j. RaptorRes. 33(3):207-217 ¸ 1999 The Raptor Research Foundation, Inc. NEST DISPERSION, DIET, AND BREEDING SUCCESS OF BLACK KITES (MILVUS MIGRANS) IN THE ITALIAN PRE-ALPS FABRIZIO SERGIO Edward GreyInstitute of Field Ornithology,Department of Zoology,South Parhs Road, OxfordOX1 3PS, U.K. ALBERTO BOTO Dipartimentodi BiologiaAnimale, Piazza Botta 9, 27100 Pavia, Italy ABSTRACT.--Westudied a population of Black Kites (Milvus migrans)from 1992-96 in a 100-km')study area in the Italian pre-Alps around Lake Lugano. Population densityincreased from 24 territorial pairs per 100 km'• in 1992 to 38 in 1996. Nearest neighbor distanceswere variable, averaging1288 m for solitary pairs (N = 24) and 306 m for colonial ones (N = 151). Regular spacing of nest siteswas the rule within colonies, but inter-nest distance for solitary breeders increased as new pairs settled in the area. Nests occurred both in trees (58%, N = 84) and on cliffs (42%); 23% and 5% (N = 84) of the nestswere originally built by Common Buzzards (Buteobuteo) and Ravens (C0rvuscorax), respectively. Mean laying date was 25 April (N = 42), mean clutch size was 2.29 eggs (N = 42) and mean number of fledged young was 0.97, 1.11, and 1.78 young per territorial, reproductive, and successfulpairs, respectively(N = 143, 95, 78). The percentageof successfulterritorial pairswas 55% (N = 143). Diet was dominated by fish and birds, which accounted for 62% and 28% of 307 identified prey items, respectively.Compared with other European populations, this population showedan intermediate den- sityand an averageclutch sizebut the lowestfledging and breeding successever recorded for the species. Reasonsfor the low successwere unclear, but may have been related to low food availability or water pollution and consequentpesticide contamination. KEYWO•DS: BlachKite,, Milvus migrans; coloniality;density; diet;, breeding success; Italy. Dispersionde nidos, dieta, y exito reproductivode Milvus migransen los pre-AlpesItalianos R•SWMEN.--Estudiamosuna poblaci6n de Milvus migransdesde 1992-96 en un frea de 100 kms en los pre-Alpes italianos alrededor del Lago Lugano. La densidad poblacional aument6 de 24 parejas terri- toriales por 100 kms en 1992 a 38 en 1996. La distanciamas pr6xima entre vecinosfu6 variable y promedi6 1288 m entre parejassolitarias (N = 24) y 306 m entre las parejascoloniales (N = 151). Los espaciosregulares de los nidos fueron la regla entre las colonias, pero la distanciaentre nidos para los reproductoressolitarios se increment6 a medida que se asentaronnuevas parejas en el firea. Los nidos se encontraron tanto en firboles(58%, N = 84) como en riscos(42%), 23% y 5% (N = 84) de los nidos fueron originalmente construidospor Buteobuteo y Corvuscorax, respectivamente. La media del dia de la puestafu6 el 25 de abril (N = 42). La media del tamafio de la nidada fu6 2.29 huevos (N = 42) y la productividadmedia fu6 0.97, 1.11, y 1.78juveniles por territorio. Las parejasproductivas territori- almente exitosasrepresentaron el 55% (N = 53). La dieta fue principalmente dominada por pecesy aves (62% y 28% respectivamente)de los 307 items de presasidentificadas. A1 comparar con otras poblacioneseuropeas, estas poblaciones mostraron una densidadintermedia y una nidada promedio, pero al mismo tiempo el mas bajo 6xito reproductivo y reclutamiento registrado para la especie. Las causasde esto, son poco claras y pudieron estar relacionadascon la poca disponiblidad de comida o con la poluci6n del agua y la consecuentecontaminaci6n por pesticidas. [Traducci6n de C6sar Mfrquez] The Black Kite (Milvus migrans)is a medium- 1990) and as one of the most numerous and suc- sized accipitrid that is distributed throughout the cessfulbirds of prey in the world (Brown and Ama- Old World and Australasia. It has been defined as don 1968). Despite its local abundance, the Black an adaptable and opportunistic feeder (Geroudet Kite was recently classifiedas a declining and vul- 1965, Delibes1975, Arroyo 1978,Jones and Manez nerable speciesin Europe (Vifiuela and Sunyer 207 208 SERGIO AND BOTO VOL. 33, NO. 3 1994). Although populationswere reported to be pice system,Matthews 1989) on a 20-30 yr rotation basis. stable or increasing in western Europe between Single mature trees are often kept into the next rotation as seedbearers (coppicewith standardssystem, Matthews 1970-90 (e.g., Bustamanteand Hiraldo 1993,Dou- 1989). Thus, even though single mature trees were pres- meret 1995), pronounced decreaseshave been ob- ent almost everywhere,most woodlotshad been thinned served in Portugal, eastern Europe, and Russia or clearfelled in recent times (<40 yr), resulting in a during the sameperiod (Vifiuela and Sunyer1994, homogeneouscover of secondgrowth forest. Following the recent decline in coppice management throughout Bijlsma1997). Recently,recorded cases of pesticide Europe (Matthews1989), somewoodlots were being con- contamination (Hernandez et al. 1986, Jenni-Eier- verted to mature forest; nevertheless, mature woodland mann et al. 1996), nest robbing (Garcia Ferr6 and wasconfined to a few steep and often inaccessibleslopes. de Juan 1983) and water pollution (e.g., Ceschiet Cultivated fields, mainly grassland and maize (Zea al. 1996) in the species' core distribution areas mays)fields, were located on the valley floors and ac- counted for 3% of the land. Sixteen small villages,all point to an urgent need for quantitativemonitor- confined to the valley floors, covered 13% of the study ing of breeding populations (Vifiuela and Sunyer area. Thus, except for forestry operations, human activ- 1994). ities were mainly confined to lowland and were virtually In Italy, both local population increasesand de- absent from the mountain slopes.The Lake Lugano wa- ter surfaceaccounted for 6% of the studyarea which also clines have been reported (Petretti 1992, De Gia- included two smallerlakes (17 and 25 ha); overall,aquat- como et al. 1993), but information is fragmentary ic habitat covered7% of the studyarea. and mainly nonquantitative (Vifiuela and Sunyer Climate was temperate continental, with wet springs 1994). During the 20th century, the speciesde- and dry summers(Pinna 1978). Annual rainfall ranged clined dramatically in the Po Plain. For example, from 1600-2100 mm, with two peaks, one in the spring and one in the autumn, and the former more pro- one colony in BoscoFontana, near the city of Man- nounced than the latter (Belloni 1975). tova, declined from over 100 pairs in the 1930s (Ar- rigoni degli Oddi and Moltoni 1931) to <10 pairs METHODS in the 1970s (Micheli 1990). The largestItalian We censused Black Kite nests from 1992-96. The ear- populationsare currently concentratedin the pre- liest territorial individualswere observed in the study Alps (Micheli 1990). No quantitative estimatesof area on 18 March. As soon as kites settled on their breed- density,diet or productivityhave been published ing territories, they startedto perch in highly prominent positions, refurbish nests and carry out conspicuousae- for these populations. rial displaysto signal territory ownershipor to attract a In this paper,we report the resultsof a 5-yr study mate (Vifiuela 1993). Nestswere censusedby progressive, on a Black Kite population in a pre-Alpine area of complete searchesas new pairs arrived from migration northern Italy. The aim of the researchwas to pro- and settled on territories. No pairs settled on territories after the middle of April, when somepairs were already vide quantitative data on Black Kite density,nest laying eggs.All nestswere found during the pre-incuba- dispersion,colony size, diet and productivity,and tion period, by watchingthe residentpair's territorial dis- to compare them with estimatesfrom other pop- playsand nest material transfers.This also allowedus to ulations. censusthe nonbreeding sector of the territorial popula- tion (i.e., pairs that failed to lay eggs). STUDY AREA Nest dispersionwas analyzed by means of the G-statistic (Brown 1975), calculatedas the ratio of the geometricto The studyarea was a 100-km'• plot located along the the arithmetic mean of the squarednearest neighbor dis- Italian coastof Lake Lugano. It included two pre-Alpine tances (NNDs) Values ranged from 0-1 with valuesrang- valleys:the CeresioValley along the lake and the Ganna ing from 0.65-1.00 indicatinga regular dispersionof nest Valleyfurther inland from the lake. Altitude rangedfrom sites (Brown 1975). Pairswere defined as solitarywhen 275-1125 m. The landscapewas characterized by pre-Al- they nested>700 m from their nearestneighbor. How- p•ne mountains, mainly of sedimentaryorigin, ranging ever, five nestsbelonging to two territories were defined from 526-1125 m in peak elevation. Mountain slopes as solitarydespite having a NND <700 m. These were were coveredby continuousdeciduous woodland which located on opposite sidesof a mountain and the birds accountedfor 77% of the studyarea and waslocally in- could not see their nearestneighbor while in their nest terrupted by a few open areas.These open areasresulted area; also,they were not seento interact with the neigh- from human activities (regular burning or sheep graz- boring pairs during each of three 1-hr observation ses- ing) and were coveredby dry grassland,ferns, or various sionsduring the pre-layingperiod (seeBrown and Brown bushes,mainly common hazel (Corylusaveliana). Domi- 1996). Apart from thesefive cases,all pairs<700 m from nant tree speciesincluded sweetchestnut (Castaneasati- their nearest neighbor were defined as colonial, and all va), downy oak (Quercuspubescens),