j RaptorRes. 33 (2) :110-116 ¸ 1999 The Raptor ResearchFoundation, Inc. SELECTION OF NEST CLIFFS BY BONELLI'S EAGLE (HIETUS FASCIATUS) IN SOUTHEASTERN SPAIN DIEGO ONTIVEROS Departamentode Biologia Animal y Ecolog•a,Facultad de Ciencias,Universidad de Granada,E-18071 Granada, Spain PmSTP•CT.---Atotal of 119 nestsand 52 cliffsoccupied by 32 Bonelli'sEagle (Hieraaetusfasciatus)pairs wasstudied during 1995-97 in southeasternSpain. Mean numberof nestsbuilt by pairsexceeded that reportedin previousstudies (i = 3.7; N = 32) and therewas a trend amongeagles to build their nests with a southeasternorientation. Breeding density was directly related to the availabilityof cliffs.Eagles occupiedhigher cliffs (i = 52.9 m; N = 32), locatedon steeperslopes (i = 34.7ø;N = 31) than was available.Occupied cliffs were highly heterogeneousdue to the fact that use of differentareas by Bonelli's Eagleswas dependent on human disturbance.Thus, occupied cliffs with the shortestlinear distanceto pavedroads were higher than occupiedcliffs far from pavedroads. Selection of high cliffs located on steep slopeswith southern orientationsmay have been associatedwith the additionallift providedeagles, since these types of nest sitesenhanced the possibilityof thermal and slopesoaring. 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'sEagle,, Hieraaetus fasciatus; southeastern Spain; cliff selection; breeding density. Selecci6nde losroquedos de nidificaci6ndel Aguila Perdicera (Hieraaetus fasciatus) en el Surestede Espafia R•SUMEN.--119nidos y 52 roquedosocupados por 32 parejas de Aguila Perdicera (Hieraaetusfasdatus), fueronanalizados en el periodo1995-97 en el surestede Espafia.E1 nfimero medio de nidosconstruidos por pareja fue mayor que el descrito por otros autores (i = 3,7; N = 32). Los resultadosrevelan una tendenciade las /tguilasde construirsus nidos hacia la orientaci6n sureste.La densidadde parejas reproductorasestuvo directamente relacionada con la disponibilidadde roquedos.Los roquedosselec- cionadospara nidificar fueron de mayor altura (i = 52,9 m; N = 32), y ubicadossobre laderasde mayor pendiente (i = 34,7ø;N = 31), que la media disponible.Existi6 una gran versatilidadentre parejasen cuantoal tipode roquedoocupado, debido a queel AguilaPerdicera nidific6 en fireas muy diferentesen funci6n de la presi6n humana. De estaforma, los roquedosocupados m/rs pr6ximos a carreterastuvieron una altura mayor que los que se encontrabanlejos de las mismas.La selecci6nde roquedosde gran altura, situadossobre pendientes elevadas, y con orientaci6nsur, podria estarrela- cionadocon la faltade sustentaci6nen vuelo del Aguila Perdicera, al favorecereste tipo de roquedos la formaci6n de t6rmicasy el vuelo de ladera. La preservaci6nde los roquedosde nidificaci6nlibres de la influenciaantr6pica, podria ser la medidamils esencial requerida para la conservaci6ndelAguila Perdicera en el/trea de estudio. [Traducci6n de Autores] Among Mediterranean raptors, the Bonelli's Ea- has decreased25% from 1980-90 (Arroyo et al. gle (Hieraaetusfasdatus) has suffered one of the 1995). Consequently,this specieshas been cata- most severepopulation declinesin Spain (Fern•tn- logued as Vulnerable in Spain (Blanco and Gon- dez and Insausti1990, Real et al. 1991), Portugal z•lez 1992), and high-priority conservation has (Palma et al. 1984), France (Cugnase 1984, Chey- been urged (De Juana 1992). lan and Simeon 1985) and Greece (Hallmann Information concerning habitat is fundamental 1985) that have resulted in its being listed as an for the management of raptor populations(Mosh- Endangered European Raptor (Rocamora 1994). er et al. 1987). Raptorsare amongthe few groups Recent data indicate that the principal European of birds whose numbers can be limited by the avail- breeding population (80%) is located in Spain ability of appropriate nesting places (Newton (Real et al. 1997), where the nestingpopulation 1979). In Spain, Bonelli'sEagles most frequently 110 JUNE 1999 CLIFFSELECTION BY BONELLI'S EAGLES 111 Table 1. Variables used to characterize Bonelli's Eagle nest-sites. CLIFFNEST--number of cliffs with nestsbuilt by a pair NESTBUIL•number of nestsbuilt by a pair DISTNEST--greatest distancebetween nestsbelonging to the same pair (m) HEIGBAS•height from the base of the cliff to the nest (m) NEIGDIST--nearest-neighbor distancebetween adjacent pairs of Bonelli's Eagles (km) AVACLIFF--availabilityof clifi• (percentageof 1 km9 squareswith suitablecliffs for nestingin each territory) nest in cliffs and rarely in trees (Arroyo et al. A total of 119 nests located on 52 cliffs that were used 1995). While some aspectsof the biology of this by 32 different pairs of Bonelli's Eagle was studied. Rap- tors frequently build more than one nest and use them raptor are well-studied, nest-siteselection has re- alternately in different years (Newton 1979). Thus, all ceived only limited study.The two main studiesin nests (regardlessof whether they were occupied or not Spain (Gil-S•tnchezet al. 1996, Sgnchez-Zapataet during the present study) were considered equally for al. 1996) refer to the selection and characteristics the analysisif they were in occupied territories. of used and unused territories. No detailed infor- The variablesused in the analysisof nest-sitecharac- teristicsare defined in Table 1. The nearest-neighbordis- mation is available concerning the choice of nest tance method from the last nest used was used to est•- siteswithin territories or characteristicsof nesting mate breeding densityof the pairs (Newton et al. 1977). cliffs (Donfizar et al. 1989). For the analysisof cliff selection, 32 occupied cliffs The aim of my study was to determine which (last cliff used for nesting by each pair) were compared cliffs in each territory were used for nestingof Bo- with 32 unoccupied cliffs within the territories (one cliff per territory). The comparativeanalysis was performed nelli's Eagles, to describe characteristicsof cliff with variables to characterize the cliffs and human dis- nestingsites, and to determine how human activity turbm•ce in surrounding cliffs (Table 2). Because most affectsthis selectionin southeasternSpain. pairs built nestsin the highest cliff of each territory, the comparison was made with the highest unoccupied cliff STUDY AREA AND METHODS suitable for nestingwithin each territory. I considereda The studywas conducted in the province of Granada, cliff suitable for nesdngwhen there were suitable cavities southeasternSpain (36ø45'-37ø49'N,2ø40'-4ø13'W) from and ledges for nesting, when it was located at <1500 m 1995-97. The area is largely mountainouswith altitudes elevation (the distributional limit of the Spanishpopu- ranging from 0-3482 m, and highly variable tempera- lation, Arroyo et al. 1995), when it was higher than 10 tures and rainfall. The vegetation includes different spe- m, and farther than 500 m from an urban center (min- cies of pines (Pinus spp.) and evergreen oaks (Quercus imal distancesfound for the population studied). I chose •/ex) mixed with cultivated areas,mainly with olive trees the unoccupiedcliffs within each territory to eliminate (Olea europaea)and cereals (Rivas-Martinez1985). the possibilitythat limited prey availabilitywas the reason Table 2. Variablesused to characterizeoccupied and unoccupied cliffs in territories used by Bonelli's Eagles. ALTITUDE--height abovesea level measuredin the middle of the cliff (m) HEIGCLIFF--cliff height (m) HEIGVAL-•height from the bottom of the valleyto the baseof 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 valleyat the baseof the cliff (m) SLOPE--inclination of the slope located at the base of the cliff (ø) TOPIND--topographic irregularityindex (total number of 20 m contour lines, cut by two lines equivalentto 2 km designedon topographic1:50 000 maps,in directionsN-S and E-W, and crossedat the location of the cliff) DISVIL•distance from cliff to nearest urban center (m) DISPAVROAD--distancefrom cliff to the nearestpaved road (m) DISUNPAVROAD--distancefrom cliff to the nearestunpaved road passableby vehicle (m) DISINHABUIL---distance from cliff to the nearestinhabited building (m) DISCULT1V--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 tionship between variables. For occupied and unoccu- variables characterizing nest sites. 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 VARIABLE MEAN SD RANGE 1997),a stepwisediscriminant function analysis was con- CLIFFNEST 1.6 0.9 1-5 ducted (STATISTICA statsoftInc. 1993). The 0.05% level of significancewas used for including variablesin each NESTBUIL 3.7 3.6 1-18 step of the analysis.Because the sample size could not be DISTNEST 774.1 897.7 1-2800 increased to three times the number of variables mea- HEI GBAS 29.8 18.3 5-90 sured (Willians and Titus 1988), a jackknifed classifica- NEIGDIST 10.0 3.2 5.8-16 tion wasobtained for the analysis. AVACLIFF 10.0 4.3 4.7-22 RESULTS Most of the cliffs occupied by Bonelli's Eagles the cliff was unoccupied given that food availabilitydi- (96%, N = 50) were in river valleysand