as juveniles in Saskatchewan. Sherrod et al. (1976) estimated that fewer than 10% of fledglings survived to breeding age on Amchitka Island, Alaska, and suggested that about 5.4% of the adult population died each year. Buehler et al. (1991b) recently estimated 100% survival for 39 ra- diotagged Bald Eagles through their first year of life in the Chesapeake Bay area. Similarly, Me- June 1999 Bald Eagle Movements 85 Month (a) Number of flights Figure 2. Mean number (±SE) of nonadult (immature, subadult and near adult) Bald Eagles recorded in weekly helicopter surveys of the PRSA, 1983-84 and 1987-91, shown by month. Clelland et al. (1996) reported 10 of 11 (91%) ju- venile Bald Eagles fledged from nests at Glacier National Park, Montana, surviving their first win- ter. Our estimated survival rate of 76.9% (10 of 13) suggested a high degree of juvenile survival for the PRSA Bald Eagle population. Our survival estimate was consistent with the present growth of the breeding population of Bald Eagles in California. A population model recently reported by Jenkins (1996) using this value and an empirically-derived annual adult survival value of 94.6% indicates a 6— 7% increase in the California breeding population, which is consistent with observed population growth during the past 15 yr (Jenkins et al. 1994). It was unclear whether subadult eagles migrated like fledglings in their second and subsequent years. The number of nonadult birds in the PRSA was low from June through late summer and fall. It seemed likely that subadults had less affinity to the PRSA at this time of year, but it was not clear if this involved a regular and repeated migration or simply an increased tendency to wander. The fact that radiotagged eagles were not detected dur- ing fixed-wing aircraft surveys of larger areas of northern California, suggested that immature ea- gles undertopk extensive movements during this period. Our data indicated the movements of immature Bald Eagles were highly nomadic and variable with only some fledglings returning to their natal areas. The tendency of some birds to concentrate their movements around PRSA may have resulted from a sampling bias due to increased monitoring in the PRSA; subadult movements may have been even more extensive than indicated by our data. Exten- sive movements give subadults an opportunity to visit various water bodies across northern Califor- nia and familiarize themselves with other breeding territories and potential habitat throughout the re- gion. Acknowledgments We thank the Pacific Gas and Electric Company for sponsoring long-term Bald Eagle research in the Pit River drainage from which these data derive. D. Driscoll and G. Beatty assisted with trapping and radiotagging eagles J.R. Smith assisted in fixed-wing aerial surveys and B. Shandley piloted the helicopter surveys. We thank the U.S. Fish and Wildlife Service and the California Depart- ment of Fish and Game for issuing permits to conduct these studies, and the California Bald Eagle Working Team for its support. Our paper was improved by com- ments on earlier drafts by D.W. Anderson, A.R. Harmata, B.R. McClelland, C. Van Riper, K. Steenhof and C.M White. Literature Cited Bortolotti, G.R. 1984. Sexual size dimorphism and age- related size variation in Bald Eagles. J. Wildl. Manage 48:72-81. Broley, C.L. 1947. Migration and nesting of Florida Bald Eagles. Wilson Bull. 59:1-68. Buehler, D.A., T.J. Mersmann, J.D. Fraser and J.K.D Seegar. 1991a. Differences in distribution of breed- 86 Jenkins et al. VoL. 33, No. 2 ing, nonbreeding and migrant Bald Eagles of the northern Chesapeake Bay. Con5 kg) were assigned an arbitrary estimate of 2.5 kg biomass contribution, assuming that eagles obtained only a portion of each carcass. June 1999 California Bald Eagle Prey 89 C n [ G G N / Eureka \ \ V / ■c c-j / \ \ '■ .I'.' I •a •• •■ » Redding \\i iJ*" ' . . j ** ' ' ' ^ ' .. rf ^Atturas ✓ • 4 HRS y ' - \ -. r'.v - "i- . . : • .!•- • Ur.. N 7' " v<- l.t- ‘ ' ■ ’ » ‘”f b‘/.' • SusanviHe » ' *♦' T- .1 -.1 JO » 4 iV 'V V • _J* ^ : s' ■ ' ' ' L, s’ »■ •G- ■ * b‘‘. V 1 C. 1 h ■> Sj - ★ O I A N 40 Km Sacramento I Figure 1. Location of the study area in northern California. Lakes indicated in parentheses were not included in the study. Our analysis of prey use was biased in that it was based exclusively on prey remains. Previous studies comparing the analysis of Bald Eagle prey remains with observations of prey deliveries to the nest (Todd et al. 1982, Dugoni et al. 1986, Knight et al. 1990, Hunt et al. 1992a, 1992c, Grubb 1995) indicated that while prey remains tend to show all taxa used by eagles, in most cases small, soft- boned fish (e.g., trout) were underrepresented, and large, bony fish (e.g., carp and catfish) and birds were generally overrepresented in remains. The fish scale col- lections from nest linings helped mitigate this potential bias. With a few exceptions and catfish which have no scales, the relative number of scales found in nests re- flected our fish bone analysis (i.e., large numbers of scales accompanied large numbers of conspecific bones) . Results Diet. We identified 2351 individual prey items representing 1637 kg of biomass from 56 nesting territories in our study area (Table 1) . Nesting Bald Eagles utilized 20 species of fish, 41 bird, 15 mam- 90 Jackman et al. VoL. 33, No. 2 Table 1 . Number of individuals and estimated biomass (kg) of prey identified from remains collected in and below 56 Bald Eagle nests in northern California from 1983-92. Species Number (%) Biomass (%) FISH (Osteichthyes) Brown bullhead 817 (34.8) 214.2 (13.1) {Ameiurus nebulosus)^ Sacramento sucker 285 (12.1) 290.3 (17.7) ( Catostomus ocddentalis) Common carp {Cyprinus carpio)^ 122 (5.2) 368.9 (22.5) Tui chub {Gila bicolor)'^ 110 (4.7) 57.5 (3.5) Hardhead 80 (3.4) 48.1 (2.9) {Mylopharodon conocephalus) Trout (Salmonidae) 32 (1.4) 15.5 (0.9) Sacramento squawfish 30 (1.3) 25.9 (1.6) {Ptychocheilus grandisY Channel catfish 21 (0.9) 17.8 (1.1) {Ictalurus punctatus)'^ Crappie {Pomoxis spp.Y 19 (0.8) 3.6 (0.2) Tule perch {Hysterocarpus traski) 15 (0.6) 0.6 (trace) Sacramento blackfish 14 (0.6) 26.5 (1.6) ( Orthodon microlepidotus) Rainbow trout {Oncorhynchus mykiss) 14 (0.6) 8.1 (0.5) Largemouth bass 6 (0.3) 4.1 (0.2) {Micropterus salmoidesY Sacramento perch 5 (0.2) 6.6 (0.4) {Archoplites interruptusY Tahoe sucker {Catostomus tahoensis) 5 (0.2) 6.5 (0.4) Other sunfish (Centrarchidae)"^’^ 61 (2.6) 38.7 (2.4) Other catfish (Ictaluridae)'^’^ 46 (2.0) 13.1 (0.8) Unidentified minnows (Cyprinidae) 30 (1.3) 9.9 (0.6) Unidentified trout/salmon 18 (0.8) 4.1 (0.2) (Salmonidae) Unident, suckers (Catostomidae) 5 (0.2) 4.0 (0.2) Other fishs 3 (0.1) 1.5 (0.1) Subtotal fish 1738 (73.9) 1165.5 (71.2) BIRDS (Aves) American Coot {Fulica americana) 120 (5.1) 69.3 (4.2) Mallard {Anas platyrhynchos) 53 (2.3) 51.6 (3.2) Western Grebe 25 (1.1) 33.2 (2.0) {Aechmophorus ocddentalis) Mountain Quail {Oreortyx pictus) 21 (0.9) 4.4 (0.3) American Wigeon {Anas americana) 15 (0.6) 10.2 (0.6) Northern Pintail {Anas acuta) 14 (0.6) 12.7 (0.8) Gull (LarM5spp.)^ 17 (0.7) 7.7 (0.5) Northern Shoveler {Anas clypeata) 12 (0.5) 6.6 (0.4) Western Meadowlark 12 (0.5) 1.1 (0.1) {Sturnella neglecta) Pied-billed Grebe 11 (0.5) 4.4 (0.3) {Podilymbus podiceps) Cinnamon Teal {Anas cyanoptera) 11 (0.5) 3.8 (0.2) Eared Grebe {Podiceps nigricollis) 10 (0.4) 2.7 (0.2) Ruddy Duck {Oxyura jamaicensis) 9 (0.4) 4.4 (0.3) Gadwall {Anas strepera) 8 (0.3) 6.6 (0.4) June 1999 Caufornia Bald Eagle Prey 91 Table 1. Continued. Species Number (%) Biomass (%) Band-tailed Pigeon 8 (0.3) 2.8 (0.2) (Columba fasciata) Canada Goose (Branta canadensis) V (0.3) 21.2 (1.3) Common Merganser (Mergus merganser) 7 (0.3) 9.3 (0.6) Other puddle ducks (Anatinae)* 52 (2.2) 32.6 (2.0) Unident, grebes (Podicipedidae) 26 (1.1) 17.3 (1.1) Other diving ducks (Aythyinae)J 21 (0.9) 15.2 (0.9) Other perching birds 15 (0.6) 2.5 (0.2) (Passeriformes) Other Anatidae^ 10 (0.4) 18.7 (1.1) Other birds'" 39 (1.7) 34.5 (2.1) Subtotal Birds 523 (22.3) 372.8 (22.8) MAMMALS (Mammalia) Muskrat ( Ondatra zibethicus) 13 (0.6) 13.7 (0.8) Mule deer (Odocoileus hemionus) 10 (0.4) 25.0 (1.5) Ground squirrels 10 (0.4) 2.7 (0.2) (Spermophilus spp.)" Rabbits (Leporidae)° 8 (0.3) 9.4 (0.6) Western gray squirrel 7 (0.3) 5.4 (0.3) (Sciurus griseus) Other SciuridaeP 15 (0.6) 11.9 (0.7) Other mammals'! 25 (1.1) 30.6 (1.9) Subtotal Mammals 88 (3.7) 98.7 (6.0) REPTILES (Reptilia) Western pond turtle 1 (trace) 0.2 (trace) (Clemmys marmorata) INVERTEBRATES Crayfish (Crustacea) 1 (trace) 0.1 (trace) GRAND TOTAL 2351 (100.0) 1637.3 (100.0) “ Introduced fish species in California. Native to California, introduced into Almanor and Mtn. Meadows reservoirs. Native to California, introduced into Eel River/Pillsbury Reservoir. Native to California, introduced into Almanor Reservoir. 45 unidentified, 10 bass {Micrapterus , 4 sunfish (Lepomiss.Yi'p.Y, 1 smallmouth bass {Micropterus dolomieuiY 2 avd 1 bluegill {Lepomis macro chirus) ^20 bullheads (Ameiurus spp.)®, 2 white catfish {Ameiurus catusY and 24 unidentified, s 1 American shad (Alosa sapidissima)^, 1 golden shiner {Notemigonus chrysoleucas) and 1 unidentified fish. ^ Includes at least 2 California Gull (Larus californicus) and 2 Ring-billed Gull (Larus delawarensis) . ‘ 4 Green-winged Teal (Anas crecca), 2 Wood Duck (Aix sponsa) and 46 unidentified. j 3 Common Goldeneye (Bucephala clangula), 3 Scanp (Aythyaspp.) , 2 Ring-necked Duck (Aythya collaris), 1 Redhead (Aythya americana), 1 Bufflehead (Bucephala albeola) and 11 nnidentified. 3 Steller’s Jay (Cyanocitta stelleri), 2 Black-billed Magpie (Pica pica), 1 Common Raven (Corvus corax), 1 American Crow (Corvus brachyrhynchos) , 1 blackbird (Emberizidae) and 7 unidentified. *2 Snow Goose (Chen caerulescens) , 1 Tnndra Swan (Cygnus columbianus) , 1 Greater White-fronted Goose (Anser albifrons), 1 goose (Anserinae) and 5 unidentified. 4 Double-crested Cormorant (Phalacrocorax auritus) , 2 Ring-necked Pheasant (Phasianus colchicus) , 2 Western Screech-Owl ( Otus kennicottii) , 2 Belted Kingfisher (Ceryle alcyon), 2 Northern Flicker (Colaptes auratus), 2 Acorn Woodpecker (Melanerpes formicivorus) , 1 Great Blue Heron (Ardea herodias), 1 Rock Dove (Columba livia), 1 pigeon (Columbidae) , and 22 unidentified. "At least 2 California ground squirrel (Spermophilus beecheyi) and 1 Belding’s ground squirrel (Spermophilus beldingi). ° 2 jackrabbits (Lepusspp.), 1 black-tailed jackrabbit (Lepus californicus) and 5 nnidentified. P 2 yellow-bellied marmot (Marmota flaviventris) , 1 chipmunk (Tamias %p.) and 12 unidentified. “ 1 4 rodents (Rodentia) , 3 voles (Microtus spp.) , 2 raccoons (Procyon lotor) , 2 nngulates (Artiodactyla) , 1 domestic cow (Bos taurus) , 1 striped skunk (Mephitis mephitis), 1 western spotted skunk (Spilogale gracilis), 1 broad-footed mole (Scapanus latimanus) and 10 unidentified. 92 Jackman et al. VoL. 33, No. 2 Table 2. Mean standard length of fish species commonly selected as prey by nesting Bald Eagles in northern Cali- fornia as measured from prey remains collected at nests. Species N Mean Standard Length (mm) Range (mm) SE Trout 18 321 185-498 24 Common carp 85 510 244-854 13 Hardhead 64 330 194-527 8 Sacramento squawfish 28 418 278-631 14 Tui chub 98 282 180-341 4 Sacramento sucker 228 392 131-587 4 Channel catfish 17 368 251-551 24 Brown bullhead 456 240 129-356 2 mal and one each of reptile and invertebrate spe- cies. Fish accounted for >70% of overall prey num- bers and biomass, while birds contributed approximately 20% and mammals <10% to both number and biomass totals. Mean standard lengths of most commonly taken prey fishes were greater than 300 mm, except for tui chub and brown bull- head (Table 2). Common carp showed both the greatest mean length and the widest range of lengths. Regional Differences in Prey Utilization. Bald Eagle food habits varied widely between drainage and habitat groups (Table 3). Both the numbers and biomass of fish (x^ = 383.3, df = 18, P < 0.001; = 415.8, df = 18, P < 0.001), birds (x^ = 306.2, df = 18, P < 0.001; x^ = 283.8, df = 18, P < 0.001), and mammals (x^ = 77,6, df = 18, P < 0.001; x^ = 105.3, df = 18, P < 0.001) differed between 19 study locations as grouped in Table 3. Overall, fish dominated the diet (>50% of biomass and prey numbers) at most locations. Exceptions included Basin Reservoirs, Trout-managed reser- voirs and Lost River, where birds and, to a lesser extent mammals, exceeded fish as prey. Anatids were most prevalent in bird remains at the majority of sites; however, American Coots were more abundant in remains collected at the Lost River sites, Almanor, Butt Valley, and Pillsbury res- ervoirs. Gulls (Laridae) and grebes (Podicipedi- dae) were the predominant avian prey at Union Valley Reservoir and Lahontan sites, respectively. All Bald Eagles nesting along the Pit River relied primarily on native Sacramento suckers (31-55% of prey biomass and 18-42% of prey numbers at all sites). Introduced ictalurids (catfish) were im- portant only at Fall River Valley and Britton Res- ervoir nests (25% and 17% prey numbers, 14% and 8% biomass, respectively). Native cyprinids were important at all Pit River sites (17-22% prey numbers, 12-19% biomass) except Baum Reser- voir (5% prey numbers, 4% biomass). Tui chub were the predominant native minnow taken in the Fall River Valley, and we found mostly hardhead in remains of cyprinids from nests along the rest of the Pit River drainage. Native Sacramento blackfish and introduced common carp (38% and 34% biomass, 26% and 21% prey numbers, respectively) were the primary prey fish species of eagles at Shasta Reservoir. At the inflow of the North Fork Feather River to Oro- ville Reservoir, one eagle pair captured relatively large numbers of another native cyprinid, Sacra- mento squawfish (18% hiomass, 15% prey num- bers), although catfish were their primary prey (34% biomass, 37% prey numbers). A diversity of mosdy introduced fish species populate both Shas- ta and Oroville Reservoirs. Eagles nesting at reservoirs along all portions of the Feather River relied heavily on catfish (36-87% of prey individuals, 5-73% biomass for all sites). Common carp were the primary prey at Butt Valley Reservoir (86% biomass, 38% prey numbers), where catfish numbers were high (36% prey num- bers), but their biomass (5%) was unimportant by comparison. With the exception of the Oroville Reservoir pair mentioned above. Feather River ea- gles captured very few native fishes. Although pres- ent throughout the Feather River system, Sacra- mento suckers were taken rarely, except at Oroville Reservoir (15% biomass and 14% prey numbers). June 1999 California Bald Eagle Prey 93 Table 3. Percent biomass of major prey groups and total number of prey items utilized by California breeding Bald Eagles as calculated from analysis of prey remains for 19 waterway territory groups. Waterwa\5 (N territories) Troli Carp Min- nows Suckers Cat- fish Sunfish Other Fish Fish Total Birds Mam- mals N Pit R.: Fall River Valley (3)^ 1.5 3.3 12.4 30.5 13.9 4.4 0.0 66.0 25.1 8.9 178 Pit R.: Baum Res. /Hat Cr. (1) 9.0 0.0 3.5 54.5 3.4 0.0 0.0 70.4 20.6 9.0 36 Pit R.: Britton Reservoir (6) 0.9 5.0 18.8 49.4 8.2 1.6 0.4 84.3 11.2 4.5 414 Pit R.: Pit 4, 5, 6 res. (4) 2.2 7.5 15.0 46.0 0.0 0.3 0.1 71.3 22.0 6.7 121 Shasta Reservoir (6)’’ 0.0 34.2 37.7 0.7 7.9 2.6 0.0 83.0 13.5 3.5 58 NFFR'=: Oroville Reservoir (1) 1.3 9.1 18.3 14.9 34.4 5.1 0.0 83.1 16.9 0.0 52 NFFR; Mtn. Meadows Res. (2) 0.1 0.0 0.9 0.1 73.0 2.1 0.0 76.3 22.1 1.6 316 NFFR; Almanor Reservoir (4) 1.3 10.3 13.0 7.7 34.6 10.8 0.0 77.8 18.8 3.4 182 NFFR: Butt Valley Res. (2) 1.0 85.5 0.1 1.8 5.0 0.8 0.0 94.2 5.2 0.6 155 NFFR: East Fork (3)^ 0.0 20.4 0.0 9.7 34.5 6.5 0.2 71.3 28.7 0.0 97 Middle Fork Feather R. (3)*^ 1.1 19.3 0.0 0.0 18.7 6.5 0.0 45.6 41.8 12.6 267 South Fork Feather R. (1)^ 0.0 41.2 9.1 0.0 15.9 0.0 0.0 66.1 33.9 0.0 22 American River (l)s 0.6 0.0 0.0 47.8 0.0 17.6 0.0 66.0 27.0 7.0 28 Eel River/Pillsbury Res. (1) 1.9 3.6 31.6 27.9 0.0 18.2 0.0 83.1 7.4 9.5 37 Lahontan System (5)^ 4.1 0.0 35.7 9.8 3.4 1.4 0.0 54.4 42.4 3.2 97 Trinity R./ Clair Engle Res. (2) 1.8 0.0 0.0 9.1 57.6 10.4 0.0 78.9 18.8 2.3 72 Lost River (2)' 0.0 0.0 0.7 3.7 0.3 0.0 0.0 4.7 70.6 24.7 42 Basin Reservoirs (5)J 1.4 0.0 0.0 0.0 16.9 1.9 0.0 20.2 62.0 17.8 72 Trout-managed res. (4)*^ 9.8 0.0 0.3 4.4 0.0 0.2 0.9 15.6 63.0 21.4 103 All Sites (57) 1.7 22.5 10.3 18.4 15.0 3.2 0.1 71.2 22.8 6.0 2349 ^ Fall R., Tule R., Big Lk., Fall R. Res.; Sacramento R., McCloud R., Pit R.; North Fork Feather R.; Snake Lk., Round Valley Res I, Antelope Res.; Davis Res., Frenchman Res.; Little Grass Valley Res.; s Union Valley Res.; ^ Eagle Lk., Stampede Res.; ‘ Clear Lk Res., Willow Cr., Lower Klamath Lk., Tule Lk.; J Orr Lk., Res. F, Littie Egg Lk., Round Valley Res. II, McCoy Flat Res.; ’‘McCloud Res., Iron Canyon Res., Macumber Res., Bucks Lk. The most common cyprinid taken by eagles at Al- manor Reservoir was the tui chub (13% biomass, 12% prey numbers), a species native to most areas and introduced into the reservoir. Sacramento sucker was a primary prey of Bald Eagles nesting on two reservoirs along the Ameri- can and Eel rivers: Union Valley and Pillsbury Res- ervoirs (48% and 28% biomass, 36% and 30% prey numbers, respectively). Sacramento squawfish, in- troduced into the Eel River, was also a primary prey fish at Pillsbury Reservoir (32% biomass, 35% prey numbers). Our examination of scales in nest lin- ings suggested that both eagle pairs took more sal- monids than indicated by the bone samples. Eagles at both reservoirs captured more centrarchids (sunfish) than other California eagles (both 18% biomass and 14% numbers). All centrarchids iden- tified in this study were introduced; the only sun- fish native to California and taken by eagles, the Sacramento perch, was planted in Almanor Res- ervoir. Within the Lahontan system, our studies focused principally on Eagle Lake (Table 3) . The native tui chub was the primary prey of Lahontan eagles (36% biomass, 44% of prey individuals), and we found some use of native Tahoe suckers (10% bio- mass, 5% prey numbers). Birds, especially grebes, were also important (42% biomass, 41% numbers). At Clair Engle Reservoir on the Trinity River, we identified catfish as the primary prey of nesting Bald Eagles (58% biomass, 72% prey numbers). Salmonids and centrarchids were probably under- estimated in our analysis as evidenced by the rela- tively high numbers of their scales found in nest linings. Prey remains and scale samples indicated no use of native suckers. In the Klamath Basin, Lost River Bald Eagles re- lied mosdy on birds (i.e., American Coots; 71% 94 Jackman et al. VoL. 33, No. 2 biomass, 76% of prey numbers) and mammals (25% biomass, 12% prey numbers). Fish, including native suckers and tui chub, were used infrequentiy (5% biomass, 12% numbers). Like the eagles at Lost River, those nesting at Basin and Trout-managed reservoirs also captured many birds, mostly anatids (62% and 63% biomass, 53% and 57% prey numbers, respectively) and mammals. In addition. Basin and Trout-managed reservoir eagles exploited catfish (brown bullhead, 17% biomass and 33% numbers) and salmonids, respectively. Statewide, salmonids were important prey to Bald Eagles only at these Trout-managed reservoirs (18% prey numbers, 10% biomass) and at Baum Lake near a trout hatchery (14% num- bers, 9% biomass). Also, salmonid use by eagles was likely even greater at these reservoirs, judging from the large number of scales collected. Bald Ea- gles captured hatchery-released trout, as evidenced by tag recoveries at Macumber Reservoir and Eagle Lake nests. Prey Selection and Eagle Productivity. To evalu- ate the relative ecological benefit to eagles of ex- ploiting mostly native versus mostly introduced prey fishes, we compared the productivity of all Bald Eagle pairs nesting on the Pit River with that of eagles nesting on the Feather River. Prey selec- tion of fish groups differed significantly between drainages (x^ = 633.8, df = 5, P< 0.001): Pit River eagles consumed mostly native suckers and native cyprinids while pairs on the Feather River took mostly introduced catfishes and carp. Mean pro- ductivity during 1983-92 on the Pit River (x = 0.93 young/occupied year, N = 121 occupied years in 14 territories, SE = 0.08) was nearly identical to that on the Feather River (x = 0.95 young/occu- pied year, = 112 occupied years in 16 territories, SE = 0.08; t = 0.11, df = 231, P = 0.46). The annual success rates (successful years/occupied years, 1983-92) for the two areas were also similar: 55% {N — 121, SE 5%) on the Pit River and 60% (N = 112, SE = 5%; / = 0.68, df = 231, P = 0.25) on the Feather River. Discussion Fisheries. Like other populations of Bald Eagles (Swenson et al. 1986, Hunt et al. 1992a), those nesting in northern California exhibited a high de- gree of versatility in exploiting prey types that var- ied within and between drainage systems. Several studies have positively correlated the abundance of fish (measured by gill-netting) in open water hab- itats (i.e., estuary, reservoir and natural lake) with the diets of Bald Eagles (Gerrard and Bortolotti 1988, Mersmann 1989, Vondracek et al. 1989, Hunt et al. 1992c). We, however, did not sample fish pop- ulations for this study. Fish predominated in the diets of eagles at most locations, except in the Lost River area and at Basin and Trout-managed reser- voirs. These reservoirs tended to be isolated from major river drainages and, hence, from large stand- ing populations of fish. Many Basin Reservoirs pe- riodically underwent drawdowns (e.g., from drought) with a resultant loss of fish. Past irriga- tion farming practices adversely affected native sucker populations in the Lost River system (Moyle 1976a). At Trout-managed reservoirs, salmonid populations were usually nonsustaining and were stocked annually. In these areas with depleted or unstable fish populations, eagles relied more on birds. Overall, native fishes contributed substantially to the diets of California’s Bald Eagles where exotics were absent (e.g., Eagle Lake), within regulated and unregulated riverine habitats (Hunt et al. 1992c) , and where circumstances or adaptations al- lowed native fish to compete with introduced spe- cies within reservoirs. For example, pool fluctua- tions and low retention time due to hydroelectric operations of the relatively narrow Pit River reser- voirs discourage spawning success and skew opti- mum temperatures for introduced centrarchids that prey on native fishes (Vondracek et al. 1989). By interviewing local fisheries biologists, we learned that native prey fish populations were rel- atively uncommon in reservoirs where eagles cap- tured mostly exotics (R. Decoto, R. Flint, P. Chap- pel pers. comm.). Our research does not reveal how habitat modifications and reductions in native fish populations have affected Bald Eagles in a his- torical perspective. Bald Eagles in California readily exploit the large populations of introduced fishes in reser- voirs, and, indeed, most pairs (74%) are associated with reservoirs (Detrich 1989). Despite the destruc- tive effects of carp on aquatic systems (Moyle et al. 1987), the species provided the greatest caloric contribution to breeding eagles overall, although they were not found as prey at many locations. In- troduced species can fill human-created niches un- suitable for native fishes and thus provide prey for eagles in otherwise unsuitable habitat. For exam- ple, bullheads endure low temperatures and re- duced oxygen conditions associated with low water June 1999 California Bald Eagle Prey 95 levels (Moyle 1976a), an adaptation allowing them to persist in intermittent or widely fluctuating wa- ter bodies such as Mountain Meadows Reservoir. Annual stocking of salmonids at higher elevation, oligotrophic lakes (e.g., Bucks Lake) no doubt in- creases foraging opportunities for Bald Eagles. Management Issues. Historically, Bald Eagle management activities focused on manipulating forest stands and restricting human activities at breeding sites (Dzus and Gerrard 1993). Bald Ea- gle prey species are now being considered more frequently when alterations to fish fauna or hydro- logical systems and wetlands are contemplated. For example. Hunt et al. (1992c) provided flow rec- ommendations which benefitted both Bald Eagle foraging and trout fishing on the regulated Pit Riv- er. There has been concern that efforts to convert to or restore salmonid fisheries by poisoning non- game fishes with rotenone may depress Bald Eagle productivity. Poisoning of Macumber Reservoir (1977) was followed by two years of no production for the single pair of nesting Bald Eagles, then suc- ceeded by 14 years of reproduction averaging 1.3 young per occupied year. More recently at Reser- voir F and Frenchman Reservoir, both eagle pairs were successful for the two years subsequent to ro- tenone treatment (R. Jurek, G. Studinski pers. comm.) . Certain conditions were implemented to limit impacts of these treatments to nesting Bald Eagles, including timing eradication^-outside the breeding season and the immediate and continued generous stocking of salmonids following eradica- tion. Waterbirds were also readily available to ea- gles to supplement their diet. If managed properly, salmonid restoration apparently has minimal im- pact on Bald Eagle productivity. Both natural and human-related factors such as spawning stress, powerhouse tailrace kills, reservoir fluctuations stranding fish, hatchery trout releases and angling mortality contribute to carrion avail- ability, which eagles habitually exploit (Hunt et al. 1992c, Stalmaster and Plettner 1992). We periodi- cally observed substantial numbers of dead fish floating in reservoirs, including suckers at Britton Reservoir (Hunt et al. 1992c) and bullheads at Mountain Meadows Reservoir, where each species was prominent in the prey of eagles nesting at those respective sites. Prior to altering existing op- erations at water facilities, managers should consid- er potential impacts on carrion availability for Bald Eagles. Acknowledgments We gratefully acknowledge R.N. Lehman and D.E. Dris- coll for their climbing expertise. Other field personnel assisting in prey collections included C. Himmelwright, T. Newman, T. Bertram, L. Spiegel, K. Austin, D. Garce- lon, S. Hawks, T. Brumley, W. Lehman, J. Driscoll, E. Ar- menta, G. O’Conner, K. Bennet, G. Wilson, R. Hagger- son, D. Sterrett, and L. Smalljackman. We are grateful to Ron Cole for assistance at the University of California at Davis Department of Wildlife and Eisheries museum collection. We thank Drs. P.B. Moyle, D.M. Baltz, and B. Vondracek of the University of California at Davis for guidance in preparing fish reference collections and identification procedures. This study was part of a senes of management-related studies funded by the Pacific Gas and Electric Company and managed by C.G. Thelander. M. Engbring and N. Farwell helped with graphics. We thank P. Moyle, T. Brown, L. Culp, G. Bortolotti, T Grubb, and M. Miller for helpful comments on earlier drafts. Literature Cited Adorjan, A.S. and G.B. Kolenosky. 1969. A manual for the identification of hairs of selected Ontario mam- mals. Ontario Ministry of Natural Resources Research Report (Wildlife). No. 90. Bagenal, T.B. and F.W. Tesch. 1978. 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Pages 751-756 in B.-U. Meyburg and R.D. Chancellor [Eds.], Raptor conservation today. World Working Group for Birds of Prey, Pica Press, London, U.K. Knight, R.L., P.J. Randolf, G.T. Allen, L.S. Young and R.G. Wigen. 1990. Diets of nesting Bald Eagles, Hal- iaeetus l&ucocephalus, in western Washington, Can, Field- Nat. 104:545-551. Lagler, C.F. 1940. Lepidological studies: scale character- istics of the families of Great Lakes fishes. Trans. Amer Micros. Soc. 66:149-162. McConnell, W.J. 1952. The opercular bone as an indi- cator of age and growth of the carp ( Cyprinus carpio) . Trans. Am. Fish. Soc. 81:138-149. Mersmann, T.J. 1989. Foraging ecology of Bald Eagles on the north Chesapeake Bay with an evaluation of tech- niques used in the study of Bald Eagle food habits. M.S. thesis, Virginia Polytechnic Institute, Blacksburg, VA U.S.A. Moore, T.D., L.E. Spence and C.E. Dugnolle. 1974. Identification of the dorsal guard hairs of some mam- mals of Wyoming. Wyoming Game and Fish Dept Bull. No. 14, Cheyenne, WYU.S.A. Moyle, P.B. 1976a. Inland Fishes of California. Univ. Ca- lif. Press, Berkeley, CA U.S.A. . 1976b. Fish introductions in California: history and impact on native fishes. Biol. Conserv. 9:101-118. , H.W. Ll and B.A. Barton. 1987. The Franken- stein effect: impact of introduced fishes on native fish- es in North America. Pages 415-426 in R.H. Stroud [Ed.], Fish culture in fisheries management. Ameri- can Fisheries Society, Bethesda, MD, U.S.A. Stalmaster, M.V. and R.G. Plettner. 1992. Diets and foraging effectiveness of Bald Eagles during extreme winter weather in Nebraska./. Wildl. Manage. 56:355- 367. Steenhof, K. 1983. Prey weights for computing percent biomass in raptor diets. Raptor Res. 17:15-27. Swenson, J.E., K.L. Alt and R.L. Eng. 1986. Ecology of Bald Eagles in the Greater Yellowstone ecosystem Wildl. Monographs 95. Todd, C.S., L.S. Young, R.B. Owen, Jr. and E.W. Gram- LICH. 1982. Food habits of Bald Eagles in Maine. / Wildl. Manage. 46:636-645. Vondracek, B., D.M. Baltz, L.R. Brown and P.B. Moyle 1989, Spatial, seasonal, and diel distribution of fishes in a California reservoir dominated by native fishes Fish. Res. 7:31-53. Received 6 August 1997; accepted 30 January 1999 /. Raptor Res. 33(2):97-101 © 1999 The Raptor Research Foundation, Inc. BALD EAGLE RESPONSE TO BOATING ACTIVITY IN NORTHCENTRAL FLORIDA Petra Bohall Wood West Virginia Cooperative Fish and Wildlife Research Unit, Biological Resources Division, USGS and West Virginia University Division of Forestry, Morgantown, WV 26506-6125 US. A. Abstract. — I examined the effects of weekend and weekday boating activity on Bald Eagle {Haliaeetus leucocephalus) use of three lakes in northcentral Florida during 1988-89. On Lake Lochloosa, which had the highest number of boats of the three lakes, boating activity significantly reduced the numbers of all age classes of eagles using the lake {P < 0.025). Increased boating activity on Lake Wauberg was not related to use by eagles {P = 0.06) likely because boating activity was concentrated during midday while eagles typically foraged early and late in the day. On Newnan’s Lake, the number of eagles observed also was not different between weekends and weekdays {P = 0.20). Weekend boating activity did not relate to perch use, habitat use, interactions or age distribution indicating no alteration of eagle behavior patterns. Flush distance did not vary between weekends and weekdays (P = 0.96) , but did vary by month (P = 0.0001), with a greater flush distance during months with highest boating activity. Minimal flush distances {x = 53m) and lack of measurable effects on behavior suggested that eagles in my study area were tolerant of boat disturbance. Key Words: Bald Eagle, Haliaeetus leucocephalus; boat disturbance, Florida-, human activity. Respuesta de Haliaeetus leucocephalus a actividades de canotzye en el centronorte de Florida Resumen. — Examine los efectos de canotaje durante los fines de semana y durante la semana en aguilas calvas {Haliaeetus leucocephalus) en tres lagos del centronorte de Florida durante 1988-89. En el lago Lochloosa el cual tiene el mayor numero de botes de los tres lagos, las actividades de canotaje redujeron significativamente los numeros de todas las clases de edad de las aguilas que utilizaron el lago (P < 0.025) . El incremento en el canotaje del lago Wauberg no fue relacionado con el uso por parte de las aguilas (P = 0.06), debido a que las actividades de canotaje se concentraron durante el medio dia, mientras que las aguilas forrajeaban temprano en la manana o tarde durante el dia. En el lago Newnan, el numero de aguilas observadas no fue diferente entre los fines de semana y entre semana (P = 0.20). Las actividades de canotaje durante el fin de semana no estuvieron relacionadas con la utilizacion de perchas, uso de habitat, interacciones o distribucion de edades lo que indico que no hubo alteraciones en los patrones de comportamiento de las aguilas. Las distancias a las cuales las aguilas levantaban el vuelo no variaron entre fines de semana y entre semana (P = 0.96), pero si entre meses (P = 0.0001), con una mayor distancia de levantamiento de vuelo durante los meses con mayor canotaje. La distancia de levantamiento de vuelo minima (x = 53 m) y la falta de efectos medibles en el comportamiento sugieren que las aguilas en mi estudio son tolerantes a la perturbacion de los botes. Boating activity can modify foraging patterns of Bald Eagles {Haliaeetus leucocephalus) by reducing or even precluding use of foraging areas (Steenhof 1976, Stalmaster and Newman 1978, Knight and Knight 1984, Chester et al. 1990, McGarigal et al. 1991, Brown and Stevens 1997). McGarigal et al. (1991) concluded that boating activities restrict use of certain foraging areas by breeding eagles and ultimately may affect productivity. Since Bald Ea- gles are easily disturbed when foraging (Grubb and King 1991) and adults are more sensitive to distur- [Traduccion de Cesar Marquez] bance than younger eagles (Stalmaster and New- man 1978), increasing recreational use of lakes in Florida may pose problems for breeding eagles. In addition, the dense eagle population in Florida that exists in close proximity to high levels of hu- man activity provided an opportunity to determine if eagles habituate to human activity. I conducted a study to examine the effects of boating activity on the use of lake shorelines by Bald Eagles and addressed the following objectives: (1) to determine if the number of Bald Eagles ob- 97 98 Wood VoL. 33, No. 2 Table 1. Mean number of Bald Eagles and boats on weekend (WE) and weekday (WD) counts on shoreline surveys of three lakes in northcentral Florida, 1988 and 1989. Lake Year Number of Eagles t P Number of Boats t P X WE (Range) X WD (Range) X WE (Range) X WD (Range) Lochloosa 1988 10 3.2 (0-10) 6.1 (1-10) -3.36 0.005 28.7 (4-50) 5.8 (4-31) 4.06 0.005 1989 10 6.9 (0-11) 10.6 (3-20) -2.38 0.025 17.9 (12-24) 7.8 (3-16) 5.37 0.005 Newnans 1988 12 4.2 (1-11) 5.3 (1-9) -0.95 0.20 18.5 (5-36) 11.5 (2-23) 5.02 0.005 1989 12 8.2 (3-20) 8.3 (2-14) -0.19 0.30 8.6 (2-16) 5.5 (0-13) 2.94 0.01 Wauberg 1988 10 2.5 (1-6) 4.2 (0-11) -1.66 0.06 6.7 (1-19) 3.5 (0.10) 1.61 0.06 1989 14 5,6 (1-11) 6.1 (1-11) -0.43 0.25 12.8 (2-32) 4.7 (0-8) 3.08 0.01 * N = number of paired surveys. served on a shoreline differed between high and low boat use days, (2) to determine if differences existed in response to boat disturbance by differ- ent age classes of eagles, (3) to determine if dis- tance perched from the shoreline or distance flushed by a boat differed between high and low boat use days, and (4) to determine if differences in activity, habitat use, perch use, or interactions occurred between high and low boat use days. Study Area and Methods Data on effects of boating activity on eagles were ob- tained at Lochloosa, Newnans, and Wauberg lakes in Ala- chua County, Florida. Lochloosa and Newnans lakes are large fishing lakes in the region and the majority of boat- ing activity involved fishing from stationary boats. Waub- erg is a small lake with restricted access for gasoline-pow- ered boats but heavily used for recreational activities, primarily canoeing, sailboating, and occasionally fishing from small boats equipped with electric motors (Wood 1992). Newnan’s Lake is a hyper-eutrophic lake of 2433 ha (Shannon and Brezonik 1972) with a mean depth of 1,5 m (maximum = 4.0 m). Lake Lochloosa is a 2235 ha meso-eutrophic lake with a mean depth of 2.9 m. Lake Wauberg is a 101-ha eutrophic lake with a mean depth of 3.8 m (maximum = 5.2 m). The lakes are rimmed primarily with baldcypress ( Taxodium distichum) and hard- wood swamps with adjacent pine (Pinus spp.) forests. Lakes were surveyed every 2 wks on Sundays and Mon- days from 28 February-9 May 1988, and on Sundays and Tuesdays from 11 December 1988-4 April 1989, to com- pare days with high human use (Sundays) to low-use days (weekdays). On Newnans and Lochloosa lakes, we sur- veyed a route of approximately 7 km from a johnboat by driving slowly (about 3-5 knots) approximately 100 m from the shoreline. Because Wauberg Lake is much smaller, we surveyed the entire shoreline from an an- chored boat at the center of the lake. Each sampling day was divided into morning, midday, and late-day and a sampling schedule was devised so that each lake rotated through these periods throughout the season. On a given sampling day, we conducted 6 surveys; 2 successive sur- veys were conducted on a lake before moving to the next lake. We began the first survey shortly after dawn, and finished the last of 6 surveys near dusk. We began each of the 6 surveys at the same time on the two paired sam- ple days. Data recorded for each eagle observed included loca- tion, age class, activity, habitat, perch type, distance perched from edge, interactions, and with whom the in- teraction occurred. Age classes were based on plumage characteristics (McCollough 1989) and included adults (all white head and tail), late subadults (some brown in head and tail) , early subadults (no white in head or tail) , immatures (first year eagles) , subadults (birds that could not be classed as early or late), and unknowns, Locations of eagles and boats were plotted on topographic maps. In 1989, the distance from our boat at which a perched eagle flushed was estimated with periodic verification us- ing a Lietz range finder. 1 first determined the sample size needed to test the hypothesis that boating activity was reducing eagle use of lakeshores using the prespecified variance method (Gil- bert 1987:51-52). Data were analyzed separately for 1988 and 1989 for each lake with a paired difference t-test to avoid problems with temporal changes in eagle and boat abundance. 1 used a t-test to examine the effect of boating activity on the distance eagles perched from the edge of the shoreline and on the estimated flush distance. Analysis of variance was used to examine month and age varia- tions in flush distance. I used contingency tests (Wink- ler and Hays 1975: 825-829) to examine the distributions for age of eagles observed, habitat use, perch types, ac- tivity, and interactions on weekends versus weekdays. Results On Lochloosa Lake, boats were more abundant on weekends than on weekdays in both years (Ta- ble 1), while more eagles were observed on week- days than on weekends. The maximum number of boats generally was greater in 1988, while the high- est maximum number of eagles occurred in 1989. Likewise on Newnans Lake, boats were more abun- dant on weekends than on weekdays in both years (Table 1), although the mean difference was not as large in 1989. The number of eagles observed June 1999 Eagle Response to Boat Activity 99 Table 2. Mean distance (m) Bald Eagles were perched from the edge of the shoreline (1988 and 1989) and flush distance (1989) on weekdays (WD) and weekends (WE) during shoreline surveys of Lochloosa, Newnans, and Wauberg lakes in northcentral Elorida. Vari- AB1.E X SE Range t P Distance to edgi e WD 256 5.6 0.56 0-50 -2.52 0.01 WE 193 8.5 0.99 0-75 Flush distance WD 32 53.9 12.32 5-200 0.05 0.96 WE 27 52.9 12.73 5-200 N = number of Bald Eagle observations. was not significantly different on weekdays than on weekends in either year. Maximum counts of boats were lower on Newnans Lake in 1989 compared to 1988, but maximum counts of eagles were higher. Fewer boats may affect a smaller portion of the shoreline available to eagles. On Lake Wauberg, there was no difference in the number of boats or eagles observed on weekends versus weekdays in 1988 (Table 1). In 1989, more boats were observed on weekends, but the number of eagles observed did not differ. Of 816 eagles observed, the majority (47.9%) were adults. The age distribution of eagles sighted on weekends did not differ from that on weekdays (X^ = 4.01, P = 0.55). Wfeekend boating activity, therefore, was related to eagle numbers regardless of age class. Eagles perched farther from the shore- line edge on weekends (Table 2) when boating ac- tivity was higher than on weekdays with less boating activity, although the difference was only 3 m. Of 517 eagle sightings in 1989, eagles flushed in response to our boat in 59 instances. Flush dis- tance did not differ between weekends and week- days (Table 2), but differed by month (Table 3; F = 10.46, P = 0.0001). Eagles were flushed by boats at a greater distance in January and February, when boating activity typically increased with win- ter tourism. Flush distance did not differ by age class (F = 1.23, P = 0.32). During the shoreline surveys, I identified five types of perches: snags, pines, cypress, hardwoods, and palms. There was no difference in the distri- bution of eagles using these perch types on week- ends (high boat use days) compared to weekdays (X^ = 5.74, P = 0.33). The mcjority of the 489 Table 3. Number of flushes (N) and mean estimated flush distance (m) by month for Bald Eagles sighted on shoreline surveys of Lochloosa, Newnans and Wauberg lakes in northcentral Florida, 1989. Means with the same letter are not significantly different (Waller-Duncan K- ratio ^-test) . Month N X SE Range December 5 5 B 0.0 5-5 January 13 80 A 21.9 5-200 February 20 99 A 14.2 5-200 March 5 5 B 0.0 5-5 April 13 5 B 0.0 5-5 May 3 5 B 0.0 5-5 sightings of perched birds occurred in cypress (51.9%) or hardwood (21.1%) trees. I also distinguished six habitat types used by ea- gles; cypress, hardwood, pinewoods, marsh, lake, and developed (Wood 1992). Eagles used these habitats in the same proportion on weekends as on weekdays (x^ = 3.85, P = 0.57) . The most frequent- ly used habitat was cypress (46%). Pinewoods (22%) and lake (21%) also were commonly used habitats but only 21 (3%) eagles occurred in de- veloped habitats. Six categories of interactions were observed and involved 332 sightings of eagles: chasing or being chased (31%), perched together (44%), flying to- gether (20%), and stooping on, hitting, and talon locking (5%). Because of small sample sizes, stoop- ing on, hitting, and talon locking were combined into one category for analysis purposes. Boating ac- tivity did not change the distribution of eagles en- gaged in the various interactions (x^ = 3.56, P = 0.31). Discussion Boat and eagle numbers were negatively related on Lochloosa Lake with boat use highest on week- ends and eagle use highest on weekdays. On Newn- ans Lake, the mean difference in the number of boats was not as large as that observed for Loch- loosa Lake, particularly in 1989. The small differ- ence in boating activity between weekends and weekdays, although significantly different, may not have been a true measure of boating effects on eagles because of the overall low number of boats present. On Newnans Lake, the maximum number of boats counted on the 7-km segment of shoreline (1988 = 36, 1989 = 16) was much less than on 100 Wood VoL. 33, No. 2 Lochloosa Lake (1988 = 50, 1989 = 24). There may be a threshold number of boats required on a lake before eagles avoid an area. In contrast, McGarigal et al. (1991) reported a reduction in the use of highly used foraging areas in response to a single stationary boat. No relationship was detected between boat and eagle numbers on Lake Wauberg. Because this lake is used primarily for recreational activities other than fishing (sailing and canoeing), boat distur- bance is concentrated during early afternoon with little disturbance in the early morning and late evening. This allowed eagles to forage undisturbed on the lake for several hours when foraging by ea- gles generally reaches a peak (Mersmann 1989). Further, Lake Wauberg has restricted access for boats powered with gasoline engines, so distur- bance created by a fishing boat is less than that on other lakes. The type of boat and timing of boating activity both can affect response by eagles (Grubb and King 1991, Grubb et al. 1992, Stalmaster and Kaiser 1998). Human disturbance could at times alter behav- ior patterns or differentially affect individual age classes (Stalmaster and Newman 1978). In my study, boating activity did not affect eagle activity, perch use, habitat use, interactions, or the age dis- tribution of eagles observed. Thus, eagles likely were not displaced from preferred perching or for- aging areas and were not differentially affected by age class. In my study, flush distance did not differ be- tween weekends and weekdays. In contrast, Stal- master and Kaiser (1998) found shorter flush dis- tances on weekends. I generally found that when eagles responded to boat disturbance the primary response was to avoid the lakes. Similarly, Steenhof (1976) and McGarigal et al. (1991) found that it was more common for eagles to entirely avoid ar- eas where boats were present. I found no differ- ence in flush distance between age classes. Knight and Knight (1984) and Buehler et al. (1991) re- ported no age-specific differences in flush dis- tance, whereas Stalmaster and Kaiser (1998) de- tected longer flush distances by subadults. Stalmaster and Newman (1978) reported that adults were more sensitive to disturbance than younger eagles and preferred areas with lower hu- man activity. Mean flush distance of 53 m was less than that reported in other studies (Knight and Knight 1984: 152 m; Buehler et al. 1991: 175 m in summer and 265 m in winter; McGarigal et al. 1991: 197 m; Stal- master and Kaiser 1998: 111-293 m). Buehler et al. (1991) suggested that the difference in winter and summer flush distances observed on the Chesa- peake Bay might be a difference in response by the northern migrant eagles inhabiting the Chesa- peake in the winter, compared to the southern mi- grants and Chesapeake eagles present in summer. Because flush distance in my study was very low, particularly after high boat disturbance in January and February, it is possible that eagles habituated to boat disturbance in Florida which contributed to the low summer flush distance observed on the Chesapeake Bay. Knight and Knight (1984) re- ported a decreased tendency for eagles to flush in response to a canoe, but could not conclusively at- tribute the response to habituation. Stalmaster and Kaiser (1998) found decreased flush responses over the winter season, but no change in flush dis- tances suggesting some habituation to disturbance. In summary, boating activity reduced the num- ber of eagles using the shoreline on only one of the three lakes studied, did not influence flush dis- tance, and increased the distance perched from the shoreline by only 3 m. Thus, at this time, there was no evidence that recreational boating activity negatively affected eagle use of these lakes. The minimal flush distances and the lack of measurable effects on eagle behavior and activity patterns sug- gested that many of these birds may have become habituated to boating disturbance, although they still show some avoidance behavior, Acknowledgments Funding for this study was provided by the Nongame Wildlife Program of the Florida Game and Fresh Water Fish Commission through a grant to the University of Florida, Department of Wildlife and Range Sciences. Lo- gistical support was provided by the Florida Cooperative Fish and Wildlife Research Unit. Research assistants Myra Noss and Rick Sullivan spent numerous hours assisting with boat surveys. D.A. Buehler, M.W. Collopy, T.G. Grubb, and J. Kaufman provided helpful comments on this manuscript. This is Scientific Journal Article #2689 of the West Virginia University Agricultural and Forestry Experiment Station. Literature Cited Brown, B.T. and L.E. Stevens. 1997. Winter Bald Eagle distribution is inversely correlated with human activity along the Colorado River, Arizona./. Raptor Res, 31: 7-10. Buehler, D.A., TJ. Mersmann, J.D. Fraser and J.K.D. Seegar. 1991. Effects of human activity on Bald Eagle June 1999 Eagle Response to Boat Activity 101 distribution on the northern Chesapeake Bay./. WildL Manage. 55:282-290. Chester, D.N., D.F. Stauffer, TJ. Smith, D.R. Luukko- NEN AND J.D. Fraser. 1990. Habitat use by nonbreed- ing Bald Eagles in North Carolina. J. Wildl. Manage. 54: 223-234. Gilbert, R.O. 1987. Statistical methods for environmen- tal pollution monitoring. Van Nostrand Reinhold Co., New York, NY U.S.A. Grubb, T.G., W.M. Bowerman, J.R Giesyand G.A. Daw- son. 1992. Responses of breeding Bald Eagles, Hal- iaeetus leucocephalus, to human activities in northcen- tral Michigan. Can. Field-Nat. 106:443-453. AND R.M. King. 1991. Assessing human distur- bance of breeding Bald Eagles with classification tree models./. Wildl. Manage. 55:500-511. Knight, R.L. and S.K. Knight. 1984. Responses of win- tering Bald Eagles to boating activity./. Wildl. Manage. 48:999-1004. McCollough, M.A. 1989. Molting sequence and aging of Bald Eagles. Wilson Bull. 101:1-10. McGarigal, K., R.G. Anthony and F.B. Isaacs. 1991. In- teractions of humans and Bald Eagles on the Colum- bia River estuary. Wildl. Monogr. 115:1-47. Mersmann, TJ. 1989. Foraging ecology of non-breeding Bald Eagles on the northern Chesapeake Bay, Mary- land. M.S. thesis, Virginia Polytechnic Institute and State Univ., Blacksburg, VA U.S.A. Shannon, E.E. and P.L. Brezonik. 1972. Limnological characteristics of north and central Florida lakes. Lim- nol. Oceanogr. 17:97—110. Stalmaster, M.V. and J.L. Kaiser. 1998. Effects of rec- reational activity on wintering Bald Eagles. Wildl. Mon- ogr. No. 137. AND J.R. Newman. 1978. Behavioral responses of wintering Bald Eagles to human activity. /. Wildl. Man- age. 42:506-513. Steenhof, K. 1976. The ecology of wintering Bald Eagles in southeastern South Dakota. M.S. thesis, Univ. of Missouri, Columbia, MO U.S.A. Winkler, R.L. and W.L. Hays. 1975. Statistics: probability, inference, and decision. Second edition. Holt, Rine- hart and Winston, New York, NY U.S.A. Wood, P.B. 1992. Habitat use, movements, migration pat- terns, and survival rates of subadult Bald Eagles in north Florida. Ph.D. dissertation, Univ. Florida, Gainesville, FL U.S.A. Received 1 July 1998; accepted 30 January 1999 J Raptor Res. 33 (2): 102-1 09 © 1999 The Raptor Research Foundation, Inc. THE GOLDEN EAGLE (AQUILA CHRYSAETOS) IN THE BALE MOUNTAINS, ETHIOPIA Michel Clouet 16 Avenue des Charmettes, 31500 Toulouse, France Claude Barrau 38 Chemin des Cotes de Peek David, 31400 Toulouse, France Jean-Louis Goar 11330 Villerouge Termenes, France Absract. — ^We studied Golden Eagles {Aquila chrysaetos) in the afro-alpine area (elevation 3500-4000 m) of the Bale Mountains in Ethiopia. We monitored seven territories from 1-5 successive years for a total of 26 territory-years. Home ranges varied from only 1.5-9 km^, the smallest size recorded for the species. This was probably due to the abundance of prey, mainly hares and grass rats, that made up 50% and 30% of prey, respectively. Despite this, productivity of these Golden Eagles was quite low averaging only 0.28 young per occupied territory {N = 25). This was due to a large number of unmated territorial adults and poor breeding performance by pairs (0.4 young per pair per year, N = 17). The high density and frequent interspecific interactions with Verreaux’s Eagles {Aquila verreauxii) were key factors affecting the dynamics of this Golden Eagle population. The unusual coexistence of these two closely related species was a novel component of the rich predator guild in the area that included five other wintering or resident eagle species. This richness was related to the high density of rodents and lagomorphs, a characteristic of the Ethiopian afro-alpine ecosystem. Key Words: Golden Eagle; Aquila chrysaetos; Verreaux’s Eagle; afro-alpine habitats; Ethiopia; prey abundance, interspecific competition; productivity. El aguila dorada {Aquila chrysaetos) en las montanas Bale de Etiopia Resumen. — Estudiamos el aguila dorada {Aquila chrysaetos) en el area afro-alpina (elevacion 3500-4000 m) de las montanas Bale en Etiopia. Monitoreamos siete territorios de 1-5 anos continuos para un total de 26 territorios/ano. Los rangos de hogar variaron entre 1.5-9 km^ Los mas pequenos reportados para la especie. Esto probablemente debido a la abundancia de presas, principalmente liebres y ratas de pastizales, las cuales representan el 50% y el 30% de las presas respectivamente. A pesar de esto la productividad de las aguilas fue muy baja con un promedio de 0.28 juveniles por territorio ocupado {N = 25) . Esto se debio al gran numero de adultos territoriales sin pareja y al pobre desempeno reprod- uctivo de las parejas (0.4 juveniles por pareja por ano, N = 17). La alta densidad y las frecuentes interacciones intraespecificas con las aguilas Verreaux {Aquila verreauxii) fueron factores que afectaron la dinamica de esta poblacion de aguilas doradas. La inusual coexistencia de estas dos especies estre- chamente relacionadas es un componente novedoso de la estructura de depredadores en el area que incluye otras cinco especies migratorias o residentes de %uilas. Esta riqueza estuvo relacionada con la alta densidad de roedores y lagomorfos, una caracteristica del ecosistema afro-alpino Etiope. [Traduccion de Cesar Marquez] The recent discovery of a Golden Eagle {Aquila chrysaetos) population in Ethiopia (Clouet and Bar- rau 1993) has provided a unique opportunity to study this species in a new biogeographical area. The population occurs in the Bale Mountains lo- cated in the southern part of the Ethiopian high plateau, east of the Rift Valley. It occurs at the afro- alpine region which supports the largest mountain moorland and grassland habitat on the continent. This rich and unique ecosystem (Dorst and Roux 1972, Hillman 1986) supports an avian community dominated by a predator-scavenger guild (Clouet et al. 1995). The Golden Eagle is part of a unique assemblage of eagle species and coexists with the 102 June 1999 Golden Eagle in Ethiopia 103 Figure 1. Location of the Bale Mountain study area in Ethiopia. Verreaux’s Eagle {Aquila verreauxit) . To our knowl- edge, this assemblage that includes the southern- most extent of the Golden Eagle has never before been studied. Study Area and Methods The study area was in Bale Mountains National Park in the upper Web River Valley and a portion of its trib- utaries above tree line (3500-4000 m) (Fig. 1). The area consists of a continuous network of cliffs stretching from the north bank of the Web River to the top of the Massif (Sanetti plateau). The afro-alpine habitat has a tropical climate tempered by the altitude and characterized by an alternating wet season that lasts from March-October and a dry season that lasts from November-February. We searched for breeding raptors in a 200 km^ area of potential nesting and hunting habitat over the course of seven expeditions covering five successive breeding sea- sons: August 1993, May 1994, March-November 1995, March 1996, and February-August 1997. One to three observers walked transects through Golden Eagle terri- tories. Observations were made continuously for periods of 2-1 1 hr, recording the activities of the eagles and any interspecific behavior involving other raptors species. All eagle flights were plotted on a map to calculate their distances and areas covered. The total observation time for the seven expeditions was 210 h. We estimated diet by observing kills and collecting prey items in nests during the fledging period. Diet diversity was calculated using a Shannon Index (Delibes et al. 1975, Clouet 1981, Fernandez 1991). Productivity (number of young per occupied territory) was calculated by observing young in nests that were older than 51 d of age (Steenhof 1987) (except in one case when a nestling was only about 35-d old) or in flight with adults during the postfledging stage (August 1993, May 1994, and August 1997). Results We identified seven territories occupied by Gold- en Eagles. We assumed that territories were cen- tered on the occupied nest or, when the pair was CD valley ftoot Off CD Plateau Figure 2. Distribution of Golden and Verreaux’s Eagles in the 200-km^ study area in the Bale Mountains National Park, Ethiopia, 1993-97. not breeding or when it was occupied by an un- paired bird, we used the arithmetic center of the known unoccupied nests in the territory. The av- erage distance between centers was 4.7 km (range = 2.5-7 km) (Fig. 2). Individual territories were monitored for 1-5 yr totaling 26 territory-years (Table 1 ) . Single adults were observed in three out of the seven monitored territories and during 4 of the 5 yr of the study (i.e., seven (27%) of the 26 territory-years) . Nonterritorial single birds were re- corded only twice (1 adult and 1 immature eagle). Topographically, the territories included a sec- tion of cliff where perches and nests were located and a part of the neighboring plateau. The slope located at the foot of the cliff was comprised of scree, grassland, and bushes, and it provided a va- riety of potential prey including hyraxes {Procavia capensis), hares {Lepus strarki) , and francolins {Fran- colinus spp.). In the valley bottom, there were also colonies of mole rats (Tachyoryctes macrocephalus) and grass rats (Arvicanthis spp. and Lophuromys spp.). We observed nine successful kills by Golden Ea- gles. All were made either on low altitude flights by eagles close to the slopes at the foot of cliffs (once) or within 200—2000 m of perches (eight times) . Golden Eagles were also seen robbing prey from an Augur Buzzard {Buteo augur), Pallid Har- 104 Clouet et al. VoL. 33, No. 2 Table 1. Territory occupancy and productivity of Golden Eagles in the Bale Mountains National Park, 1993-97. Territory Year 1 2 3 4 1993 (6-13 August) Single adult Adult pair 1 young flying Single adult — 1994 (5-17 May) Adult pair Adult pair Adult pair Single adult 1995 (21 March-5 April) Single adult 1 young flying Adult pair Adult pair Single adult 1996 (4-15 March) Single adult Adult pair Adult pair Single adult 1997 (3-12 February) (9-18 August) Adult pair Adult pair Adult pair 1 5-wk-old young Adult pair rier {Circus macrourus), Lanner Falcon {Falco biar- micus) , and Steppe Eagle {Aquila nipalensis) . Undulating display flights and attacks on intrud- ers of other raptor species were performed by both paired or unpaired Golden Eagles, both near and km from perches, cliffs and nests. In the case of one unpaired territorial adult, undulating flights accounted for up to 51% of the total flying time (115 min) in November at the beginning of the breeding season suggesting that the function of this flight was to display its territory. The mean area of the home ranges, estimated from observations of hunting forays and territorial flights, such as undulating displays and attacks on intruders, was 3.6 km^ (range = 1.5-9 km^, N = 7). Home ranges were smaller for single adults (1- 1.5 km^, N = 2) and appeared to be larger where there was no rodent colony or where scrub {Erica spp.) was extensive (estimated to be about 9 km^). Observations of kills {N = 9) and identification of prey items brought to nests {N = 41) showed a predominance of mammals which accounted for 86% of the prey: 25 hares (50%), 3 hyraxes (6%), 2 giant mole rats (4%), 13 grass rats (26%), 5 Ar- vicanthis blicki (10%), 1 Lophuromys melanonyx (2%), and 7 unidentified prey (14%). Small species were probably underestimated because they were often entirely consumed by eagles. Birds accounted for a smaller part of the diet (14%) : 4 Moorland Fran- colins {Francolinus psilolaemus, 8%), 2 Blue-winged Geese {Cyanochen cyanopterus, 4%), and 1 uniden- tified bird. Mammals accounted for 90% of the to- tal prey biomass and hares alone made up 78% of the biomass. Other breeding raptors were observed in every Golden Eagle territory we observed, including one or two pairs of Augur Buzzards, one pair of Lanner Falcons and one pair of Common Kestrels {F. tin- nunculus). Frequent mobbing behavior of these raptors triggered attacks by eagles. We observed Tawny Eagles (A. rapax) which were residents throughout the year and Greater Spotted Eagles (A. clanga), Lesser Spotted Eagles (A. pomarina), and Steppe Eagles which were either migrating through or spent the winter in the area flying over Golden Eagle hunting areas. The latter were very numerous and on several occasions approximately 30 were recorded simultaneously in flight in Feb- ruary. Steppe Eagles were observed attacking Gold- en Eagles on their prey (3 cases). Golden Eagles were also seen chasing off intruding Tawny and Steppe Eagles (3 cases). The survey area contained seven territories of Verreaux’s Eagles, all occupied by pairs. All pairs raised at least one young during the study period. The average distance between occupied nests was 5.2 km (range = 3-7 km). Observations at six ter- ritories showed that Verreaux’s Eagles traveled from 12-13 km^ on hunting forays and territorial flights and they established common boundaries between their territories. The mean distance be- tween the nearest occupied nest or territory center of a Golden Eagle and a Verreaux’s Eagle was 3.4 km {N = 12, range — 0.65-6.0 km). Territories of the two species were mutually exclusive. Interac- tions we recorded most often were attacks by Gold- en Eagles on Verreaux’s Eagles {N= 21). Only one attack by a Verreaux’s Eagle on a Golden Eagle was June 1999 Golden Eagle in Ethiopia 105 Table 1. Continued. Territory 5 6 7 — Adult pair 1 10-wk-old young Adult pair 1 7-wk-old young Adult pair 1 7-wk-old young Adult pair Adult pair 1 9-wk-old young incubating Adult pair Adult pair 1 young flying with adults observed and it involved an attempted piracy at the boundary of two territories. We recorded three types of behavior by the Gold- en Eagle toward Verreaux’s Eagles {N = 26) . One behavior we classified as tolerance. It occurred when one or a pair of Verreaux’s Eagles flew at high altitudes (>500 m) {N = 5) above Golden Eagles. However, we observed birds of each species perched out of sight of each other on the same cliff at dis- tances of <400 m. Flight and undulating display flight behaviors were triggered by a Verreaux’s Ea- gle approaching within 500-1000 m of a Golden Ea- gle nest or perch {N — 7) . Aggressive flight behavior toward an intruding Verreaux’s Eagle occurred when the intruding eagle came within 500 m of an occupied nest or perch used by a Golden Eagle {N = 14). Aggressive flights usually ended when the intruder withdrew. In two cases, the interaction re- sulted in grappling of talons. The most frequent and most intense interspecif- ic encounters observed occurred when two Golden Eagle territories were located between two Ver- reaux’s Eagle territories and where inter-nest dis- tances were short. In this situation, the number of Golden Eagle flights per hour of observation (38 hr) was 1.74. Territory defense flights and undu- lating flights made up 32% and 24% of all flights (N = 66), respectively. When the species were far- ther apart {N = 4 territories), the number of flights per hour of observation (44 hr) was 1.98 and the number of undulating flights accounted for only 6% of the total number of flights observed (N= 87). Golden Eagles began building nests in Novem- ber and unsuccessful breeding pairs often contin- ued building until February. We observed five nests containing a single eaglet between March and May and immatures were seen flying together with adults in May and August (Table 1 ) . Judging from the age of the young we observed, we esti- mated that laying occurred from mid-November to mid-January, and that fledgling occurred from mid-March through the end of May. The begin- ning of the breeding season corresponded with the beginning of the dry season and the young left the nest at the beginning of the rainy season. This phe- nology corresponded to the breeding seasons of most other nesting raptors in the Bale Mountains. The nesting season for Verreaux’s Eagles started a few weeks earlier than that of Golden Eagles and the first females began incubation in November. Young Verreaux’s Eagles left their nests from March-June. Productivity of Golden Eagles was 0.42 young per territorial pair per year {N = 19, Table 2). Discussion Our observations of Ethiopian Golden Eagles showed similarities with holarctic populations but Table 2. Golden Eagle productivity in the Bale Mountains National Park and locations of neighboring Verreaux’s Eagles. Distance from Locations of Neighboring Neighboring Years Productivity Verreaux’s Eagle Verreaux’s Eagle Territory OF Study (Young/Year) Pair(s) (km) Pair(s) 2.7 different valley 0.65 same cliff 3.0 same valley 1.2 different valley 4.0 same valley 5 3 1.0 106 Clouet et al. VoL. 33, No. 2 differed in terms of their use of space, population regulation, and the type of predator community into which they integrated. Even though our prey sample was limited, Ethiopian Golden Eagles ap- peared to select terrestrial prey of similar size to that taken by Golden Eagles in European and North American populations (Brown and Watson 1964, Murphy 1975, Delibes et al. 1975, Clouet and Goar 1980, Haller 1982, Steenhof and Kochert 1988, Fernandez and Purroy 1990). The number of small-sized prey (<2 kg) such as grass rats, which was likely underestimated by our sampling meth- od, showed that these Golden Eagles were oppor- tunistic predators taking small prey items when they became available in sufficient quantities (De- libes et al. 1975, Fernandez 1991, Watson et al. 1993). Because hares were predominant prey, diet diversity was low {H' — 1.44) in comparison with Golden Eagles in the Pyrenees {H' — 2.77) (Clouet 1981). The density of Golden Eagles in the Bale Moun- tains matched the highest figures recorded in the western highlands of Scotland (one pair per 38 km^, Watson et al. 1992) and in North America (one pair per 29-36 km^, Phillips et al. 1990). The home range size we recorded was the smallest re- ported for the species. In Europe, territory size can range from 40-160 km^ (Clouet 1988). In the Swiss Alps, the home range of four breeding pairs ranged from 22-48 km^, and core areas used for hunting that ranged from 6-16 km^ (Haller 1982) were larger than the areas used in Ethiopia. Variation in density be- tween different areas in Europe was associated with food availability (Tjernberg 1985, Watson et al. 1992). The home range size documented in North America was influenced by the amount of favor- able prey habitat and became smaller where high quality prey habitat was abundant (Dixon 1937, Collopy and Edwards 1989, Marzluff et al. 1997). The abundance of prey and its availability through- out the year in the Bale Mountains was probably an important factor in explaining the high density and the small home ranges we observed. Both hares and rodents occurred in high numbers in the afro-alpine moorland and grassland of the Bale Mountains. Their abundance was estimated by Got- telli and Sillero-Zubiri (1990) during their study of the Ethiopian wolf (Canis simiensis). Hare density was 32 individuals/ km^ for a biomass of 120 kg/ km‘^. The biomass of mole and grass rats was esti- mated at 3000-4000 kg/km^ (Gottelli and Sillero- Zubiri 1990) . These values are more than 20 times the available biomass of rodents above the tree-line on the northern slopes of the Pyrenees where the size of Golden Eagle territories is roughly 20 times larger than in Ethiopia (Clouet 1991). Prey abundance influences breeding perfor- mance both in Europe and in North America (Murphy 1975, Smith and Murphy 1979, Clouet 1981, Haller 1982, Thompson et al. 1982, Tjern- berg 1983, Jenny 1992, Watson et al. 1992, Steen- hof et al. 1997). Productivity of Golden Eagles ranging from 0.79-0.82 young per territorial pair per year have been reported in long-term studies in North America (Phillips et al. 1990, Bats and Moretti 1994, Steenhof et al. 1997). Productivity has been lower in most European studies ranging from 0.48-0.53 young per territorial pair per year in the Alps (Haller 1996) and Pyrenees (Clouet 1988) . Watson (1997) found a significant negative correlation between breeding success and diet di- versity. Where Golden Eagles had a narrow feeding niche they tended to breed more successfully than where the niche was broad. In Ethiopia, despite high prey abundance and low diet diversity, pro- ductivity was low. This was probably due to tbe fact that territorial pairs did not breed successfully ev- ery year and only raised a single eaglet per brood and, in some years, certain territories were occu- pied by only single adults (27% of the total 26 ter- ritory-years studied) . The latter may have resulted from a lack of surplus birds in the population. Sur- plus birds have often been noted in Golden Eagle populations (Haller 1982, 1996, Marzluff et al. 1997) and can compose up to 30% of the popu- lation in the Alps (Clouet and Couloumy 1994). The lack of surplus birds in the Bale Mountains could have been due to human persecution, but we have no data to support this. It may also have been due to the absence of recruitment of individ- uals from outside the Bale Mountain range. Be- cause the population is sedentary, there is little op- portunity for interaction with populations located 2000 km further to the north (Thiollay and Du- hautois 1976). No Golden Eagles have been ob- served on the banks of the Red Sea during migra- tory movements (Bruun 1985, Welch and Welch 1989) . Finally, insufficient productivity could ex- plain the absence of surplus birds. Our estimated 0.42 young fledged per territorial pair per year is very low for a Golden Eagle population. In other African eagle populations, breeding success is not related to food availability but is highly density de- June 1999 Golden Eagle in Ethiopia 107 pendent (Thiollay and Meyer 1978, Gargett 1977, 1990, Simmons 1993). The effect of density has also been documented in Golden Eagle popula- tions in Europe (Haller 1982, 1996, Jenny 1992). The reduced productivity (0.48 young per pair per year) recorded in the Swiss Alps has been inter- preted as the consequence of a density-dependent regulation process. The major limiting factor in breeding success in the Alps is the frequency of interactions with unpaired nonterritorial surplus birds causing a negative feedback effect. Regular interactions with settled birds increases the pro- portion of nonbreeding pairs and depresses suc- cessful incubation (Haller 1982, 1996, Jenny 1992). Perhaps, in the Bale Mountains, interactions be- tween Verreaux’s and Golden Eagles have the same negative effect on reproduction as surplus Golden Eagles in the Alps. Productivity of Golden Eagle pairs appeared to be influenced by the proximity of neighboring Verreaux’s Eagle pairs. For the clos- est nests (650 m apart on the same cliff), neither species succeeded in breeding simultaneously. Such interactions with other species of eagles are exceptional, making the situation in Ethiopia rath- er unique. Golden Eagle coexistence with other eagle spe- cies such as with Bonelli’s Eagles {Hieraaetus fascia- tus) has been reported in the Mediterranean re- gion. Apparently, the two species become eco- logically isolated through their territoriality and diets (Brosset 1961, Cheylan 1977, Jordano 1981, Clouet and Goar 1984, Parellada et al. 1984, Fer- nandez and Insausti 1986, Bahat 1989). In Israel, apparently competition between the two species re- sults in a lower density of Golden Eagles in areas where Bonelli’s Eagles are abundant and where both species maintain exclusive home ranges (Ba- hat 1989). In Ethiopia, the Golden Eagle is integrated into the largest eagle assemblage known which includes five other Aquila species. Here, ecological isolation between Tawny Eagles (resident) and Steppe, Less- er, and Greater Spotted Eagles (winter only) de- velops through specialized use of habitat (less rocky than that of the Golden Eagle), temporal separation of breeding periods and partly through diet. The Verreaux’s Eagle is morphologically and ecologically very similar to the Golden Eagle and it is generally regarded to be its African equivalent. Both species are of similar size, have a similar pre- dation potential (Voous 1970, Brooke et al. 1972), feed on terrestrial prey, use similar nesting habitat and breed at the same time of the year. Evidently, the two species escape from interspecific competi- tion by establishing mutually exclusive ranges and they use undulating display flights to advertise their territory boundaries (Harmata 1982, Collopy and Edwards 1989). The only niche parameter which actually distinguishes the two species is diet. Dietary studies in Africa show that Verreaux’s Ea- gles prey almost exclusively on rock hyraxes which represent up to 98% of their food intake (Brown et al. 1982, Gargett 1990). The prey remains that we collected under Verreaux’s Eagle nests con- firmed that hyraxes also predominate in the diet of Verreaux’s Eagles in the Bale Mountains. On the other hand, hyraxes represented only a very small part of the diet of Golden Eagles. We believe that the unusual coexistence of Gold- en and Verreaux’s Eagles is possible because the Ethiopian afro-alpine region supports a rich and dense rodent community (Yalden 1988) that makes possible the assemblage of the most diverse guild of raptors ever found at such a high altitude. Acknowledgments We would like to thank Alex Clamens and Jeff Watson for comments on a first draft of this paper, Elisabeth Goar for the English translation and Mike Kochert, David Ellis, and Gary R. Bortolotti for their stimulating remarks. Literature Cited Bahat, O. 1989. Aspects in the ecology and biodynamics of the Golden Eagle {Aquila chrysaetos homeyeri) in the arid regions of Israel. M.S. thesis, Tel Aviv Univ., Tel Aviv, Israel. Bats, J.W. and M.O. Moretti. 1994. Golden Eagle {Aqut- la chrysaetos) population ecology in eastern Utah. Great Basin Nat. 54:248-255. Brooke, R.K., J.H. 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SELECTION OF NEST CLIFFS BYBONELLTS EAGLE (HIERAAETUS FASCIATUS) IN SOUTHEASTERN SPAIN Diego Ontiveros Departamento de Biologic Animal y Ecologia, Facultad de Ciendas, Universidad de Granada, E-18071 Granada, Spain Abstract. — total of 119 nests and 52 cliffs occupied by 32 Bonelli’s Eagle {Hieraaetus fasdatus) pairs was studied during 1995-97 in southeastern Spain. Mean number of nests built by pairs exceeded that reported in previous studies {x = 3.7; N = 32) and there was a trend among eagles to build their nests with a southeastern orientation. Breeding density was directly related to the availability of cliffs. Eagles occupied higher cliffs {x = 52.9 ra, N = 32), located on steeper slopes (x = 34.7°; N = 31) than was available. Occupied cliffs were highly heterogeneous due to the fact that use of different areas by Bonelli’s Eagles was dependent on human disturbance. Thus, occupied cliffs with the shortest linear distance to paved roads were higher than occupied cliffs far from paved roads. Selection of high cliffs located on steep slopes with southern orientations may have been associated with the additional lift provided eagles, since these types of nest sites enhanced the possibility of thermal and slope soaring. Preservation of nest cliffs free from disturbances should be undertaken to ensure the survival of Bonelli’s Eagle in this area of Spain. Keywords: Bonelli’s Eagle, Hieraaetus fasciatus; southeastern Spain-, diff selection-, breeding density. Seleccion de los roquedos de nidificacion del Aguila Perdicera {Hieraaetus fasdatus) en el Sureste de Espana Resumen. — 119 nidos y 52 roquedos ocupados por 32 parejas de Aguila Perdicera {Hieraaetus fasdatus), fueron analizados en el periodo 1995-97 en el sureste de Espana. El numero medio de nidos construidos por pareja fue mayor que el descrito por otros autores (x = 3,7; N = 32). Los resultados revelan una tendencia de las aguilas de construir sus nidos hacia la orientacion sureste. La densidad de parejas reproductoras estuvo directamente relacionada con la disponibilidad de roquedos. Los roquedos selec- cionados para nidihcar fueron de mayor altura (x = 52,9 m; N = 32), y ubicados sobre laderas de mayor pendiente {x = 34,7°; N = 31), que la media disponible. Existio una gran versatilidad entre parejas en cuanto al tipo de roquedo ocupado, debido a que el Aguila Perdicera nidifico en areas muy diferentes en funcion de la presion humana. De esta forma, los roquedos ocupados mas proximos a carreteras tuvieron una altura mayor que los que se encontraban lejos de las mismas. La seleccion de roquedos de gran altura, situados sobre pendientes elevadas, y con orientacion sur, podria estar rela- cionado con la falta de sustentacion en vuelo del Aguila Perdicera, al favorecer este tipo de roquedos la formacion de termicas y el vuelo de ladera. La preservacion de los roquedos de nidihcacion libres de la influencia antropica, podria ser la medida mas esencial requerida para la conservacion del Aguila Perdicera en el area de estudio. [Traduccion de Autores] Among Mediterranean raptors, the Bonelli’s Ea- gle {Hieraaetus fasdatus) has suffered one of the most severe population declines in Spain (Fernan- dez and Insausti 1990, Real et al. 1991), Portugal (Palma et al. 1984), France (Cugnase 1984, Chey- lan and Simeon 1985) and Greece (Hallmann 1985) that have resulted in its being listed as an Endangered European Raptor (Rocamora 1994). Recent data indicate that the principal European breeding population (80%) is located in Spain (Real et al. 1997), where the nesting population has decreased 25% from 1980-90 (Arroyo et al. 1995). Consequently, this species has been cata- logued as Vulnerable in Spain (Blanco and Gon- zalez 1992), and high-priority conservation has been urged (De Juana 1992). Information concerning habitat is fundamental for the management of raptor populations (Mosh- er et al. 1987). Raptors are among the few groups of birds whose numbers can be limited by the avail- ability of appropriate nesting places (Newton 1979). In Spain, Bonelli’s Eagles most frequently no June 1999 Cliff Selection by Bonelli’s Eagles 111 Table 1. Variables used to characterize Bonelli’s Eagle nest-sites. CLIFFNEST — number of cliffs with nests built by a pair NESTBUIL — number of nests built by a pair DISTNEST — greatest distance between nests belonging to the same pair (m) HEIGBAS — height from the base of the cliff to the nest (m) NEIGDIST — nearest-neighbor distance between adjacent pairs of Bonelli’s Eagles (km) AVACLIFF — availability of cliffs (percentage of 1 km^ squares with suitable cliffs for nesting in each territory) nest in cliffs and rarely in trees (Arroyo et al. 1995). While some aspects of the biology of this raptor are well-studied, nest-site selection has re- ceived only limited study. The two main studies in Spain (Gil-Sanchez et al. 1996, Sanchez-Zapata et al. 1996) refer to the selection and characteristics of used and unused territories. No detailed infor- mation is available concerning the choice of nest sites within territories or characteristics of nesting cliffs (Donazar et al. 1989). The aim of my study was to determine which cliffs in each territory were used for nesting of Bo- nelli’s Eagles, to describe characteristics of cliff nesting sites, and to determine how human activity affects this selection in southeastern Spain. Study Area and Methods The study was conducted in the province of Granada, southeastern Spain (36°45'-37“49'N, 2°40'-4°13'W) from 1995-97. The area is largely mountainous with altitudes ranging from 0-3482 m, and highly variable tempera- tures and rainfall. The vegetation includes different spe- cies of pines {Pinus spp.) and evergreen oaks {Quercus ilex) mixed with cultivated areas, mainly with olive trees {Olea europaea) and cereals (Rivas-Martinez 1985). A total of 119 nests located on 52 cliffs that were used by 32 different pairs of Bonelli’s Eagle was studied. Rap- tors frequently build more than one nest and use them alternately in different years (Newton 1979). Thus, all nests (regardless of whether they were occupied or not during the present study) were considered equally for the analysis if they were in occupied territories. The variables used in the analysis of nest-site charac- teristics are defined in Table 1. The nearest-neighbor dis- tance method from the last nest used was used to esti- mate breeding density of the pairs (Newton et al. 1977). For the analysis of cliff selection, 32 occupied cliffs (last cliff used for nesting by each pair) were compared with 32 unoccupied cliffs within the territories (one cliff per territory). The comparative analysis was performed with variables to characterize the cliffs and human dis- turbance in surrounding cliffs (Table 2). Because most pairs built nests in the highest cliff of each territory, the comparison was made with the highest unoccupied cliff suitable for nesting within each territory. 1 considered a cliff suitable for nesting when there were suitable cavities and ledges for nesting, when it was located at <1500 m elevation (the distributional limit of the Spanish popu- lation, Arroyo et al. 1995), when it was higher than 10 m, and farther than 500 m from an urban center (min- imal distances found for the popnlation studied) . I chose the unoccupied cliffs within each territory to eliminate the possibility that limited prey availability was the reason Table 2. Variables used to characterize occupied and unoccupied cliffs in territories used by Bonelli’s Eagles. ALTITUDE — height above sea level measured in the middle of the cliff (m) HEIGCLIFF— cliff height (m) HEIGVAL — height from the bottom of the valley to the base of the cliff (m) HEIGHILEV — height from the upper edge of the cliff to the summit of a hill located on the cliff (m) WIDTHVAL — ^width of the valley at the base of the cliff (m) SLOPE — inclination of the slope located at the base of the cliff (°) TOPIND — topographic irregularity index (total number of 20 m contour lines, cut by two lines equivalent to 2 km designed on topographic 1 :50 000 maps, in directions N-S and E-W, and crossed at the location of the cliff) DISVIL — distance from cliff to nearest urban center (m) DISPAVROAD — distance from cliff to the nearest paved road (m) DISUNPAVROAD — distance from cliff to the nearest unpaved road passable by vehicle (m) DISINHABUIL — distance from cliff to the nearest inhabited building (m) DISCULTIV — distance from cliff to the nearest cultivated field (m) KMPAVROAD — km of paved roads in the circular sampling area to the nearest 2 km KMUNPAVROAD — km of unpaved roads in the circular sampling area to the nearest 2 km 112 Ontiveros VoL. 33, No. 2 Table 3. Means, standard deviations (SD) and ranges of variables characterizing nest sites. Variable Mean SD Range CLIFFNEST 1.6 0.9 1-5 NESTBUIL 3.7 3.6 1-18 DISTNEST 774.1 897.7 1-2800 HEIGBAS 29.8 18.3 5-90 NEIGDIST 10.0 3.2 5.8-16 AVACLIFF 10.0 4.3 4.7-22 the cliff was unoccupied given that food availability di- rectly limits the distribution of some raptors (Newton 1979). The territory of each pair was considered to be a radius equal to half the average distance between nests of neigh- boring pairs, based on the last nest occupied during the study (Howell et al. 1978, Bednarz and Dinsmore 1981, Gilmer and Stewart 1984, Rich 1986, Gonzalez et al. 1992). The orientation of nest cliffs was compared with the distribution of all available cliffs within territories (N = 172). Cliff orientation was determined using a compass to the nearest 5°. To determine a mean angle of a cir- cular distribution, a simple calculation of an arithmetic mean of the observed angles is inadequate. Thus, specific methods for circular statistics were used for analyses of preference in nest placement orientation (Fisher 1995), Other variables were measured with an altimeter (VZ Per- formance; precision ±1 m), theodolite (Pentax PTH 20; precision ±10"), clinometer, compass and 1:50 000 to- pographic maps prepared by the Spanish Army Carto- graphic Service. A Pearson coefficient was used to determine the rela- tionship between variables. For occupied and unoccu- pied cliffs, the mean values of the variables were com- pared using paired t-tests. As is usual in this type of analysis (Gonzalez et al. 1992, Penteriani and Faivre 1997), a stepwise discriminant function analysis was con- ducted (STATISTICA StatSoft Inc. 1993). The 0.05% level of significance was used for including variables in each step of the analysis. Because the sample size could not be increased to three times the number of variables mea- sured (Willians and Titus 1988), a jackknifed classifica- tion was obtained for the analysis. Results Most of the cliffs occupied by Bonelli’s Eagles (96%, N — 50) were in river valleys and the nests were either in cavities (46.2%) or on ledges (53.8%). The remaining 4% of cliffs were sur- rounded by plains. The number of nests built by a pair (Table 3) appeared to be dependent on nest-site availability since nests were built on the majority of suitable ledges and cavities. One pair had a surprising 18 nests with a maximum distance of only 350 m be- tween them. The pairs with the highest availability of cliffs were closer to the nearest-neighbor pair (Table 1; r = -0.46, P = 0.009, N = 32). There- fore, breeding density was directly related to the availability of cliffs. Occupied and unoccupied cliffs differed signifi- cantly in height and slope at the base of the cliff (Table 4) . Nests were built on the highest cliffs with the steepest slopes. In fact, most of the pairs (84%) built nests on the highest suitable cliff in Table 4. Features of the cliffs analyzed. Mean, standard deviation (SD), and results of the Student’s t-tests. An asterisk indicates those tests that remained significant (P < 0.05) after Bonferroni sequential correction (Rice 1989). Variable Occupied Cliffs {N= 32) Mean ± SD Unoccupied Cliffs {N = 32) Mean ± SD t P ALTITUDE 937.9 -t- 324.2 969.4 -h 332.3 -1.37 0.18 HEIGCLIFF 52.9 ± 27.8 37.6 -h 19.2 4.85 0.00003* HEIGVAL 135.9 63.6 130.1 65.9 0.48 0.63 HEIGHILEV 147.5 -1- 255.1 165.9 -+- 190.9 -0.78 0.44 WIDTHVAL 573.2 -1- 410.0 698.3 -h 582.1 -1.01 0.32 SLOPE 34.7 8.2 30.3 5.9 3.63 0.001* TOPIND 56.9 15.9 54.4 -1- 15.0 1.35 0.18 DISVIL 3362.5 2047.6 3654.7 H- 2304.3 -1.46 0.15 DISPAVROAD 1640.6 1273.1 1856.2 + 1473.7 -1.24 0.22 DISUNPAVROAD 493.7 “h 342.9 554.7 + 1017.2 -0.35 0.73 DISINHABUIL 917.2 669.8 1092.2 H- 822.3 -1.22 0.23 DISCULTIV 1040.3 1198.4 1219.1 H- 1138.8 -0.72 0.47 KMPAVROAD 3.7 + 3.6 3.4 3.5 0.53 0.59 KMUNPAVROAD 5.2 2.5 6.0 ± 2.4 -1.80 0.08 June 1999 Cliff Selection by Bonelli’s Eagles 113 184 ® NESTING CUFFS E AVAILABLE CLIFFS Figure 1. Orientation for Bonelli’s Eagle nesting cliffs {N = 52) and available cliffs within territories (N = 172). Sample sizes are indicated in each direction and the mean orientation is indicated by arrows. their territory. In the stepwise discriminant analy- sis, occupied and unoccupied cliffs were best dis- tinguished by the following relationship: Occupied cliffs = -15.0262 + 0.6842SLOPE -H 0.0934HEIGCLIFF Unoccupied cliffs = - 10.8732 + 0.5938SLOPE -I- 0.0663HEIGCL1FF. Using these equations, 65.6% of occupied cliffs and 75.0% of unoccupied cliffs were correctly clas- sified. A jackknife classification reduced the cor- rect classification of occupied cliffs to 65.1% and unoccupied cliffs to 74.1%. The mean orientations and angular deviations (equivalent to SD) obtained with the trigonometric method (Fisher 1995) were 184° ± 74° and 324° ± 81° for nesting cliffs {N = 52) and available cliffs {N = 172), respectively (Fig. 1). There were signif- icant differences between study samples (Watson test: Y 2 = 4.96, P = 0.02). The analysis of 119 nests revealed a trend toward a southeastward orienta- tion (Rayleigh test: r — 0.178, P = 0.02; mean ori- entation = 121° ± 70°). Due to the height of nest cliffs near paved roads, I compared these with the other nest sites. Nest cliffs closer to paved roads than 1859 m (mean val- ue for the 52 cliffs with nests) were higher than nest cliffs located farther from paved roads (x = 59.4 ± 32.4 m, A = 30; X = 33.3 ± 20.8 m, N = 22, respectively; t = 3.31, P = 0.001). Discussion The results obtained for the elevational distri- bution of the pairs coincided with those of the overall Spanish population (Arroyo et al. 1995), but the number of nests built by pairs and their orientation differed from those observed in the Si- erra Morena region (Jordano 1981). This was probably due to smaller sample size {N =10 pairs) and lower availability of cliffs in the Sierra Morena area. In the Sierra Morena, a trend toward a north northwest orientation and an average of 1.8 nests per pair were observed. As in other raptor species, changes in nest orientation may be correlated with changes in latitude and elevation, which are both indicators of local temperature and insolation reg- imens (Mosher and White 1976). Nevertheless, a difference in the number of nests built was still found when the two pairs in this population with more than 10 nests were removed from the analysis (x= 2.9 ± 2.1). Some pairs occupied irregular cliffs with many cavities and ledges and built a large number of nests. In raptors, maintaining more than one nest is an obvious advantage, since pairs can shift nests if they are disturbed, if the nest has been taken over by another species, or if their first breeding at- tempt failed early (Newton 1979). Moreover, use of many nests may help in avoiding parasites which remain in nests (Winberger 1984) and kill young already weakened by starvation (Seidensticker and Reynolds 1971, Beecham and Kochert 1975). My results indicated that breeding density should be highest in uneven terrain. A similar trend was found by Ceballos and Donazar (1989) in a population of Egyptian Vultures {Neophron perc- nopterus) and by Donazar et al. (1993) for the Bearded Vulture ( Gypaetus barbatus) , both cliff-nest- ing raptors. They found breeding density to be di- recdy related to the availability of cliffs. Overheating and sunstroke are two factors that direcdy can limit the distribution of Bonelli’s Ea- gles due to their morphology which makes them agile and swift but limits the amount of lift they can generate (Parellada et al. 1984). This would explain why Bonelli’s Eagles were not found at >1500 m and why higher cliffs and steeper slopes were selected. Such nest-site selection improves the possibilities for thermal bubbles frequently used by Bonelli’s Eagle (Cheylan 1979, Parellada et al. 1984) and favors slope soaring, a common tech- nique in raptors with low aspect ratio wings such 114 Ontiveros VoL. 33, No. 2 as the Bonelli’s Eagle (Janes 1984, Parellada et al. 1984). Because the southeastern area of Spain is rather cold during the Bonelli’s Eagle breeding season, use of cliffs oriented toward the south, where the thermal bubbles are frequent, may be important for the reproductive success of Bonelli’s Eagles. Selection of higher than average cliffs has also been demonstrated in the Bearded Vulture (Donazar et al. 1993), which inhabits cold moun- tain climates where lift problems are similar to those of the Bonelli’s Eagle (Hiraldo et al. 1979, Brown 1988). The discriminant function correctly classified 65.6% of the occupied cliffs. Lack of a higher dis- crimination was apparently due to the heteroge- neity of the cliffs selected by Bonelli’s Eagles and their potential for human disturbance. Paved roads were frequently located in river valleys inhabited by Bonelli’s Eagle pairs. Human activity has been shown to influence the selection of nest sites in several species of raptors (Fyfe 1969, Hickey and Anderson 1969, Kumari 1974, Newton 1976, Sher- rod et al. 1977), and for some, the minimum ac- ceptable height of a cliff varies inversely with the degree of wilderness available (Newton 1979). Therefore, eagles can occupy lower cliffs far from paved roads, while in zones of heavy human use, higher cliffs must be used. The abandonment of some nests (N = 5) located on low cliffs in areas with high levels of human disturbance corroborat- ed this finding (Ontiveros 1997). A previous study of this same population ana- lyzed the habitat selection of Bonelli’s Eagle with and without competition from Golden Eagles {Aq- uila chrysaetos) (Gil-Sanchez et al. 1996). Several au- thors have doubted that Bonelli’s Eagles compete with Golden Eagles (Brosset 1961; Cheylan 1979; Jordano 1981; Clouet and Goar 1984). Rejecting such competition, Gil-Sanchez et al. (1996) found differences between occupied and unoccupied ter- ritories that only occurred in habitats undergoing cereal crop cultivation. This pattern of habitat se- lection and the results of my study on cliff selection within territories show that nest cliffs are the most important resource for habitat selection in Bonel- li’s Eagle, regardless of food supply. The availability of adequate nesting areas direct- ly influences habitat selection in raptors (Newton 1979, Janes 1985). My data indicate that, for Bo- nelli’s Eagles, suitable nest sites may be a more lim- iting resource than in other raptors, causing terri- tories of this eagle to overlap frequendy with human-populated areas (Brown 1976, Cheylan 1981, Parellada et al. 1984). Human activity in ter- ritories can negatively affect Bonelli’s Eagles and might account for the decline in the Mediterra- nean population in recent years when other rap- tors have recovered in Spain (Arroyo et al. 1990, 1995). The preservation of nest cliffs and protec- tion from surrounding disturbances (Cade 1974), is essential to ensure the survival of the Bonelli’s Eagle. Acknowledgments I wish to thank B. Arroyo, G.R. Bortolotti and J.M. Ple- guezuelos, for reviewing the original manuscript provid- ing valuable suggestions, and J.M. Gil by their data. R. Morales kindly helped with statistical treatment of the data. Literature Cited Arroyo, B., E. Ferreiro and V. Garza. 1990. El Aguila Real {Aquila chrysaetos) en Espaita. ICONA, Serie Tec- nica, Madrid, Spain. , and , 1995. El Aguila Perdicera {Hieraaetus fasciatus) en Espaha. Censo, reproduccion y conservacion. ICONA, Serie Tecnica, Madrid, Spain. Beecham,J.J. and M.N. Kochert. 1975. Breeding biology of the Golden Eagle in southwestern Idaho. Wilson Bull. 87:503-513. Bednarz, J.C. AND JJ. Dinsmore. 1981. Status, hatitatuse and management of Red-shouldered Hawks in Iowa. J. Wildl. Manage. 45:236-241. Blanco, J.C. and J.L. Gonzalez. 1992. Libro rojo de los vertebrados de Espaha. ICONA, Coleccion Tecnica, Madrid, Spain. Brosset, A. 1961. Ecologie des oiseaux du Maroc orien- tal. Trav. Inst. Sci. Cherif., Rabat, Morocco. Brown, C.J. 1988. A study of the Bearded Vulture Gypae- tus barbatus in southern Africa. Ph.D. dissertation, Univ. Natal, Pietermaritzburg, Natal. Brown, L. 1976. Eagles of the world. David and Charles, London, U.K. Cade, T.J. 1974. Plans for managing the survival of the Peregrine Falcon. Pages 89-104 in F.N. Hamerstrom, B.E. Harrell and R.R. Olendorff [Eds.], Management of raptors. Raptor Res. Rep. No. 2, Vermillion, SD U.S.A. Ceballos, O. and J.A. Donazar. 1989. Factors influenc- ing the breeding density and nest-site selection by the Egyptian Vulture {Neophron percnopterus) . J. Ornithol. 130:353-359. Cheylan, G. 1979. Recherches sur I'organisation du peu- plement de vertebres d'une montagne Mediterra- neenne: la Sainte Victoire (Bouches-du-Rhone) . These, Universite Pierre et Marie Curie, Paris, France. . 1981. Sur le role determinant de Labondance des ressouces dans le succes de reproduction de June 1999 Cliff Selection by Bonelli’s Eagles 115 I'aigle de Bonelli Hieraaetus fasciatus en provence. Ra- paces Mediterraneens, P.N.R.C. et annales du C.R.O.P. 1: 95-99. AND D. Simeon. 1985. La reproduction de I'aigle de Bonelli en Provence. Bulletin du Centre de Recherches Ornithohgiques de Provence 6:36-37. Clouet, M. and J.L. Goar. 1984. Relation morphologie- ecologie entre I'aigle royal {Aquila chrysaetos) et I'aigle de Bonelli {Hieraaetus fasciatus). Especes sympatriques dans le midi de la France. Rapaces Mediterraneens 2: 109-119. CUGNASE, J.M. 1984. L'aigle de Bonelli Hieraaetus fasciatus en Languedoc-Roussillon. Nos Oiseaux 37:22^-2^2. De Juana, E. 1992. Algunas prioridades en la conserva- cion de aves en Espana. Ardeola 39:73-84. DonAzar, J.A., O. Ceballos AND C. FernAndez. 1989. Fac- tors influencing the distribution and abundance of seven cliff-nesting raptors: a multivariate study. Pages 545-552 in B.-U. Meyburg and R.D. Chancellor [Eds.], Raptors in the modern world. World Working Group for Birds of Prey, London, U.K. , F. Hiraldo and J. Bustamante. 1993. Factors in- fluencing nest-site selection, breeding density and breeding success in the Bearded Vulture ( Gypaetus bar- batus).J. Appl. Ecol. 30:504—514. Fernandez, C. and J.A. Insausti. 1990. Golden Eagles take up territories abandoned by Bonelli 's Eagles in Northern Spain./. Raptor Res. 24:124—125. Fisher, N.L 1995. Statistical analysis of circular data. Cambridge Univ. Press, London, U.K. Fyfe, R. 1969. The Peregrine Falcon in northern Canada. Pages 101-114 mJ.J. Hickey [Ed.], Peregrine Falcon populations: their biology and decline. Univ. Wiscon- sin Press, Madison, W1 U.S.A. Gil-SAnchez, J.M., F. Molino-Garrido and G. Valenzue- la-Serrano. 1996. Seleccion de habitat de nidifica- cion por el Aguila perdicera {Hieraaetus fasciatus) en Granada (SE de Espana). Ardeo/ 0.05 in all cases). A total of 108 nests was studied (26 in 1987, 12 in 1988, 18 in 1989, 39 in 1990, and 13 in 1991). The Cornalvo area had 69 nests, with the remainder located in La Se- rena (18), Llanos de Badajoz-La Albuera (17), and Lla- nos de Caceres (4) , respectively. All nests were in cereal crops; 67 (63%) in barley, 23 (21%) in wheat, 9 (8%) in oats, and 9 (8%) in mixed barley-oat crops. Harrier col- onies were monitored regularly (once a week) from the beginning of the breeding cycle (early April); visits in- creased (every 3-4 d) during the nestling period (15 May-10 July). During the first visit, 88% of nests (N = 95) were in the incubation period, with the remainder (N = 13) in the nestling stage. Each breeding season, all the nests in some colonies were treated as controls (N = 36) and no conservation measures were applied to these nests. At the other nests (N = 72), management mea- sures included the removal of young (or eggs) from nests prior to harvesting and baling, and their subsequent re- turn to the nest after harvesting. It took <1 hr to remove eggs and young and return them to nests in all cases so artificial incubation and nursing of young were not nec- essary. There were no differences in laying date or clutch size between managed and unmanaged clutches (Mann- Whitney U test, F = 0.64 and F — 0.26). In order to assess seasonal patterns in breeding per- formance, laying dates were classified in 10-d periods starting from 11 April (all years pooled), resulting in five phenological classes in relation to the onset of egg laying (Class 1; 11-20 April, 6 clutches; Class 2; 21-30 April, 20, Class 3: 1-10 May, 26; Class 4: 11-20 May, 10; and Class 5; >20 May, 10). For Montagu’s Harrier, harvesting ap- pears to be the main determinant of breeding success in arable farmland such as that in the study area (Corbacho et al. 1997). Hence, we classified clutches according to their breeding stage at harvesting time; this resulted in pairs at incubation stage (N= 13), pairs at nestling stage (rearing young, N = 65), and pairs with Hedgings (N = 12). At the same time, we separated nestling pairs ac- cording to the age of broods at harvest time; early age (1-10 d, N = 11), middle age (11-20 d, N = 26) and late age (>20 d, N = 16). Analysis of reproductive output were performed using: hatching success as the number of eggs that hatched versus the number of eggs laid, in- cluding clutches deserted or preyed upon; nestling mor- tality rate (% of young that died versus the number hatched); proportion of successful pairs (the number of pairs with at least one nestling fledged versus total num- ber of pairs that laid eggs); and breeding success (the mean number of fledglings per laying pair). Results are shown as arithmetic mean ± standard deviation or %, with sample size indicated in each case. Nonparametric methods and two-way analysis of variance with interaction was used to test for differences (Zar 1996), with the exact method used in each case indicated. Results We found the reproductive success of Montagu’s Harriers to be strongly dependent on management measures undertaken at harvest time. They result- ed in an important decrease in nestling mortality rate (28% in managed pairs vs. 67% in control ones; G-test, P < 0.001; Ni = 55, N 2 — 18) which in turn resulted in a higher proportion of success- ful pairs (75% vs. 29%; G-test, P < 0.001) as well as breeding success (2.04 ± 1.53 vs. 0.77 ± 1.28; Mann-Whitney 1/ test, P = 0.001) in managed (N = 68) than in unmanaged (N = 34) pairs. A two-way Anova showed that breeding success was highly dependent on both laying date (P4 56 = 4.45, P = 0.003) and management performed (Pi 56 — 8.51, P = 0.005) (Fig. 1). Hatching success, which was unaffected by management actions, showed a large seasonal decline (Spearman rank correlation test, = —0.57, P< 0.001, N= 59, all data pooled). Nestling mortality rate increased as the laying date was increased in unmanaged broods (r^ — 0.69, P = 0.009, N — 14), and high mortality was recorded in late broods (80% in Class 3, 100% in Classes 4 and 5). However, this attribute showed no significant seasonal increase in managed clutches (17% in Class 1, 58% in Class 2, June 1999 Conservation Measures for Montagu’s Harrier 119 Figure 1. Variation in the percent successful pairs and breeding success of Montagu’s Harriers in agricultural areas of southwestern Spain according to laying date and management measures undertaken. Laying date categories were determined as follows: day 1 = 10 April; I = 1-10; II = 11-20; III == 21-30; IV = 31-40; V = >40 days after 10 April. 41% in Class 3, 33% in Class 4, and 50% in Class 5; — 0.16, P = 0.40, N = 30). As a result, man- aged clutches had a high proportion of successful pairs as well as breeding success; these values were maintained until well into the breeding season (Fig. 1) . In contrast, unmanaged pairs showed high breeding failure, because only early clutches (Clas- ses 1 and 2) had good reproductive output (Fig. 1). However, the effect of laying date was not lin- ear, even in the absence of management. This was due to the variations in timing of farming activities (especially the harvest) either between localities or from one year to another. The different crops se- lected as nesting habitats in any one year or locality were also involved for the same reason. It was better to relate reproductive output to the breeding stage of each pair at harvest time than to relate it to laying date. In our study, all pairs at incubation stage during harvesting {N =13) failed because of total hatching failure. In contrast, all pairs that raised young before harvest (pairs that fledged young) showed a high breeding success both in managed (3.14 ± 0.69, N = 7) and un- managed clutches (3.00 ± 0.00, N = 5). Two-way Anova (breeding stage-management) showed that the stage at harvest time was the only factor deter- mining significant differences in breeding success (^ 2,71 ~ 20.62, P < 0.001). Therefore, breeding stage at this critical moment appeared to be the main factor affecting reproductive output. In this sense, conservation measures were clearly effective at the nestling stage. First, management actions greatly increased survival of young (30 to 63%; G- test, P < 0.001, Ni = 11, N 2 = 37), which in turn resulted in a higher proportion of successful pairs (82% vs. 47%; G-test, P< 0.004, = 39, N 2 = 13) and breeding success (2.08 ± 1.40, N = 39 vs. 1.16 ± 1.34, N = 19; Mann-Whitney U test, P = 0.007) in managed versus unmanaged pairs. Second, man- aged pairs at nestling stage had a reproductive suc- cess for both attributes that was similar to that of pairs at fledging stage (G-test, P = 0.09, and Mann- Whitney U test, P = 0.30, respectively). The older the broods were, the greater the breeding success for both managed (Spearman rank correlation test; = 0.84, P = 0.001, N = 14) and unmanaged broods (r^ = 0.68; P = 0.002; N = 19) . Pairs with broods in the early-age group failed completely, while all pairs in the late-age group were successful, with high breeding success for both managed (2.60 ± 0.55, N = 8) and unman- aged (2.14 ± 0.93, N = 8) broods (Mann-Whitney U test, P = 0.18). Conservation measures appeared to be effective only for middle-age broods, which had higher scores in managed than in unmanaged pairs for both proportion of successful pairs (75% 120 CORBACHO ET AL. VoL. 33, No. 2 Clutch size CD B (D g. 3' CO 0 ) c o o (D (/> 0 > Figure 2. Variation in percent of successful pairs and breeding success of Montagu’s Harrier in agricultural areas of southwestern Spain according to clutch size and management measures undertaken. vs. 14%; G-test, P < 0.001) and breeding success (1.25 ± 0.96 vs. 0.29 ± 0.76, = 16 and = 8; Mann-Whitney U test, P = 0.045) , respectively. Nesting habitat selection affected the success of pairs, because of variations in timing of harvest among the different crops. No differences in mean harvesting date were found among wheat (14 June), oats (14 June), or mixed wheat-oats (12 June) (Kruskal-Wallis test, P = 0.82), and thus these data were pooled. However, the timing of harvest for barley (26 June) was significandy later than for the rest of the cereal crops (Mann-Whit- ney U test, P < 0.001), and this in turn affected the age of young at harvest time. The later the harvest occurred, the older were the broods (Spearman rank correlation, = —0.60, P < 0.001, N = 57). Thus, in barley crops at harvest age of broods (24.31 ± 10.83, N = 13) was older than in both wheat and/or oat crops (15.16 ± 8.48, N = 44) (Mann-Whitney U test, P = 0.008). Similarly, the number of pairs with fledglings at harvest time in barley crops (35.4%) was higher than in the other crops (9.30%) (G-test, P = 0.04, df = 2). However, with respect to breeding success, the few data avail- able (only two clutches) for unmanaged pairs nest- ing in barley crops made it impossible to test the influence of nesting habitat selection. In managed clutches, no differences in reproductive output were observed between pairs nesting in wheat-oats and barley crops with regard to breeding success (1.53 ± 1.46 vs. 1.88 ± 1.58; Mann-Whitney C/test, P = 0.48) and proportion of successful pairs (66.67% vs. 70.59%; G-test, P = 0.33; = 30 and A ^2 = 1^7 in both cases). This pointed out the effectiveness of conservation measures on the breeding of Montagu’s Harriers in farmlands. Breeding performance showed clear trends that depended on clutch size but these differences were independent of management measures per- formed. No seasonal decrease in clutch size was associated (Spearman rank correlation test, P > 0.05; Corbacho et al. 1997). The greater the num- ber of eggs laid, the greater the hatching success (Spearman rank correlation; = 0.32; P = 0.002; N = 92) that was realized. Clutches of 1, 2, and 3 eggs yielded a lower reproductive output for all var- iables in comparison to 4- and 5-egg clutches in both managed and unmanaged clutches (Fig. 2). Nevertheless, for a given clutch size, managed clutches resulted in a higher proportion of success- ful pairs and greater breeding success than un- managed clutches (Fig. 2). Thus, both clutch size (^ 4.82 ~ 9.31, P < 0.001) and management actions (^ 1,82 ~ 5.68, P = 0.019) had decisive influences on breeding success. Discussion Breeding success of Montagu’s Harrier in Med- iterranean areas, where breeding occurs chiefly in cereal crops, appears to be highly dependent on June 1999 Conservation Measures for Montagu’s Harrier 121 conservation measures (Corbacho et al. 1997). We showed that the effectiveness of management ac- tions had some constraints. Laying date was the main factor affecting it, by its influence on breed- ing stage and brood-age at harvest time. Thus, nei- ther hatching success nor mortality rate of young increased with the use of conservation measures in very delayed clutches. This happened because of the helpless situation of clutches (eggs in un- hatched clutches) or broods (young <10-d old) during and after harvesting as they became more vulnerable to predation and/ or theft, or desertion by females. The longer the eggs and broods re- mained in a defenseless situation, the lower the reproductive success. On average, all pairs that de- layed laying until after 15 May were unsuccessful, regardless of any management applied. However, breeding failure occurred in unmanaged clutches even when laying occurred in the beginning of May. Although this 15-d period may not be impor- tant, it occurs at a time when a considerable num- ber of pairs start their clutches (33%, Corbacho et al. 1997). A comparative analysis in the western Palearctic showed that although breeding season starts earli- er in the Mediterranean area than in northern Eu- rope (Schipper 1979, Corbacho et al. 1997), the timing of harvest in cereal crops in southern lati- tudes also occurs earlier. Consequently, the overlap of the breeding cycle with the postharvest period is greater in southern populations, with the result that the number of pairs that finish breeding be- fore harvesting decreases southwards (10-40%, Berthemy et al. 1983; 70%, Pandolfi and Giacchini 1991; 40%, Area 1989; over 50%, Arroyo 1995; 18%, Castano 1997; 16%, this study). In our study, the number of pairs affected by farming practices (either at incubation or brooding stages) varied from high (>80%) to complete (100%). The in- creasing use of early varieties of cereal crops may constitute a further threat to breeding harriers in arable lands. Breeding condition of the pair may also play a role in determining reproductive output (Schipper 1979, Newton 1979), but in agricultural environ- ments this factor is overshadowed by farming prac- tices. If clutch size is an indicator of pair quality (Drent and Daan 1980), low quality breeders achieved a low reproductive output despite man- agement (Corbacho et al. 1997, this study) . In conclusion, conservation measures offset the negative influence of intensified mechanization of farming practices on Montagu’s Harrier nesting in cereal fields, with the result that breeding success of managed populations in agricultural environ- ments of Mediterranean areas was similar to that of northern European breeders in natural habitats (Corbacho et al. 1997). However, predation and theft of young was more pronounced in managed (50%) than in unmanaged nests (30%), suggesting that management activities attract attention from predators and humans (Area 1989). Therefore, al- though cultivated areas in southern Europe seem to provide suitable habitat for the species, integral conservation measures are urgendy needed, espe- cially because that region provides refuge to one of the largest populations of Montagu’s Harrier in the western Palearctic (Berthemy et al. 1983, Tuck- er and Heath 1994, Ferrero 1996). In addition to conventional conservation schemes (removal of young to increase survival), other actions such as delayed harvesting or setting sheltering areas around main colonies should be considered. Such actions could be accompanied by subsidies for farmers in order to compensate for any economic losses incurred. Environmental education pro- grams should be carried out in the areas near the sites of the main colonies, because of the large number of nests that are unsuccessful because of theft and destruction at harvest time. Acknowledgments A. Lopez, A. Fernandez, J.L. Perez, P. Corbacho, and the Guard Service of Direccion General de Medio Am- biente of Junta de Extremadura assisted in field studies. We thank A. Munoz del Viejo and two anonymous ref- erees for useful comments on a earlier version of this manuscript. Thanks also to J. McCue and R. Moran for English translations. Literature Cited ArcA, G. 1989. La conservazione dell’Albanella minore Circus pygargus nelle aree agricole della Maremma Tosco-Laziale. Atti Conv. Ital. Ornitol, Rome, Italy. Arroyo, B. 1995. Breeding ecology and nest dispersion of Montagu’s Harrier in central Spain. Ph.D. disser- tation. Oxford Univ., Oxford, U.K. Berthemy, B., P. Dabin and M. Terrasse. 1983. Recense- ment et protection d’une espece protegee: le Busard cendre. Le Courier de la Nature 83:10-16. Castano, J.P. 1995. Efecto de la actividad de siega y cau- sas de fracaso reproductivo en una poblacion de aguilucho cenizo Circus pygargus L. en el SE de Ciudad Real. Ardeola 42:167-172. . 1997. Fenologia de puesta y parametros reprod- uctivos en una poblacion de aguilucho cenizo Circus pygargus en el Campo de Montiel. Ardeola 44:51—59. 122 CORBACHO ET AL. VOL. 33, No. 2 CoRBACHO, C., J.M. Sanchez and A. Sanchez. 1997. Breeding biology of Montagu’s Harriers ( Circus pygar- gus) in agricultural environments of the southwestern Iberian Peninsula and comparison with other popu- lations in the Western Palearctic. Bird Study 44:166- 175. Cramp, S. and K.E.L. Simmons. 1980. The birds of the western Palearctic. Vol. II. Oxford Univ. Press, Ox- ford, U.K. Drent, R.H. and S. Daan. 1980. The prudent parent: energetic adjustements in avian breeding. Ardea 68: 225-252. Ferrero, J.J. 1996. La poblacion iberica del aguilucho cenizo Circus pygargus. Alytes 7:539-560. Newton, I. 1979. Population ecology of raptors. T. & A.D. Poyser, London, U.K. Pandolfi, M. and P. Giacchini. 1991. Distribuzzione e successo riproduttivo di Albanella minore, Circus py- gargus, nelle Marche. Riv. Ital. Omitol. 61:25—32. Perez-Chiscano, J.L. and M. Fernandez-Cruz. 1971. So- bre Crus grus y Circus pygargus en Extremadura. Ardeo- la (vol. esp.):509— 574. Schipper, W.J.A. 1979. A comparison of breeding ecology in three European harriers {Circus). Arrfm 66:77-102. Tucker, G.M. and M.F. Heath. 1994. Birds in Europe. Their conservation status. Bird Life Conservation Ser. No. 3. Bird Life International, Cambridge, U.K. Zar, J.H. 1996. Biostatistical analysis. 3rd Ed. Prentice Hall, Princeton, NJ U.S.A. Received 29 December 1997; accepted 4 February 1999 J Raptor Res. 33 (2): 123-1 27 © 1999 The Raptor Research Foundation, Inc. COOPERATIVE HUNTING OF JACKDAWS BY THE LANNER FALCON (FALCO BIARMICUS) Giovanni Leonardi^ Avian Science and Conservation Centre, Macdonald Campus of McGill University, 21, 111 Lakeshore Road, Ste. Anne de Bellevue, Quebec, H9X 3V9 Canada Abstract. — Cooperative hunting has been recorded for several subspecies of Tanner Falcon (Falco biar- micus). On average, the success rate for pairs is higher than for single birds. During 1988-90, 1 collected data on the success of five Tanner Falcon pairs that cooperatively hunted Jackdaws {Corvus monedula) in western Sicily. Fifty-three percent of attacks were aimed at larger groups of Jackdaws. Males made most of the initial attacks (74%) but prey captures were mainly made by females (87%). Pairs tended not to share prey and used visual contact to coordinate chases. Most attacks were by partial surprise (60.8%), followed by nonsurprise (21.6%), and surprise attacks (17.6%). Surprise attacks tended to involve small flocks of Jackdaws, whereas partial surprise tended to involve large flocks. Key Words: Lanner Falcon] Falco biarmicus; cooperative hunting, Sicily; Jackdaw; prey group size. Caza cooperativa de Corvus monedula por Falco biarmicus Resumen. — Ta caza cooperativa de Falco biarmicus ha sido registrada para varias subespecies a lo largo de su distribucion. En promedio la tasa de exito por pareja es mas alta que la individual. Durante 1988- 90 recolecte datos sobre el exito de la caza cooperativa de Corvus monedula de cinco parejas en el oeste de Sicilia. Cincuenta y tres porciento de los ataques fueron dirigidos a grandes grupos de Corvus monedula. Tos machos efectuaron los ataques iniciales (74%) pero la captura de presas fue efectuada por las hembras (87%). Tas parejas tendian a no compartir la presa y utilizaron contactos visuales para coordinar las persecusiones (60.8%), seguidas de ataques sin sorpresa (21.6%), y ataques sorpresivos (17.6%). Tos ataques por sorpresa involucraron pequenas parvadas de Corvus monedula, mientras que los parcialmente sorpresivos involucraron parvadas mas grandes. [Traduccion de Cesar Marquez] Cooperative hunting is a social foraging behav- ior where predators coordinate their movements to increase efficiency of capture (Ellis et al. 1993), Prey may be shared among members according to social organization, prey size and individual func- tional role (Bednarz 1988, Ellis et al. 1993). Pair hunting is cooperative when participants perform separate roles. In certain species and under certain circumstances, cooperative hunting is more suc- cessful than solitary foraging (Hector 1986, Thiol- lay 1988, Yosef 1991, Ellis et al. 1993). Cooperative hunting in the genus Falco seems to be restricted to bird-eating species, such as Lanner Falcons {Falco biarmicus), Aplomado Falcons {F. fe- moralis), and Red-headed Falcons {F. chicquera), which inhabit semi-open savannas and desert and Mediterranean scrub (Mebs 1959, Osborne 1981, ^ Present address: Via Santagelo Fulci, 28, 1-95127 Ca- tania, Italy. Hector 1986). Southern Mediterranean Peregrine Falcons {F. peregrinus hrookei) hunt cooperatively in areas where prey density is low (Thiollay 1988). Cooperative hunting in Lanner Falcons has been recorded for several subspecies throughout the species’ geographic range (Cramp and Simmons 1980, Tarboton and Allan 1984, Leonard! et al. 1992) . Lanner Falcon pairs pursue swift flying prey (e.g., swifts [AjbM.s spp,]) along parallel paths (Mir- abelli 1982, Bijlsma 1990) . They hunt flocks of gre- garious small birds (e.g., swallows spp.]) working together with repeated stoops upon the same individual (Mirabelli 1982). In contrast, for larger perched prey (e.g., shorebirds and pigeons [Columbasp.]) , one falcon flushes the quarry while it is taken by the mate (Mebs 1959, Massa et al. 1991, Yosef 1988). Partners have distinct roles. Males usually attack and direct prey toward females (Yosef 1991) and females tend to pursue large prey (Brossett 1961, Tree 1963, Kemp 1993). Success 123 124 Leonardi VoL. 33, No. 2 rates when hunting in pairs (20-25%) are higher than that of single birds hunting alone (15-40%) (Bijlsma 1990, Yosef 1991, Kemp 1993). This paper describes my observations of coop- erative hunting in Tanner Falcons nesting in west- ern Sicily. In this region, pairs frequently attack co- lonial nesting Jackdaws {Corvus monedula). This provided an opportunity to compare success rates among attacks on different sized flocks, as it relat- ed to sex of pursuers and attack strategies utilized. Study Area I studied Tanner Falcons on the island of Sicily in the central Mediterranean. I observed five pairs during the breeding season: two breeding pairs near the northern periphery of the Sicilian distribution and three pairs in a southern area where the species was studied previously by Mascara (1986). The climate of the northern study area is temperate- wet with 600-800 mm of rainfall and an average annual temperature of 12-14°C. The southern study area has a subarid climate (<600 mm of rainfall and temperature >16°C) (Instituto Geografico De Agostini 1987). Land use in the study areas was predominately farming and pasture. Cereal farming and pasturelands covered by olive (Olea europaea) and prickly pear {Opuntia ficus- mdica) cultivation dominated northern open spaces. The southern study area was largely in a wheat monoculture with interposing spots of xeric Mediterranean vegetation and small Eucalyptus plantations. Within both study areas, lanners nested on clay-sand and calcareous cliffs with heights of 50-1150 m (Massa et al. 1991). Methods I visited breeding sites 21 times during two prerepro- ductive periods (November-January 1988-90). Each breeding site was visited 10 times for 55 total H. 1 watched Tanner Falcons hunting in pairs from 200-600 m with 8 X 40 and 10 X 40 binoculars. Age and sex of observed falcons was recorded for each sighting accord- ing to criteria in Cramp and Simmons (1980) and Porter et al. (1981). Attacks were defined as very rapid flights or stoops to- ward one or more clearly observed prey (an individual or group of specific prey species) (Cresswell 1994, 1996). First attacks were defined as the first, fast approach by falcons toward potential prey. During each attack, 1 re- corded the following data: position and sex of each fal- con at the start of the attack, size of the prey flock, and type of attack strategy. 1 placed attack strategies into three categories: surprise attacks, partial surprise attacks, and nonsurprise attacks. In surprise attacks, Tanner Falcons first approached close to Jackdaws from behind rock cliffs. In partial surprise attacks, one of the two attacking falcons was visible to prey while the other falcon attacked by surprise. In partial surprise attacks, two perched fal- cons would depart at different times (Yosef 1991, Kemp 1993). In nonsurprise attacks, both falcons were visible at the onset of attacks, then they tried to encircle Jackdaw flocks (Cresswell 1994, 1996). In nonsiirprise attacks, one falcon stooped on prey after soaring while the other at- tempted to flush prey (Mebs 1959, Massa et al. 1991, Kemp 1993, Jenkins 1995). I observed Tanner Falcons cooperatively hunting both Rock Pigeons {Columba livia) and Jackdaw flocks near their nests. Both prey species nest on cliffs 100-300 m from Tanner Falcon nests (Sodhi et al. 1990, Suhonen et al. 1994). For evaluating the importance of cooperative hunting, I only investigated hunts of Jackdaws. Prelimi- nary observations indicated that single female Tanner Falcons initiated nearly all pursuits of pigeons. Also, <5% of the total attempts on pigeons (N = 32) were per- formed by males. Cooperative hunting, and necessarily, participation by males, was more common in hunts of Jackdaws. In addition, Jackdaws consistently comprised a large percentage of dietary biomass for lanners in Sicily (Massa et al. 1991, Leonardi et al. 1992, Leonardi 1994) Finally, Jackdaws responded to attacking falcons with in- tricate forms of mobbing behavior. This provided an op- portunity to investigate interactions between cooperative hunting and antipredator defense behavior (Kenward 1978, Caraco et al. 1980, Turner and Pitcher 1986, Cres- swell 1994). The number of Jackdaws present was estimated daily by counting the maximum number of birds seen simul- taneously. Jackdaw colonies typically contained 20-70 in- dividuals. During an attack, I estimated the size of each flock attacked by assuming the members to be all birds within 25 m of each other (Cresswell 1994, 1996). At times entire colonies behaved as a single flock. Under these circumstances, I counted the number of individuals in the group first attacked (Kenward 1978). For statistical comparisons, I placed Jackdaw flocks into three size cat- egories according to previous studies of predation on prey groups (Kenward 1978, Cresswell 1994, 1996): 2-10, 11-30 and 31-50 individuals. I assessed the validity of the above flock size classes for this study through preliminary observations of flocking reactions measured for single and paired Tanner Falcons (Leonardi 1991, Leonardi un- publ. data). I compared F-frequencies of hunting strategies and success rates among different flock sizes and strategies using chi-squared tests and Gtests (Zar 1984). I used Cochran’s corrected chi-square test for differences be- tween males and females using a 2 X 2 contingency table (Zar 1984). Results In 52 cooperative hunts, I detected no vocaliza- tions which might have functioned to coordinate pursuits. Females alone ate 70% of prey captured in cooperative hunts (captures N = 10). In only 2 of 16 cases (12%), males fed on prey captured in cooperative hunts after the departure of females. Although Tanner Falcons preferred to attack larg- er flocks (Table 1; = 12.33, df = 2; P < 0.001), hunting success was inversely proportional to flock size (G = 10.7, df = 2; P < 0.005). Female Tanner Falcons initiated attacks less of- ten than did males (26% vs. 74%). Although males preferred to pursue larger prey (87% of 52 pur- June 1999 Cooperative Hunting by Lanner Falcons 125 Table 1. Distribution of Lanner Falcon attacks on flocks of Jackdaw by cooperative hunting in Sicily. Jackdaw Flock Size Class 2-10 11-30 31-50 Attempts 10 14 27 Kills 2 5 9 Total 12 19 36 Captures (%) 20 35.7 33.3 suits) and larger flocks more often than did fe- males (Table 2; G = 13.9; df = 2; P< 0.001), male hunting success rates tended to be lower than those of females (Table 2; ~ 2.86, df = 2; P < 0 . 10 ). banners attacked Jackdaws by partial surprise (60.8%) much more frequently than they did by nonsurprise (21.6%) and surprise attacks (17.6%) {N — 52). Although degree of surprise is one of the most important factors in improving the suc- cess of raptor attacks, lanners used this technique in only nine of 52 attempts. Also, open attacks give time for antipredatory behavior by prey. Neverthe- less, partial surprise was used significantly more of- ten (x^ = 17.40, df — 2; P < 0.001). In addition, lanner pairs captured more prey using nonsurprise attacks (Table 3; x^ = 11.90, df = 2; P < 0.01). Cooperative hunting techniques were not uni- formly distributed among prey flock classes. Hunt- ing success in relation to prey flock size was signif- icant for partial surprise on larger groups (22%; Table 3; P 0.01) and nonsurprise attacks on me- dium flocks (27%; x^ = 14.40, df = 2; P < 0.01). Discussion Evidence of coordinative signaling among hunt- ing predators is indicative that hunts are coopera- tive (Hector 1986, Ellis et al. 1993). Male Aplo- mado Falcons initiate attacks and then vocalize a “chip” call (Keddy-Hector pers. comm.). Although I detected no vocalizations among hunting lan- ners, Thomsett (1987) reported that pairs of lan- ners hunting bats gave chupping calls. Mebs (1959), however, failed to mention any calls given by cooperative hunting lanners in Sicily. Partici- pants in hunts, however, can coordinate pursuits without vocal signals. Massa et al. (1991) suggested that partners monitor their movements by visual contact. Predators should avoid vocalizations dur- ing surprise attacks, which would reveal their pres- Table 2. Capture success (%) from first attacks of Lan- ner Falcons cooperatively hunting flocks of Jackdaws in Sicily. Jackdaw Flock Size Class 2-10 11-30 31-50 Total Males First attack 2 12 24 38 Kills 0 1 1 2 Captures (%) 0 0.08 0.04 Females First attack 8 2 3 13 Kills 2 5 7 14 Captures (%) 0.25 2.5 2.3 ence. In partial surprise attacks, flying Lanner Fal- con males from outside the colony area would suddenly stoop on Jackdaws. Prey capture percentage of this study was lower (31%) than that observed for other lanner subspe- cies (50%; Yosef 1991, Kemp 1993) and Aplomado Falcons (45%; Hector 1986). Sicilian lanners pur- sued small- and medium-sized prey with solitary hunting strategies and used cooperative hunting for large-size prey like Jackdaws. Nevertheless, this low percentage may have been due to Jackdaw an- tipredator behavior. Large Jackdaw flocks frequent- ly used mobbing (43%, N = 58) against lanners. This active defense, combined with the dilution ef- fect of individuals in a flock, can improve predator avoidance by prey. The dilution effect is an advan- tage because individuals are less likely to be taken by predators when in a flock (Turner and Pitcher 1986). Morgan and Godin (1985) reported that the rate of predator attack per individual prey is inversely proportional to group size. Although examples of role reversal are known (Mebs 1959, Mirabelli 1982, Massa et al. 1991), the Table 3. Percent hunting successes (kills/attempts; total of 52 attempts, 16 kills) by Lanner Falcons hunting co- operatively on flocks of Jackdaws in Sicily. Jackdaw Flock Size Class Attack Type 2-10 11-30 31-50 Partial 0 6 22 surprise Surprise 11 11 0 Nonsurprise 9 27 9 126 Leonardi VoL. 33, No. 2 male success rate of <1% was irrelevant in com- parison to the 50% reported. This was probably because of the strong reversed sexual dimorphism (RSD) of this species and its tendency not to share prey. In other words, females physically dominated males during hunts and feedings. RSD may also account for divergences in hunting and prey choice. Males of F. b. feldeggi weigh 69% that of females and capture prey which average 45% the size of the female’s prey (Leonardi et al. 1992). It is likely that RSD favors cooperative hunting, since it allows the hunting of a wide range of prey and also the use of different hunting strategies. Data on flock size choice showed that lanners prefer to attack larger groups. In previous studies of flocking behavior and hunting, hunting success has been shown to be inversely proportional to flock size (Kenward 1978, Turner and Pitcher 1986, Cresswell 1994, Krause and Godin 1995). Krause and Godin (1995) suggested that flock conspicu- ousness, rather than flock size per se, influenced predator choice. Flock conspicuousness lends to repeated attacks in a single chase, thereby increas- ing success (Krause and Godin 1995). In Jackdaws, antipredator defense is based on the group’s con- spicuousness (which determines the encounter rate) and on the total number of individuals in the group (dilution effect; Turner and Pitcher 1986). In my study, the effect of group conspicuousness on rates of encounter with falcons may have been immaterial because Jackdaws lived so close to nest- ing lanners (Pitcher 1986, Krause and Godin 1995). As in my study, partial surprise was the strategy most commonly used by cooperative hunting Tan- ner Falcons in South Africa (Kemp 1993, Jenkins 1995). Sicilian lanners frequently use this strategy (60.8%) in capturing Jackdaws only. In South Af- rica, nonsurprise attacks were aimed at small prey (Kemp 1993, Jenkins 1995). Previous Sicilian stud- ies described nonsurprise attacks as frequent co- operative techniques used against larger prey (Mebs 1959, Massa et al. 1991). My data indicated a subordinate use of this strategy in comparison with the partial surprise attack. Inversely, lanners using nonsurprise attacks had good hunting suc- cess rates. This technique was probably used be- cause it involved energetically inexpensive soaring and resulted in relatively high hunting success (Jenkins 1995). The surprise attack was reported as the most im- portant factor in Peregrine Falcon and Merlin {Fal- co columbarius) hunting success when pursuing small flocks of birds (Cresswell 1996). In this study, sur- prise attacks were less successful than were other strategies (17.6%) and were employed mostly for attacks of pigeons. In South Africa, surprise attacks from fast, low coursing flight were principally aimed at small birds and doves {Streptopelia spp.; Kemp 1993). In my study in Sicily, surprise attacks on Jackdaws caused intense confusion inside flocks. This confusion, and the dilution effect, pro- duced an abatement effect; Tanner Falcons had difficulty attacking the group repeatedly, decreas- ing capture chances (Leonardi 1991, Turner and Pitcher 1986, Krause and Godin 1995). Acknowledgments Special thanks are due to Alan Kemp, C.M. White, and R. Yosef for their invaluable suggestions and critical read- ing of the manuscript. Dean P. Keddy-Hector, Steve Sher- rod, David Ellis, and Daniel Varland made extensive com- ments that improved the paper. Literature Cited Bednarz, J.C. 1988. Cooperative hunting in Harris’ Hawks {Parabuteo unicinctus) . Sdewce 239:1525-1527. Bijlsma, R.G. 1990. Predation by large falcons on winter- ing waders in the Banc d’Arguin, Mauritania. Ardea 78:75-82. Brossett, a. 1961. Ecologie des oiseaux du Maroc ori- ental, Faucon Lanier. Trav. Inst. Sci. Cherif. Ser. Zool 22:32-33. Caraco, T., S. Martindale and H.R. Pulliam. 1980. Avi- an flocking in the presence of a predator. Nature 285 400-401. Cramp. S. and K.E.L, Simmons, [Eds.] 1980. The birds of the western Palearctic, Vol. 2. University Press, Ox- ford, UK. Cresswell, W. 1994. Flocking is an effective antipredator strategy in Redshanks, Tringa totanus. Anim. Behav. 47 433-442. . 1996. Surprise as a winter hunting strategy in Sparrowhawks Accipiter nisus, Peregrines Falco peregn- nus and Merlins E columbarius. Ibis 138:684-692. Ellis, D.H., J.C. Bednarz, D.G. Smith and S.P. Flemming. 1993. Social foraging classes in raptorial birds. Bio- Science 43:14-20. Hector, D.P. 1986. Cooperative hunting and its relation- ship to foraging success and prey size in an avian predator. Ethology 73:247-257. Instituto Geografico De Agostini. 1987. Atlante ge- nerale metodico. IGDA, Novara, Italy. Jenkins, A.C. 1995. Morphometries and flight perfor- mance of southern African Peregrine and Lanner Fal- cons. J. Avian Biol. 26:49-58. Kemp, A.C. 1993. Breeding biology of Lanner Falcons near Pretoria, South Africa. Ostrich 64:26-31. June 1999 Cooperative Hunting by Lanner Falcons 127 Kenward, R.E. 1978. Hawks and doves; factors affecting success and selection in Goshawk attacks on pigeons. J. Anim. Ecol. 47:449—460. Krause, J. and J.-G.J. Godin. 1995. Predator preferences for attacking particular prey group sizes: consequenc- es for predator hunting success and predation risk. Anim. Behav. 50:465-473. Leonardi, G. 1991. Osservazioni preliminari sull’eco-eto- logia del Lanario Falco biarmicus feldeggi in Sicilia. Suppl. Ric. Biol. Selvaggina 17:147-149. . 1994. The home range of the lanner Falco biar- micus: influences of territory composition. Pages 153- 155 in B.-U. Meyburg and R.D. Chancellor [Eds.], Raptor conservation today. WWGBP and Pica Press, Berlin, Germany. , A. Longo and G. Corpina. 1992. Ecology and behavior of the Lanner Falcon. GLE Publications, Ca- tania, Italy, Mascara, R. 1986. Consistenza e note sulla biologia ri- produttiva del Lanario Falco biarmicus, nella Sicilia meridionale. Riv. Ital. Ornitol. 56:203-212. Massa, B., F. Lo Valvo, M. Siracusa and A. Ciaccio. 1991. II Lanario {Falco biarmicus feldeggi Schlegel) in Italia: status, biologia e tassonomia. Naturalista Sicili- ano 15:27-63. Mebs, T. 1959. Beitrag zur Biologie des Feldeggsfalken (Falco biarmicus feldeggi) . 80:142-149. Mirabelli, P. 1982. Biologia del Lanario Falco biarmicus in Calabria; confronti con la biologia del Falco Pel- legrino Falco peregrinus. Atti Conv. Ital. Ornitol. 1:149- 154. Morgan, JJ- AND J.-G.J. Godin. 1985. Antipredator ben- efits of schooling behavior in a cyprinodontid fish, the barred killfish {Fundulus diaphanus) . Z. Tierpsychol. 70: 247-264. Osborne, T.O. 1981. Ecology of the Red-necked Falcon Falco chicquera in Zambia. Ibis 123:289-297. Pitcher, T.J. 1986. Functions of shoaling behavior in tel- eosts. Pages 294-337 mT.J. Pitcher [Ed.], The behav- ior of teleost fishes. Chapman and Hall, London, U K Porter, R.F., I. Willis, S. Christensen and N.B. Nielsen 1981. Flight identification of European raptors. T. & A.D. Poyser, Cal ton, U.K. Sodhi, N.S., A. Didiuk AND L.W. Oliphant. 1990. Differ- ences in bird abundance in relation to proximity of Merlin nests. Can. f. Zool. 68:852-854. Suhonen,J., K. Nortdahl AND E. Korpimaki. 1994. Avian predation risk modifies breeding bird community on a farmland area. Ecology 75:1626-1634. Tarboton, W. AND D. Allan. 1984. The status and con- servation of birds of prey of Transvaal. Transvaal Mus Monogr. No. 3, Pretoria, South Africa. Thiollay, J.-M. 1988. Prey availability limiting an island population of Peregrine Falcons in Tunisia. Pages 701-710 in T.J. Cade, J.H. Enderson, C.G. Thelander and C.M. White, [Eds.], Peregrine Falcon popula- tions: their management and recovery. Peregrine Fund, Boise, ID U.S.A. Thomsett, S. 1987. Bat hunting by Lanner Falcons in Kenya. Gabar 2:7-8. Tree, A.J. 1963. Grey Hornbill Tockus nasutus as prey of the Lanner Falcon Falco biarmicus. Ostrich 34:179. Turner, G.F and TJ. Pitcher. 1986. Attack abatement, a model for group protection by combined avoidance and dilution. Am. Nat. 128:228-240. Yosef, R. 1988. Observations on Lanner Falcons Falco biarmicus in the Sede Boqer area. Torgos 7:68-73 (in Hebrew) . . 1991. Foraging habits, hunting and breeding suc- cess of Lanner Falcon (Falco biarmicus) in Israel./. Rap- tor Res. 25:77-81. Zar, J.H. 1984. Biostatistical analysis. Prentice-Hall Inc., Englewood Cliffs, NJ U.S.A. Received 6 August 1997; accepted 30 January 1999 J. Raptor Res. 33(2):128-133 © 1999 The Raptor Research Foundation, Inc. METHODS FOR GENDER DETERMINATION OF CRESTED CARACARAS Joan L. Morrison^ Department of Wildlife Ecology and Conservation, P. O. Box 1 1 0430, University of Florida, Gainesville, FL 32611 US. A. Mary Maltbie^ Department of Biological Sciences, Texas Tech University, Box 43131, Fubbock, TX 79409-3131 US.A. Abstrac:t. — ^We report details of a method that is reliable for gender determination of Crested Caracaras {Caracara plancus) . Using the microsatellite probe Poly(dA-dG)- (dC-dT), we detected sex-specific (female only) high-molecular weight restriction fragments in DNA from blood samples collected in the field. This method correctly identified the gender of 14 known-sex captive caracaras and was subsequently used to identify gender for 28 wild adults. We also evaluated morphometric measurements for these 42 individuals to determine whether any single characteristic or combination could be used in the field to make reliable gender determinations. No morphometric measurements were found that were reliable for gender deter- mination in adult caracaras. Bill depth and wing length tended to be larger for females, but there was considerable overlap among the sexes for all measurements. Bill depth and wing length were retained in a model developed using multiple logistic regression analysis, but the model’s overall predictive capability for indicating gender was poor. Because young caracaras are only 80% of adult size at fledging, their morphometric measurements are not usable for gender determination. Genetic methods are likely the only reliable methods suitable for determining gender in Crested Caracaras. Key Words; Crested Caracara; Caracara plancus; DNA; blood sampling, gender determination. Metodos para la determinacion de genero de Caracara plancus Resumen. — Reportamos detalles de un metodo confiable para la determinacion de genero de Caracara plancus. Mediante la utilizacion de la exploracion de microsatelite Poll (dA-dG)- (dC-dT) detectamos altos fragmentos moleculares de peso restringidos (en hembras solamente) en el ADN proveniente de muestras tomadas en campo. Este metodo identifico en forma correcta el genero de 14 caracaras cautivos de los cuales se conocia el sexo. Subsecuentemente fue utilizado para la identificacion de 28 adultos silvestres. Tambien evaluamos las medidas morfometricas de estos 42 individuos para determinar si alguna caracter- istica o combinacion puede ser usada en campo para la determinacion de genero. Ninguna medicion morfometrica fue confiable paia la identificacion de genero en caracaras adultos. La profundidad del pico y la longitud del ala tendian a ser mayores en las hembras, hubo coincidencias en las medidas de ambos sexos. La profundidad del pico y la longitud del ala fueron utilizados para la aplicacion de un analisis de regresion multiple, pero en general la capacidad predictiva del modelo para la identificacion del genero fue pobre. Debido a que los caracaras jovenes son un 80% del tamafio adulto cuando son pichones, sus medidas morfometricas no son utilizables para la determinacion de genero. Los metodos geneticos pa- recen ser los ilnicos confiables para determinar el genero de los caracaras crestados. [Traduccion de Cesar Marquez] The ability to identify the gender of birds is crit- ical to ecological studies for addressing a variety of topics including population and brood sex ratios, ^ Present address: Department of Biology, Colorado State University, Fort Collins, CO 80523 U.S.A. ^ Present address: Life Sciences Division, Mailstop M888, Los Alamos National Laboratory, Los Alamos, NM 87545 U.S.A, sex-ratio manipulation and gender-related differ- ences in dispersal, habitat use, site fidelity, and sur- vival. However, gender determination can be dif- ficult for young birds and sexually monomorphic species. Discriminant function and logistic regres- sion analyses have been used along with morpho- metric data to determine gender for breeding adults in a variety of avian species (Edwards and 128 June 1999 Gender Determination of Crested Caracaras 129 Kochert 1987, Clark et al. 1991, Smith and Wie- meyer 1992), but rates of misclassification can ex- ceed 20%, particularly for species with consider- able overlap in measurements, and these methods cannot be used on young individuals. Hormone immunoassays and genetic methods such as kar- yotyping and flow cytometry have been used for sexing birds but with varying success (Tiersch et al. 1991) and only to a limited extent for raptors (Ivins 1975). The usefulness and reliability of mi- cro- and minisatellite probes for gender identifi- cation in birds, including members of Falconifor- mes, has been well-documented (Longmire et al. 1991, 1993, Epplen et al. 1991, Delehanty et al. 1995, Fleming et al. 1996). The Crested Caracara ( Caracara plancus) is a lit- tle-known tropical raptor with a limited distribu- tion in North America. Although distinct plumage differences can be recognized among age groups (Wheeler and Clark 1995, Morrison 1996), there are no distinguishable gender-related plumage dif- ferences. Snyder and Wiley (1976) reported a low index of dimorphism (2.2) for this species, and considerable overlap in measurements exists be- tween males and females for all subspecies (Mor- rison 1996). Both males and females incubate, so both have a brood patch (Morrison 1996). In Florida, the Crested Caracara occurs as an iso- lated population in the southcentral peninsula (Stevenson and Anderson 1994). Because of its small size, restricted range, and apparent vulnera- bility to habitat changes, this population is listed as Threatened by the U.S. Fish and Wildlife Service and by the state of Florida. Recent studies on the ecology and dynamics of this population have been limited by the inability to determine the gender of individuals. The objective of this study was to identify tech- niques suitable for gender determination in Crest- ed Caracaras. We investigated the feasibility of us- ing genomic DNA obtained from blood samples to look for the presence of high-molecular weight, fe- male-specific, micro satellite fragments, following Longmire et al. (1993). We also examined external morphological characteristics for known-sex indi- viduals. Reliable gender determination using ge- netic methods would facilitate our ability to evalu- ate the use of morphometric measurements for gender determination in the field. Although sur- gical examination also provides gender informa- tion, this was not a viable option for this study be- cause of the cost, difficulties of use in the field. lack of usefulness for sexing juveniles and concern regarding use of an invasive technique on a Threat- ened Species. Study Area and Methods We studied Crested Caracaras in southcentral Florida (27°10'N, 81°12'W). Nesting territories were located in eight counties: Highlands, Glades, Okeechobee, Osceola, DeSoto, Polk, Hendry, and Indian River. This region con- stitutes most of the species’ current breeding range in Florida. During 1994-96, we took blood samples and morpho- metric measurements from 42 Crested Caracaras, includ- ing 14 captives and 28 wild, breeding adults that were captured throughout the study area (Morrison and McGehee 1996). Using laparoscopy, we determined gen- der of the 14 captive individuals, which were subsequent- ly used for genetic gender determination. We used blood samples obtained from the 14 known- sex captive caracaras (5 M, 9 F) to identify a suitable probe for gender identification. Approximately 0.2 pi of blood was collected from a brachial vein of the wing and transferred immediately into 5 ml of lysis buffer (Arctan- der 1988). Samples were sent to the laboratory for pro- cessing without information on the known gender of the captive individuals. Genomic DNA was isolated using a modified proce- dure from Longmire et al. (1991). The concentration of each DNA sample was estimated using a UV spectropho- tometer. Approximately 10 pg of DNA from each sample was digested with the enzymes Hae III and Hinfl in two separate reactions using reaction conditions recommend- ed by the supplier (New England Biolabs, Beverly, MA U.S.A.). Digested samples were electrophoresed in a 0.8% agarose gel at 40 volts for 36 hr. Restriction frag- ments were transferred to positively charged nylon mem- branes (Amersham, Arlington Heights, IL U.S.A.) using a modification of Southern (1975). Membranes were then baked for 2 hr at 65°C. Hybridization procedures followed the protocol of Longmire et al. (1993) except that prehybridization and hybridization solutions were 6 X SSC, 0.01 M EDTA pH 8.0, 10 X Denhardts solution and 1% (w/v) SDS. The probe used to identify gender was Poly(dA-dG) • (dC-dT) (supplied by Pharmacia, Pis- cataway NJ U.S. A.). Post hybridizations, membranes were washed twice for 5 min each in 2 X SSC, 0.1% SDS at 22°C and twice for 5 min each in 0.05 M NaCl, 0.1% SDS at 42°C. Membranes were then exposed to film (Amer- sham Hyperfilm MP) overnight in cassettes with two in- tensifying screens. Radiographs were examined for pres- ence of gender-specific high-molecular-weight bands (Longmire et al. 1993). Blood samples from wild caracaras were collected and processed for gender determination, as above. We deter- mined gender of the 28 wild caracaras by applying the genetic method after it was validated using the 14 cap- tives. We took five external morphometric measurements on all 42 Crested Caracaras. The captive individuals had originally come from the wild as either immatures or adults, so we assumed that their measurements were not different (due to being in captivity) from those of other 130 Morrison and Maltbie VoL. 33, No. 2 Hinf I Hae III Figure 1. Representative autoradiograms of four female (F) and four male (M) Crested Caracaras. Genomic DNA was digested with the enzymes Hinf I (left panel) and Hae III (right panel). Both sets of digestions are hybridized with the probe Poly(dA-dG)- (dC-dT) . To the side of each autoradiogram are the molecular weight markers in kilobases. wild individuals. Four of the 14 (1 M, 3 F) were in Basic I plumage (Wheeler and Clark 1995) so were at least one year old. We assumed their measurements were not dif- ferent from those of adults (Morrison unpubl. data). Measurements followed the North American Bird Band- ing Techniques Manual (1984). All the following were tak- en in mm: wing length (length of the unflattened wing from the bend to the tip of the longest primary) , culmen (from the cere edge to the tip of the bill), tarsus length (back of the intertarsal joint to the lower edge of the last complete scale before the toes), bill depth (cere edge to the bottom of the lower mandible at the deepest point), and bill width (maximum measurement at the posterior part of the bill) . We did not measure tails because we con- sidered this measurement unreliable. Caracaras regularly walk on the ground while foraging, and their tails incur considerable wear and breakage distally. Mass was not con- sidered reliable because many captured wild individuals had engorged crops. We compared measurements (x ± 1 SE) between males and females for all 42 caracaras using unpaired, 2- tailed t-tests. We also used multiple logistic regression (MLR) on the entire set of measurements. MLR relates several explanatory variables, in this case morphological measurements, to a dichotomous dependent variable, in this case, gender. MLR is more appropriate than discrim- inant function analysis for these comparisons, particularly when assumptions of multivariate normality are violated (Press and Wilson 1978). The multiple logistic function is the probability of an individual belonging to one par- ticular group, in this case, the probability of any individ- ual being female. Probabilities below a threshold value (assigned as 0.50) indicate male and higher probabilities indicate female. MLR was conducted using SigmaStat ver. 2.0 (Jandel Scientific, Inc. 1995). Results Genetic Method. We correctly identified the gender of all 14 captive caracaras using the micro- satellite probe. All females {N = 9) exhibited 2 fragments larger than 23 kb. All males exhibited only a single band in this same size range (Fig. 1). Thus, female caracaras were distinguishable by the presence of another band at or in excess of 23 kb, in comparison to males, which did not show this second band. In the Hae III digested samples, the June 1999 Gender Determination of Crested Caracaras 131 Table 1. Comparison of five external morphological characteristics of 42 (14 captive and 28 wild) Crested Caracaras m southcentral Florida, 1994—96. All measurements in mm. Male {N = 23) Female {N = 19) t P Characteristic Mean SE Range Mean SE Range Wing length 392.41 2.99 345.0-408.0 404.68 2.64 384.0-430.0 3.07 0.004 Tarsus 103.77 0.45 98.6-107.8 103.89 0.98 94.6-113.0 0.18 0.91 Culmen 33.12 0.20 31.5-35.0 33.56 0.55 24.6-35.6 0.74 0.46 Bill depth 23.56 0.13 22.5-24.9 24.2 0.17 22.9-25.6 2.97 0.005 Bill width 13.86 0.13 12.2-15.0 14.04 0.12 13.0-15.0 0.98 0.35 marker was above 23 kb. The marker in the Hinfl digested samples started at about 23 kb (Fig. 1). Morphometric Analyses. Only bill depth and wing length differed between males and females (Table 1), though considerable overlap was noted even for these two characteristics (Table 2). Only bill depth and wing length were retained in the logistic regression model: probability of being fe- male = —55.79 -I- (0.064*wing length) + (1.263*bill depth) (x^ = 40.81, P< 0.001) . Predic- tive capability of the model was poor, however, re- sulting in a mean misclassification rate of 59%. Table 2. Percentage of male and female Crested Cara- caras in each measurement group for wing length and bill depth. All measurements in mm. Wing Length No. OF Males % No. OF Eemales % 345-350 2 0.09 0 0.00 386-390 6 0.26 2 0.10 391-395 4 0.17 2 0.10 396-400 7 0.30 3 0.16 401-405 1 0.04 2 0.10 406-410 3 0.13 4 0.21 411-415 0 0.00 3 0.16 416-420 0 0.00 3 0.16 Total 23 19 Bill No. OF No. OF Depth Males % Females % 22.5-23.0 5 0.22 1 0.05 23.1-23.5 6 0.26 3 0.16 23.6-24.0 8 0.35 5 0.26 24.1-24.5 2 0.09 4 0.21 24.6-25.0 2 0.09 2 0.11 25.1-25.5 0 0.00 3 0.16 25.6-26.0 0 0.00 1 0.05 Total 23 19 Most females were correcdy identified but 86% of males were incorrectly classified as females. Discussion Genetic analyses correctly identified the gender of all known-sex Crested Caracaras. This technique can also be used for gender determination in hatch-year (HY) and after hatch-year (AHY) cara- caras. Even at fledging, young caracaras cannot be sexed reliably using morphometric measurements because they are only approximately 80% of over- all adult size (Morrison 1996). A variety of techniques have been used for gen- der identification in birds, reviewed by Ellegren and Sheldon (1997). Recently, a set of universal primers were published that will identify gender in all groups of birds except for ratites (Griffiths et al. 1996). Use of these universal primers would have been the technique of choice if we had sought only gender information for individual ca- racaras. An advantage of using microsatellite fin- gerprint analysis for gender identification is that these fingerprint patterns can also be used to ex- amine population-level parameters such as genetic diversity within the study group. Membranes ob- tained from our analyses can be rehybridized with other mini- and microsatellite probes to obtain in- formation on the frequency of polymorphic frag- ments. Hypervariable DNA fragment patterns have been successfully used in other population studies of birds (Longmire et al. 1991, 1992). We did not detect any adverse effects of blood sampling and handling procedures on sampled in- dividuals. Because adults were year-round residents within their territories, all individuals sampled were resighted numerous times following handling and blood sampling. We also sampled over 100 HY caracaras that were fitted with radiotransmitters. The resighting rate at independence (2 months 132 Morrison and Maltbie VoL. 33, No. 2 postfledging) for these individuals was 100%. Even nestlings that were sampled at 4—6 wk of age did not appear to incur any adverse effects of the sam- pling procedures. Although the caracara does exhibit some slight sexual dimorphism in external characteristics, morphometric measurements proved to be unre- liable indicators of gender even for adults. Larger overall size, particularly larger wing length and bill depth, was generally indicative of females. Consid- erable variation and overlap existed among all characters measured, however, so reliable gender identification of birds in the field from observation alone is not possible. Because of the multivariate nature of these differences, no single combination of characters was found that could reliably predict gender in breeding adults. Acknowledgments We thank S. McGehee, V. Dreitz, L. Todd, and C. Pages for assistance with fieldwork. B. Mealey and B. Bowen provided blood sampling supplies. We thank R. Collins, B. Mealey, G. Straight, D. Wrede, and K. Wrede for per- mitting us to take blood samples from their captive ca- racaras. We are grateful for logistic support from the MacArthur Agro-Ecology Research Center of Archbold Biological Station; the Department of Wildlife Ecology and Conservation, the Florida Agricultural Experiment Station, and the Institute of Food and Agricultural Sci- ences at the University of Florida; R.J. Baker at Texas Tech University; and J.L. Longmire at Los Alamos Na- tional Laboratory. Comments from 2 anonymous review- ers improved the manuscript. Most importantly, we thank landowners throughout southcentral Florida, who gen- erously provided access to their lands. This research was funded by the Nongame program of the Florida Game and Fresh Water Fish Commission, the Florida Ornitho- logical Society, the Avon Park Air Force Range, the South Florida Water Management District, and the U.S. Fish and Wildlife Service. This is MacArthur Agro-Ecology Re- search Center contribution No. 44 and Florida Agricul- tural Experiment Station journal series R-06604. 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Chandler and D. Nak- amura. 1991. The use of flow cytometry for rapid identification of sex in birds. Auk 108:206-208. Wheeler, B.K. and W.S. Clark. 1995. A photographic guide to North American raptors. Academic Press, San Diego, CA U.S.A. Received 17 May 1998; accepted 2 February 1999 J. Raptor Res. 33(2):134-142 © 1999 The Raptor Research Foundation, Inc. WHY DO GRASS OWLS ( TYTO CAPENSIS) PRODUCE CLICKING CALLS? D. Crafford and J.W.H. Ferguson Department of Zoology and Entomology, University of Pretoria, 0002 Pretoria, South Africa A,C. Kemp Bird Department, Transvaal Museum, PO. Box 413, 0002 Pretoria, South Africa Abstract. — Flying Grass Owls ( Tyto capensis) continuously produce double clicks and trains of single clicks with an emphasized frequency of 1.9 kFIz. Double clicks have a click rate of seven per second while click trains have a rate of 32 single clicks per second. We examined the possible role that clicking could play in echolocation or in prey capture. The owls did not increase clicking when no moonlight was available. In most cases the birds landed at the roost without clicking. Spectral analysis using a dead Grass Owl showed that the facial mask was directionally insensitive to sounds at 2 kHz. An echolocative function was thus unlikely. Neither of the prey rodents (Otomys angoniensis and Mastomys natalensis) reacted to recorded Grass Owl clicks. The clicks, therefore, probably did not play a role in prey capture. We present evidence that clicks are involved in social communication between Grass Owls. Key Words: Grass Owl] Tyto capensis; echolocation] prey location] communication] territoriality. Porque Tyto capensis emite vocalizaciones “click?” Resumen. — Tyto capensis continuamente produce “clicks” dobles y seriados de un solo “click” con una frecuencia de 1.9 kHz. Los “clicks” dobles tienen una tasa de siete por segundo mientras que los seriados tienen una tasa de 32 “clicks” individuales por segundo. Examinamos el posible papel de las vocalizaciones “click” con la ecolocalizacion o en la captura de presas. Las lechuzas no aumentaron estas vocalizaciones sin luz de luna. En la mayoria de los casos las aves llegaron a las perchas sin producir sonido. El analisis del espectro utilizando un Tyto capensis muerto demostro que el disco facial fue direccionalmente insensible a sonidos de 2 kHz. Por lo tanto la funcion de ecolocalizacion fue descar- tada. Tampoco los roedores presa ( Otomys angoniensis y Mastomys natalensis) reaccionaron a las graba- ciones de vocalizaciones “click” de Tyto capensis. Por lo tanto las vocalizaciones “click” probablemente no juegan un papel en la captura de presas. Presentamos evidencias que las vocalizaciones “click” estan involucradas en la comunicacion social de las lechuzas. [Traduccion de Cesar Marquez] The Grass Owl ( Tyto capensis) is a Red Data Book Species which inhabits grasslands, usually in long grass and often in the vicinity of water (Steyn 1982). Although it is mainly nocturnal, it occasion- ally hunts during daylight (Steyn 1982). Tytonid owls produce loud bill snapping or clicking sounds under conditions of fear or aggression (Campbell and Lack 1985). Walker (1974) and Bunn et al. (1982) found breeding Barn Owls {Tyto alba) using a peculiar rapid vocal clicking call and suggested that this may be connected with courtship, excite- ment, or intimidation. Litde is known of Grass Owl vocalizations but they emit sharp clicking calls dur- ing flight, presumably by repeatedly flicking the tongue against the palate (Steyn 1982, Kemp and Calburn 1987, Erasmus 1992). Grass Owls are ex- ceptional among the owls in that these calls, which have never been described quantitatively, are emit- ted almost continuously in flight. This requires ex- planation. There are three hypotheses explaining these clicking sounds. The first is that the clicks are used for echolocation. Since the owls cannot see in absolute darkness and have to rely on a de- tailed knowledge of local topography during dark nights (Campbell and Lack 1985, Martin 1986), clicking sounds enable them to echolocate obsta- cles (Kemp and Calburn 1987). Curtis (1952) (cit- ed in Payne 1971) found the performance of Barn Owls in avoiding obstacles to be dependent on available light and concluded that Barn Owls do 134 June 1999 Grass Owi. Clicking Calls 135 not echolocate. Both Oilbirds {Steatornis caripensis) and Cave Swiftlets {Aerodromus spp.) perform echo- location by means of clicking sounds (Schnitzler and Henson 1980) associated with obstacle avoid- ance (Medway 1967, Fenton 1975, Schnitzler and Henson 1980). There are two types of echolocative sounds: broadband clicks and more complex calls (broadband or narrowband, Fenton 1980). Broad- band clicks are used by Oilbirds comprising a rapid burst of sound impulses lasting up to 25 ms. Some swiftlets and megachiropteran fruit bats emit dou- ble clicks with an internal interval of 15-40 ms. The mask and external ear of tytonid owls have several adaptations which increase auditory acuity (Bunn et al. 1982) and which could potentially aid in echolocation. Payne (1971) investigated the acoustic abilities of Barn Owls, and concluded that asymmetrically-placed ear flaps, feathers that are modified to reflect sound and held in a tightly packed and almost parabolic wall, and even the characteristic position in which the head itself is held (downward tilting) are all adaptations in- volved in hearing. Grass Owls share these charac- teristics. Payne (1971) conducted playback experi- ments to dead Barn Owls and found a positive relationship between directional sensitivity and in- creasing frequency. However, these experiments only took into account the external structure of the facial mask and not the neural basis of hearing which may, in itself, strongly affect owl hearing and which may assist echolocation. The second hypothesis for clicking calls is that they are used for prey stimulation. The clicking calls of Grass Owls could be a means of stimulating rodents into activity, causing them to reveal their whereabouts (Kemp and Calburn 1987). Given the well-developed auditory power of owls (Campbell and Lack 1985), this would facilitate the capture of prey. The majority of studies on the influence of owl activity on rodents concern owl foraging be- havior and rodent use of microhabitat (Abramsky et al, 1996, Thompson 1982, Brown et al. 1988, Longland and Price 1991). However, none of these studies measured the initial reaction of rodents to owl-generated cues but rather at the longer-term activity patterns of the rodents in response to pre- dation. A third hypothesis suggests that clicking calls are used for intraspecific communication. Erasmus (1992) noted that Grass Owls often click when in the vicinity of their breeding site. This gives rise to the hypothesis that the clicks are used as signals between Grass Owls. The aims of this study were, firstly, to give a quantitative description of the clicking call of Grass Owls and, secondly, to test the three hypotheses. Methods During March and April 1997, recordings were made on 22 occasions (1800-2300 H) at Rietvlei Dam Nature Reserve, Pretoria (25°54'S, 28°18'E) using a Sony TC- D5M cassette recorder with a Sony ECM-1035 directional microphone. The frequency response of the recording system was 30 Hz-18 kHz within 4 dB. Most of the re- cordings were made at two Grass Owl roosts. The first was located in a temporary marshland and inhabited by a Grass Owl pair. The second roost, from which only a single Grass Owl was flushed, was located in a permanent marsh at least 1 km from the first roost. During recording sessions the observer sat approximately 15 m from the roost. Visual observations of the owls were made when possible. Three different light classes were identified us- ing the phase of the moon: (1) full moon, waxing and waning gibbous, (2) waxing and waning crescent, first and last quarter, and (3) no moon. The number of click sequences heard per observation hour was calculated for each of the three light classes. We characterized the spectral and temporal properties of each recording using Canary 1.2 (Cornell Laboratory of Ornithology) on a Power Macintosh 7100/66 comput- er. Except for some click trains which were too short in duration, we performed 30 measurements of each of the six parameters (Fig. 1 and Table 1) for a particular re- cording. The means of these values were used for de- scribing the clicks and for comparing clicks emanating from owls at the two main roosts. Recorded calls were usually in the form of click pairs or as trains of single clicks. Since the amplitude of the recorded clicking calls varied depending on the distance between the micro- phone and the owl, detail of spectral range also varied. For this reason the emphasized (peak) frequency was the only spectral characteristic measured (Table 1). To measure the directional hearing characteristics of Grass Owls, we played sounds to a dead Grass Owl; an undamaged road casualty. Due to the protected nature and rarity of this species, other carcasses could not be obtained. Measurements were conducted in an anechoic chamber provided by the South African Bureau of Stan- dards (SABS). We connected a Bruel and I^aer (B 8c K) 1405 noise generator to a B & K 1617 filter; the latter was, in turn, connected to a B & K 2706 amplifier which drove a Philips ADI 1400 tweeter loudspeaker (LS) through which pink noise of % octave was played to the carcass (2.0, 10.0, and 12.5 kHz, respectively). We mount- ed the LS on a flat metal baffle on a tripod. We used a B & K 4165 calibration microphone, calibrated by means of a B & K 4230 calibrator, to measure the frequency response of the LS. We then determined the frequency response of a G-196 miniature electret microphone (Mat- sushita Corporation). The weakest response was at 12.5 kHz where the signal-to-noise ratio was better than 11 dB. This microphone and an OP07 buffer amplifier were im- bedded in resin and placed in the dead owl’s head from 136 Crafford et al. VoL. 33, No. 2 Figure 1 . Graphical representations of Grass Owl clicking calls. (A) Oscillogram depicting (i) double clicks recorded at roost 1, (ii) double clicks recorded at roost 2 and (iii) a click train recorded at roost 2 indicating call durations and the temporal characteristics of sound amplitude. (B) Spectrogram of the same sounds. (C) Frequency spectrum of clicks in parts (i) and (ii), above, indicating a single emphasized frequency just below 2 kHz with no significant energy between 2 kHz and 10 kHz. Energy below 1 KHz, resulting from background noise, has been filtered out. Analysis of Fig. la, b: FFT size 1024 points; frequency grid size 21.53 Hz. Analysis for Fig. Ic: FFT size = 2048 points, frequency grid size = 10 Hz. The important parameters measured for these calls are indicated on this figure. Double clicks from roost 1 and from roost 2 differ in the durations of single clicks (SCD) , the presence of clear harmonics and many other characteristics (Table 1 ) . above so that the diaphragm of the microphone occu- pied the position formerly taken by the tympanum of the right ear. The owl was strapped to a mount on a tripod in such a position that the microphone was 1 m from the LS. The microphone was connected to a B &: K 2610 measuring amplifier from which the output was mea- sured in microvolts and transformed to relative sound pressure values in dB. Readings of the microphone out- put were taken through angular increments of 5° in the horizontal plane of the owl head, starting from OO" with respect to the forward orientation. Rodent trapping was performed close to the owl roost sites used for sound recordings, enabling us to decide on suitable rodent species for playback experiments. During May 1997, 100 Sherman live traps were set for 1000 trap nights in the vicinity of roost 1, where owls were regularly observed flying parallel to the marsh. Four trap lines, each with 25 traps 10 m apart, were arranged into two June 1999 Grass Owl Clicking Calls 137 Table 1. Properties of clicks recorded near roost 1 (single bird), roost 2 (a pair) and three other roosts. Rightmost column gives results of a Mann-Whitney t/-test, comparing the values for roost 1 and roost 2. The data for other roosts are not analyzed since these comprise observations at a collection of other sites in the study area. Number of observations varies between 20-30 per roost. Property Symbol & Roost 1 Roost 2 Other roosts t/-TEST Roosi's 1 & 2 Description Units Mean SD Mean SD Mean SD P Emphasized frequency: audio frequency with the highest amplitude EE (Hz) 1916 68 1865 95 1945 110 <0.001 Duration of double click, from start of 1st click to end of 2nd click DCD (ms) 48 27 43 6 42 10 <0.03 Duration of an individual click SCD (ms) 14 4 10 3 14 5 <0.001 Time duration from end of a click to the start of subse- quent click II (ms) 17 5 25 3 18 5 <0.001 Time duration from end of 2nd click of a double click to start of 1st click of subse- quent double click IBC (ms) 114 35 93 13 104 19 <0.001 Time duration from start of 1st click of a double click to start of 1st click of following double click CRM (ms) 155 35 123 42 147 19 <0.001 grids of two trap lines per grid. Grids were 500 m apart and the lines within each grid were 50 m apart. Peanut butter with oats was used as bait and alternated with a mixture of raisins and oats in consecutive traps along a trap line. Traps were cleared twice daily at 0700 H and 1700 H. Trapped rodents were sexed, marked using toe clipping and released. Density, by species, was estimated using the Petersen density estimate (Caughley 1977) of the resulting mark-recapture data for the two grids com- bined. Animals were found to move between trap lines within a grid (50 m) . The area covered by a grid was thus calculated as the length of the transect line 250 m and 150 m wide, thereby assuming the animals moved into the grid from at least 50 m distant. This translated to a capture area of 7.5 ha for both grids combined. In the laboratory, rodents were subjected to recorded owl clicks. Recorded owl clicks were played to five vlei rats (Otomys angoniensis) and four multimammate mice (Mastomys natalensis) removed from the trapping site at the end of the survey. These species were used because they were the two most common nocturnal rodents with vlei rats also being a favored food item of Grass Owls (Kemp and Calburn 1987). Calls of Crowned Plover {Ste- phanibyx coronatus) and a recording of traffic in a busy street were used as control sounds, respectively, repre- senting sounds to which the rodents were accustomed to in the field and sounds which were foreign to them. These three sounds alternated during consecutive play- back events and each of the sounds was 25 sec in dura- tion, separated by a silent interval of 15 sec. This se- quence was recorded twice onto a four-min endless loop tape. Two glass tanks (surface 150 cm X 70 cm) were used to hold test animals. The floor of each tank was covered with white sand. In one corner was an artificial burrow, while food, water and a passive infrared detector were positioned on the other side of the tank. This area was kept clear. A rodent was placed in each tank. While one animal was tested the other was given time to settle down (>24 hr). Two time switches regulated a 12L: 12D cycle, while a pair of red light bulbs remained switched on for the entire duration of the experiment. At night these provided light to record data on a Panasonic AG- 455 ME video recorder. When the mouse triggered the infrared detector, a computer switched on the video cam- era which recorded for 90 sec. After the video camera had been recording for 10 sec, the computer activated a Panasonic RQ-L305 tape player positioned above the tank. The tape played for 40 sec (7.5 sec silence, 25 sec sound, 7.5 sec silence), after which it stopped. The video camera recorded for a further minute before it was de- activated. Each rodent’s response to the three test sounds was recorded at least 10 times. Six reaction categories were identified from observation of the video recordings; (1) no movement, (2) rodent moved less than half of length of tank, (3) moved at least half of length of tank, (4) moved to opening of burrow but didn’t enter, (5) 138 Crafford et al. VoL. 33, No. 2 Gibbous Crescent Dark Ught Class Figure 2. The frequency of Grass Owl clicking (number of owls heard per observation hour) at Rietvlei Dam as a function of the amount of moonlight. There is no trend towards an increase of clicking when no moonlight is available. Bars indicate standard deviations of observa- tions. ran into burrow but emerged within 10 sec or while sound still played, and (6) ran into burrow and remained there for the duration of the 10 sec or playing time. A reaction was noted for the first 10 sec of the playback (i.e., initial reaction) and also for the entire playing du- ration (ED) of the sound (i.e., overall reaction). Results Field Observations. During 28 nights, we made 64 observations on Grass Owls. When landing at the roost (four observations) , the owls did not click at all. On two of these occasions, the owls clicked while approaching the roost but not when landing. When taking off from the roost, the owls produced the clicking call once. While perched on the roost, they clicked on four occasions. On two occasions, owls were seen flying, then stopped clicking and landed, before almost immediately taking off again and resuming clicking. On two other occasions, two owls appearing to chase each other produced click trains. Grass Owls also answered clicks pro- duced by other individuals. This was observed on five occasions though only one bird was visible. The owls tended to increase their clicking activity when ample light was available (Fig. 2). However, the difference in clicking activity between the three light classes is not statistically significant (Kruskal- Wallis ANOVA, P = 0.654). Spectrographic Analysis. Double clicks, compris- ing pairs of single clicks, were recorded during 30 observation periods. Click trains, comprising more than two single clicks following in close succession, were recorded seven times (Fig. 1, Table 1). Dou- ble clicks had an emphasized frequency of around 2 kHz (Table 1). The mean value for click trains was 1891 ±144 Hz {N — 7), similar to that of dou- ble clicks. The single clicks within double clicks ex- hibited an internal interval (II) of some 20 ms (Ta- ble 1), compared to 20.4 ± 6.6 ms {N — 7) for the internal interval within click trains. The click rate measurements (CRM) for the double clicks and click trains were 123—155 ms (Table 1) and 31.6 ± 5.0 ms (N — 7), respectively. This corresponded to approximately seven double clicks per sec and 31.5 click train clicks per sec. The click trains, however, had a mean duration of only 275 ms {N = 7) , Dou- ble clicks had a duration (DCD) of 42-48 ms (Ta- ble 1) and an interval between double clicks (IBC) of 93-114 ms. Single clicks within double clicks and within click trains had similar durations, re- spectively 10-14 ms (Table 1) and 11.9 ± 4.5 ms (N = 7) . A Mann-Whitney G-test indicated signifi- cant differences in all the click properties pro- duced at roost one (a single bird) compared with those emanating from roost two (a pair, Table 1). Playback to Dead Owl. At all three playbacks to dead owls, the experimental frequencies (2.0, 10.0, and 12.5 kHz) showed a decline in amplitude of the incoming sound toward 90° (i.e., as the right ear, in which the microphone had been placed was turned away from the loudspeaker; Fig. 3) . The mi- crophone was thus shielded from the loudspeaker by the owl’s head. Playbacks at 2.0 kHz indicated no clear amplitude peaks or nulls at various ori- entations (Fig. 3). Three such peaks were mea- sured at 10.0 kHz. The highest was at —15° with two smaller peaks at —60° and 75°, respectively and a distinct null at —45°. Readings taken at 12.5 kHz had a distinct peak at 20° and nulls at —60° and 85°. A 7-dB difference in amplitude existed be- tween the highest peak and the clearest null at 10 and 12.5 kHz. Rodent Trapping. Six mammal species were trapped. Their densities (animals per ha ± S.E.M., based on the Peterson estimators for the two grids) were 28 ± 2.9 for the diurnal striped mouse {Rhab- domys pumilio), 13.3 ± 2.6 for the multimammate mouse (Mastomys natalensis) , 2.7 ± 2.5 for the an- June 1999 Grass Owl Clicking Calls 139 Figure 3. The directional sensitivity of the facial mask of a dead Grass Owl towards pink noise of % octave at 2 kHz, 10 kHz and 12.5 kHz. No clear peaks and nulls were evident at 2 kHz, indicating no directional sensitivity at 2 kHz, but which was evident at the higher audio-fre- quencies. goni vlei rat (Otomys angoniensis) , 2.4 ± 2.1 for swamp musk shrew {Croddura mariqumsis) , 1.3 ± 0.6 for the forest shrew {Myosorex varius), and 0.27 ±0.1 for the grey climbing mouse {Dendromus me- lanotis). The striped mouse was diurnal, the other species nocturnal or crepuscular. Rodents Subjected to Owl Clicks. For both the 10 sec and entire duration categories, the reactions of the rodents did not differ significantly between the three different treatments (Fig. 4, X'< 11.36; df = 10 for each of the nine individuals tested, P > 0.35) . Most of the rodents either did not move (reaction category one) , or they reacted by moving only a short distance (reaction category two) . On a few occasions the animals reacted to plover and traffic recordings by running into their burrows (reaction category six, Fig. 4). This reaction was never exhibited in response to the Grass Owl clicks. Discussion The repetitive broadband clicks of Grass Owls have a structure which is potentially useful for echolocation. Buchler and Mitz (1980) argued that the signal-to-noise ratio of a signal can be in- creased by the integration of successive pulses into (/> 0) 2 mice and cached the last mouse (Franklin et al. 1996). Regurgitated pellets were collected from 1 April-20 August 1991-95, which encompassed the breeding peri- od from incubation to the fledging of young (Gutierrez et al. 1995). Pellets were collected opportunistically be- low owl roosts and nests. Although no random or system- atic survey design was used to collect pellets, we assumed the prey remains we identified reflected the true diet composition of the owls. We combined pellets collected on the same day from the same site into one sample un- less some pellets were markedly older; in which case old- er pellets were separated from more recent pellets. We used skull, appendicular skeletons, beaks, and feathers to identify mammalian and avian remains. Re- mains were identified using keys in Findley et al. (1975), Hoffmeister (1986) and Dalquest and Stangl (1983), and by comparison with collections at the Humboldt State University Vertebrate Museum and the Museum of South- western Biology (MSB) at the University of New Mexico. We estimated the number of prey items in a sample by counting pairs of mandibles, skulls, or appendicular re- mains, whichever gave the highest count (Forsman et al. 1984). We used mandibles, legs, and exoskeletons to identify and enumerate insects. We estimated diet composition for each owl site by multiplying counts of each prey species by species-specific body mass. Mean body mass of individual species was es- timated from known weights of specimens at the MSB. Most MSB specimens we used were collected within the counties of the study areas or adjacent counties. We at- tempted to use at least 50 museum specimens for each prey species to estimate mean weight. Comparison with reference collections indicated rabbit (Sy/wi/ogii^ spp.) re- mains were probably all small individuals or juveniles. Thus, we used an average weight of juvenile rabbits from the MSB. We did not attempt to age other prey items. We used an estimate of 1.0 g for each insect. Diet of individual owls probably varies owing to differ- ences in territory composition (vegetation and prey), competition, sex, breeding status, and possibly learned or inherent individual preference. Biases in dietary pat- terns likely are introduced by lumping prey remains across individuals or territories which have unequal sam- ple sizes. To avoid such bias, we estimated owl diet com- position by considering diet composition on a territory by territory base, or by using the aggregate percentage of individual prey remains. The aggregate percentage equaled the proportion of an individual prey species from an individual territory averaged over all territories (Swanson et al. 1974). We compared the frequency of occurrence of the most important prey groups in the diet (arbitrarily defined as groups that comprised >10% of the diet by number or weight) between the study areas using i-tests (Zar 1984), using each territory in each year as the sample. For inclusion into the analysis, we only considered territories with >20 prey remains in a year. We examined the relationship between owl reproduc- tive success and diet following two approaches. The two null hypotheses we tested were: (1) Hq’- There was no population response in reproductive output to composi- tion of the diet, and (2) Hq: There was no individual (territory) response in reproductive success to composi- tion of the diet. We used the aggregate percentages of white-footed mice (all Peromyscus species) and woodrats (all Neotoma species) by year and study area as the sam- ples for the population approach. We used the percent- ages of white-footed mice and woodrats for individual ter- ritories by year as the samples for the individual approach. We only considered white-footed mice and woodrats because they were the only two prey items that comprised >10% of the dietary biomass on both study areas. We arbitrarily used 20 prey remains within a year as the cutoff point for a territory to enter the analyses. We used analysis of covariance (ANCOVA; Zar 1984) to test for a population response, with the mean number of young fledged by pairs as the dependent variable, study area as a categorical factor and the aggregate per- centages of white-footed mice, and woodrats as the covar- iates. We used logistic regression (Hosmer and Leme- show 1989) to test for an individual response in reproductive success to diet. The response variable was divided into unsuccessful territories (zero young fledged) and successful territories (>1 young fledged). The pre- dictor variables were year, study area, the proportion of the diet comprised of white-footed mice, and the pro- portion of the diet comprised of woodrats. We tested the significance of predictor variables using the Wald statistic (Hosmer and Lemeshow 1989). We excluded territories occupied by single (unpaired) owls for both analyses Results We identified 16 species of mammals, 13 species of birds, and 3 families of insects among 3793 prey June 1999 Mexican Spotted Owl Diet and Reproduction 145 remains from 44 and 41 Spotted Owl territories in AZ and NM, respectively (Table 1). Mammals com- prised 69.2% of owl diet by number and 91,9% by mass. The most important mammalian prey groups were woodrats (16.1% by frequency and 47.8% by mass), white-footed mice (38.6% and 17.0%), northern pocket gopher {Thomomys bottae, 3.6% and 11.5%), and rabbits (1.8% and 10.1%). Birds comprised 5.4% of the diet by frequency and 7.5% by mass (Table 1). No single bird species account- ed for >2.0% of the diet. Insects accounted for 25.4% of the diet hy frequency and 0.6% by mass. Mean prey mass for both study areas combined was 42.5 g. Woodrats {t = 2.60, df = 76, P = 0.01), rabbits {t ~ 2.10, df = 76, P = 0.04) and insects {t = 2.10, df = 76, P = 0.04) occurred more frequently in NM owl diets while gophers (f = 2.17, df = 76, P = 0.03) and birds {t — 2.05, df = 76, P — 0.04) occurred more frequently in AZ owl diets. The fre- quency of occurrence of white-footed mice was not different {t = 1.59, df = 76, P = 0.12) between study areas. Mean prey mass was 36.3 g for AZ and 47.3 g for NM. We collected 20 or more prey remains in a year from 32 territories in AZ and 46 territories in NM. There was no population response in reproductive output to composition of the diet (ANCOVA mod- el T = 0.85, df = 3,6, P — 0.52). There was no indication of a pattern among the individual terms in the model (study area F = 0.06, df = 1,6, P = 0.82; white-footed mice P — 1.72, df — 1,6, P = 0.24; woodrats F = 0,50, df = 1,6, P = 0.51). To test for an individual response in reproduc- tive success to composition of the diet, the basis for calculating log odds for the logistic model were AZ for the study area effect and 1991 for the year effect. The logistic model adequately fit the data based on a goodness-of-fit test (x^ = 76.59, df = 70, P = 0.28). There were differences in individual territory reproductive success among years, but not between study areas or in relation to the propor- tion of the diet composed of white-footed mice or woodrats (Table 2). Discussion Mexican Spotted Owls in our study took a wide variety of prey, but concentrated on small mam- mals, especially woodrats, similar to the Northern and California Spotted Owl subspecies (Forsman et al. 1984, Verner et al. 1992). However, except in the canyonlands of southern Utah, the Mexican Spotted Owl appeared to depend more on small rodents such as white-footed mice and voles {Mi- crotus spp.; Ganey 1992, Young et al. 1997) than the other Spotted Owl subspecies (Gutierrez et al. 1995). Regional differences in diet have been noted within the ranges of all three subspecies (Forsman et al. 1984, Ganey 1992, Verner et al. 1992), and dietary differences between our study areas may reflect differences in prey abundance, prey avail- ability, or prey selection. We could not address the latter two possibilities given the nature of our study. However, the habitat preferred by gophers (gentle topographic relief with deeper soils) was more abundant in AZ while habitat preferred by rabbits (pihonjuniper woodland) was more abun- dant in NM (Findley 1975, Hoffmeister 1986). The Mexican woodrat {N. mexicana) has been associat- ed with montane coniferous forest and rock out- crops (Comely and Bakei 1986, Hoffmeister 1986), and in New Mexico reaches its highest abundance in montane mixed-coniferous forests (Findley et al. 1975). There appeared to be an abundance of montane coniferous forest on both study areas. In contrast to previous studies of California and Northern Spotted Owls (Barrows 1987, Thrailkill and Bias 1989, White 1996), breeding owls in our study did not consume larger prey than nonbreed- ing owls. There were three possible reasons for this difference. First, the Mexican Spotted Owl may be different ecologically from the two coastal subspe- cies. The Mexican subspecies may depend on the overall abundance of prey within the landscape to successfully reproduce, or may respond to other environmental cues such as predator abundance, or intra- or interspecific competition. A second possible explanation is that the prey remains we collected did not accurately reflect the true diet of the owls. This should not have contrib- uted to the differences in findings because our protocol for pellet collection was similar to those for studies of the two coastal subspecies. A further concern was that reproducing males may have tak- en larger prey back to the nest and consumed smaller prey at the point of capture. Under such a scenario, prey remains in pellets might not repre- sent the general diet. Bull et al. (1989) found such a pattern for Great Gray Owls (5. nebulosa). How- ever, based on pellet egestion rates, we believe that pellets collected below Spotted Owl roosts and nests accurately depicted overall diet. In an exper- 146 Seamans and Gutierrez VoL. 33, No. 2 Table 1. Mexican Spotted Owl diet composition in central Arizona and westcentral New Mexico, 1991-95. Prey Species or Group Arizona New Mexico Mass (g)=^ N Number % Mass % N Number % Mass % Sylvilagus spp. 232.4 7 0.4 2.8 63 2.9 14.3 Spermophilus lateralis 173.9 1 0.1 0.3 0 0.0 0.0 Tamiasciurus hudsonicus 218.5 5 0.3 1.8 3 0.1 0.6 Neotoma mexicana 121.3 143 8.8 29.3 288 13.3 34.2 N. albigula 146.9 21 1.3 5.2 22 1.0 3.2 Neotoma spp. 134.1 38 2.3 8.6 100 4.6 13.1 Neotoma total 202 12.4 43.2 410 19.0 50.5 Thomomys bottae 114.4 89 5.5 17.2 49 2.3 5.5 Eutamias spp. 63.2 4 0.2 0.4 7 0.3 0.4 Microtus mogollonensis 30.8 47 2.9 2.4 82 3.8 2.5 M longicaudus 34.9 4 0.2 0.2 43 2.0 1.5 Microtus spp. 32.9 3 0.2 0.2 87 4.0 2.8 Peromyscus maniculatus 16.9 403 24.7 11.5 409 18.9 6.8 P boylii 21.4 141 8.6 5.1 148 6.8 3.1 P diffidlis 22.0 0 0.0 0.0 96 4.4 2.1 Peromyscus spp. 20.1 82 5.0 2.8 186 8.6 3.7 Peromyscus total 626 38.4 19.4 839 38.8 15.6 Zapus princeps 25.3 0 0.0 0.0 1 0.0 0.0 Sorex spp. 5.3 7 0.4 0.1 14 0.6 0.1 Eptesicus fuscus 16.4 1 0.1 0.0 8 0.4 0.1 Lasionycleris noctivagans 9.0 0 0.0 0.0 4 0.2 0.0 Myotis spp. 6.7 6 0.4 0.1 5 0.2 0.0 Unidentified bats 10.7 5 0.3 0.1 2 0.1 0.0 Mammal total 1007 61.7 88.2 1617 74.8 94.0 Cyanocitta stelleri 107.7 8 0.5 1.5 10 0.5 1.1 Colaptes auratus 132.9 5 0.3 1.1 4 0.2 0.5 Accipiter striatus 128.8 1 0.1 0.2 0 0.0 0.0 Unidentified large avian 123.1 15 0.9 3.1 16 0.7 1.9 Myadestes townsendi 31.6 0 0.0 0.0 1 0.0 0.0 Otus flammeolus 50.0 3 0.2 0.3 2 0.1 0.1 Glaucidium gnoma 62.5 3 0.2 0.3 4 0.2 0.2 Unidentified medium avian 48.1 43 2.6 3.5 23 1.1 1.1 Parus gambeli 11.0 0 0.0 0.0 8 0.4 0.1 Dendroica coronata 12.3 0 0.0 0.0 1 0.0 0.0 Junco hy emails 19.2 0 0.0 0.0 3 0.1 0.1 Sialia mexicana 24.5 2 0.1 0.1 2 0.1 0.0 Sitta carolinensis 17.5 0 0.0 0.0 4 0.2 0.1 Tachycineta spp. 16.7 0 0.0 0.0 1 0.0 0.0 Catharus guttatus 25.4 1 0.1 0.0 0 0.0 0.0 Unidentified small avian 17.9 26 1.6 0.8 20 0.9 0.4 Aves total 107 6.6 10.9 99 4.6 5.6 Cerambycidae 1.0 263 16.1 0.4 275 12.7 0.3 Gryllacridae 1.0 47 2.9 0.1 77 3.6 0.1 Scarabacidae 1.0 4 0.2 0.0 16 0.7 0.0 Unidentified insect 1.0 203 12.4 0.3 78 3.6 0.1 Insect total 517 31.7 0.9 446 20.6 0.4 No. prey items 1631 2162 ■* Prey weights were estimated from specimens at the Museum of Southwestern Biology, Albuquerque, New Mexico. June 1999 Mexican Spotted Owl Diet and Reproduction 147 Table 2. Results of logistic regression for test of individual Mexican Spotted Owl territory response in reproductive success to diet. Data from central Arizona and westcentral New Mexico, 1991-95. Predictor Variable Parameter Estimate SE OF Parameter Wald Value P Study Area -0.508 0.565 4.58 0.03 Year 1992 1.677 1.722 2.05 0.15 1993 2.254 1.163 3.75 0.05 1994 3.019 1.215 6.18 0.01 1995 3.125 1.352 5.34 0.02 White-footed mice 1.693 2.292 0.55 0.46 Woodrats 1.298 1.579 0.67 0.41 imental study of Barred Owls {S. varia), Duke et al. (1980) estimated that pellet egestion occurred on average 16.24 hr (SD = 3.48) after meal con- sumption. Thus, pellets collected below Spotted Owl roosts probably represented food consumed from the previous night, regardless of where it was consumed. In addition, we observed nesting fe- males egesting pellets away from nests, often in the vicinity of male roosts, making pellets collected at roosts of nesting owls a reflection of the pair’s diet. A third possible reason is the different statistical methods used to compare owl diets. We used an aggregate percentage method to estimate popula- tion level responses to diet and territories to esti- mate individual pair response to diet. Previous studies have lumped all prey remains across terri- tories before estimating diet. Thus, there was little information on the contribution of individual ter- ritories to the total number of prey remains. Con- sequently, the observed patterns may have been the result of one or a few territories consuming unique prey items and contributing most of the prey remains to the final tally (Swanson et al. 1974). A difficult question in estimating diet following our methods is what should be the required min- imum number of prey items for including a terri- tory in the analysis. We arbitrarily chose 20 prey remains for the cutoff, but more prey remains would have led to a higher precision in diet esti- mates. For example, if white-footed mice comprise 20% of the diet (p = 0.20), the estimated coeffi- cient of variation (CV) given a sample size of 20 is 45%, and given a sample size of 200 the CV is 14%. Although the latter CV estimate is obviously pre- ferred, collecting 200 prey remains from a territory during the breeding season would be nearly im- possible due to logistical constraints. Our choice of 20 prey remains was a tradeoff between precision of diet estimates and sample size considerations. However, simulations using different cutoff points revealed our results were somewhat unstable. An increase or decrease of 10 prey remains from our cutoff point (10 or 30 prey remains to be included in the analysis) resulted in significant associations between reproductive success and white-footed mice in the diet. Because of this instability, we chose to infer no patterns of associations, recog- nizing that future studies with larger samples or more sophisticated analyses might detect such pat- terns. Thus, future studies should first consider the sampling effort required to obtain sufficient sam- ples to adequately describe variation among indi- viduals. Acknowledgments We would like to thank D. Olson, E. Forsman, D. Holt, A. Franklin, Z. Peery, K. White, and P. Carlson for helpful comments. We thank D. Olson, C. May, and our many field assistants who collected pellets. We also thank an anonymous reviewer for one of the most cogent and helpful reviews we have received. B. Gannon at the Mu- seum of Southwestern Biology and T. 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The California Spotted Owl: general biology and ecologi- cal relations. Pages 55-78 in]. Verner, K.S. McKelvey, B.R. Noon, R.J. Gutierrez, G.L Gould, Jr. and TW Beck [Eds.], The California Spotted Owl: a technical assessment of its current status. Gen. Tech. Rep. PSW- GTR-133, Pac. Southwest For. Range Exp. Stn., USDA For. Serv., Albany, CA U.S.A. Ward, J.R, Jr., R.J. Gutierrez and B.R. Noon. 1998. Hab- itat selection by Northern Spotted Owls: the conse- quences of prey selection and distribution. Condor 100:79-92. White, K. 1996. Comparison of fledging success and sizes of prey consumed by Spotted Owls in northwestern California./. Raptor Rs. 30:234—236. Young, K.E., PJ. Zwank, R.V. Valdez, J.L. Dye and L.A Tarango. 1997. Diet of Mexican Spotted Owls in Chi- huahua and Aguascalientes, Mexico./. Raptor Rs. 31. 376-380. Zar, J.H. 1984. Biostatistical analysis. Prentice-Hall, Inc., Englewood Cliffs, NJ U.S.A. Received 23 May 1998; accepted 30 January 1999 J Raptor Res. 33{2):149-153 © 1999 The Raptor Research Foundation, Inc. PHILOPATRYAND NEST SITE REUSE BY BURROWING OWLS: IMPLICATIONS FOR PRODUCTIVITY R. Scott Lutz Department of Wildlife Ecology, 226 Russell Labs, 1630 Linden Drive, Madison, WI 53706 U.S.A. David L. Plumpton^ Department of Range and Wildlife Management, Texas Tech University, Lubbock, TX 79409 U.S.A. Abstract. — ^We examined demographics of an annually migratory population of western Burrowing Owls {Athene cunicularia hypugaea) in Colorado from 1990-94. We banded 555 Burrowing Owls (60% of the known population on the study area) as adults or as nestlings. Five hundred thirteen banded owls (92%) were never reencountered after the year in which they were banded. Forty-two banded owls (8%) returned to the area in >1 year following banding, and used the area for 2-4 years. Males and females banded as adults returned at similar {P = 0.45) rates (19% and 14%, respectively); 5% of banded nestlings returned. Adult males and females nested in formerly used sites at similar rates (V5% and 63%, respectively; P = 0.71). We found no difference in productivity between philopatric adults (those returning to any portion of the study area) and presumed new adults. However, past brood size was greater for females that returned to former nest sites (x = 4.9 ± 0.69) than for females that changed nest .sites in sub.sequent years {x = 2.2 ± 0.79; ti4 = —2.52, P = 0.02). Females banded as nestlings that returned as adults always did so after a 1-yr absence from the study area. Conversely, males banded as nestlings that did return, with one exception, returned first in the year following hatch. Fledge rate from 167 nests ranged from 0-9 young per nest {x = 3.62 ± 0.19). Nest density increased with the number of years sites were used by breeding owls, but density did not affect mean fledge rate. Key Words: Burrowing Owl; Athene cunicularia; demography, migration-, nest-site reuse, philopatry, reproduc- tion. Filopatria y reutilizacion de sitios de anidacion por Athene cunicularia hypugaea: implicaciones para su productividad Resumen. — Examinamos la demografia de una poblacion migratoria anual de Athene cunicularia hypugcwa del oeste en Colorado entre 1990-94. Anillamos 555 lechuzas (60% de la poblacion conocida en el area de estudio) adultos y pichones. Quinientos trece lechuzas anilladas (92%) nunca fueron encontradas despues del ano en que fueron anilladas. Cuarenta y dos lechuzas anilladas (8%) regresaron al area despues del ano en que fueron anilladas y utilizaron el area por 2-4 anos. Los machos y hembras anilladas como adultos retornaron en tasas similares (19% y 14% respectivamente; P = 0.45); 5% de los pichones anillados regresaron. Los machos y hembras adultos anidaron en sitios previamente utili- zados, con tasas similares (75% y 63% respectivamente; P = 0.71). No encontramos diferencias en la productividad entre adultos filopatricos (aquellos que regresaron a alguna porcion del area de estudio) y los presumibles nuevos adultos. Sinembargo, los pasados tamanos de la nidada fueron mayores para las hembras que retornaron a los sitios de anidacion anteriores {x = 4.9 ± 0.69) que para las hembras que cambiaron de sitio en los anos subsecuentes (x = 2.2 ± 0.79; ^4 = —2.52, P = 0.02). Las hembras anilladas como pichonas regresaron despues de un ano de ausencia al area de estudio. Opuestamente, los machos anillados como pichones que regresaron al area de estudio lo hicieron al ano de haber eclosionado con una sola excepcion. La tasa de pichones de 167 nidos oscilo entre 0-9 por nido (x = 3.62 ± 0.19). La densidad de nidos se incremento con el numero de sitios/ano utilizados por las lechuzas en reproduccion, pero esta densidad no afecto la tasa de pichones. [Traduccion de Cesar Marquez] 1 Present Address: H.T. Harvey and Associates Ecological Consultants, 906 Elizabeth Street, P.O. Box 1180, Alviso, CA 95002 U.S.A. 149 150 Lutz and Plumpton VoL. 33, No. 2 The Western Burrowing Owl {Athene cunicularia hypugaea) is a species of concern throughout much of its range in the U.S. (Rich 1984) and Canada (Ratcliff 1986, Johnsgard 1988). Campaigns against burrowing mammals that provide nest sites for Burrowing Owls (Butts 1973, Zarn 1974) and hab- itat loss to development by humans (Zarn 1974, Millsap and Bear 1997) are principal factors sus- pected in population declines. In Colorado, Bur- rowing Owls depend chiefly on black-tailed prairie dogs {Cynomys ludovicianus) for nesting burrows, and often return to nesting areas used previously (Plumpton and Lutz 1993a). Philopatry and nest burrow reuse by Burrowing Owls have been well- documented (Martin 1973, Gleason 1978, Rich 1984), However, litde is known about demographic parameters and the effects of prior reproductive success on site fidelity. Study Area We conducted fieldwork on the Rocky Mountain Ar- senal National Wildlife Refuge (RMANWR), located 16 km from Denver, CO in southwestern Adams County. This 6900 ha area is vegetated primarily by weedy forbs, cheatgrass {Bromus tectorum) , perennial grasses and crest- ed wheatgrass {Agropyron cristatum) . Shrubs include yuc- cas {Yucca spp.), sand sagebrush {Artemisia filifolia) , and rubber rabbitbrush ( Chrysothamnus nauseosus) that occur in patches throughout the area. Methods We captured and banded Burrowing Owls during the breeding seasons (1 April-31 July) from 1990-94. We used primarily Sherman and Tomahawk traps to capture nesting Burrowing Owls and their young (Plumpton and Lutz 1992, 1993b). We banded owls with color-anodized aluminum leg bands engraved with unique alpha/nu- meric combinations (Acraft Sign and Nameplate Co., Ltd., Edmonton, Alberta Canada) and classified owls as either young of the year or adult (^1 yr). We surveyed the study site daily during the breeding season to locate nest burrows, count young and trap owls. Our surveys consisted of driving roads and using spotting scopes to identify nesting and previously banded Burrowing Owls. We also walked prairie dog towns inspecting burrows for signs of occupancy by Burrowing Owls (whitewash, cast- ings and prey remains). We defined mated pairs as those that used a single burrow and attempted to nest. We de- fined successful nesting attempts as those in which Sil young fledged (Steenhof 1987). We estimated minimum brood size as the maximum number of young seen at each burrow prior to fledging. We classified Burrowing Owls that returned to RMANWR after residency in any prior year as philopatric. We calculated the rate of philopatry by dividing the num- ber of owls banded in any year by the number that re- turned in one, two, three, or four subsequent years. To explore the relationship between reproductive perfor- mance and philopatry, we tested the null hypothesis that brood sizes of philopatric Burrowing Owls and broods from those owls nesting for only a single season were equal. Because we did not measure individual territory sizes, we defined nest-site fidelity subjectively to include those owls that nested within the same or an adjacent Vl6 section (0.162 km^) site used in any previous year. We included adjacent sites because roads surround all sec- tions at RMANWR, often bisecting contiguous prairie dog towns, and Burrowing Owls commonly nest along roadsides (Plumpton and Lutz 1993a, 1993b). Therefore, consecutive nest attempts in adjacent sites were often m close proximity and within the same prairie dog town. The rate at which adults returned to previous nest sites was the proportion of banded Burrowing Owls that re- turned to previous nest sites, or those banded as nestlings that returned as adults to nest within the same or adja- cent sites. To determine whether nest outcome influ- enced future returns to nest sites, we tested the hypoth- esis that brood sizes from the prior year for returning Burrowing Owls and broods from owls that changed nest sites in successive years were similar. We banded owls as nestlings and as adults. Banded nestlings encountered in subsequent years could be aged to a specific year class. Owls banded as unknown-aged adults and encountered in subsequent years were as- signed an age class by adding the number of years since initial capture to >1. We excluded owls initially captured in the last two years of study to minimize bias in estimat- ing returns. We tested whether Vl6 section sites that were used m more years supported more nests, and whether the den- sity of breeding pairs influenced the average productivity of nests. For all paired analyses, we used Hests when data were normally distributed and Wilcoxon 2-sample tests (z) to make comparisons when data were nonnormally distrib- uted. For comparisons involving >2 samples, we used Rruskal-Wallis H tests (SAS Inst. Inc. 1988). For small sample size tests for differences in proportion (e.g., re- turn rates between sexes), we used pooled t-tests. All sta- tistical tests were conducted at a significance level of a = 0.05. Means are expressed ± SE. Results We banded 555 Burrowing Owls from 1990-94, providing 4 consecutive years of potential return to RMANWR involving 514 individuals (those banded before 1994). During all nesting years, 201 of 334 nesting adults (60%) were known individu- als (banded or band-resighted; Table 1). We esti- mated that this population fledged 533 owlets pri- or to 1994, of which we banded 369 (69%). Of the 514 Burrowing Owls banded prior to 1994, 42 (8%) returned in >1 year after the year of banding. The return rate of banded owls was highest in the year immediately following banding for both sexes and age classes (Table 2) . Males and females banded as adults returned at similar rates (19% and 14%, respectively; P = 0.45). June 1999 Burrowing Owl Demography 151 Table 1 . Burrowing Owls banded or band-resighted (percentage of breeding population^) at Rocky Mountain Ar- senal Wildlife National Wildlife Refuge, Colorado, 1990-94. Age/Sex Year 1990 1991 1992 1993 1994 Total Adult/F 15 (56) 26 (68) 22 (56) 32 (76) 14 (67) 109 Adult/ M 19 (70) 21 (55) 21 (54) 22 (52) 9 (43) 92 Nestling/Unk. 61 (56) 114 (69) 57 (37) 137 (85) 36 (51) 405 Total 95 161 100 191 59 606 banded or identified from banding in a previous year/# breeding. Of the 369 Burrowing Owls banded as nestlings prior to 1994, 18 (5%) returned in one or more years after hatch. Of these, 13 (72%) were male and 5 (28%) were female. None of the females re- turned in the year following their hatch; all re- turned after a 1-yr absence from RMANWR. Con- versely, all but one of the males banded as nestlings that returned in any year, returned first in the year following hatch. Brood sizes of philopatric owls were not different from those of single-season nest- ers for males (philopatric males: N = 16, x = 4.2 ± 0.66; single season males: N - 43, x = 3.7 ± 0.43; z = 0.47, P = 0.64) or females (philopatric females: N = 15, x = 3.7 ± 0.63; single season fe- males: N = 69, X = 3.8 ± 0.28; z = —0.22, P = 0.83). Of the owls that returned to RMANWR, 75% of the males banded as adults returned to previously used nest sites, while 63% of females returned to former nest sites {P — 0.71). Adult males that re- turned to nest sites supported broods in the pre- vious year (x = 3.9 ± 0.81) no different in size from returning males that changed nest sites (x — 5.0 ± 1.08, t -14 = 0.69, P = 0.49). However, pro- ductivity in the preceding year was greater for fe- Table 2. Philopatry rate (%) of Burrowing Owls banded at Rocky Mountain Arsenal National Wildlife Refuge, Colorado, 1991-94. Age at Banding Wars Post- banding Adult Nestling Male Female N{%) N{%) N{%) Both N{%) 1 12 (3) 11 (19) 12 (14) 23 (16) 2 7 (3) 2 (5) 2 (4) 4 (4) 3 3 (1) 0 (0) 1 (3) 1 (2) 4 1 (1) 0 (0) 0 (0) 0 (0) males that returned to former nest sites (x = 4.9 ± 0.69) than for females that changed nest sites in subsequent years {x = 2.2 ± 0.79; ^^4 = —2.52, P = 0 . 02 ). Five hundred thirteen owls (92%) banded at the RMANWR were encountered in only the year of banding. Excluding the last two years of study, of those encountered during at least one year after banding, males {N =17) occupied RMANWR for 2 or 3 yr, and females {N =13) for 2-4 yr. The longest-lived owls we encountered were females; one was banded as a nestiing and encountered during its fourth year, and one was at least one year old when banded, and encountered three years thereafter, in at least its fourth year (Fig. 1). The Figure 1. Age classes of banded Burrowing Owls reen- countered (including multiple reencounters for some in- dividuals) at Rocky Mountain Arsenal National Wildlife Refuge. 152 Lutz and Plumpton VoL. 33, No. 2 0123456789 Number of years occupied of 5 Mean # nests/5 years /?4 = 42.48, P < 0.0001 Mean productivity ■ ■ ■ ■ = 1 .58, P = 0.8 Brood size Figure 2, Productivity of 167 Burrowing Owl nests at the Rocky Mountain Arsenal National Wildlife Refuge. Figure 3. The effects of increasing annual site reuse on mean Burrowing Owl nest-site density and productivity at the Rocky Mountain Arsenal National Wildlife Refuge, 1990-94. median number of years that owls of both sexes were reencountered at the RMANWR was two. From 1990-94, 167 nests were observed. At least 31 nests (18%) failed to produce a single chick. Nest success ranged from 0-9 young fledged (x = 3.62 ± 0.19; Fig. 2). The V|g section sites were occupied from 0 to all 5 yr of this study (0/5: N = 361, 1/5: N = 19, 2/5: N= 18, 3/5: N - 4, 4/5: A - 6, 5/5: A - 4). The study area was not homogeneous, and not all sites were suited for occupancy by owls. The mean number of nests/site increased with the number of years of five that the site was occupied (1/5: x = 1.2 ± 0.12, 2/5: x = 2.5 ± 0.12, 3/5: x = 4.25 ± 0.75, 4/5: X = 7.33 ± 0.61, 5/5: x - 8.5 ± 0.87; = 42.48, P < 0.0001; Fig. 3). However, the mean fledging rate did not differ among the 5 levels of annual reuse (1/5: x = 3.8 ± 0.61, 2/5: x = 4.1 ± 0.37, 3/5: x = 3.4 ± 0.69, 4/5: x = 3.4 ± 0.28, 5/ 5: X = 3.7 ± 0.24; = 1.58, P - 0.8; Fig. 3). Discussion Traditionally, differences in philopatry between sexes have been explained as mechanisms to en- hance reproductive success (Greenwood 1980). In our study, males and females were equally philo- patric and returned to nest sites at an equal rate, but females obtained a reproductive advantage in this behavior by increasing their productivity. How- ever, the relationships we observed between phil- opatry and reproductive success suggested that, for females, a former mate is not as important to re- productive success as is the former nest site. The actual criteria used by females to choose mates are not known for most species (Wittenberger 1983). In our earlier work, we found only weak relation- ships between morphological characteristics in mated owl pairs (Plumpton and Lutz 1994) and only moderate differences between nesting bur- rows used and those available, but unused by nest- ing Burrowing Owls (Plumpton and Lutz 1993a). Assuming female selection, male Burrowing Owls may be chosen for the nesting territories they hold preferentially over other criteria. For Florida Burrowing Owls {A. c. floridand) , Mill- sap and Bear (1997) observed much higher reen- counter rates for both sexes of adults, and for owls banded as nestlings. They also observed that male adults reused former nest territories most frequent- ly. As Millsap and Bear (1997) observed for the Flor- ida subspecies, we observed two pairings between a female and her offspring from the previous year. We concur that migration would tend to separate breed- ing pairs, and that returns to natal sites by yearling males, combined with nest-site fidelity by their mothers, could contribute to such mother-son pair- ings. Millsap and Bear (1997) also reported adult male Burrowing Owls excavated burrows for them- selves on their prior territories, while allowing their sons to occupy their own natal burrow for nesting. They attributed this behavior to reproductive advan- tages gained by the male in instances where female June 1999 Burrowing Owl Demography 153 selection favors a mate with previous site familiarity, in this instance her son. We offer as an alternative (though not mutually exclusive) , hypothesis that the father of the yearling male may increase his inclu- sive fitness by guaranteeing his son a nest territory and mate, while assuring a territory for himself, and presumably not reducing his own direct fitness. Density of breeding pairs appeared to be unrelated to brood size in our study, so a male sharing a ter- ritory with his son may not suffer decreased direct fitness as a result. Millsap and Bear (1997) also indicated the pos- sibility that there may be little advantage, in terms of retained site familiarity, conferred upon migra- tory owls. Our population consisted of complete annual migrants, and yet we did observe nest-site reuse in successive years. Therefore, migration may lessen the advantages gained by previous experi- ence on a nest site, but may not eliminate them entirely. Because we lack band returns from else- where in the migratory cycle, we do not know whether owls that failed to return to the study area were killed or migrated elsewhere. Unlike results from Millsap and Bear (1997) our study found that nest-site reuse by females was more often preceded by above-average brood sizes. In our work, broods from previous years were sig- nificantly larger for females that reused a site than for those that selected a new nest site. Acknowledgments Funding and other support for this work was provided by the U.S. Army and U.S. Fish and Wildlife Service at the Rocky Mountain Arsenal National Wildlife Refuge, for which D.R. Gober, J.M. Lockhart, and J.G. Griess de- serve special thanks. D.J. Buford, L.S. Pezzolesi, K.J. Rat- tray, J.M. Schillaci, and T.M. Sproat assisted in trapping, marking, and resighting owls. J. Belthoff, B. Millsap, and K. Steenhof provided helpful reviews of earlier drafts of this manuscript. Capture, handling, and marking of Bur- rowing Owls followed a plan approved by the Texas Tech University animal care and use committee. Literature Cited Butts, K.O. 1973. Life history and habitat requirements of Burrowing Owls in western Oklahoma. M.S. thesis, Oklahoma State Univ., Stillwater, OK U.S. A. Gleason, R.S. 1978. Aspects of the breeding biology of Burrowing Owls in southeastern Idaho. M.S. thesis, Univ. Idaho, Moscow, ID U.S. A. Greenwood, RJ. 1980. Mating systems, philopatry and dispersal in birds and mammals. Anim. Behav. 28 1140-1162. JOHNSGARD, P.A. 1988. North American owls: biology and natural history. Smithsonian Institution Press, Wash- ington, DC U.S. A. Martin, D.J. 1973. Selected aspects of Burrowing Owl ecology and behavior. Condor 75:446—456. Mu I. .SAP, B.A. AND C. Bear. 1997. Territory fidelity, mate fidelity, and dispersal in an urban-nesting population of Florida Burrowing Owls. Raptor Res. Report 9'.^\—99> Plumpton, D.L. and R.S. Lutz. 1992. Multiple-capture techniques for Burrowing Owls. Wildl. Soc. Bull. 20: 426-428. AND . 1993a. Nesting habitat use by Bur- rowing Owls in Colorado./. Raptor Res. 27:175-179. AND . 1993b. Influence of vehicular traffic on time budgets of nesting Burrowing Owls. /. Wtldl. Manage. 57:612—616. AND . 1994. Sexual size dimorphism, mate choice and productivity of Burrowing Owls. Auk 111: 724-727. Ratclife, B.D. 1986. The Manitoba Burrowing Owl sur- vey 1982-1984. Blue Jay 44:31-37. Rich, T. 1984. Monitoring Burrowing Owl populations: implications of burrow re-use. Wildl. Soc. Bull. 12:178- 180. SAS Institute, Inc. 1988. SAS/STAT user’s guide: statis- tics. SAS Institute, Inc., Cary, NC U.S.A. Steenhof, K. 1987. Assessing raptor reproductive success and productivity. Pages 157-170 in B.A. Giron Pen- dleton, B.A. Millsap, K.W. Cline and D.M. Bird [Eds ], Raptor management techniques manual. Natl. Wildl. Fed., Washington, DC U.S.A. Wittenberger, J.F. 1983. Tactics of mate choice. Pages 435-447 in P. Bateson [Ed.], Mate choice. Cambridge Univ. Press, Cambridge, U.K. Zarn, M. 1974. Burrowing Owl {Speotyto cunicularia hypu- gaea). Habitat management series for unique or en- dangered species, U.S. Bur. Land Manage. Tech. Note 242. Denver, CO U.S.A. Received 1 July 1998; accepted 1 February 1999 J Raptor Res. 33(2): 154-1 59 © 1999 The Raptor Research Foundation, Inc. USE OF RAPTOR MODELS TO REDUCE AVIAN COLLISIONS WITH POWERLINES Guyonne F.E. Janss Estacion Biologica de Donana, Consejo Superior de Investigaciones Cientificas, Department of Applied Biology, Avda. de Maria Luisa s/n, 41013 Sevilla, Spain Alfonso Lazo Asistencias Tecnicas Clave, s.L, Progreso 5, 41013 Sevilla, Spain Miguel Ferrer Estacion Biologica de Donana, Consejo Superior de Investigaciones Cientificas, Department of Applied Biology, Avda. de Maria Luisa s/n, 41013 Sevilla, Spain Abstract. — We evaluated the use of raptor models to decrease bird mortalities caused by collisions with powerlines. One realistic statue of a Golden Eagle {Aquila chrysaetos) and two Accipiter silhouettes were placed on top of utility towers. Flight behavior of both resident and migrating birds near these power structures was compared to flight behavior we observed at towers where models were not installed. Overall, the number of flocks, number of crossings, and flight altitudes were not affected by the models. Our results indicated that the models did not in any way reduce the risk of collisions. Potential collision victims such as waterfowl, storks, and lapwings were generally indifferent to the models. Most reactions were shown by raptors primarily because the eagle model provoked abundant attacks. We felt that, due to the intensity of attacks on the eagle model, it may have actually increased the possibility of collisions by raptors with powerlines. Key Words; avian collisions-, mortality, avoidance models', powerlines. El uso de modelos de rapaces para reducir la colision de aves con tendidos electricos Resumen. — La eficacia de modelos de rapaces para disminuir la colision de aves contra tendidos elec- tricos fue comprobada. Un modelo realista de un aguila real {Aquila chrysaetos) (estatua) y dos siluetas de halcones {Accipiter sp.) fueron colocados en lo alto de torres electricos. El comportamiento de las aves cerca del tendido fue comparado entre un tramo tratado y un tramo control y entre aves migratorias y residentes de dos areas de estudio. En conjunto, el numero de bandos, el numero de cruces y la altura de vuelo fueron independientes de los tramos. Estos resultados indicaron que los rnodelos no cambiaron el comportamiento de las aves en la manera que pudiera reducir el riesgo de colision. La composicion de especies mostraba dependencia de tramos. Las potenciales victimas de colision en las areas (aves acuaticas, cigiienas, avefrias) parecian, en general, indiferentes ante los modelos. La mayoria de las reacciones fueron registradas en rapaces, porque el modelo de aguila real provoco ataques de otras rapaces. Por ello, un mayor uso de los tramos tratados fue registrado. En consecuencia la probabilidad de una colision podria incluso aumentar. [Traduccion de Autores] Collisions with powerlines can be an important cause of death for some species of birds, especially those in unstable populations (Crivelli et al. 1988, Morkill and Anderson 1991). Species that fly in flocks (e.g., waterfowl) and species with high wing loading (e.g., storks [Ciconia spp.] and cranes [Grw5 sp.]) (Bevanger 1994, 1998) most frequently collide with and die at power structures. Measures tested to decrease collision mortality have mainly focused on the use of wire markers to increase the visibility of powerlines. Wire markers have been shown to reduce mortality by 50-80% (Alonso et al. 1994, Brown and Drewien 1995, Janss and Fer- rer 1998). In some areas where bird collisions are a prob- lem, the use of models of raptors has been sug- gested as a useful mitigation measure. However, the effectiveness of these models in decreasing col- 154 June 1999 Raptor Models as Mitigation Measures 155 lisions has not been tested (Heijnis 1980, Brown 1993, APLIC 1996). If effective, they might have other applications such as at airports and along highway corridors where they might frighten birds away before collision accidents can become a prob- lem (Solman 1973, Burger 1985, Hernandez 1988, Dolbeer et al. 1993, Work and Hale 1996). Habit- uation of birds to raptor models is a potential prob- lem since it would make them only effective (Brown 1993) along migratory pathways where ex- posure to the models would only occur once or twice a year (Brown 1993, APLIC 1996). We assessed the effectiveness of three different raptor models in reducing bird flights near power structures in two study areas. We discuss the effec- tiveness of these models in reducing collision mor- tality on powerlines for both migratory and resi- dent birds. Methods We used three models of raptors. Model A was a real- istic statue of an “oversized” Golden Eagle (Aquila chry- saetos; height 70 cm, length 120 cm, about 130% of nor- mal size) on a perch made of hberglass. Models B and C were flat, brown and white silhouettes of Acdpiters made of wood. Model B simulated an Accipiter (height 30 cm, length 40 cm) on a perch and model C an Accipiter in flight (wingspan 105 cm, length 50 cm) (Heijnis 1980). The models were placed on top of powerpoles or other utility structures. The first study area was in the south of Cadiz (southern Spain), near the Straits of Gibraltar, where large numbers of birds from Europe pass through when migrating to Africa (Bernis 1980, Einlayson 1992). The high-voltage powerline (400 kV) used was under construction and was without conductors or static wires (Fig. la, b). Towers were about 40-m high and about 400 m apart. We tested all three models in this migration area. The second study area was in the Dohana National Park (southwest Spain) . Two powerpoles were erected in marshland and scrub ecotone, where both wintering and breeding birds concentrated at the end of winter. The poles were not connected with any wire or conductor. The poles were about 10 m high and were 150 m apart, as in a distribution powerline (Fig. Ic). In this resident area only model A was tested. Species we expected to be most susceptible to colli- sions in the study areas were waterfowl, pigeons ( Columba spp.). White Storks {Ciconia ciconia) and Lapwings {Va- nellus vanellus) (Fiedler and Wissner 1980, Bevanger 1994, Janss and Ferrer 1998). Our observation periods were designed to coincide with periods when birds would be most abundant in each of the study areas. In the migration area, observations were made during the postnuptial migration period from 10 July-20 August 1996. In the resident area, observa- tions were made from 12 February-13 March 1997. This period coincided with the end of the winter period and the start of the breeding period. All observations started immediately after the models were installed. Observa- tions were made almost daily in sessions which lasted at least 2 hr (60 sessions on model A, 62 on models B and C in the migration area, and 24 sessions on model A in the resident area). Observation sessions covered all day- light hours and several sessions were conducted on the same day. We analyzed the total number of flocks (i.e., bird groups) we observed because individuals in the same flock could not be considered as independent observa- tions. Numbers of flocks were compared between utility towers with raptor models and adjacent towers where rap- tor models were not installed. Sections were further di- vided into subsections with one central tower and two lateral subsections which ended at the center of the spans (left and right from the tower subsection) (Fig. 1). Using a telescope and binoculars, birds were recorded simultaneously at both types of sections from a fixed ob- servation point centered between the two types of sec- tions (approximately 200 m away). All birds and flocks that flew within 100 m of the structures were recorded For each observation of a bird or flock, we recorded the subsection where the bird came closest to the powerline, the flight altitude at this minimum distance, if the bird (flock) crossed the powerline, and any reactions to the raptor models (e.g., changes in flight direction or alti- tude either toward or away from the model, any aggres- sive behavior and vocal reactions) . Three levels of flight altitude were recorded in the migration area: 0-20 m (under powerlines), 20-60 m (powerline level) and >60 m (above powerlines). Because utility towers differed m height in the resident area, flight altitude was assigned to two levels: 0-20 m (near poles) and >20 m (above poles). In the resident area, observations recorded at the cen- tral subsections were omitted because of the small dis- tance between the poles (Fig. 1). Observations in the treated sections where models B and C were placed, were compared with the same control section, which was sit- uated in between both treated sections (Fig. lb). The number of flocks per subsection and per flight altitude category, and the number of flocks crossing vs. those not crossing over powerlines were compared using either chi-square or R X C tests of independence (Sokal and Rohlf 1995). This way we tested the homogeneity of the distribution of numbers (i.e., if proportions of birds near the towers were independent of treated and control sections). We used Yates’s correction when necessary (So- kal and Rohlf 1995). Based on the experimental design (which had fixed control and treatment sections), we chose a significance level of P < 0.01. This way we low- ered the probability of drawing wrong conclusions due to random effects. Although we planned to evaluate the use of the models to reduce collisions, we used two-tailed tests because we suspected that models could be both able to attract and scare off birds. Distributions of the number of flocks in tower subsections vs. lateral subsec- tions (the sum of left and right) per taxonomic group, per flight altitude category and birds crossing sections vs not crossing sections were compared between treatments and controls. We analyzed the number of flocks indepen- dent of species as well as pooled by taxonomic groups (Appendix 1). 156 Janss et al. VoL. 33, No. 2 SECTIONS TREATED CONTROL SUBSECTIONS LEFT T 1 MODEL A 1 RIGHT T LEFT C 1 CONTROL 1 RIGHT C a. I "0° SECT. TREATED CONTROL TREATED SUBS. LEFT T 1 MODEL B 1 RIGHT T LEFT C 1 CONTROL 1 RIGHT C LEFT T 1 MODEL C 1 RIGHT T 40 m b. 400 m Figure 1. Experimental setting where raptor models were tested in reducing avian collisions with powerlines, (a) utility towers and study sections for model A in the migration area, (b) utility towers and study sections for models B and C in the migration area, and (c) powerpoles and study sections for model A in the resident area. Results Model A in Migration Area. During 120 hr of observations, we recorded 466 flocks (2738 individ- uals) of 30 bird species that came within 100 m of the powerline sections (Table 1, Appendix 1). Number of flocks observed did not differ by sub- sections (x^ = 0.98, df = 1, P = 0.322); however, species composition did differ by section (x^ = 119.00, df = 4, P< 0.001). At sections where mod- els were installed 41.9% of the birds observed were raptors (119 records) while, at sections without models, raptors represented only 20.9% of the birds observed (43 records). Flocks also used the second and third flight altitude categories (20-60 and >60 m) more frequently (x^ 11.66, df = 2, P — 0.003) at sections that were equipped with rap- tor models. All taxonomic groups tended to be more frequent in higher altitude levels, but this was not significant. Only Griffon Vultures ( Gyps ful- vus) were observed more frequently at flight level June 1999 Raptor Models as Mitigation Measures 157 Table 1. Number of flocks per taxonomic groups within 100 m of subsection of powerline tested. Flocks in lateral subsections (left and right) were summed (Lat A, B, C indicate the numbers in lateral subsections of models A, B and C, respectively; Lat X indicates the numbers in the lateral subsections of corresponding control sections) . Species per group are indicated in Appendix 1. Model Lat Lat Migration Area A A Control X Ciconiiformes 20 34 15 14 Vultures 10 14 19 24 Raptors 79 40 19 19 Gulls 3 5 1 0 Other birds 31 36 20 34 Passerines 3 9 10 4 Corvids 0 0 1 2 Total 146 138 85 97 Model Lat Lat Model Lat Migration Area B B Control X C C Ciconiiformes 24 47 25 32 22 18 Vultures 9 6 8 20 13 15 Raptors 17 17 23 22 49 27 Other birds 10 4 22 24 48 33 Passerines 7 2 4 8 0 3 Corvids 0 0 1 1 0 1 Total 67 76 83 107 132 97 Model Lat Lat Resident Area A A Control X Ciconiiformes 45 60 17 81 Waterfowl 22 16 6 8 Raptors 51 13 15 10 Lapwings 21 19 31 23 Other birds 4 1 4 0 Corvids 20 4 5 5 Total 163 113 78 127 >60 m at sections with raptor models (83.3%) compared to sections without raptor models (53.5%, = 4.74, df = 1, P= 0.030). The number of flocks crossing vs. those not crossing was inde- pendent of section (x^ = 1.70, df = 1, P — 0.161). In 32 cases (6.9%), birds reacted to the models. Nearly all of the reactions were by raptors (90.6%) . Fifteen of these we identified as “curiosity,” 10 were “attacks,” six were “vocal” reactions and one was “scared off.” Black Kites {Milvus migrans) showed the highest reaction rate (33.8% of the rec- ords) , followed by the Common Buzzard {Buteo hu- teo, 16.7%), There was no relationship between the number of days since the model was installed and. the number of reactions per observation session (Spearman’s r, = —0.18, P — 0.463; N — 18). A Common Kestrel {Falco tinnunculus) actually perched twice in the tower with model A installed at a lower level and it was apparently not bothered by the model. Models B and C in Migration Area. In 124 hr of observations, we recorded 562 flocks (4062 individ- uals) of 24 bird species within 100 m of the sec- tions (Table 1, Appendix 1). As in the former case, number of flocks observed did not differ by sub- section (model B, x^ = 0.33, df = 1, P = 0.565; model C, x^ — 0.10, df = 1, P = 0.756). Flocks per taxonomic group did differ by section for both models, but no clear pattern was shown (model B, X2 = 12.01, df = 3, P = 0.007; model C, x^ = 13.17, df = 3, P = 0.004) . The number of flocks per flight altitude category also did not differ by section (model B, x^ = 2.15, df = 2, P = 0.341; model C, X^ = 5.54, df = 2, P = 0.063), nor did the propor- 158 Janss et al. VoL. 33, No. 2 tions of flocks crossing vs. not crossing powerlines (model B, ~ 4.34, df = 1, P = 0.037; model C, = 3.59, df = 1, P = 0.058). We felt that birds reacted to these models in only four cases (0.7%; three toward model C and one toward model B). These reactions were recorded for two raptors and two vultures and were classified either as “changes in flight direction” (three rec- ords) or “curiosity” (one record, model C). Three of these reactions were recorded on the first 2 d after the models were installed. The fourth reac- tion was recorded 7 d after installation. Birds also perched on the utility towers with the models 10 times (five times near model B and five times near model C). These were kestrels {Falco tinnunculus and F. naumanni), Short-toed Eagles (Circaetus gallicus) and a Spanish Starling (Sturnus unicolor) . Model A in Resident Area, In 98 hr of observa- tions, we recorded 481 flocks comprising 1288 indi- viduals of 31 bird species (Table 1). The number of flocks observed varied between subsections (x^ = 22.14, df = 1, P = 0.001). Over 33% of the obser- vations were made at subsections with raptor models, while only 16.2% were recorded near control subsec- tions. The number of flocks per taxonomic group also varied by section (x^ = 25.93, df = 5, P < 0.001) . Waterfowl, raptors and corvids were more often re- corded near treated sections (13.8%, 23.2% and 8.7%, respectively) than near control sections (6.8%, 12.2% and 4.9%, respectively). Number of flocks was independent of altitude category (x^ = 5.34, df = 1, P = 0.021). Flocks crossing vs. not crossing over pow- erlines was also independent of section (x^ = 1.74, df = 1, P = 0.187). In 59 cases (8.6%), we felt that a bird reacted to models. These reactions were mainly out of “curi- osity” (21 records) but 19 birds were “scared off,” 13 birds “attacked,” and six showed “vocal reac- tions.” Raptors seemed most curious or aggressive while waterfowl and storks were scared off by the model. Black Kites were recorded only four times near the structures equipped with models and in all of the cases the kite attacked model A. Marsh Harriers (Circus aeruginosus) approached model A 71.1% (N = 31) of the time it was observed. Both kites and harriers breed in the area. The Grey Her- on (Ardea cinerea) was most frequently “scared off’ (9.8%). Again, no correlation was found between the number of reactions and the days passed after the model was installed (Spearman’s r^ = —0.43, P = 0.086, N= 17). Discussion We found that the installation of raptor models on utility structures in Spain had no effect on de- creasing the number of flocks or the types of birds that came near powerlines. Neither did we find that the number of birds in the highest flight alti- tude category increased over sections equipped with raptor models nor that there were fewer flocks that crossed over treated sections. In general, raptors were responsible for the dif- ferences that we found. The eagle model (model A) had more effect on bird behavior (although not the intended effects) than the Accipiter silhouettes. This suggested that models designed to deter birds from approaching powerlines need to be as real as possible. Visible reactions toward the models such as attacks, curiosity or being scared off occurred only 10% of the time. Resident raptor species were more persistent in attacking models. Black Kites and Marsh Harriers had high reaction rates and we did not observe an accommodation toward the models. Although raptors are seldom recorded as collision casualties (Olendorff et al. 1981, Olen- dorff and Lehman 1986, Bevanger 1994), their re- actions to the models suggested that models should not be used to deter raptors near power- lines because the possibility of collisions could ac- tually increase. Based on our results, we concluded that the raptor models we tested would not reduce avian collisions with powerlines. None of the mod- els had a significant effect in scaring off birds and, in the case of raptors, models even attracted birds toward the power structures. Acknowledgments We are indebted to M. Castro, J. Balbontin, and H. Lefranc for field observations, J. Sanchez (Asistencias Tecnicas Clave, S.L.) coordinated the field work. R.R. Lehman, S. H. Anderson, and A.M.A. Holthuijsen made useful suggestions and corrections on an earlier version of the manuscript. The study was financed by the Utility Company RED ELECTRICA DE ESPANA. We thank J. Roig and V. Navazo of the department of environment of this firm for the supervision and technical arrange- ments. Raptor models were provided and placed by RED ELECTRICA DE ESPANA. Literature Cited Alonso, J.C., J.A. Alonso and R. Munoz-Pulido. 1994. Mitigation of bird collisions with transmission lines through groundwire marking. Biol. Conserv. 67:129- 134. Avian Powerline Interaction Committee (APLIC). 1996. Suggested practices for raptor protection on powerlines; the state of the art 1996. Edison Electric June 1999 Raptor Models as Mitigation Measures 159 Institute/Raptor Research Foundation, Washington DC U SA. Bernis, F. 1980. La Migracion de las Aves en el Estrecho de Gibraltar. Vol. I: Aves Planeadoreas. Univ. Complu- tense, Madrid, Spain. Bevanger, K. 1994. Bird interactions with utility struc- tures; collision and electrocution, causes and mitigat- ing measures. Ibis 136:412-425. . 1998. Biological and conservation aspects of bird mortality caused by electricity powerlines: a review. Biol. Conserv. 86:67-76. Brown, W.M. 1993. Avian collisions with utility structures: biological perspectives. Pages 12.1-12.13 m E. Colson and J.W. Huckabee [Eds.], Proceedings of the inter- national workshop on avian interactions with utility structures, Miami (Florida) . Electr. Power Res. Comm, and Avian Power Line Interactions Commit- tee, Palo Alto, CA U.S.A. and R.C. Drewien. 1995. Evaluation of two pow- erline markers to reduce crane and waterfowl colli- sion mortality. Wildl. Soc. Bull. 23:217-227. Burger, J. 1985. Factors affecting bird strikes on aircraft at a coastal airport. Biol. Conserv. 33:1-28. Crivelli, A.J., H. Jerrentrup and T. Mitchev. 1988. Elec- tric powerlines; a cause of mortality in Pelecanus crispus Bruch, a world endangered bird species in Porto- Lago, Greece. Colon. Waterbirds 11:301-305. Dolbeer, R.A., J.L. Belant and J.L. Siblings. 1993. Shooting gulls reduces strikes with aircraft at John F. Kennedy international airport. Wildl. Soc. Bull. 21: 442-450. Fiedler, G. and A. Wissner. 1980. Freileitungen als tod- liche Gefahr fiir WeiBstorche {Ciconia ciconia). Okol. Vogel 2(Sonderheft):59-109. Finlavson, C. 1992. Birds of the Strait of Gibraltar. Poy- ser, London, U.K. Heijnis, R. 1980. Vogeltod durch Drahtanflrige bei Hochspannungs-Leitungen. Okol. Vogel 2(Sonder- heft):l 11-129. Hernandez, M. 1988. Road mortality of the Little Owl {Athene noctua) in Spain./. Raptor Res. 22:81-84. Janss, G.F.E. and M. Ferrer. 1998. Rate of bird collision with powerlines; effects of conductor-marking and static wire-marking./. Field Ornithol. 69:8-17. Morrill, A.E. and S.H. Anderson. 1991. Effectiveness of marking powerlines to reduce Sandhill Crane colli- sions. Wildl. Soc. Bull. 19:442—449. Olendorff, R.R. and R.N. Lehman. 1986. Raptor colli- sions with utility lines: an analysis using subjective field observations. Pacific Gas and Electric Company, Sacramento, CA U.S.A. , A.D. Miller and R.N. Lehman. 1981. Suggested practices for raptor protection on powerlines: the state of the art in 1981. Raptor Research Report No. 4. Raptor Research Foundation, Provo, UT U.S.A. SoKAL, R.R. and FJ. Rohlf. 1995. Biometry, 3rd ed. W.H. Freeman and Company, New York, NY U.S.A. SOLMAN, V.E.F. 1973. Birds and aircraft. Biol. Conserv 5‘ 79-86. Work, T.M. and J. Hale. 1996. Causes of owl mortality in Hawaii, 1992 to 1994. J. Wildl. Dis. 32:266-273. Received 17 May 1998; accepted 3 February 1999 Appendix 1. Species observed in each taxonomic group for all experiments. Taxonomic GROUPS Species Ciconiiformes Waterfowl Vultures Raptors Lapwings Gulls Other birds Passerines Corvids Ardea cinerea; Bubulcus ibis-, Ciconia ciconia-, Egretta garzetta; Platalea Imcorodia Anas clypeata-. Anas platyrhynchos', Anser anser, Himantopus himantopus; Limosa limosa-, Numenius arquatcr, Tringa totanus Gyps fulvus Acdpiter nisus-, Athene noctua-, Buteo buteo; Circus aeruginosusr. Circus cyaneus-, Circus pygargus-, Cir- caetus gallicus-, Falco naumanni; Falco peregrinus-, Falco tinnunculus-, Hieraaetus pennatus; Milvus migransr, Milvus milvus-. Neophron percnopterus Vanellus vanellus Larus cachinnans Apus apus; Apus caffer ; Coccothraustes coccothraustes] Columba livia-, Columba palumbuy, Delichon urbica; Hirundo rustica-, Lanius senator-, Merops apiaster, Streptopelia turtur-, Upupa epops Alauda arvensis; Carduelis cannabina; Carduelis carduelis-, Galerida cristata; Miliaria calandra; Saxi- cola torquata; Sturnus unicolor, Sylvia melanocephala-, Turdus merula CoTvus corax, Corvus corone corone Short Communications J. Raptor Res. 33(2): 160-1 63 © 1999 The Raptor Research Foundation, Inc. Winter Diet of the Barn Owl ( Trro alba) and Long-eared Owl (Asio otus) in Northeastern Greece: A Comparison ELvralambos Alivizatos 1 Zaliki 4, GR-1 1524 Athens, Greece Vassilis Goutner Department of Zoology, Aristotelian University of Thessaloniki, GR-54006 Thessaloniki, Macedonia, Greece Key Words: Barn Owl, Tyto alba; Long-eared Owl, Asio otus; diet, Greece. There have been several comparative studies of the di- ets of Barn ( Tyto alba) and Long-eared {Asio otus) Owls (Marti 1974, Amat and Soriguer 1981, Mikkola 1983, De- libes et al. 1983, Marks and Marti 1984, Cramp 1985, Capizzi and Luiselli 1996). Dietary information has been useful in documenting the trophic relationships in the areas where the two species are sympatric (Herrera and Hiraldo 1976, Marks and Marti 1984). Greece is within the breeding and wintering areas of these species. Infor- mation on the diet of Barn Owl in Greece has come mainly from islands and parts of central and western Greece (Bohr 1962, Cheylan 1976, Pieper 1977, Nietham- mer 1989, Tsounis and Dimitropoulos 1992). Only a sin- gle study has provided information on the diet of these two species on Euboea Island (Akriotis 1981). This study compares the winter diet of the Barn Owl and the Long- eared Owl in a Greek wetland area. Study Area and Methods Our study was conducted in northeastern Greece near Porto Lagos (40°99'N, 25°32'E) in an area with an exten- sive coastal wetland complex including lagoons, salt- marshes, mudflats, reedbeds, open cultivated and uncul- tivated land, small villages, and pinewood plantations. Pellets of Long-eared Owls were collected at a large com- munal, winter roost in a pinewood and those of Barn Owls were collected in neighboring ruined buildings in February and early March of 1987. Prey were identified according to Brown et al. (1987), Chaline (1974), and MacDonald and Barrett (1993). Mean prey weights were taken mainly from Perrins (1987) for birds, MacDonald and Barrett (1993) for mammals and from our own data for insects. We estimated the trophic diversity of birds and mam- mals in the owl diets at the generic level and that of insects at a class level using the antilog of the Shannon Index (NB = expi/', where H' = ~l,pfn.p„ where pi rep- resents the proportion of prey items of each genus in the sample. To standardize diversity for comparison between species, we calculated evenness (£)(N 2 j = (Ng — 1 )/(N 2 — 1), where = exp/7' and Ng = l/2pf) (Alatalo 1981, Marks 1984). In order to compare the dietary overlap between species in each wetland, we used Pianka’s Index (1973), multiplied by 100 to express it as a percentage. Results The diets of both owls contained small mammals, birds, and insects, in descending order of importance (Table 1). Small mammals made up 92% of the Barn Owl diet by number and 85% by biomass. At least 10 mammal species were eaten. The most important of them were Mus spp. (40% by number and 32% by biomass), Microtus epiroticus (20% and 28%), Apodemus spp. (7% and 10%), and Crocidura suaveolens (19% and 8%). Birds of at least five species formed 6% of the diet by number and 15% by biomass. Insects (orthopterans) were a minor diet con- stituent (2% by number and less than 1% by biomass). The average prey weight was 14.7 g (range 0.5-70 g) Prey diversity was 5.19 and evenness 0.67. Mammals made up 89% of the diet by number and 85% by biomass of Long-eared Owls. We identified at least 12 mammalian species in the diet but the main mammalian prey were Mus spp. (48% by number and 35% by biomass), Apodemus spp. (23% and 28%), and M. epiroticus (13% and 15%). Birds (at least 16 species) formed 11% of the diet by number and 15% by biomass, while insects (orthopteran, Tettigoniidae) were less than 1% by both number and biomass. The average prey weight was 16.5 g (range 2-80 g). Prey diversity and even- ness values were 4.29 and 0.56, respectively, both being lower than these of the Barn Owl. The proportions of all mammalian prey, in terms of number and biomass, were very similar in both owl spe- cies. Nevertheless, the proportions of the four most im- portant genera {Mus, Apodemus, Microtus, and Crocidura) differed significantly (y^ = 208.83, df = B, P < 0.0001) Crocidura were much more abundant in the Barn Owl’s diet while Apodemus was more common in the Long-eared Owl’s diet. Although fewer birds were taken by the Barn 160 June 1999 Short Communications 161 Table 1 . Diet of Barn and Long-eared Owls in Porto Lagos. Barn Owl Long-eared Owl Prey Number % Number % Biomass Number % Number % Biomass Insects 7 2.3 0.2 2 0.2 0.1 Tettigoniidae 1 0.3 0.1 2 0.2 0.1 Gryllidae 6 1.9 0.1 — — — Birds 18 5.8 14.7 102 10.6 15.5 Alcedo atthis — — — 1 0.1 0.3 Lullula arborea — — — 2 0.2 0.4 Alauda arvensis — — — 1 0.1 0.2 Galerida cristata — — — 9 0.9 1.8 Phylloscopus spp. — — — 3 0.3 0.2 Erithacus rubecula — — — 4 0.4 0.5 Turdus spp. — — — 3 0.3 1.5 Aegithalos caudatus — — — 6 0.6 0.3 Parus caeruleus 1 0.3 0.2 2 0.2 0.1 Parus spp. — — — 4 0.4 0.3 Sturnus vulgaris 3 1.0 4.6 2 0.2 0.9 Emberiza spp. 1 0.3 0.5 2 0.2 0.3 Miliaria calandra 5 1.6 4.4 — — — Eringilla coelebs — — — 11 1.1 1.4 Carduelis chloris — — — 3 0.3 0.6 Carduelis spp. — — — 2 0.2 0.2 Serinus serinus — — — 3 0.3 0.2 Passer spp. 3 1.0 1.6 10 1 1.6 Unident. 5 1.6 3.3 34 3.5 4.3 Mammals 286 92.0 85.1 857 89.2 84.5 Crocidura leu codon 6 1.9 1.1 3 0.3 0.2 Crocidura suaveolens 60 19.3 7.9 3 0.3 0.1 Suncus etruscus 2 0.6 0.1 1 0.1 <0.1 Talpa europaea — — — 2 0.2 0.9 Rhinolophus ferrumequinum — — — 1 0.1 0.1 My Otis sp. — — — 1 0.1 0.1 Pipistrellus sp. 1 0.3 0.1 — — — Tadarida teniotis — — — 1 0.1 0.2 Microtus epiroticus 63 20.3 27.6 121 12.6 15.2 Arvicola terrestris 1 0.3 1.3 — — — Micromys minutus 2 0.6 0.2 — — — Apodemus spp. 23 7.4 10.1 219 22.8 27.6 Rattus rattus 3 1.0 3.9 — — — Rattus norvegicus — — — 1 0.1 0.4 Rattus spp. — — — 1 0.1 0.4 Mus spp. 125 40.2 31.6 464 48.3 35.1 Unident. Muridae — — — 33 3.4 3.1 Unident. Rodentia — — — 6 0.6 1.1 Total 311 100 100 961 100 100 Owl, some larger-sized species (Sturnus, Miliaria) were proportionally more common, so bird biomass was simi- lar in the diet of both owls. Average prey weights were similar. Both the total prey overlap and mammalian prey overlap of the two owl species were 86%. Discussion We found small mammals to be the most important prey of both Barn and Long-eared Owls in northeastern Greece. In other Greek areas, Barn Owls have also been reported to prey mainly on small mammals (4—15 spe- 162 Short Communications VoL. 33, No. 2 cies), mice (Mus or Apodemus) being the most important prey by number and usually also by biomass (Akriotis 1981, Bohr 1962, Cheylan 1976, Tsounis and Dimitro- poulos 1992). On some islands such as Crete and Corfu, a diverse spectrum of bat species was taken but in low overall proportions (Bohr 1962, Pieper 1977). In com- parison to the Barn Owl’s diet in Euboea (Akriotis 1981), we found higher biomass proportions of birds (15% vs. 3%) and C. suaveolens (8% vs. 1%) but similar propor- tions of Apodemus (10% vs. 11%). In contrast, the diet of the Long-eared Owls we studied had higher proportions of birds (32% vs, 15% by biomass) and Apodemus (34% vs 28%) but those of C. suaveolens Vf ere low (both <1%). In Euboea, Long-eared Owls preyed upon some mammal species not found in our study. While owls probably differ in terms of the species of mammals they eat in various habitats (Akriotis 1981, MEHPW 1986), they seem to con- sistently use mammals as their most common prey source. In Europe and the Canary Islands, both owl species are also mainly mammal predators. As in Greece, in some areas the Long-eared Owl’s diet can become heavily re- liant on birds (Mikkola 1983, Amat and Soriguer 1981, Delgado et al. 1986). Mice and voles, where abundant, are often the main prey of both species, but their relative proportions in diets vary greatly among areas (Cramp 1985, Taylor 1994). In the U.S., both owl species are pri- marily mammalian predators but the Long-eared Owl tends to prey on Microtus spp. in lower proportions than the Barn Owl, taking fewer birds than in Europe (2% vs. 14% by biomass) (Marti 1976, Marks and Marti 1984). Bunn et al. (1982) have described the Barn Owl as an unspecialized predator of small mammals while Taylor (1994) suggested that it shows a definite preference for Microtus because they are of small size and easy to cap- ture. Long-eared Owls seem to concentrate on relatively few mammal species regardless of habitat type or location they are found (Marti 1974). There is controversy wheth- er Microtus are selected or simply taken according to their availability (Mikkola 1983, Cramp 1985). As far as the availability of small mammals in our area is concerned, among 93 individuals snap-trapped at Porto Lagos area between June 1984— October 1986, 48% were Mus spp. (41% M. abbotti), 38% Croddura suaveolens, 11% Apodemus sylvaticus, and 3% Microtus epiroticus (Vohralik and Sofian- idou 1992). Trapping results may not reflect the true pro- portions of small mammals in their habitats (Yom-Tov 1991, Blem et al. 1993), but we felt they were a good indicator of the relative abundance of small mammals in our study area. They indicated that mice Mus were mostly taken by both owls probably because they were plentiful. M. epiroticus was somewhat preferred by both and Croci- dura suaveolens was generally avoided, especially by the Long-eared Owl that seemed to prefer Apodemus. Al- though shrews are in general distasteful to many preda- tors, including the Long-eared Owl, Barn Owls have been found to take them in large numbers, a fact frequently related to this prey’s local availability (Bunn et al. 1982, Mikkola 1983). The average prey weight of the Barn Owl in Porto La- gos was within the limits of the European populations (range = 12.8-25 g, Taylor 1994). That of the Long- eared Owl was much lower than that of the rest of Eu- rope (37.4 g, Marti 1976), where Microtus spp. (average weight range = 30-35 g) make up a larger percentage of the diet (41.5% vs. 12.6% in our study). The lighter prey weight in our study was primarily due to the preponder- ance of Mus spp. in the diet which weighed only 12 g. Average prey weight in the U.S. is even higher than that in Europe for both species reflecting the availability of larger-sized prey species (Taylor 1994). The higher aver- age prey weight of Barn Owls in the U.S. may also simply be due to its larger size than its European relative (Marti 1974, Marks and Marti 1984, Mikkola 1983). Dietary overlap of the two species varied greatly in six studies in the U.S. ranging from 56-90% (Marks and Marti 1984). In Spain, overlap was much higher in winter (89%, Delibes et al. 1983) than in summer (69%, Delibes et al. 1983; 78%, Amat and Soriguer 1981). The trophic diversity (H') of Barn Owls in our area was 0.32 and even- ness (£) was 0.29 (calculated according to Herrera, on a prey class level) . Both values were much lower than those reported in Spanish studies (Herrera 1974) suggesting that Barn Owl in northeastern Greece have a more sten- ophagic diet and that, unlike the Mediterranean region, prey in Greece, especially some small mammals, are prob- ably not in short supply for owls. The high dietary over- lap we found between the two owl species, coupled with the similarity in average prey weights, suggested that the two species are grouped along their food dimension and belong to the same trophic guild of owls wintering in this area. Also in other areas, where the Barn and Long-eared Owl are syntopic. Barn Owls have been shown to have a broader diet (Marti 1976, Amat and Soriguer 1981, Veiga 1981, Capizzi and Luiselli 1996). This probably results from the high dietary overlap between them and it may facilitate their coexistence in areas of syntopy (Marks and Marti 1984). The noticeable difference in the bird spe- cies composition in the diets of the two owl species in our area may simply have been related to differences in their hunting habits. Although both forage in the open, Long-eared Owls also hunt under tree canopy (Cramp 1985) (which may also account for the higher proportion of Apodemus taken) and they also raid bird roosts in bush- es and trees to a much greater extent than Barn Owls. Resumen. — En Porto Lagos (noreste de Grecia), las die- tas de invierno de Tyto alba y Asia otus consistieron basi- camente de pequenos roedores (en am bos 85% de la biomasa). Ratones {Mus y Apodemus) y ratas {Microtus epi- roticus) fueron las presas mas importantes para ambos bu- hos. Las musaranas {Croddura) fueron importantes sola- mente para Tyto alba (8% de la biomasa). Las June 1999 Short Communications 163 proporciones de las presas de los cuatro mamiferos mas abundantes fueron significativamente diferentes entre los buhos. Mas especies de aves fueron capturadas por Asia otus (16 vs. 5) pero la contribucion a la biomasa fue similar para los dos (15%). El promedio del peso de las presas fue similar {Tyto alba: 14.7 g; Asia otus: 16.5 g), la diversidad de presas fue mayor en Tyto alba (5.19 vs. 4.29). Las dietas coincidieron en un 86%. [Traduccion de Cesar Marquez] Literature Cited Akriotis, T. 1981. Food ecology of five sympatric owls. First Degree dissertation, Wolfson College, Oxford Univ., Oxford, U.K. Amat, J.A. AND R.C. SORIGUER. 1981. Analyse conparative des regimes alimentaires de I’Effraie Tyto alba et du Moyen-Duc Asio otus dans L’Ouest de L’Espagne. Alauda 49:112-120. Alatalo, R.V. 1981. Problems in the measurements of evenness in ecology. Oikos 37:199-204. Blem, C.R., L.B. Blem, J.H. Felix and D.W. Holt. 1993. Estimation of body mass of voles from crania in Short- eared Owl pellets. Am. Midi. Nat. 129:281-286. Bohr, H.-J. 1962. Zur kenntnis der Vogelwelt von Korfu. Bonn. Zool. Beitr. 13:50—114. Brown, R., J. Ferguson, M. Lawrence and D. Lees. 1987. Tracks and signs of the birds of Britain and Europe. Helm, London, U.K. Bunn, D.S., A.B. Warburton and R.D.S. Wilson. 1982. The Barn Owl. T. 8c A.D. Poyser, Carlton, U.K. Capizzi, D. and L. Luiselli. 1996. Feeding relationships and competitive interactions between phylogenetical- ly unrelated predators (owls and snakes). Acta Oecol. 17:265-284. Chaline, J. 1974. Les proies des rapaces. DOIN Editeurs, Paris, France. Cheylan, G. 1976. Le regime alimentaire de la chouette effraie Tyto alba en Europe Mediterraneenne. Rev. Ecol. (Terre Vie) 4:565—579. Cramp, S., [Ed.]. 1985. The birds of the western Palearc- tic. Vol. 4. Oxford Univ. Press, Oxford, U.K. Delgado, G., V. Quills, A. Martin and K. Emmerson. 1986. Alimentacion del Buho Chico {Asio otus) en la isla de Tenerife y analisis comparativo con la dieta de Tyto alba. Donana, Acta Vert. 143:87-93. Delibes, M., P. Brunet-LaCompte and M. Manez. 1983. Datos sobre la alimentacion de la lechuza comun (Tyto alba), el buho chico {Asio otus) y el mochuelo {Athene noctua) en una misma localidad de Castilla la Vieja. Ardeola 30:57-63. Herrera, C.M. 1974. Trophic diversity of the Barn Owl Tyto alba in continental western Europe. Ornis Scand 5:181-191. AND F. Hiraldo. 1976. Food-niche and trophic re- lationships among European owls. Ornis Scand. 7.29— 41. MacDonald, D. and P. Barrett. 1993. Mammals of Brit- ain and Europe. Harper Collins, London, U.K. Marti, C.D. 1974. Feeding ecology of four sympatric owls. Cowrfor 76:45-61. . 1976. A review of prey selection by the Long- eared Owl. Condor 78:331-336. Marks, J.S. 1984. Feeding ecology of breeding Long- eared Owls in southern Idaho. Can.]. Zool. 62:1528- 1533. AND C.D. Marti. 1984. Feeding ecology of sym- patric Barn Owls and Long-eared Owls in Idaho. Orms Scand. 15:135-143. Ministry of Environment, Housing and Public Works (MEHPW). 1986. Project for delineation of Ramsar Convention wetlands. Wetiand: Lake Vistonis-Porto Lagos. MEHPW, Athens, Greece (in Greek). Mikkola, H. 1983. Owls of Europe. T. & A.D. Poyser, Carlton, U.K. Niethammer, j. 1989. Gewolinhalte der Schleiereule {Tyto alba) von Kos und aus Siidwestanatolien. Bonn Zool. Beitr. 40:1-9. Perrins, C.M. 1987. Collins new generation guide to the birds of Britain and Europe. Collins, London, U.K. PlANKA, E.R. 1973. The structure of lizard communities. Annu. Rev. EcQl. Syst. 4:53-74. PiEPER, H. 1977, Fledermause aus Schleiereulen-Gewol- len von der Insel Kreta. Z. Sdugetierkd. 42:7-12. Taylor, I. 1994. Barn Owls: predator-prey relationships and conservation. University Press, Cambridge, U.K. Tsounis, G. and a. Dimitropoulos. 1992. Seasonal vari- ation of the feeding of Barn Owl, Tyto alba (Scopoh 1769) in mount Hymettus, Attica, Greece. Biol. Gallo- hell. 19:29-36. Veiga, J.P. 1981. Variacion anual de regimen alimenticio y densidad de poblacion de dos estrigiformes: sus cau- sas. Donana Acta Vert. 8:159-175. Vohralik, V. and T.S. Sofianidou. 1992. Small mammals (Insectivora, Rodentia) of Thrace, Greece. Acta Univ Carol. 36:341-369. Yom-Tov, Y 1991. Character displacement in the psam- mophile Gerbillidae of Israel. Oikos 60:173-179. Received 15 April 1998; accepted 6 February 1999 164 Short Communications VoL. 33, No. 2 J Raptor Res. 33 (2): 164-1 67 © 1999 The Raptor Research Foundation, Inc. SiBLICIDE, SpLAYED-TOES-FLIGHT DISPLAY, AND GRAPPLING IN THE SAKER FaLCON David H. Ellis USGS Patuxent Wildlife Research Center, HCR 1, Box 4420, Oracle, AZ 85623 U.S.A. Peter L. Whitlock P.O. Box 325, Eastham, MA 02642 U.S.A. P. Tsengeg Mongolian State University, Ulaanbaatar—46, P.O. Box 137, Ulaanbaatar, Mongolia R. Wayne Nelson 4218 63 Street, Camrose, AB T4V 2W2 Canada Key Words: Saker Falcon; Falco cherrug; social display; Grappling, siblicide, cannibalism. Here we report three types of novel aggressive behavior for the Saker Falcon {Falco cherrug). The first concerns sibilicide, never before directly witnessed for the genus Falco (see Newton 1979:117). The remainder concern ag- gressive behavior of adults, including a new social display we call Splayed-toes-flight and observations of Grappling (wherein two birds lock feet) and Whirling. On 16 June 1997, we visited a Saker Falcon nest in an elm tree (Ulmus spp.) containing three nestlings in Sookhbaatar Aimag (Axe Hero Province), eastern Mon- golia. When we went back the next day, we saw one young tearing at prey. Upon climbing to the nest, we discovered that the healthy-looking nestling (age ca. 14 d; compared with photographs of known-aged Prairie Falcons [F] mex- icanus]: Moritsch 1983) had both feet locked onto its still alive but bloodied and emaciated sibling (Fig. 1). The aggressor repeatedly tore at the wing of its sibling and the whole lateral surface of the right wing was tattered and sodden with blood. After the larger sibling delivered a long series of blows and tugs, it rested, but its feet re- mained clutching its nestmate. Its lack of responsiveness to our nearness was a surprise and likely reflected that it was starving. After a few moments, the victim gave a series of hoarse, quiet peeps, then lay silent. It seemed very near death and the tattered condition of its wing evidenced that the larger sibling had already consumed a small portion of it We were unable to find remains from the third sibling observed on the previous evening. Holthuijzen et al. (1987) and Court et al. (1988) concluded for their study areas, that missing falcon chicks had likely been con- sumed. Based on the difficulty the older sibling was hav- ing tearing at its nestmate, we concluded that, if the third nesding was consumed since the previous night, it must have been with the assistance of an adult. After about 15 min, we left the nest with the older sibling still locked onto its victim. Although siblicide is common in the family Accipitri- dae (Ingram 1959), Newton (1979) concluded that it was unknown for the genus Falco. The best substantiation (but no eye witness account) of siblicide for a falcon is by Ristow et al. (1983), Ristow and Wink (1985), and Wink et al. (1993) for Eleonora’s Falcon {F. eleonorae). They reported many cases of the youngest nestling (in broods of three) being wounded or killed and partly eat- en. They clearly documented cannibalism, but they never saw the actual killing. They were confident that the wounds they observed were due to sibling attacks rather than rat {Rattus rattus) predation or adult attacks (i.e., infanticide). Cade (1960:208) recorded a probable case of siblicide and cannibalism in the Gyrfalcon {F. rustico- lus), and Tordoff and Redig (1998) reported a possible case of Peregrine Falcon {F. peregrinus) siblicide. We has- ten to state that the behavior we observed, and what we believe these authors inferred in other falcons, is prob- ably not the Cainism common in Accipitridae, but rather the concerted attempt of hungry young to eat their nest- mates. Cannibalism is also apparently rare (or seldom report- ed) for species in the genus Falco, except for the Eleo- nora’s Falcon and the American Kestrel (F. sparverius) (Bortolotti et al. 1991). A small number of clear records of cannibalism exist for the Peregrine Falcon and Prairie Falcon (Ratcliffe 1980:142, Holthuijzen et al. 1987, Court et al. 1988). Another novel observation of aggressive behavior oc- curred on 23 June 1997, also in Sookhbaatar Amag. Our observations began when a lone adult male Saker Falcon performed a ledge-display bout, including the saker ho- molog of the Eechup-call (Herbert and Herbert 1965), June 1999 Short Communications 165 Figure L Two-week-old Saker Falcon in process of killing and eating its sibling. at a nest that had failed earlier that year. Later, this bird performed various courtship displays at a larger cliff about 1.5 km away from the failed nest and within the crater of a dormant volcano. There followed a 23-min aggressive encounter between this bird and a pair of adults within the crater. Deciphering the interactions that follow was possible because all three sakers were physically very different. The two males were conspicuously smaller than the fe- male. The lone male was extremely light and was also readily distinguished from the paired male by its differ- ent molt pattern. The female exhibited spot-belly plum- 166 Short Communications VoL. 33, No. 2 age, very unlike the males. Our first observations were made unconcealed about 60 m from the 1997 nest. The later observations were made from about 400 m away. The social encounter began at about 1335 H when the lone male saker flew to an old eagle nest on the crater wall and Eechup-called (the saker version is a monosyl- labic Chup; T.J. Cade pers. comm.) while watching the adult female flying away. As the female continued flying away, the lone male began Cliff-racing (Nelson 1977), fly- ing very fast back and forth in front of the cliff and pe- riodically landing on the cliff. After about 3 min, the lone male perched, watched westward, and then flew rapidly up and west as the second male stooped from the west and dove three times at it. At 1340 H, the two males Grappled (i.e., locked feet) about 10 m from the ground, Whirled two revolutions, then separated just before or just as they hit the ground. They then immediately flew off in different directions. About 500 m from the eagle nest, the lone male settled on a grassy hillside, still within the crater. Next began a 4-min attack wherein the paired sakers stooped many times at the lone male on the hillside. The frequency and high velocity of the stoops effectively kept it grounded. In response to many of the stoops, the lone male would leap into the air, flip upside down, and thrust its feet upwards to ward off the attacks. No exact count was made of the number of stoops during this flurry of activity, but we estimate that the lone male flipped up about 20 times to fend off about 30 stoops. About half of the stoops were by the female, but it appeared that each time contact was made, it was the paired male that had stooped. Contact between the two males probably occurred about five times and definitely occurred twice (i.e., once when they Grappled and Whirled and once when feathers were dis- lodged) . At 1344 H, the lone male flew rapidly toward the eagle cliff, but was immediately attacked by the pair. In re- sponse, the lone male sought refuge amidst boulders at the mouth of a small cave about 50 m below the lower eagle nest. At this juncture, the pair circled many times about 50-100 m above the cave. During this low-level soaring bout, both members of the pair performed an obvious social display which we called Splayed-toes-flight. This display differed from normal soaring flight in that the feet were held about 3 cm below the ventral contour feathers and the toes were spread apart and held slightly below horizontal. From careful, but distant, scrutiny, we estimated that the angle between the outer and inner toes was about 45°. At 1350 H, the male of the pair flew into the upper eagle nest and Chup-called several times. At 1353 H, the lone male flew up the slope a short distance to some large caves below the lower eagle nest. This action prompted more stoops by the pair, but at 1356 H, the male of the pair circled higher and drifted over the rim of the crater. At 1357:30 H, the lone male flew from the big cave. At 1358 H, the pair stooped very fast and pur- sued the lone male over the crater rim and east out of sight. At 1406 H, the female returned and circled in the crater. Then at 1408 H, the female landed high on the cliff, then flew west out of sight. At 1420:30 H, the male of the pair returned, circled, landed on the upper cliff, Chup-called and then circled with the female. Both per- formed Splayed-toes-flight but for neither bird were the legs so conspicuously down as at 1344 H. By 1426 H, all birds were gone from the crater. At 1427 H, both members of the pair returned, circled in the crater and again performed the less intense form of Splayed-toes-flight. After a few moments, we left the cra- ter. Talon Grappling has previously been observed in at least six species of falcons (Herbert and Herbert 1965, Simms 1975, Balgooyen 1976:12, Nelson 1977:35-36, 121-122, Newton 1979:153, Ellis 1992). We know of no previous record of talon Grappling for the saker. Splayed-toes-flight has been observed for two other sa- kers, both in central Mongolia. On 2 July 1998, an adult female performed two bouts of this display, each about 10 sec in duration, while circling about 200 m from us as we approached a nest containing four nestlings. The display was initiated when a fifth, and already fledged, young flew from the vicinity of the nest to the female’s position. The hrst observation of Splayed-toes-flight was made 22 May 1997 when an adult female saker dangled one leg and repeatedly circled our team during our climb to the nest that contained four young about 12 d of age. To our knowledge, Splayed-toes-flight has never been previously reported as a social display for any raptor. Leg- lowered flight may serve a thermoregulatory (cooling) function in some raptors (T. Fleming and an anonymous reviewer pers. comm.), but cooling fits the context of only one of our saker bouts. Splayed-toes-flight is very different from the leg-dangle displays found in the Com- mon Buzzard {Buteo buteo: Weir and Picozzi 1975) and others of Accipitridae. The only published falcon display involving lowered legs is the Mutual-floating-display de- scribed by Platt (1989), wherein mated Gyrfalcons, with wings partly furled and tails spread, perform a slow, par- allel descent. Nothing like Splayed-toes-flight was men- tioned by either Monneret (1974), Weick (1989), or Nel- son (1970, 1977) in their ethograms for the Peregrine Falcon. Resumen. — Proveemos informacion sobre un pichon de dos semanas de Falco cherrug matando a su companero de nido, siendo este el primer reporte de fratricidio en el genero Falco. Tambien reportamos combates aereos entre tres adultos de Falco cherrug. Se incluyen observa- ciones de aferramientos, volteretas y talones extendidos, un comportamiento social previamente no descrito. [Traduccion de Cesar Marquez] June 1999 Short Communications 167 Ackn o wledgments Philanthropic contributions from Mr. Howell Wynne and an anonymous donor made our 1997 travels possible. Our expeditions are part of a long-term monitoring pro- gram funded by the National Aeronautics and Space Ad- ministration-Goddard SFC. The manuscript benefitted by reviews from C.M. White and T.J. Cade. Literature Cited Balgooyen, T.G. 1976. Behavior and ecology of the American Kestrel {Falco sparverius'L.) in the Sierra Ne- vada of California. Univ. Calif. Publ. Zool. 103:1-87. Bortolotti, G.R., K.L. Wiebe and W.M. Iko. 1991. Can- nibalism of nestling American Kestrels by their par- ents and siblings. Can. J. Zool. 69:1447-1453. Cade, T.J. 1960. Ecology of the peregrine and Gyrfalcon populations in Alaska. Univ. Calif. Publ. Zool. 63:151— 290. Court, G.S., C.C. Gates and D.A. Boag. 1988. Natural history of the Peregrine Falcon in the Keewatin Dis- trict of the Northwest Territories. Arctic 41:17-30. Ellis, D.H. 1992. Talon grappling by Aplomado Falcons and by Golden Eagles, f Raptor Res. 26:41-42. Herbert, R.A. and K.G.S. Herbert. 1965. Behavior of Peregrine Falcons in the New York City region. Auk 82:62-94. Holthuijzen, A.M.A., P.A. Duley, J.C. Hager, S.A. Smith and K.N. Wood. 1987. Piracy, insectivory and canni- balism of Prairie Falcons {Falco mexicanus) nesting in southwestern Idaho./. Raptor Res, 21:32-33. Ingram, C. 1959. The importance of juvenile cannibalism in the breeding biology of certain birds of prey. Auk 76:218-226. Monneret, R.-J. 1974. Repertoire comportemental du faucon pelerin Falco peregrinus hypothese explicative des manifestations adversives. Alauda 42:407-428. Moritsch, M.Q. 1983. Photographic guide for aging nes- tling Prairie Falcons. Bureau of Land Management, Snake River Birds of Prey Project, Boise, ID U.S.A. Nelson, R.W. 1970. Some aspects of the breeding behav- iour of Peregrine Falcons on Langara Island, B C M.S. thesis, Univ. Calgary, Calgary, AB Canada. . 1977. Behavioral ecology of coastal peregrines {Falco peregrinus pealei). Ph.D. dissertation, Univ. Cal- gary, Calgary, AB Canada. Newton, I. 1979. Population ecology of raptors. Buteo Books, Vermillion, SD U.S.A. Platt, J.B. 1989. Gyrfalcon courtship and early breeding behavior on the Yukon North Slope. Sociobiology 15. 43-69. Ratcliffe, D, 1980. The Peregrine Falcon. Buteo Books, Vermillion, SD U.S.A. Ristow, D. and M. Wink. 1985. Breeding success and conservation management of Eleonora’s Falcon. Pag- es 147-152 in I. Newton and R.D. Chancellor [Eds ], Conservation studies on raptors, ICBP Technical Pub- lication No. 5. Pica Press, London, U.K. , C. Wink and M. Wink. 1983. Biologie des Eleo- norehfalken {Falco eleonorae): 11. Die Anpassung des Jagdverhaltens an die vom Wind abhangigen Zugvo- gelhaufigkeiten. Die Vogelwarte 32:7-13. Simms, C. 1975. Talon-grappling by Merlins. Bird Study 22: 261. Tordoff, H.B. and P.T. Redig. 1998. Apparant siblicide in Peregrine Falcons./. Raptor Res. 32:184. Weick, F. 1989. Zeichenstudien zur Morphologic und zum Verhalten des Wanderfalken. Ornithologische Jah- reschefte fur Baden-Wurttemberg 5:1-75. Weir, D. and N. Picozzi. 1975. Aspects of social behav- iour in the buzzard. Brit. Birds 68:125-141. Wink, M., H, Biebach, F. Feldmann, W. Scharlau, I. SwATSCHEK, C. Wink and D. Ristow. 1993. Contribu- tion to the breeding biology of Eleonora’s Falcon {Fal- co eleonorae). Pages 59—72 in M.K. Nicholls and R. Clarke [Eds.], Biology and conservation of small fal- cons. The Hawk and Owl Trust, London, U.K. Received 1 July 1998; accepted 30 January 1999 168 Short Communications VoL. 33, No. 2 J Raptor Res. 33(2):168-169 © 1999 The Raptor Research Foundation, Inc. Improving the Success oe a Mounted Great Horned Owl Lure for Trapping Northern Goshawks Jon T. McCloskey^ and Sarah R. Dewey USDA, Forest Service, Ashley National Forest, Vernal Ranger District, 355 North Vernal Avenue, Vernal, UT 84078 U.S.A. Key Words: Great Horned Owl; Bubo virginianus; North- ern Goshawk, Accipiter gentilis; trapping. Dho-gaza nets with live Great Horned Owl {Bubo vir- ginianus) lures are one of the most effective ways of cap- turing raptors during the breeding season (Bloom et al. 1992, Steenhof et al. 1994). However, the rigors of field conditions, stress on the owl during handling and trans- port and risk of injury or death to both the owl and in- tended capture bird should be considered and may pre- clude use of a live owl. Mounted Great Horned Owls have been used to capture raptors but they are typically not as effective as live owls (Bloom 1987). Using a mounted owl, Gard et al. (1989) reported that the lack of movement resulted in a less aggressive response by breeding Amer- ican Kestrels {Falco sparverius) . ] 2 iCohs (1996) constructed a mechanical owl using a remote control unit to capture three species of hawks. Although his method was rela- tively successful, he did not provide detailed assembly in- structions or mention factors (e.g., cost or mechanical failure) that may limit the use of this technique. Here, we describe a simple technique to improve the success of mounted Great Horned Owl lures and report the success of this method for trapping breeding Northern Gos- hawks {Accipiter gentilis) . Methods Trapping was conducted at 14 goshawk nest sites with- in Ashley National Forest located in northeast Utah. We used a modified dho-gaza (as described by Clark 1981) with a taxidermic mount of a Great Horned Owl as a lure to capture breeding goshawks during the nestling period. We placed the dho-gaza (net size 139.5 cm high X 256.5 cm long with 4.5 cm mesh) within 30 m of nests, selecting areas where natural vegetation provided flyways that would funnel goshawks into the net. Subsequently, one person laid face up on the ground <1 m in front of and toward the center of the net (between the net and the nest) covered by camouflaged netting. This individual held the owl upright on their chest and after the crew was out of sight, voiced the 5-note territorial hoot of the ^ Present address: Caesar Kleberg Wildlife Research In- stitute, Campus Box 218, Texas A&M University-Kings- ville, Kingsville, TX 78363 U.S.A. Great Horned Owl while moving the owl with their hands. Once an adult goshawk was captured, we reset the dho-gaza and attempted to capture the mate. During the nestling period, male goshawks frequently forage away from the nest for extended periods. In con- trast, females remain relatively close to nests and will ag- gressively defend against potential predators (Palmer 1988). For these reasons and because we did not always attempt to capture mates, we separated success rates into two categories: (1) birds caught first at each nest site and (2) capture of the mate. Trapping success was deter- mined by calculating captures per attempt and we con- sidered multiple trapping attempts at the same nest site as one attempt (see Bloom et al. 1992, Jacobs 1996) When attempting to capture mates, the individual with the owl was not placed under the net until we heard or observed the bird, thus minimizing discomfort to the in- dividual. Because we were evaluating the effectiveness of the lure (not the net) to incite the bird to stoop at the net, we considered it a success even if a goshawk hit the net and escaped (i.e., this was our failure, not that of the lures) . Results Between 24 June-4 July 1995, we captured a total of 15 adult goshawks. We had an 86% (12/14) success rate for capturing goshawks during our initial attempts (cat- egory 1) and a 60% (3/5) success rate for subsequent attempts to capture mates (category 2). All but one (11/ 12) of the initially captured birds were females and all (3/3) subsequently captured birds were males. The re- maining birds that were not captured during initial at- tempts vocalized but never stooped at the lure. Of the five attempts to capture mates, two were actually caught, one bounced out of the net, one vocalized but never stooped and one never appeared. Overall, there was a 79% (15/19) success rate using our technique to capture nesting goshawks. Discussion Using a mechanical owl to capture breeding Red-shoul- dered Hawks {Buteo lineatus), Cooper’s Hawks {Accipiter cooperii) and Sharp-shinned Hawks {Acdpiter striatus) , ]?i- cobs (1996) reported a 54% (15/28), 60% (3/5), and 77% (48/62) success rate, respectively. Gard et al. (1989) reported 21 of 24 (87%) American Kestrels either vocal- June 1999 Short Communications 169 ized or dove at a live Great Horned Owl placed 10m and 50 m from nests. The same study reported that only 8 of 24 (33%) kestrels responded aggressively to a mounted owl placed at the same distance from the nest. Our tech- nique enables trappers to simulate the natural movement of the owl while hooting from the same location. These factors improved our success compared to techniques that used a mechanical or mounted owl alone. Bloom et al. (1992), using three independent trapping studies of breeding goshawks, reported a 76% (41/54), 54% (27/50), and 67% (68/102) success rate using a live Great Horned Owl as a lure. These results are territory trapping success (TTS) rates (see Bloom et al. 1992 for definition) and are not directly comparable to our defi- nition of success. Using their definition of raptor trap- ping success (RTS), our success increases to 93% (14/ 15) or 100% (if we include the one male that escaped). Thus, our definition is more conservative than RTS and more comparable to TTS. We feel our definition is ap- propriate, considering we targeted both sexes at only 5 of 14 territories. Perhaps a more precise method of eval- uating trapping success would be to include time spent for each capture (Steenhof et al. 1994). Bloom (1987) reported two incidents where female goshawks locked talons with bait owls. With other hawk species he suggests that injury to the live owl lure or attacking hawk is rare. In any case, we agree with Schulz (1990) who suggests that we not forget our moral and ethical responsibility, which includes respect, sen- sitivity, and compassion for the animal being manipu- lated. Because our trapping success equals or exceeds those reported by Bloom et al. (1992) and considering the ease of maintaining and transporting a mounted owl compared to a live owl, we suggest that a live owl be used only when absolutely necessary. The mechanical owl built by Jacobs provides a safe and effective alternative to a live owl. Using our method of placing an individual covered with camouflaged netting (and hooting) at the location where the owl is placed, or using taped vocalizations placed near the owl, may fur- ther improve the success of the mechanical owl. However, weather conditions, condensation, wet vegetation, and other logistical considerations (i.e., cost, maintenance, and difficulty of construction) may prevent proper func- tioning or practical use of a mechanized decoy. Our tech- nique is an easy, safe, and effective method for capturing breeding goshawks. This method should be effective for capturing other raptors that aggressively defend their nest, but it has not been evaluated. We recommend this method or Jacob’s mechanical owl, in lieu of a live owl, for capturing breeding Northern Goshawks. Resumen. — Buhos {Bubo virginianus) vivos, disecados o mecanicos han sido utilizados como senuelos para me- jorar la captura de las redes dho-gaza y atrapar aves ra- paces. Los buhos vivos han sido los mas efectivos, pero existen ciertos riesgos para el bnho como para la rapaz Los buhos disecados son menos efectivos debido a la falta de monvimiento y vocalizaciones. Los buhos mecanicos son efectivos pero carecen de vocalizaciones, no funcion- an adecuadamente bajo ciertas condiciones de campo y son dificiles de construir y mantener. En este articulo, describimos una tecnica simple y segura que permite vo- calizacion y movimiento de un buho disecado. Documen- tamos el exito de este metodo para atrapar a Acdpiter gentilis. Nuestra tecnica fue tan exitosa como la de un buho vivo pero sin riesgos y mejor que un simple buho disecado o uno mecanico. [Traduccion de Cesar Marquez] Acknowledgments Funding for this project was provided by the USDA Forest Service, Ashley National Forest. We thank K. Pau- lin and L. Welch for their support and J. Cook, J. Coop, K. Johnson, S. Lewis, L. Ortiz, D. Roberts, and D. Sedla- chek for their field assistance. The manuscript benefitted from reviews by P.H. Bloom, F. Chavez-Ramirez, S. De- Stefano, and J.E. Thompson. Literature Cited Bloom, P.H. 1987. Capturing and handling raptors. Pag- es 99—123 in B.G. Pendleton, B.A. Millsap, K.W. Cline and D.A. Bird [Eds.], Raptor management techniques manual. Natl. Wildl. Fed., Washington, DC U.S.A. , J.L. Henckel, E.H. Henckel, J.K. Schmutz, B. WooDBRiDGE, J.R. Bryan, R.L. Anderson, P.J. De- TRiCH, T.L. Maechtle, J.O. McKinley, M.D. McCrary, K. Titus and P.F. Schempf. 1992. The dho-gaza with Great Horned Owl Lure: an analysis of its effective- ness in capturing raptors. / Raptor Res. 26:167-178 Clark, W.S. 1981. A modified dho-gaza trap for use at a raptor banding station. J. Wildl. Manage. 45:1043- 1044. Card, N.W., D.M. Bird, R. Densmore and M. Hamel 1989. Responses of breeding American Kestrels to live and mounted Great Horned Owls. J. Raptor Res. 23. 99-102. Jacobs, E.A. 1996. A mechanical owl as a trapping lure for raptors. / Raptor Res. 30:31-32. Palmer, R.S. 1988. Handbook of North American birds. Vol. 4. Diurnal raptors (Part 1). Yale Univ. Press, New Haven, CT U.S.A. Schulz, T.A. 1990. Raptor restraint, handling and trans- port methods. Pages 97-115 in D.R. Ludwig [Ed], Wildlife rehabilitation 8. Steenhof, K., G.P. Carpenter and J.C. Bednarz. 1994. Use of mist nets and a live Great Horned Owl to cap- ture breeding American Kestrels. /. Raptor Res. 28. 194-196. Received 1 July 1998; accepted 4 February 1999 170 Short Communications VoL. 33, No. 2 / Raptor Res. 33(2): 170-1 72 © 1999 The Raptor Research Foundation, Inc. Prey Size Matters at the Upper Tail of the Distribution; A Case Study in Northcentral Chile David P. Santibanez and Fabian M. Jaksic Departamento de Ecologia, Pontificia Universidad Catolica de Chile, Casilla 114-D, Santiago, Chile Key Words: prey selection'. Barn Owl, Tyto alba; Great Horned Owl; Bubo virginianus; Chile; semiarid ecosystem. Sympatric raptors are known to consume different prey species, often cueing on the abundance, size, mor- phology, or behavior of the prey (Kotler 1985, Kotler et al 1988, Jaksic 1989). Less known is how raptor preda- tion applies on different size or age classes of a given prey species (Fulk 1976, Marti and Hogue 1979, Zamo- rano et al. 1986, Vargas et al. 1988, Longland andjenkins 1987, Dickman et al. 1991), but abundance, size, and be- havior of age classes have also been postulated as the cues used for hunting them. Castro and Jaksic (1995) showed that sympatric Barn Owls {Tyto alba) and Great Horned Owls {Bubo virgini- anus) at a study site in northcentral Chile (Auco) did not take different sizes of their most frequently shared prey, the leaf-eared mouse {Phyllotis darwini) . The larger Great Horned Owl (1200 g) preyed on average on 50-g mice, while the Barn Owl (300 g) consumed 54-g mice. The lack of statistical difference resulted from both owls prey- ing across all size/ age classes of their shared prey. Because leaf-eared mice in Auco average 47 g, which IS close to the mean prey size for the Barn Owl in Chile (45.1 g, Marti et al. 1993), we decided to investigate pre- dation on a prey species shared by both Great Horned and Barn Owls that exceeds the mean prey size for the Great Horned Owl in Chile (72.8 g, Marti et al. 1993). The species studied was the 182-g chinchilla rat {Abrocoma bennetti), the second largest rodent species at our study site in northcentral Chile (Jaksic et al. 1992, Jaksic 1997). Material and Methods Las Chinchillas National Reserve (31°3TS, 71°06'W) at Auco is located approximately 300 km north of Santiago, Chile. This site has a semiarid climate, mean annual pre- cipitation of 157 mm, elevations ranging from 400-1700 m and slopes with vegetation dependent on solar expo- sure. On equator-facing slopes, vegetation is dominated by cacti, bromeliads and a few evergreen shrubs; on po- lar-facing slopes, evergreen shrubs are the dominant spe- cies. More details about this site may be found in Castro and Jaksic (1995). From March 1993— February 1996, we collected pellets of Great Horned and Barn Owls under perches, roosts, and nests in Auco. At least one pair of Great Horned and four pairs of Barn Owls inhabited the study area. Prey remains in pellets (mostly small mammals) were deter- mined to species level. More details about procedures may be found in Castro and Jaksic (1995). Whole cranial remains of chinchilla rats found in owl pellets were set apart and measured. According to the morphometric characters of each cranium, we estimated the body mass by regression analysis. The relationship between cranial measurements and body mass was cal- culated from specimens of known mass in the Museo Na- cional de Historia Natural (Santiago, Chile). Three cra- nial dimensions were measured with calipers at 0.5 mm precision: width of the zygomatic arch (cf. Green and Jameson 1975), minimum distance between upper inci- sor and first molar (upper diastema, cf. Blem et al. 1993) and length of the upper tooth row. We used bilateral Kolmogorov-Smirnov tests (Sokal and Rohlf 1981) to compare the size distribution of chin- chilla rats preyed upon by each species of owl. Although estimates of body mass derived from cranial measure- ments were computed to 1 g, we preferred to group individuals into 20-g increment classes because of the in- herent statistical error contained in making extrapola- tions based on regressions. We pooled data obtained dur- ing the entire study period of 36 mo. (March 1993- February 1996). Results and Discussion The three cranial measurements were good estimators of chinchilla rat body mass (r > 0.949), but tooth row length was chosen because of its better fit {r = 0.978, P < 0.05), and because it could be measured in 97% of the cranial remains (256 out of 264). The equation was: body mass (g) = antilog (2.341953 -I- 3.386149 log tooth row length in mm) . On average, Barn Owls consumed chinchilla rats weighing 145 ± 73 g (± SD, N = 182), whereas those in the diet of Great Horned Owls weighed 178 ± 70 g (N = 73). This difference in prey weight consumed was sig- nificant at P = 0.00119 (Kolmogorov-Smirnov D = 0.28005). Nevertheless, the prey weight ranges consumed overlapped considerably: 31-332 g for Barn Owl and 47- 348 g for Great Horned Owl (Fig. 1). How a 300-g Barn Owl can take such large-sized chinchilla rats eludes us, unless our equation overestimates prey weights based on cranial measurements. We would like to emphasize that chinchilla rats >290 g were preyed upon only sporadi- cally by Barn Owls (Fig. 1). On the other hand, it is not surprising that 1200-g Great Horned Owls preyed on June 1999 Short Communications 171 49 69 89 109 129 149 169 189 209 229 249 269 289 309 329 349 Weight of Abrocoma bennetti (g) Figure 1. Body mass frequency distribution of chinchilla rats {Abrocoma bennetti) consumed by Great Horned Owls {Bubo virginianus; N = 73 rats) and Barn Owls {Tyto alba-, N = 182 rats) in Auco, northcentral Chile, March 1993- February 1996. <49-g chinchilla rats because this owl is known to con- sume even smaller rodents at the study site (Castro and Jaksic 1995). It should also be noted that chinchilla rats comprise only a minor part of the diet of Barn Owls at the study site (x = 2.2% by prey numbers throughout 1988-90; see Jaksic et al. 1992), whereas they are the sec- ond most common mammal consumed by Great Horned Owls {x = 22.7% throughout 1988-90; Jaksic et al. 1992). Tyto (300 g) and Bubo (1200 g), which differ in body weight by a factor of 4, and by 23% in mean prey size (145 vs. 178 g, respectively), were able to exploit a single prey species ranging over one order of magnitude in mass (31-348 g) . This suggested that the Barn Owl was able to handle, even if infrequently, prey of 50% its own body weight which is remarkable. The equivalent figure for the Great Horned Owl would be 15%, well within its handling power (Marti et al. 1993). Our results indicated that small prey such as the 47-g leaf-eared mouse does not allow segregation by size be- tween these two owls, likely because of its limited size range (Castro and Jaksic 1995). However, the two owls did show segregation by size when preying on larger prey such as the 182-g chinchilla rat, likely because of the greater opportunity afforded by its ample size range. These observations support Wilson’s (1975) assertion that prey size matters to predators chiefly at the upper tail of the frequency distribution. Resumen. — En un estudio previo en Chile central (Auco), se detecto que las lechuzas Tyto alba (300 g) y Bubo virginianus (1200 g) consumian individuos del roe- dor Phyllotis darxvini de peso promedio 50 y 54 g, respec- tivamente. Esta diferencia no era significativa. Debido a que este roedor esta cerca del tamano promedio de presa calculado en Chile para Tyto (45 g) y lejos del calculado para Bubo (73 g), decidimos investigar que ocurria con la depredacion de estas lechuzas sobre un roedor mucho mas grande, Abrocoma bennetti (182 g). Encontramos que Tyto consumia individuos de peso promedio 145 g y que Bubo consumia aquellos de peso promedio 178 g, una diferencia significativa de 28%. Nuestra conclusion es que cuando la presa es pequena {Phyllotis) las dos lechu- zas no alcanzan a segregarse en cuanto a los tamanos consumidos, y que esto solo ocurre cuando la presa es grande {Abrocoma). [Traduccion de Autores] Acknowledgments We thank Enrique Silva for doing laboratory work, and Juan Carlos Torres-Mura and Jose Yanez for allowing us access to the mammal collection of the Museo Nacional de Historia Natural (Santiago, Chile). Eunding was pro- vided by EONDECYT grant 196-0319 and a Presidential Chair in Science. Fred Gehlbach and Carl Marti made cogent criticisms that helped us improve this paper. Literature Cited Blem, C.R., L.B. Blem, J.H. Felix and D.W. Holt. 1993. Estimation of body mass of voles from crania in Short- eared Owl pellets. Am. Midi. Nat. 129:282-287. Castro, S.A. and F.M. Jaksic. 1995. Great Horned and Barn Owls prey differentially according to the age/ 172 Short Communications VoL. 33, No. 2 size of a rodent in northcentral Chile. J. Raptor Res. 29:245-249. Dickman, C.R., M. Prevadec and A.J. Lynam. 1991. Dif- ferential predation of size and sex classes of mice by the Barn Owl, Tyto alba. Oikos 62:67-76. Fulk, G.W. 1976. Owl predation and rodent mortality: a case study. Mammalia 40:423-427. Green, A. and D.L. Jameson. 1975. An evaluation of the zygomatic arch for separating juvenile from adult cot- ton rats {Sigmodon hispidus). J. Mammal. 56:534—535. Jaksic, F.M. 1989. What do carnivorous predators cue in on: size or abundance of mammalian prey? A crucial test in California, Chile, and Spain. Rev. Chil. Hist. Nat. 62:237-249. . 1997. Ecologia de los vertebrados de Chile. Edi- ciones Universidad Catolica de Chile, Santiago, Chile. , J.E. Jimenez, S.A. Castro and P. Feinsinger. 1992. Numerical and functional response of predators to a long-term decline in mammalian prey at a semi- arid Neotropical site. Oecologia 89:90-101. Kotler, B.P. 1985. Owl predation on desert rodents which differ in morphology and behavior. J. Mammal. 66:824-828. , J.S. Brown, RJ. Smith and W.O. Wirtz. 1988. The effects of morphology and body size on rates of owl predation on desert rodents. Oikos 53:145-152. Longland, W.S. and S.H. Jenkins. 1987. Sex and age af- fect vulnerability of desert rodents to owl predation J. Mammal. 68:746—754. Marti, C.D. and J.C. Hogue. 1979. Selection of prey by size in screech owls. Auk 96:319-327. , E. Korpimaki and F.M. Jaksic. 1993. Trophic structure of raptor communities: a three-continent comparison and synthesis. Pages 47-137 mD.M. Pow- er [Ed.], Current ornithology. Vol. 10. Plenum Pub- lishing Corporation, New York, NYU.S.A. SoKAL, R.R. and F.J. Rohlf. 1981. Biometry. W.H. Free- man and Co., San Francisco, CA U.S.A. Vargas, J.M., LJ. Palomo and P. Palmquist. 1988. Pre- dacion y seleccion intraespecifica de la lechuza co- miin {Tyto alba) sobre el raton moruno {Mus spretus) Ardeola 35:109-123. Wilson, D.S. 1975. The adequacy of body size as a niche difference. Am. Nat. 109:769-784. Zamorano, E., L.J. Palomo, A. Antunez and J.M. Var- gas. 1986. Criterios de predacion selectiva de Bubo bubo y Tyto alba sobre Rattus. Ardeola 33:3-9. Received 17 May 1998; accepted 7 March 1999. J. Raptor Res. 33(2):172-175 © 1999 The Raptor Research Eoundation, Inc. Spatial and Temporal Variations in the Diet of the Common Kestrel {Falco tinnunculus) in Urban Rome, Italy Emanuele Piattella Dipartimento di Biologia Animale e delTUomo (Zoologia), Universitd di Roma “La Sapienza” Viale deirUniversitd 32, 1-00185 Rome, Italy Luca Salvati Piazza F. Morosini 12, 1-00136 Rome, Italy Alberto Manganaro Via di Donna Olimpia 152, 1-00152 Rome, Italy Simone Fattorini Dipartimento di Biologia Animale e delTUomo (Zoologia), Universitd di Roma “La Sapienza” Viale delTUniversitd 32, 1-00185 Rome, Italy Key Words; Common Kestrel, Falco tinnunculus; diet, avi- an prey, urban area, Rome, Italy. Several studies have described the ecology of raptors m urban areas (e.g., Galeotti 1994). Common Kestrels {Falco tinnunculus) breed in many European towns, fre- quently occurring in urban areas in higher densities than in farmland areas (Village 1990, Shrtibb 1993). Never- theless, few studies have described details of the feeding ecology of kestrels in these urban areas (Quere 1990, Ro- man owski 1996). Therefore, the aim of our study was to describe the composition of the kestrel diet and any sea- June 1999 Short Communications 173 Table 1. Common Kestrel {Falco tinnunculus) diet in urban Rome, Italy. Summer Winter Prey Number (%) Mean SD Prey Biomass (%) Mean SD Prey Number (%) Mean SD Prey Biomass (%) Mean SD Stylommatophora 0.6 1.3 0.1 0.3 0.1 0.2 0 0.1 Scorpiones 0.2 0.6 0 0 0 0 0 0 Mantodea 0 0 0 0 0.5 1.1 0.1 0.4 Orthoptera 5.2 4.4 0.4 0.4 22.1 12.8 3.1 3.2 Dermaptera 0.4 1.0 0 0 1.2 2.6 0 0 Coleoptera 29.3 11.9 1.4 0.8 30.9 14.7 1.9 0.8 Hymenoptera 0.1 0.2 0 0 2.7 4.6 0 0 Unidentified insects 0.3 0.5 0 0 0.1 0.2 0 0 Sauria 15.1 6.1 7.2 4.7 9.6 7.9 5.8 4.5 Columbiformes 0.9 1.4 10.8 15.8 0.1 0.2 3.2 5.6 Apodiformes 4.8 5.4 9.4 10.6 0.3 0.9 1.1 2.9 Passeriformes 21.7 9.6 41.6 15.4 4.4 2.8 16.4 10.0 Unidentified birds 2.4 4.1 5.3 10.4 0.1 0.2 0.3 0.8 Insectivora 0 0 0 0 0.5 0.5 0.2 0.3 Chiroptera 7.0 9.7 4.1 6.5 0.6 0.8 0.4 0.6 Rodentia 12.1 6.6 19.6 12.6 26.8 8.7 67.3 15.5 Total prey 1123 16504 g 1238 11 574 g sonal variation in a Mediterranean urban area like Rome, Italy. Methods We conducted our study in urban Rome where Com- mon Kestrels occur at higher breeding densities (0.1-2. 3 pairs/km^) than anywhere else in Italy. The kestrels nest in scaffolding holes in Roman ruins and monumental buildings (Salvati and Manganaro 1997). We assessed the diet by analyzing pellets and prey remains collected from 16 sites during the years 1996 and 1997. A total of 13 and 7 pellet samples were analyzed for the spring to summer (breeding period) and winter, respectively. In the city center, pellets were collected every month from April 1996-March 1997. Pellets and prey remains were dissected in water. Prey remains were identified using diagnostic keys (Mangan- arb et al. 1990) and by comparison with museum speci- mens in the Zoology Museum, “La Sapienza” University, Rome, Italy. Mean weights for each prey taxon were es- timated using data from Mediterranean areas (Mangan- aro et al. 1990). The number of individuals (scored as minimum value) was calculated taking into account all different kinds of prey items found. Paired anatomical parts were counted as belonging to the same individual. This method allowed us to estimate the frequency of oc- currence of prey numbers (PN) and biomass (PB) for each prey category and to relate PN and PB to the habitat composition of hunting areas. A Spearman Rank Correlation was used to assess rela- tions among prey numbers for the most important prey categories and between the habitats of hunting areas and prey categories found in the diet during the breeding period, when kestrels generally feed close to nests (Vil- lage 1990). Using the mean size of hunting ranges given in Village (1990) and Shrubb (1993), habitat composition within a 1-km radius of nests (3.14 km^) was characterized as farmland, wooded, modern urban, and ancient urban. A sequential Bonferroni test (1989) was used to adjust the significance level to the number of comparisons using the same data set. A minimum probability level of P < 0.05 was accepted (all tests were two-tailed). Statistical analyses were performed using STATISTICA software (version 4.5, 1993). Results are presented as mean ± SD. Results We identified 1123 prey items at breeding diets (86.4 ± 85.9 prey per nest) and 1238 prey items in the winter diets (176.9 ± 120.3 prey per roosting site), for a total biomass of 28 078 g. The number of prey items per pellet varied from 1.6-3. 7 in summer {x = 2.8 ± 0.5), and from 2.V-4.9 in winter (x = 3.2 ± 0.7) {t = -1.45, df = 18, P = 0.165). Kestrels preyed on species ranging in size from ants {Messor sp., 0.01 g) to adult Feral Pigeons {Columba livia, 300 g) . Throughout the year, the main prey groups were insects, reptiles, birds, and mammals. Beetles (especially families widely distributed in Mediterranean areas like scarabs and tenebrionids) and birds were most common- ly consumed in summer and grasshoppers and small mammals were most common in winter (Table 1). Birds and mammals were the main prey groups by biomass Other prey included molluscs, scorpions, and ants. The 174 Short Communications VoL. 33, No. 2 □ Birds H Mammals □ Reptiles ■ Insects Figure 1. Diet of the Common Kestrel {Falco tinnunculus) in an urban area of Rome, Italy during the breeding season. number of Feral Pigeons taken was positively correlated with the the number of Swifts {Apus apus) taken (y, = 0.63, P < 0.005, N = 20). The number of passerines tak- en was negatively correlated with the number of rodents (r^ = —0.70, P < 0.001, N = 20) while the number of shrews {Suncus etruscus and Crocidura sp.) taken was pos- itively correlated to numbers of rodents taken (r^ = 0.60, P < 0.01, N = 20). The number of Swifts taken was pos- itively correlated with ancient urban areas (r^ = 0.76, P < 0.005, N = 13) and negatively with farmland areas (r^ = —0.77, P < 0.005, N = 13). By contrast, the number of rodents taken was positively correlated with farmland areas (r^ = 0.93, P< 0.001, N = 13) and negatively with ancient urban areas (r^ = —0.77, P < 0.005, N = 13). Monthly analysis of diets from pellets of a city-center nest showed a wide variation for some prey groups: insects were regularly taken throughout the year, but their bio- mass was always very low. Birds and reptiles were mainly taken in summer and small mammals in winter. The pro- portion in biomass of different prey groups varied signif- icantly (x^ = 180.3, df = 6, P < 0.00001) during the breeding season with rodents and lizards taken mostly during incubation, while birds predominated in the diet during the nestling and postfledging periods (Fig. 1). Discussion The diet of the Common Kestrel in its typical habitat that consists of farmland areas with small woodland patches is generally composed of small mammals such as voles {Microtus spp.; Village 1990, Shrubb 1993). The in- crease in predation on reptiles and insects observed in Rome was probably due to the large availability of these prey in Mediterranean areas (Village 1990). An increase in birds in the diet of Tawny Owls (Strix aluco) has also been observed in European towns (Galeotti et al. 1991), probably because of the greater availability of birds and the decreased abundance of rodents in these areas (Gal- eotti 1994). In some European cities, kestrels take prey far from their nest sites (Quere 1990, Romanowski 1996). In Rome, however, kestrels hunt near their nests during the nesting period most likely because a wide variety of prey is available both in the city center (birds, bats, and rep- tiles) and in the suburban open areas (small mammals, reptiles, and insects). Predation on birds and small mammals, the two most important prey groups, varied in relation to the distance between open areas and the city center, and prey groups with similar ecological habits were correlated to each oth- er and to the habitat types in hunting territories. Thus, both Swifts and Eeral Pigeons were caught in archeolog- ical and ancient urban areas, where they were a readily available and conspicuous food source for city-center kes- trels. By contrast, rodents and shrews were less common in these areas. Resumen. — Estudiamos la dieta del cernicalo euroasiati- co (Falco tinnunculus) por dos ahos en la Roma urbana, Italia. Identificamos un total de 2361 items de presas en egagropilas y restos de presas recolectados en 13 sitios de anidacion y 7 perchas de invierno. Los cernicalos cap- turaron una gran variedad de presas, desde pequehos insectos incluyendo hormigas hasta aves grandes como palomas. Las aves y los murcielagos predominaron dur- June 1999 Short Communications 175 ante la estacion reproductiva mientras que los pequenos mamiferos y las lagartijas fueron mas comunes en invier- no. Los insectos estuvieron presentes en la dieta a lo lar- go del ano, pero su biomasa fue muy baja. Las aves fue- ron capturadas predominantemente en areas urbanas y los roedores en areas agricolas. [Traduccion de Cesar Marquez] Acknowledgments We are grateful to G. Bogliani, P. Galeotti, and B. Massa for comments on an early draft of the manuscript, and to E. Gizzi and A. Sama for checking the English lan- guage. Two anonymous referees provided useful sugges- tions and improved data presentation. Literature Cited Galeotti, P. 1994. Patterns of territory size and defence level in rural and urban Tawny Owl (Strix aluco) pop- ulations./. Zool. London 234:641-658. , F. Morimando and C. Violani. 1991. Feeding ecology of Tawny Owls {Strix aluco) in urban habitats (northern Italy). Boll. Zool. 58:143-150. Manganaro, a., L. Ranazzi, R. Ranazzi and A. Sorace 1990. La dieta dell’allocco, Strix aluco, nel parco di Villa Doria Pamphili (Roma). Riv. Ital. Om. 60:37-52 Quere, J.P. 1990. Approche du regime alimentaire du faucon crecerelle {Falco tinnunculus L. 1758) en mi- lieu urbain (Paris intra muros) et durant la periode de reproduction. Le Passer 27:92-107. Roman OWSKI, J. 1996. On the diet of urban kestrels {Falco tinnunculus) in Warsaw. Buteo 8:123-130. Salvati, L. and a. Manganaro. 1997. Prime valutazioni su una popolazione urbana di gheppio Falco tinnun- culus. Avocetta 21:142. Shrubb, M. 1993. The kestrel. Hamlyn, London, U.K. Village, A. 1990. The kestrel. T. 8c A.D. Poyser, London, U.K. Received 1 July 1998; accepted 4 February 1999 Letters J, Raptor Res. 33(2):176 © 1999 The Raptor Research Foundation, Inc. Ospreys Incubate Goose Egg to Hatching Serial use of the same nest by Canada Geese (Branta canadensis) and Ospreys (Pandion haliaetus) has increased in recent decades. Campbell et al. (1990, The Birds of British Columbia, Royal British Columbia Museum, Victoria BC Canada) reported that 13% of the nesting Canada Geese utilized nests of Ospreys and Bald Eagles {Haliaeetus leuco- cephalus) in British Columbia. Because geese begin nesting earlier, Ospreys sometimes return to find their favored nesting site occupied by a pair of geese. Attempts to drive them from these nests vary in success (e.g., Flath 1972, Auk 89:446-447). On 1 June 1995, while trapping adult Ospreys near the mouth of the Coeur d’Alene River in northern Idaho, I found a pair of Ospreys attending a nest containing an Osprey egg and a goose egg. I intended to leave the eggs undisturbed until I heard soft vocalizations and discovered that the goose egg was already pipped. Because there was no chance of its survival at the nest, I removed the gosling from the shell in the hope of releasing it into a brood on the Coeur d’Alene River Wildlife Management Area nearby. However, John Nigh, the area manager, informed me that most local broods hatched from 15 April-15 May which limited the opportunity to foster the bird. I was advised to euthanize it, a step I carried out with much regret. I am uncertain if the Osprey egg hatched; no birds were present when I returned in midjuly to band nestlings in the area. I assume that a dispute over use of the nest occurred soon after Ospreys arrived from the south in late March or early April, before the geese had completed egg laying, A 1 June hatching date lagged behind that of most local clutches by 2-6 weeks, suggesting that the nest contained a single goose egg at the time Ospreys drove the geese off The goose egg was not incubated until an Osprey egg was laid several weeks later when the incubation of both began Alternate scenarios such as a delay in the initiation of egg laying or the reduction of a completed clutch to a single egg seem less likely. Alteration of clutch size or differences in egg size or color apparently have little influence on incubation effort by Ospreys. Fannin (1894, Auk 11:322) found a pair of Ospreys incubating a mixed clutch of goose and Osprey eggs which they continued to incubate after he reduced the clutch to a single goose egg. Reese (1977, Auk 94:202-221) found Mallard {Anas platyrhynchos) eggs in several clutches of Ospreys nesting in Maryland. In a cross-fostering experiment involving extensive replication, nesting Ospreys closely incubated 1-2 dummy Bald Eagle eggs for several weeks. All pairs later completed incubation of their clutches, which were maintained in an incubator during the experiment (E. Bizeau pers. comm.). I thank Wendy and Jon Lawrence for field assistance. Tracy Fleming provided helpful comments on the manuscript and additional literature sources. The suggestions of C.J. Henny and an anonymous reviewer also improved the manuscript. — Donald R. Johnson, Department of Biological Sciences, University of Idaho, Moscow, ID 83844 U.S.A. J. Raptor Res. 33(2):176-177 © 1999 The Raptor Research Foundation, Inc. Swainson’s Hawks in Nuevo Leon, Mexico In Mexico, the Swainson’s Hawk (Buteo swainsoni) is considered mostly a migratory species that nests mainly in the U S. and Canada. Here, we report our observations of two Swainson’s Hawk nests in the state of Nuevo Leon, Mexico Previously, Urban (1959, Birds from Coahuila, Mexico. Univ. Kansas Publ., Mus. Nat. Hist. 11(8):443-516) reported two Swainson’s Hawks in western Coahuila, Mexico. The first specimen was collected on 20 June 1952, two miles west of Jimenez. Measuring the gonads to be 6 X 4 mm. Urban concluded the bird must have been breeding. The second individual was collected at Iglesias, 24 km southwest of Sabinas on 22 August 1949. Swainson’s Hawks have also been 176 June 1999 Letters 177 recorded breeding in the following states of Mexico: Baja California, Sonora, Durango and Chihuahua (Oberholser 1974, The bird life of Texas. Vol. I. Univ. Texas Press, Austin, TX U.S.A.; AOU 1998, Check List of North American Birds. 7th edition. Washington, DC U.S.A.); Coahuila and Baja California (Urbina-Torres and Morales-Gonzalez 1996, Aves rapaces de Mexico, Centro de Investigaciones Biologicas, U.A.E.M., Mexico); Neuvo Leon, Sonora through Chihuahua, Durango and Coahuila, and extreme northern Tamaulipas (Howell and Webb 1995, A guide to the birds of Mexico and northern Central America, Oxford Univ. Press, London, U.K.; England et al. 1997, Swainson’s Hawk {Buteo swainsoni). In The Birds of North America, A. Poole and E. Gill [Eds.]. The Academy of Natural Sciences, Philadelphia, PA, and The American Ornithologists’ Union, Washington, DC U.S.A.). We have previously reported the Swainson’s Hawk primarily to be a migrant in our area that is vulnerable to deforestation for ranching (Contreras- Baleras et. al. 1995, Lista preliminar de las aves del estado de Nuevo Leon. Capitulo 3:41-54; Contreras-Balderas 1997, pages 35-44 in R.W. Dickerman [Ed.], Resumen avifaunistico de Nuevo Leon. The era of Allan R. Phillips. A Festschrift. Horizon Communications, Albuquerque, NM U.S.A.) We surveyed select trails in two areas in the northcentral part of Nuevo Leon, Mexico from January-September 1996. One area was near the municipality of Pesqueria (22.5 km northeast of Monterrey; 25°47'N, 100°06'W). Veg- etation in the area included Bumelia spiniflora, Prosopis glandulosa, Acacia farnesiana, and various species of sedges (Brouteloua spp.). The second area was near China (93.7 km southeast of Monterrey; 25°39'30"N, 99°20T5"W). Veg- etation in this area included Bumelia spiniflora, Prosopis glandulosa, Acacia farnesiana, Acacia wrightii, Leucophyllum tex- anum, Jatropha dioica, Opuntia spp., and various species of sedges {Bouteloua spp.), both in the Coastal Plain Gulf region among predominately high thorn brush. Both areas have a warm climate with an annual precipitation of 60- 100 cm and median annual temperatures in Pesqueria and China of 22 and 18°C, respectively (Institute Nacional de Estadistica, Geografia e Informatica 1981, Sintesis geografica de Nuevo Leon. Secretaria de Programacion y Presu- puesto, Nuevo Le6n, Mexico). We found two Swainson’s Hawk nests in Nuevo Leon. The first, just east of Monterrey in Pesqueria, had the following chronology: on 10 January 1996, five adults arrived at the site; on 8 April, a pair remained in the area; on 21 April, the pair established its territory; on 14 May, nest construction was observed; on 23 May, two eggs were being incubated; on 13 June, two young were observed in nest; and on 1 July, the young fledged. The nest was in a Prosopis glandulosa, 6.90 m from the ground. We found the second nest in the municipality of China on 27 July 1996. It contained one young. The nest was in a Pithecellobium flexicaule, 8.0 m above the ground. At both nests, the adults had typical light morph plumage. Our findings confirm that Swainson’s Hawks breed in Nuevo Leon, although they are probably not common. This increases the number of species that nest in the state while extending the southeasternmost boundary of the breeding range of the Swainson’s Hawk 360 km. We appreciate the English editorial skills of Laura Cholodenko, Association of Field Ornithologists’ program of editorial assistance, and Arturo Pehaflor for his help on fieldwork. We thank reviewers Stuart Houston, Peter H. Bloom, and A. Sidney England for improvements they made in the manuscript. — Armando J. Contreras-Balderas and Fernando Montiel-de la Garza, Laboratorio de Ornitologia, Facultad de Ciencias Biologicas, Universidad Autonoma de Nuevo Leon, Apartado Postal 425, San Nicolas de los Garza, Nuevo Leon, Mexied 66450. BOOK REVIEW Edited by Jeffrey S. Marks J Raptor Res. 33(2): 178-1 79 © 1999 The Raptor Research Foundation, Inc. The Raptors of Europe and The Middle East: A Handbook of Field Identification. By Dick Fors- man. 1999. T. & A.D. Poyser, London, xviii + 589 pp., 71 line drawings, 737 color photographs. ISBN 0-85661-098-4. Cloth, $45. — As stated in the subti- tie, this book is a compilation of information about the field identification of diurnal raptors. It covers the 43 species that occur regularly in Europe and the Middle East but not the vagrants to that area. The Handbook goes well beyond any of the available field guides in describing how to identify raptors as to species, age class, and in many cases, sex. New are the use of color photographs in place of color illustrations (although a photographic guide exists for North American raptors) and the thorough de- scription of molt, especially of flight feathers, and its use in aging raptors. Dick Forsman has published several books (in Swedish) and numerous articles on field identifi- cation of raptors. He has traveled throughout much of Europe and the Middle East taking pho- tos of and studying raptors in the field. After 25 years of experience, Forsnjan is well qualified to write this book. The book begins with a preface and acknowledgments, as well as an extensive glos- sary entitled “Abbreviations and Terminology.” This is followed by “How to Use the Book,” which provides a brief description of each heading in the species accounts. The first chapter, “Introduction to the Field Identification of Raptors,” begins with a detailed discussion of molt in falconiforms. Differences in molt sequences of flight feathers among members of the Accipitridae, Pandionidae, and Falconidae are covered, as is the use molt to determine age, especially for species that take more than one year to reach definitive basic plumage. Other topics cov- ered in this chapter are identification based on plumage characters, size, shape, structure, and characters of flight and movement. The chapter concludes with short discussions of variable light conditions, hybrids, and points to remember, all relating to field identification. Anyone with an in- terest in identification or molt, especially for di- urnal raptors, should study this chapter. The 43 species accounts constitute the meat of The Handbook. Each begins with short paragraphs summarizing subspecies, distribution, habitat, pop- ulation (estimates and trends), movements, and hunting and prey. This is followed by a more ex- tensive section, “Species Identification.” Measure- ments of length and wingspan are given first (most are taken from live birds) , followed by a sentence or two about the degree of difficulty of identifying the species in the field. Next is a blue-background box entitled “Identification Summary,” which is a particularly helpful feature because some discus- sions are lengthy and detailed. Then follow sections entitled “In Flight, Dis- tant,” “In Flight, Closer,” “Perched,” “Bare Parts,” and “Confusion Species,” and a section on molt by age class. The final section of each species account, “Ageing and Sexing,” includes another helpful blue-background summary. References to the photographs of each species are given for age and sex classes. The text concludes with a list of references; full citations are given in the bibliog- raphy at the end of the book. Many species ac- counts also include illustrations showing wing atti- tudes and plumage characters; some of these are in color, others in black-and-white. The heart of the book is a set of color photo- graphs that depict both perched and flying individ- uals, and for some species, birds in hand (covering all of the different plumages) . Photographs of cap- tive birds were used for at least one species. The caption for each photo gives information on age, sex (if possible), field marks, date, location, and photographer. I, too, have studied raptor identification in Eu- rope and the Middle East and have a raptor field guide in press for that area that uses color plates and a few color photos. Although it may appear that I would be somewhat biased in reviewing a book that could be considered a competitor for 178 June 1999 Book Revi£w 179 mine, please read on and reserve judgment on that issue until you have read the entire review. Somewhat at random, I have chosen four species accounts for detailed scrutiny: White-tailed Eagle {Haliaeetus albicilla), Pallid Harrier {Circus macrou- rus), Eastern Imperial Eagle {Aquila heliaca), and Sooty Falcon {Falco concolor) . While reading through the book, I had to keep in mind that En- glish is Forsman’s third language; he lives in a Swedish-speaking area of Finland. Although at times the wording is somewhat cumbersome, he still manages to describe plumages and behaviors clearly. The sets of photographs and the descrip- tions of the various age and sex classes and iden- tification points for these four species were all ac- curate and thorough, with the exception of some points mentioned below. The description of the first prebasic molt in the White-tailed Eagle is biased toward northern Eu- ropean eagles, because it is stated that they replace only “some inner secondaries,” when clearly the second-plumage eagle in plate 77 taken in Israel shows a minimum of seven new inner, outer, and middle secondaries. The field mark of uniformly dark leg feathers (called “trousers”) is mentioned but not stressed as a character of second- and third- plumage eagles not found on juveniles, whose trousers have tawny-buff spotting. In the Pallid Harrier account, the relative posi- tion of the wingtip to tail tip on perched birds, and the facial ring extending across the throat, both of which are useful field marks for distinguishing adult females from the very similar Montagu’s Har- rier (C. pygargus), are not mentioned under Con- fusion Species. However, my article in Birding World (July 1997) describing these field marks was pub- lished after The Handbook was already in press. Also not mentioned is the absence of streaking on the flanks of juvenile Pallid Harriers, which is useful for distinguishing them from juvenile Montagu’s Harriers, which always show such streaking. The adult males in plates 229 and 231, labeled “First plumage adult,” show dusky bands on the tips of the secondaries and a dark breast. I believe that these are just variants of the second-plumage male. The only item that I question in the Eastern Im- perial Eagle account is the number of immature plumages. Based on detailed examination of spec- imens and birds in the field, John Schmitt and I found that adult plumage is attained in four or five years, the same time required for all of the other large eagles, such as Golden {Aquila chrysaetos). Steppe {A. nipalensis), and White-tailed eagles. Forsman gives this as six or seven years. I think that his second and third plumages are the same, just variations in the amount of molt, and correspond with our second plumage. Likewise, his fourth and fifth plumages are the same; we consider this to be the third plumage. And his sixth plumage, the first adult plumage, is the same as our fourth plumage. Further fieldwork, particularly with marked indi- viduals, would be helpful in determining which of us is correct. I question that there are 200 pairs of Sooty Fal- cons breeding in Israel. I wonder if this is a mis- print, because I had thought that there are no more than 20 pairs there, based on my fieldwork in the mid-1980s. 1 have never seen any Sooty Fal- con, alive or as a specimen, with “nearly black” feathers as described by Forsman for some second- year birds, nor does any photograph in the book show this. Adult Barbary Falcons {F. pelegrinoides) were not mentioned in the Confusion Species sec- tion; they are also blue-gray above, only slightly larger than Sooty Falcons, and breed in the same areas. Almost all of the accounts have a complete set of photographs showing all plumages described; however, only four photos are shown for the Span- ish Imperial Eagle {A. adalberti) , three of which are of birds in captivity. Also, no photos of dark-morph Marsh Harriers {Circus aeruginosus) were included. In spite of the nit-picking comments above. The Handbook provides an excellent and nearly com- plete compilation of color photographs and infor- mation on raptor identification in Europe and the Middle East. This reflects the time and effort ex- pended by the author over many years to under- stand how to identify raptors in the field, including age and sex determination. It certainly lives up to the subtitle, A Handbook of Field Identification. I high- ly recommend The Handbook for anyone interested in raptor field identification or working on raptors in Europe and the Middle East. It is worth acquir- ing solely for the wonderful collection of photo- graphs, which is all the more remarkable given that the original photographs were misplaced, and the author had to spend more than a year assembling a new set. — William S. Clark, 7800 Dassett Court, Apt. 101, Annandale, VA 22003 U.S.A. BUTEO BOOKS The following Birds of North America Species Accounts are available through Buteo Books, 3130 Laurel Road, Shipman, VA 22971. TOLL-FREE ORDERING: 1-800-722-2460; FAX; (804) 263-4842. Barn Owl (1). Carl D. Marti. 1992. 16 pp. Boreal Owl (63). G.D. Hayward and RH. Hayward. 1993. 20 pp. Broad-winged Hawk. (218). L.J. Goodrich, S.C. Crocoll and S.E. Senner. 1996. 28 pp. Burrowing Owl (61). E.A. Haug, B.A. Millsap and M.S. Martell. 1993. 20 pp. Common Black-Hawk (122). Jay H. Schnell. 1994. 20 pp. Cooper’s Hawk (75). R.N. Rosenfield and J. Bielefeldt. 1993. 24 pp. Crested Caracara (249). Joan L. Morrison. 1996. 28 pp. Eastern Screech-owl (165). Frederick R. Gehlbach. 1995. 24 pp. Ferruginous Hawk (172). MarcJ. Bechard and Josef K. Schmutz. 1995. 20 pp. Flammulated Owl (93). D. Archibald McCallum. 1994. 24 pp. Great Gray Owl (41). Evelyn L. Bull and James R. Duncan. 1993. 16 pp. Great Horned Owl (372). C. Stuart Houston, Dwight G. Smith, and Christoph Rohner. 1998. 28 pp. Gyrfalcon (114). Nancy J. Glum and Tom J. Cade. 1994. 28 pp. Harris’ Hawk (146). James C. Bednarz. 1995. 24 pp. Long-eared Owl (133). J.S. Marks, D.L. Evans and D.W. Holt. 1994. 24 pp. Merlin (44) . N.S. Sodhi, L. Oliphant, R James and 1. Warkentin. 1993. 20 pp. Northern Saw-whet Owl (42). Richard J. Cannings. 1993. 20 pp. Northern Goshawk (298). John R. Squires and Richard T. Reynolds. 1997. 32 pp. Northern Harrier (210). R. Bruce MacWhirter and Keith L. Bildstein. 1996. 32 pp. Northern Hawk Owl (356). James R. Duncan and Patricia A. Duncan. 1998. 28 pp. Red-shouldered Hawk (107). Scott T. Crocoll. 1994. 20 pp. Red-tailed Hawk (52). C.R. Preston and R.D. Beane. 1993. 24 pp. Short-eared Owl (62). D.W. Holt and S.M. Leasure. 1993. 24 pp. Snail Kite (l7l). RW. Sykes, Jr., J. A. Rodgers, Jr. and R.E. Bennetts. 1995. 32 pp. Snowy Owl (10). David F. Parmelee. 1992. 20 pp. Spotted Owl (179). R.J. Gutierrez, A.B. Franklin and W.S. Lahaye. 1995. 28 pp. Swainson’s Hawk (265). A. Sidney England, MarcJ. Bechard and C. Stuart Houston. 1997. 28 pp. Swallow-tailed Kite (138). Kenneth D. Meyer. 1995. 24 pp. Turkey Vulture (339). David A. Kirk and Michael J. Mossman. 1998. 32 pp. White-tailed Hawk (30). C. Craig Earquhar. 1992. 20 pp. White-tailed Kite (178). Jeffrey R. Dunk. 1995. 16 pp. Buteo Books stocks all published species accounts, not only those covering raptors. Usually available from Buteo Books, the classic reference on diurnal birds of prey: Brown, Leslie and Dean Amadon. Eagles, Hawks and Falcons of the World. Country Life Books, 1968. Two volumes. First English edition in brown cloth. Fine in slipcase. $300.00 and other editions at lesser prices. I- pi A Telemetry Receiver Designed with The Researcher in Mind What youWe been waiting for! Finally* a highly sensitive 999 channel synthesized telemetry receiver that weighs less than 13 ounces* is completely user programmable and offers variable scan rates over all frequencies. For each animal being tracked* the large LCD display provides not only the frequency (to lOOHz) and channel number* but also a 7 character alphanumeric comment field and a digital signal strength meter. Stop carrying receivers that are the size of a lunch box or cost over $1500. The features and performance of the new R-1000 pocket sized telemetry receiver will impress you* and the price will convince you. Other Features Include: Factory tuned to any 8MHz wide segment in the 148-174MHz Band • Very high sensitivity of -148dBm to - ISOdBm • Illuminated display and keypad for use in low light or darkness • User selectable scan rates from 1-30 seconds in 1 second steps • Rechargeable batteries operate the receiver for 12 hours and can be replaced with standard AA Alkaline batteries in the field. Both I2vdc and llOvac chargers are included. • 6.1** (15.5cm) high, 2.6** (6.6cm) wide, 1,5” (3.8cm) deep. • 3 year warranty • 1 day delivery $ 695 . 00 , Please specify desired 8MHz wide segment in the 148174MHz band Visit our complete specifications, operating manual and .1 information on the R-1000 or call our toll-free number to order your receiver now. Try the New R-1000 Impressed! COMMUNiCATiONS SPECIALiSTS, INC. 426 West Taft Avenue • Orange, CA 92865-4296 • 1-714-998-3021 • Fax 1-714-974-3420 Entire U.SA (800) 854-0547 • Fax (800) 850-0547 • http://www.com-spec.com Handbook of the BIRDS OF THE WORL D The first work to illustrate and cover in detail ALL the species of birds in the world. Volume 5, Barn Owls to Hummingbirds, due July 1 999 Great pre-publication offer expires 31st July 1999 Volume 5, with more than 1600 birds depicted in 75 plates, and close to 400 photographs, will contain more illustrations than any of the volumes published to date. Also, such important groups of birds, as the Owls and the Hummingbirds, will be covered in detail and with all the species and distinctive subspecies illustrated for the first time. See many full reviews and several sample plates, photographs and texts of the first five volumes at http://www.hbw.com To receive a free 4-page colour brochure, please contact: LYNX EDICIONS Passeig de Gracia, 12 - E-08007 Barcelona, Spain Tel. -b34-93 301 07 77 Fax. -b34-93 302 14 75 E-mail: lynx@hbw.com •s Lynx Edicions THE RAPTOR RESEARCH FOUNDATION, INC. (Founded 1966 ) OFFICERS PRESIDENT: Michael N. Kochert VICE-PRESIDENT: Keith L. Bildsiein SECRETARY: Patricia A. Hale TREASURER: Jim Fitzpatrick BOARD OF DIRECTORS NORTH AMERICAN DIRECTOR #1 : Brian A. Millsap NORTH AMERICAN DIRECTOR #2: Petra Bohait. Wood INTERNATIONAL DIRECTOR #3; Beatrix Arroyo DIRECTOR AT LARGE #1 : Jemima ParryJones DIRECTOR AT LARGE #2: John A. Smallwood DIRECTOR AT LARGE #3: James C. Bednarz DIRECTOR AT LARGE #4: Miguel Ferrer DIRECTOR AT LARGE #5: Lloyd Kief DIRECTOR AT LARGE #6: Robert Ki nward NORTH AMERICAN DIRECTOR #3: Robert Lehman INTERNATIONAL DIRECTOR #1: Massimo Pandolfi INTERNATIONAL DIRECTOR #2: Reuven Yosef EDITORIAL STAFF EDITOR: Marc J. Bechard, Department of Biology, Boise State University, Boise, ID 83725 U.S.A. BOOK REVIEW EDITOR: JEFFREYS. Marks, Montana Cooperative Research Unit, University of Montana, Missoula, MT 59812 U.S.A. SPECIAL PUBLICATIONS EDITOR: Daniel E. Varland, Rayonier, 3033 Ingram Street, Hoquiam, WA 98550 SPANISH EDITOR: Cesar MArquez Reyes, Instituto Humboldt, Colombia, AA. 094766, Bogota 8, Colombia The Journal of Raptor Research is distributed quarterly to all current members. Original manuscripts dealing with the biology and conservation of diurnal and nocturnal birds of prey are welcomed from throughout the world, but must be written in English. Submissions can be in the form of research articles, letters to the editor, thesis abstracts and book reviews. Contributors should submit a typewritten original and three copies to the Editor. All submissions must be typewritten and double-spaced on one side of 216 X 278 mm (8% X 11 in.) or standard international, white, bond paper, with 25 mm (1 in.) margins. The cover page should contain a title, the author’s full name(s) and address(es). Name and address should be centered on the cover page. If the current address is different, indicate this via a footnote. A short version of the title, not exceeding 35 characters, should be provided for a running head. An abstract of about 250 words should accompany all research articles on a separate page. Tables, one to a page, should be double-spaced throughout and be assigned consecutive Arabic numer- als. Collect all figure legends on a separate page. Each illustration should be centered on a single page and be no smaller than final size and no larger than twice final size. The name of the author(s) and figure number, assigned consecutively using Arabic numerals, should be pencilled on the back of each figure. Names for birds should follow the A.O.U. Checklist of North American Birds (7th ed., 1998) or another authoritative source for other regions. Subspecific identification should be cited only when pertinent to the material presented. Metric units should be used for all measurements. Use the 24-hour clock (e.g., 0830 H and 2030 H) and “continental” dating (e.g., 1 January 1990). Refer to a recent is.sue of the journal for details in format. Explicit instructions and publication policy are outlined in “Information for contributors,”/. Raptor Res., Vol. 32(4), and are available from the editor. ASSOCIATE EDITORS Allen M. Fish Gary R. Bortolotti Charles J. Henny Fabian Jaksic Daniel E. Varland Cole Crocker-Bedford 1999 ANNUAL MEETING The Raptor Research Foundation, Inc. 1999 annual meeting will be held on 3-7 November in Las Pas, Mexico. For information about the meeting contact Ricardo Rodriguez Estrella, Centro de Investigaciones Biologicas del Noreste, Division de Biologica Terrestrie, KM 1 Carretera, San Juan de LaCosta, La Paz 23000, Mexico. Telephone 112-536-33, FAX 112-553-43, E-mail estrella@cibnor. mex. Persons interested in predatory birds are invited to join The Raptor Research Foundation, Inc. Send requests for information concerning membership, subscriptions, special publications, or change of address to OSNA, P.O. Box 1897, Lawrence, KS 66044-8897, U.S.A. The Journal of Raptor Research (ISSN 0892-1016) is published quarterly and available to individuals for $33.00 per year and to libraries and institutions for $50.00 per year from The Raptor Research Foundation, Inc., 14377 117th Street South, Hastings, Minnesota 55033, U.S.A. (Add $3 for destinations outside of the continental United States.) Periodicals postage paid at Hastings, Minnesota, and additional mailing offices. POSTMASTER: Send address changes to The Journal of Raptor Research, OSNA, P.O. Box 1897, Lawrence, KS 66044-8897, U.S.A. Printed by Allen Press, Inc., Lawrence, Kansas, U.S.A. Copyright 1999 by The Raptor Research Foundation, Inc. Printed in U.S.A. 0 This paper meets the requirements of ANSi/NISO Z39.48-1992 (Permanence of Paper). Raptor Research Foundation, Inc., Awards Recognition for Significant Contributions^ The Dean Amadon Award recognizes an individual who has made significant contributions in the field of systematics or distribution of raptors. Contact: Dr. Clayton White, 161 WIDE, Department of Zoology, Brigham Young University, Provo, UT 84602 U.S.A. Deadline August 15. The Tom Cade Award recognizes an individual who has made significant advances in the area of captive propagation and reintroduction of raptors. Contact: Dr. Brian Walton, Predatory Bird Research Group, Lower Quarry, University of California, Santa Cruz, CA 95064 U.S.A. Deadline: August 15. The Fran and Frederick Hamerstrom Award recognizes an individual who has contributed significantly to the understanding of raptor ecology and natural history. Contact: Dr. David E. Andersen, Department of Fisheries and Wildlife, 200 Hodson Hall, 1980 Folwell Avenue, University of Minnesota, St. Paul, MN 55108 U.S.A. Deadline: August 15. Recognition and Travel Assistance The James R. Koplin Travel Award is given to a student who is the senior author of the paper to be presented at the meeting for which travel funds are requested. Contact: Dr. Petra Wood, West Virginia Cooperative Fish and Wildlife Research Unit, P.O. Box 6125, Percival Hall, Room 333, Morgantown, WV 26506-6125 U.SA. Deadline: established for conference paper abstracts. The William C. Andersen Memorial Award is given to the student who presents the best paper at the annual Raptor Research Foundation Meeting. Contact: Ms. Laurie Goodrich, Hawk Mountain Sanctuary, Rural Route 2, Box 191, Kempton, PA 19529-9449 U.SjV. Deadline: Deadline established for meeting paper abstracts. Grants^ The Stephen R. Tully Memorial Grant for $500 is given to support research, management and conservation of raptors, especially to students and amateurs with limited access to alternative funding. Contact: Dr. Kimberly Titus, Alaska Division of Wildlife Conservation, P.O. Box 20, Douglas, AK 99824 U.S.A. Dead- line: September 10. The Leslie Brown Memorial Grant for $500-$l,000 is given to support research and/or the dissemination of information on raptors, especially to individuals carrying out work in Africa. Contact: Dr. Jeffrey L. Lincer, 1220 Rosecrans St. #315, San Diego, CA 92106 U.S.A. Deadline: September 15. ^ Nominations should include: (1) the name, title and address of both nominee and nominator, (2) the names of three persons qualified to evaluate the nominee’s scientific contribution, (3) a brief (one page) summary of the scientific contribution of the nominee. Send 5 copies of a proposal (^5 pages) describing the applicant’s background, study goals and methods, anticipated budget, and other funding.