J RaptorRes.32(3):208-214 ¸ 1998 The Raptor ResearchFoundation, Inc.

SELECTION OF SETTLEMENT AREAS BY JUVENILE BONELLI'S IN CATALONIA

SANTIMA•OSA, JOAN REA• ANt)JORO•CODINA Departamentde Biologia , Facultat de Biologia, Universitat de Barcelona,Avinguda Diagonal, 645, 08028 Barcelona,Catalonia,

ABSTRACT.--W½observed Bonelli's (Hieraaetusfasdatus) outside their breeding range in Catalonia (northeasternSpain) to identify the main dispersalareas of juvenile eaglesin the Central CatalanBasin. Autumn counts were conducted to determine the size of the nonbreeding population in the main dispersalarea and to analyze factors leading to the selection of settlement areasby juveniles. The permanenteagle population in the areawas estimated at about 18 >1-yr-oldeagles for an averagedensity of 0.90 eagles/100km s. Number of eaglesin the area

Selecci6nde zonasde asentamientojuvenil por el/tguila perdicera en Catalufia RESUMEN.--Describimosla localizaci0n y caracteristicasambientales de la principal zona de dispersion juvenil de las/tguilasperdiceras Hieraaetusfasdatus en Catalufia (Espafia).Se realizaronconteos otofiales en coche con objeto de evaluarel tamafio de la poblaci6n de/tguilas no reproductorasque utilizan esta zona,y se analizanlos factores que conducena la selecci6nde lasfireas de asentamientodentro de esta regi6n. La poblaci6n de /tguilasdispersantes est/t formada por una poblaci6n permanentede unos 18 ejemplaresde m/rsde 1 afio de edad,a una densidadde 0.90 aves/100km • , y una poblaci6nestacion- almente fluctuante de 17-94 avesde menosde un afio de edad, mS_ximaa principiosde otofio y que disminuyer/tpidamente a 1o largo del invierno. Las /tguilasseleccionan sus fireas de asentamientoen funci6n de la abundanciade perdiz y conejo, mientrasque las caracteristicasdel paisajeaparecen como un factor mucho menos importante. La gesti6n correcta de la caza menor constituyeuna pieza clave para la supervivenciade las/tguilasperdiceras j6venes en susfireas de asentamientotemporal. [Traducci6n Autores]

In long-livedbirds of prey, the acquisitionof sex- ous effectson population stability.Full understand- ual maturity is delayed for several years which ing of the habitat requirementsof long-livedrap- meansthat nonbreedingindividuals, whose behav- tors during the nonadult stagesof their lives is, ior and ecologymay differ from that of breeding therefore, necessaryto design appropiate conser- , form an important fraction of the total pop- vation measuresfor threatened or declining pop- ulation (Newton 1979, Ferrer 1993a, Omland and ulations. Hoffman 1996, Bustamante et al. 1997). In some The Bonelli's Eagle (Hieraaetusfasciatus)is such species, nonadult birds (those not in definitive a long-livedraptor. It doesnot mature sexuallyun- plumage) tend to settle in areasoutside the breed- til 2-4 yr of age (Newton 1979, del Hoyo et al. ing range in juvenile dispersalor temporary settle- 1994, Real and Mafiosa 1997) and it is an Endan- ment areaswhere they stayfor variable periodsbe- gered Species whose populations have declined fore moving to another settlementarea or joining acrossEurope (Rocamora 1994, Real and Mafiosa breeding populations(Gonzfilez et al. 1989, Ferrer 1997). The current Spanishbreeding population 1993b, 1993c). Lack of appropriate settlementar- of 675-751 pairsrepresents about 80% of the Eu- eas or reduction in their habitat qualitymay result ropean population (Arroyo 1991, Real et al. 1994, in a reduction in nonadult survival and have seri- Real et al. 1996). The nonbreeding population is

208 SEVT1998 JUWr•,•E BONELLI'SEAGLE SETFLEMENT AREAS 209

[--] Agramunt 1907 km Montclar429kml 37.3 I•_.•32.0• Atmenaras4.1•-• 2•-.-.-•aris de SantGuim 769km Si6197 km•-•185 km Granyena 1001 km Io I Prelxana 31•-.•I-•1 •---J '• Masde :3,2km 2• -L• •J sur6 I_LI Melons1324 km • 304 km _[•__• I ""', IArbeca629 km • •-•Castelldans ' I/I I I I I I I I I 1 L•431 km 10km l/I I I I i•! I I I I / I,- .... .• •/ I• I / I • I I I I I I•'"• •'"• • •q I • • I I I I I I I I I I I I • Figure 2. Areas surveyed during autumn eagle counts. J• ee• ' •'• The number of km driven and the linear index of abun- •1 I lelele'•l ' I I I I '•1 I I I I I•1 dance of eagles/1000 km are given for each sector. I{1111 e•.e IIl• I1•1 I •11 I lelelel•,eel I,I ,J I I , I/I I VI lelel I•1 t V.:.I 11 I •1 I• • I I I I l 1• :1 I 'e/I •_.•. many ornithologists as possibleon Bonelli's Eagles out- .•.I• ..'•" ';1. , ,-" •'• Mobregal side their breeding range. Resultswere plotted on 10 X • t It' 111' ,,h ,• 10 km Universal Transverse Mercator (UTM) squares •t'• •1':•,t,•;t, •I' • showing the presence or absenceof nonbreeding eagles in each square of the grid (Fig. 1). We also conducted counts of Bonelli's Eagles in the autumn and winter in central Catalonia in the main non- breeding area where eagleswere more seen. Counts were conducted for 3 yr from August-March 1991-94. They Figure 1. Breeding range (shaded area) and dispersal were made to identify the areas used by eagles and to areas (dotted squares) of Bonelli's Eagles in Catalonia estimate the number of eaglesin the region. We selected (northeastern Spain). The area depicted in Fig. 2 is out- 11 sectors (Fig. 2), each one exhibiting uniform land- lined. scape in terms of topography, land use, and vegetation cover and patterns. Sectorswere selectedin order to relY resent all typesof landscapepatterns available to eagles. estimatedat approximately700 eagles (Real and Each sectorwas driven by car once a week at a speed not Mafiosa 1997). These nonbreedersare commonly exceeding 40 km/hr. Becauseof the large area to be cov- ered on each sampling day, counts were conducted con- seen in areas unsuitable for nesting (Cramp and tinuously from sunrise to sunset. In order to obtain an Simmons 1980, Cugnasseand Cramm 1990, Real equal sample for each sectorat all times of the day, the et al. 1990, Cheylan et al. 1996), but little is known sampling sequencefor sectorswas changed each day,but concerning the exact locationsand physicalchar- travel constraints prevented perfect randomization. To analyzepotential biasesintroduced by different sampling acteristics of these settlement areas (Gonzfilez et al. schedules between sectors, counts were grouped into 1989, Ferrer 1990, Cugnasseand Cramm 1990, four time intervals: early morning (before 0700 H), Real and Mafiosa 1992, Real et al. 1994). The aims morning (0700-1200 H), afternoon (1200-1700 H), and of this study were to describe the locations and evening (after 1700 H). general characteristicsof juvenile dispersalareas of Transectswere driven by teams of two observerswith each team including at least one trained observer and a Bonelli's Eaglesin Catalonia (northeastern Spain), driver. Each team counted eagles in all of the sectorsso to estimate the size, age structure, and temporal bias among observerswas reduced. Each time a sector dynamics of the nonbreeding population in these was sampled, time of day, number of km driven, number areas, and to describefactors leading to the selec- of Bonelli's Eagles, and number of prey species (Red- legged [Alectorisrufa] and European rabbits tion of thesesettlement areas by nonadult Bonelli's [Oryctolaguscuniculus]) were recorded. When possible, Eagles. eagleswere classifiedas juvenile (•1 yr), immature (1-2 yr), subadult (2-3 yr), or adult (•3 yr) according to STUDY AREA AND METHODS plumage criteria (Parellada 1986). Although some prac- In Catalonia, the Bonelli's Eagle breeds along the tice is needed to sex Bonelli's Eagles in the field, the coastaland pre-pyreneanmountain ranges (Fig. 1). A to- most probable gender of the eagles was determined tal of 70-80 pairs, or about 10% of the Europeanbreed- based on size and color, females being larger and darker ing population, breeds in this area (Real and Mafiosa than males (Parellada 1986). When possible,eagles were 1992). From 1980-94, we compiled observationsfrom as classifiedas having full or empty crops. From these data, 210 MAF4os^ }•T ,'•L. VOL. 32, No. 3 linear indices of abundance for , rabbits, and Table 1. Names, definition, units, and source for habitat each age classof eagleswere computed for each sector. variablesrecorded in juvenile Bonelli's Eagle settlement Distance sampling (Buckland et al. 1993) was used to areas. estimate the abundance and densityof eagleswithin the sectorswhere theywere observed.Each eagleobservation was plotted on a 1:50000 map and th.e perpendicular NAME DESCRIPTION, UNITS AND SOURCE distance to the transect was measured. An overall detec- ROAD tion function was fitted to these data using the DIS- Paved roads (km/9 kmS); CORINE TANCE software (Laake et al. 1993). Because most con- TRAC Unpaved roads and tracks (km/9 kmS); tacts (93%) involvedsingle birds, the model was fit by CORINE consideringeach eagle as a single object. To obtain a URBA Urban, residential,and other developedar- more robust estimate of the detection function, obser- eas (ha/9 kmS); CORINE vations were truncated at 1.0 km after visual inspection CROP Arable land (ha/9 kmS); CORINE of the distance histogram (Buckland et al. 1993). The BUSH Low bush and dry grassland(ha/9 km2); detection function allowed the estimation of the Effective CORINE Strip Width for the countsand an overalldensity estimate WOOD Woodland (ha/9 km s) (mS); CORINE was computed from sectordensity estimates weighted by MIXE the area of each sector. Land occupied by agriculture with areas of To determine habitat selection, each sector was divided naturalvegetation (ha/9 km2); CORINE on 3 X 3 km UTM squares(900 ha or 9 kms each) on ALTI Averageelevation (m) (Minimum elevation which landscapevariables (Table 1) were measuredfrom + Maximum elevation)/2; 1:50 000 map a CORINE Land Cover 1:250000 habitat digital database RELI Relief index (total number of 20 m isolines processedby means of Arc/Info software,and 1:50000 crossedby the diagonalsof the 3 X 3 km military maps. The landscapeof every sectorwas then square [N]); 1:50000 map described using the same variables averaged over each IKARUFA sector. The area variables were measured on the 900 ha Number of partridges/1000 km; Linear counts unit squaresfrom each sector and could be considered IKACUNI as proportions,so they were better analysedby convert- Number of rabbits/1000 kin; Linear counts ing them to log-ratios (Table 1) to make them indepen- IKAFASC Number of Bonelli's Eagles/1000 km; Lin- dent from one another (Robertson et al. 1993). Spear- ear counts man rank correlation coefficients between eagle LR-WC Log-ratiowoodland-crop: In(wood/ abundance indices and partridge indices, rabbit indices, (crop+ 1) + 1) habitat variables,and land cover log-ratioswere comput- LR-BC Log-ratio bush-crop:In (bush/(crop+ 1) + 1) ed to show the cause of spatial variation in eagle abun- LR-MC Log-ratio mixed-crop: In(mixed/ dance between sectors. (crop+l) +1) LR-BW Log-ratiobush-wood: In(bush/(wood+ 1) + 1) RESULTS Log-ratio mixed-wood:In(mixed/ Location and General Characteristicsof the Ju- (wood+ 1) + 1) venile Dispersal Areas. We compiled 81 indepen- LR-MB Log-ratiowoodland-crop: In (mixed/ dent observationsof Bonelli's Eaglesoutside their (bush+l) + 1) usual breeding range in Catalonia. Some occurred along the coast, in wetland areas such as Aigua- molls de l'Empord•t,Delta del Llobregat,and Del- 7039 km. In thesecounts, Bonelli's Eagles were ob- ta de l'Ebre, but most observations came from the served on 196 occasionstotalling 211 birds. One Central Catalan Basin. Overall, 14 records were re- eaglewas observed on 183 (93%), two on 11 (6%), ported in coastalareas (2.8 recordsper 100 km2) and three on two (1%) occasions. Over 25% of and 67 in inland areas(3.19 recordsper 100 km2) eagles had wing tags that had been attached in (Fig. 1). The main juvenile dispersalarea in Caw- 1989-93 while they were nestlingsin the nearby Ionia wasa belt of extensivedry farmland, bound- Catalanbreeding population. Forty (27%) of the ed by the Pyrenees,the litoral mountains,and the juvenile, six (18%) of the immature, and four irrigated lands of central Catalonia.This is a nearly (28%) of the subadult eagles had wing tags. No fiat region, lacking cliffs and large forests, and is eagles tagged outside Catalonia (24 eagles in mainly devoted to cereal crops and to a lesserex- southeasternFrance and 60 eagles in Murcia-Ala- tent olive, almond and vineyards. cant, southeasternSpain) were seen in our survey. Origin, Sex-ratio, Age-ratio and Abundance of For eagleswhose sexes were determined (24%), Birds. Within the 11 sectors, 355 autumn counts 32 were male (63%) and 19 female (37%; Binomial were conductedalong 7528 km. Eagleswere ob- test, P = 0.093). For wing-tagged birds of known served on only 322 countsfrom nine sectorsover sex, six were male and three female. SEPT1998 JUVENILEBONELLI'S E•c,I•}• SETTI•EMENT AREAS 211

Table 2. Summarystatistics for the variablesrecorded in sectorswhere Bonelli's Eagleswere detected (N = 9) or not detected (N = 2). Mean ñ SD (minimum-maximum)

EAGLES PRESENT EAGLES ABSENT (N = 9) (N = 2) ROAD 2.1 - 0.7 (0.9-3.3) 2.3 + 2.5 (0.5-4.1) TRAC 23.3 + 6.1 (11.4-34) 26.3 ___1.0 (25.6-27.1) URBA 2.3 ___2.3 (0-6.5) 2.0 ___1.5 (0.9-3.0) CROP 640 ___195 (274-898) 618 + 151 (510-725) BUSH 49 + 70 (0-223) 7 + 7 (2-11) WOOD 23 _ 28 (0-67) 45 __+2 (43-46) MIXE 186 + 182 (213-567) 229 ---+162 (114-343) ALTI 402 ___65 (307-544) 676 + 18 (663-689) RELI 35 + 10 (21-48) 50 + 10 (43-56) IKARUFA 2582 + 1900 (182-5470) 1115 +__362 (859-1372) IKACUNI 78 - 90 (8-289) 39 + 1 (38-39) IKAFASC 26 ___17 (3-55) --

No significant difference in the linear index of and six (3%) adult eagles.Two (1%) were only. abundance of eagles was found between periods identified as > 1 yr of age. The observationrate for when the six sectorsvisited in every period were nonjuvenilesremained fairly stablearound 0.75 ea- included (Kruskall-Wallistest, x• = 0.05, df = 2, P gles/100 km during autumn and winter, while the = 0.98), or when the 10 sectors visited in both the observationrate of yearlingsreached a maximum last two periods were compared (Kruskall-Wallis of 3.2 eagles/100km in early autumn and declined test, x• = 0.006, df = 1, P = 0.94). There was also to 0.72 eagles/100km in December-March (Fig. no significant difference between the linear indi- 4). As a consequence,the age ratio of nonjuveniles ces of abundance of nonjuvenile (immatures,sub- to juvenilesincreased from 0.37 in Augustto 1.00 adults, and adults) eaglesbetween periods (Krus- after November. The seasonalvariation on eagle kall-Wallis test, x• = 1.377, df = 2, P-- 0.50 or x• abundance correlated to a similar variation in Red- = 0.006, df = 1, P = 0.94). Therefore, periods legged Partridge abundance (rs = 0.73; P = 0.015, were considered together. N = 10). Of the 198 individualswe aged, 145 (73%) were The age ratio also varied between sectors (0.91 juvenile, 33 (17%) immature, 14 (7%) subadult, in Mas de Melons, N = 42 eagles;0.67 in Almen- ara, N = 40 eagles;0.45 in Montclar, N = 16 eagles; 0.16 in Granyena,N = 29 eagles;and 0.07 in Agra- '--'60 munt, N = 59 eagles), but was not significantly z 50 related to eagle abundance (rs = 0.68; P = 0.205, N = 5) or prey abundance (rs = -0.50; P = 391, :• 40 .... N = 5). The linear index of abundance of young eagleswas positivelycorrelated with the linear in- O 30 ...... dex of abundance of mature eagles (rs = 0.63; P =

... 0.04, N = 11). >-20 " ' ' ' ".? A uniform function with two cosineadjustments

uJ10 ..... wasfound to be the best model to fit the perpen- dicular distancedata (Fig. 3). After truncation at Ill • • !..:. 1.0 km, only 196 birds remained in the analysis, u_ 100 300 500 700 900 and the Effective Strip Width was estimatedat 416 PERPENDICULAR DISTANCE (m) m (95% C.I. = 370-469). For the sectorswhere Figure 3. The number of eaglesobserved in relation to eagles were observed, we obtained an overall en- the perpendicular distanceto the transectline. The solid counterrate of 2.7 eagles/100km (95% C.I. = 1.9- line showsthe shape of the adjusted detection model. 3.8), and an averagedensity estimate of 3.2 eagles/ 212 MA•OS^ EWAL. VOL. 32, No. 3

only the nine sectorswhere eagleswere observed were considered, the linear index of abundance of E 4 Bonelli's Eagleswas found to be positivelycorre- lated with the Red-leggedPartridge linear index of abundance (rs = 0.72; P = 0.03, N = 9), the rabbit linear index of abundance (rs = 0.77; P = 0.02, N = 9), the log-ratiomixed-crop (rs = 0.87; P = 0.002, N = 9), and the log-ratio bush-woodland(r• = 0.71; P = 0.031, N = 9). Within the settlement areas, eagles were ob- 2ndAug 2nd Sep 2nd Oct 2nd Nov 2nd Dec FORTNIGHT servedfeeding on Red-leggedPartridge on one oc- casion, twice on European rabbits, and once on l-• All -•-Juvenile -*- Non-Juvenile I Quarry Guineafowl (Numida meleagris).Unsuccess- ful attackswere observedonce on feral pigeon (C0- Figure 4. The number of Bonelli's Eaglesin the central lumba livia), once on Wood Pigeon ( Columbapal- Cataloniadispersal area accordingto the time of the year expressedas the number of individualsseen/100 km of urnbus),once on pigeon (Colurnbasp.), twice on transect. Red-legged Partridges, and once on an unidenti- fied . Full cropswere observedin 43% of the eagleswhose crop contents could be determined 100 km2 (95% C.I. = 2.3-4.5). Maximum eagleden- (N = 120, 57% of all eagles).This wasprobably an sitywas attained in late September(5.2 eagles/100 overestimatesince empty crops were probably re- km2), and minimum densityin January-March(1.5 corded as unnoticed more than full crops. For eagles/100km•). The number of mature eaglesre- those sectorsin which crop contents were estimat- mainedfairly stableat about 0.90 eagles/100km •, ed for >5 eagles,those with a higher linear index while the number of young showedlarge variation of eagle abundancehad a larger proportion of ea- from late September(3.9 eagles/100km •) to late gles with full crops (rs = 0.98; P = 0.02, N = 4). winter (0.73 eagles/100km •, Fig. 4). If theseden- In some sectors,eagles were more often observed sity estimatesare extrapolated to the entire poten- perching (Plansde Si6 100%,Almenara 69%, Mas tial dispersalarea in central Catalonia (dotted area de Melons 50%) than in others (Montclar 25%, in Fig. I = 2000 km2), an estimateof 18 nonjuven- Granyena29%, Agramunt31%), but this wasnot ile eaglesand 17-94 yearlingswould be obtained related to eagle abundance(rs = -0.20; P = 0.70, for the entire area. N = 6). Of 97 perchesobserved, trees were the Selection of Settlement Areas, Eagle Behavior most frequently used (52%) followed by large and Habitat Use. Although the sequenceof sam- rocks (19%), the ground (18%), large transport pling was changed on each sampling day, differ- power poles (6%), buildings (4%), and small dis- ences in the average sampling time between sec- tribution power poles (1%). tors were detected (Kruskall-Wallis test, P • 0.000). D[SdUSS•ON However, a two-way ANOVA of the square-root transformed(x + 98)•/• index of eagleabundance Eagle abundance was related to partridge and (Zar 1984) showedno significantrelationship be- rabbit abundance in the dispersalarea. In sectors tween eagle abundance indices and time of day with larger eagle concentrations,eagles with full (F3,3,3= 0.759;P = 0.518) and a significanteffect cropswere more frequently encountered and these of sector(F10,•e• = 5.537;P • 0.000), sodifferences areas had more bush, dry grasslandor cropland in eagle abundance between sectorswere not the mixed with natural habitats relative to woodland or result of a different sampling pattern (Fig. 2). A homogeneousfarmland. These open habitatswere large variation was found in the habitat character- likely the most suitablefor eaglesbecause of their istics of the sectors where eagles were observed prey abundance and because their open habiat (Table 2). The linear index of abundance of Bo- maximized eagle foraging success(Bohall and Col- nelli's Eagleswas positively correlated with the lin- lopy 1984, Janes 1985, Preston 1990). Therefore, ear index of abundanceof Red-leggedPartridge (rs given the large variation in habitat pattern between = 0.74; P = 0.01, N = 11), and to the log-ratioof sectorswhere eagleswere observed,we concluded bush-woodland(rs = 0.73; P = 0.01, N = 11). If that nonadult Bonelli's Eagles selected settlement SE?T1998 JUVENILEBONELLI'S E^CLE S•TTL•M•NT Amws 213 areasmainly on food availabilityrather than their tribute to reducedjuvenile dispersal,decrease pre- topographicand landscapepatterns. A similarhab- adult mortality, and improve recruitment rates in itat selection strategyhas been describedin other nearby breeding areas (Gonz/tlez et al. 1989), speciesof birds,in whichfood is a proximatefactor which would be particularlyhelpful to stop the de- for habitat selection (Hild6n 1965, Hutto 1985, cline of isolatedsubpopulations or those found on Gonzffiez et al. 1989, Ferrer 1990, Gerrard et al. the edge of the speciesrange. Sensiblegame man- 1990, Heredia et al. 1991, Bustamante et al. 1997). agementis an essentialtool to achievethis objec- Although yearlingsform the bulk of the non- tive. However,reduced dispersalwill only be advan- breeding Bonelli's Eagle population in central Cat- tageousif the main mortality factorsfor eaglesin alonia, older birds form the stable fraction of the the area (Real et al. 1996, Real and Mafiosa 1997) population. Yearlingsarrive in the region in late are also eliminated. summer, following independence (Real et al. ACKNOWLEDGMENTS 1989), and probably occupy only those areas left availableby older birds which die, move to other We are indebted to Rodrigo del Arno, Joan Manel Baqu•s, Dani Diaz, Joan Estrada,Daniel Gonz•tlez,Ferran areas, or are recruited into the breeding popula- Gonz/tlez-Prat,G16ria Laviga, Montse L6pez, Marc No- tion. In thesedispersal areas, young eagles find suf- guera, Vittorio Pedrocchi and Montserrat Tortosa for ficient food and probablyavoid competitionwith contributing to field work. We also thank Dr. J. Loman, breeders. The seasonal decline of the dispersal Dr. H. KAllander,Dr. G.R. Bortolotti and Dr. M. McGrady for their useful comments on earlier versions of the population after October probably resultsfrom a manuscript.Josep Garriga, Cap del Servei d'Agents Rur- combination of factors including further innate als del Departament d'Agricultura, Ramaderiai Pescade dispersal (Horn 1983), competitive exclusion la Generalitat de Catalunya,provided accessto the COR- (Waser 1985), juvenile mortality (Real et al. 1996, INE database,and Emili Ponsahelped in the extraction Real and Mafiosa1997), or a declinein prey avail- of data from it. We especiallythank Fundaci6 Miquel Tor- res for continousfinancial support of Bonelli'sEagle con- ability (Newton 1979, Ferrer 1993b,Brodeur et al. servation in Catalonia. We also thank Caixa de Sabadell 1996) whichwas supported by the fact that the sea- for financial supportof the fieldwork on which this paper sonal decline of eagle abundancematched that of is based. The Fundaci6 Bosch i Gimpera cooperated in Red-leggedPartridges. A continuousturnover of the administration of part of the project. The first author received a postdoctoralgrant from Comissi6Interdepar- eagles with other dispersalareas, temporary re- tamental de Recercai Innovaci6 Tecnolbgica(CIRIT) de turns to natal areas, or movements to nearby la Generalitatde Catalunya(BPOST-9316). breeding areas may also occur as has been de- LITERATURE CITED scribed in the Spanish Imperial Eagle (Ferrer 1993c). AJ•oYo, B. 1991. Resultadosdel censonacional de aguila According to our wing-tagginginformation, the perdicera.Quercus 70:17. Catalan breeding population is the main sourceof BOHALL, P.G. AND M.W. COLLOY. 1984. Seasonal abun- Bonelli's Eaglesthat come to the dispersalarea in dance, habitat use, and perch sitesof four raptor spe- central Catalonia. A higher number of maleswere cies in north-central Florida. J. Field Ornithol.55:181- 189. observedin the area than females.This suggested BRODEUR,S., R. DI•CARIE, D.M. BIRD AND M. FULLER. 1996. that there may be female-biasedjuvenile dispersal Complete migration cycleof Golden Eaglesbreeding in this species,as is commonly found in raptors in northern Quebec. Condor98:293-299. (Clarke et al. 1997) or, alternatively,a sex-biased BUCKLAND,S.T., D.R. ANDERSON,K.P. BURNHAMAND J.L mortality (Ferrer and Hiraldo 1992). Given the size LAAKE.1993. Distance sampling. Chapman and Hall, and demographic parameters (Real and Mafiosa London, U.K. 1997) of the breedingpopulation in Catalonia,ev- BUSTAMANTE,J., J.A. DON•ZAa, E HIRALOO,O. CEBALLOS ery year 55 independentjuveniles, 22 immatures, AND A. TRAVA•NI. 1997. Differential habitat selection and 9 subadultsenter this eagle population. The by immature and adult Grey Eagle-BuzzardsGeranoae- tus melanoleucus. Ibis 139:322-330. dispersal area in central Catalonia can only give CHEYLAN,G., A. RAVAIROL,J.M. CUGNASSE,J.M. BILLET permanent refuge to about 35 of these birds, so ANDC. JOULOT.1996. Dispersiondes aiglesde Bonelli many must dispersefarther away.Although the Hieraaetusfasciatusjuv6nilesbagu6s en France.Alauda mechanismsof dispersaland recruitment in Bo- 64:413-419. nelli's Eaglesare only partially understood,in our CLARKE,A.L., B.E. S/ETHERAND E. ROSKAFF.1997. Sex bi- opinion increasing the carrying capacity of dis- asesin arian dispersal:a reappraisal.Oikos 79:429- persalareas adjacent to breedingareas would con- 438. 214 MA•os^ ET AL. VOL. 32, NO. 3

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