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Density, Nest Sites, Diet, and Productivity of Common Buzzards (Buteo Buteo) in the Italian Pre-Alps

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Density, Nest Sites, Diet, and Productivity of Common Buzzards (Buteo Buteo) in the Italian Pre-Alps j RaptorRes. 36(1):24-32 ¸ 2002 The Raptor ResearchFoundation, Inc. DENSITY, NEST SITES, DIET, AND PRODUCTIVITY OF COMMON BUZZARDS (BUTEO BUTEO) IN THE ITALIAN PRE-ALPS FABRIZIO SERGIO 1 Edward GreyInstitute of Field Ornitholo•, Departmentof Zoolog,South Paths Road, OxfordOX1 3PS, U.K ALBERTO BOTO, CHIARA SCANDOLAIL{, AND GIUSEPPE BOGLIANI Dipartimentodi BiologiaAnitaale, Piazza Botta 9, 27100 Pavia, Italy ABSTRACT.--Westudied a Common Buzzard (Buteobuteo) population of 32-35 territorial pairs between 1993 and 1999 in a 113-km9 plot locatedin the central Italian pre-Alps.Density progressively increased from 28 to 31 pairs/100 km2. Territorial pairs were regularlydispersed with a mean distancef?om the nearest occupied nest of 1108 m (N = 108). Eighty-onepercent of 108 used nest siteswere on cliffs, while the remaining 19% were placedon mature trees.Each year, 16-21% of the nestsbuilt by Common Buzzardswere taken over by migratory Black Kites (Milvus m}grans).Mean laying date was 9 April (earliest = 7 March, latest 30 April, N = 45). Mean clutch sizewas 2.32 (N = 19). Hatching successwas 91% (N: 33 eggst?om 14 clutches).Mean brood size at hatching was 2.14 (N = 14). Eighty-nine percentof the territorialpairs laid eggs(N = 37) and 72% raisedat leastone chickto fledging(N = 100). Mean number of fledgedyoung was 1.07 per territorialpair (N: 100), 1.11 per reproductive pair (N: 33), and 1.49per successfulpair (N = 72), with no significantdifferences among years. Diet was dominated by medium to s•nall passetines,small mammals,and snakes.Recorded densityand productivitywere comparableand often higher than those reported for other European populations. Human persecutionwas high until the 1970s,but is currentlyunimportant. Future conversion of young coppice standsto mature tbrest could further favor pre-Alpine populations of Common Buzzards. I•¾ WOP.DS: breedingsuccess; Buteo buteo; CommonBuzzard; density; diet; forestry; Italy; pre-Alps. Densidad, sitiosnido, dieta y productividadde los gavilanescomunes (Buteo lmteo) en los Pre Alpes Italianos R•suM•N.--Estudiamosuna poblaci6nde gavilanescomunes (Buteo tmteo) de 32-35 parejasterritoriales entre 1993y 1999en una parcelade 113km 9 localizadaen lospre Alpesdel centrode Italia. La densidad incremento prog7-esivamentede 28 a 31 parejas/100 km•. Las parejasterritoriales estuvieron dispersas regularmente con una distanciamedia al nido mas cercanamenteocupado de 1108 m (N --- 108). Ochenta y uno pot ciento de los 108 sitiosnidos usadosestaban en cornisas,mientras que el restante 19% estabanubicados en firbolesmadufos. Cada afio, 16-21% de los nidos construidospot gavilanes comunes tornadosen posesi6npot nfilanos negros migratorios (Milvus migrans).La fecha media de posturafue 9 de abril (los primeros = 7 marzo, los mas tardios30 de abril, N = 45). E1tamafio medio de la posturafue 2.32 (N = 19). E1 6xito en la posturafue 91% (N = 33 huevosde 14 nidadas).E1 tamafio medio de la nidada en la posturat•e 2.14 (N = 14). Ochentay nueve pot ciento de las parejas tcrritorialespusieron huevos (N = 37) y 72% sacaronadelante al menosun polluelohasta volant6n (N = 100). E1numero promediodcj6venes volantones fue 1.07 pot parejaterritorial (N = 100), l.ll pot pareja reproductiva(N = 33), y 1.49 pot pareja exitosa(N = 72), sin difercnciassignificativas entre aftos.La dieta fue dominadapot passeriformesmedianos a pequefios,pequefios mamfferos, y culebras. La densidady productividadregistradas rueton comparablesy a menudo mas altas que aquellasrepor- tadaspara otraspoblaciones europeas. La persecuci6nhumana fi•e alta hastalos 70's, actuahnenteno es importante. La futura conversi6nde los bosquecillosj6vencs a bosquesmadufos podria favorecer mayormente alas poblacionespre-alpinas de gavilanescomunes. [Traducci6n de C6sarMftrquez] • Presentaddress: Raptor ConservationResearch Unit, MuseoTridentino di ScienzeNaturali, Via Calepina14, 38100 Trento, Italy. E4nail address:[email protected] 24 M•CH 2002 COMMON BUZZARD BREEDING ECOLOGY 25 The Common Buzzard (Buteo buteo)is one of the wet springsand dry summers (Pinna 1978). Further dc- most abundant European raptors (Bijlsma 1997). tails on the area can be found in Sergio and Boto (1999) Except for a possibledecrease in Sweden(Ryttman METHODS 1994), its populations are generally stable or in- Common Buzzards were surveyed between 1993 and creasing, and in some areas still recovering from 1999. We censusedterritorial pairs during the pre-incu- declinescaused by pesticidepoisoning and human bation period, between I February and 15 April, by ob- persecution in the 1950s and 1960s (Taylor et al. servingterritorial displaysand transfersof nest material. 1988,Bijlsma 1997). Factorscurrently limiting den- Common Buzzards typically refurbish many alternate nestseach year,before selectingthe one which they even- sity,productivity, and range expansion,include low tually use (Cramp and Simmons 1980). We put effort availability of food and nest sites, direct persecu- into finding all the active alternate nestsof each pmr tion, and poisoning (Newton et al. 1982, Elliott every year. An alternate nest was defined as active when and Avery 1991, Gibbons et al. 1994, Graham et al. it contained greenery or freshlybroken branchesduring 1995). the preincubation period, and was defined as used when eggswere laid in it. In Italy, the Common Buzzard occurs from sea Whenever possible, nests were visited at least three level to an elevation of 1800 m in the Alps (Canova times: (1) about a week after the mean local laying date 1992). However, despite its abundance and wide to asscssclutch size; (2) just after hatching to estimate distribution, the species' breeding ecology and hatching success,brood size, and hatching date; and (3) when the nestlings were older than 45 d to record the population trends are virtually unknown. The few number of fledgedyoung (nestlingsusually fledge at 50- existing estimatesrefer mainly to the densityof ter- 55 d; Cramp and Simmons1980). Nestswere checkedby ritorial pairs and are usuallybased on low sample climbing the nest tree, descendingcliffs with a rope, or sizes(Canova 1992). In this paper,we present data watchingthe nest from a vantagepoint up the slopewith a 20-60X telescope.To minimize the risk of disturbance, on density,nest spacing,diet, and productivityof nest desertion, or egg/chick predation by Black Kites a sedentarypopulation of Common Buzzardsstud- (Milvus migrans)or Ravens(C0rvus corax), only neststhat ied for sevenyears in the Italian pre-Alps. could be checkedvery rapidly were visited during incu- bation/early hatching. Thus, estimates of clutch size, STUDY AREA hatching success,number of laying pairs and brood size representeda subsampleof nests.Hatching date was es- The studyarea is a 113-km2 plot locatedalong the Ital- timated by backdating from the feather development of ian margin of Lake Lugano, within the central Italian nestlingsfirst observedwhen •<15 d old, by observations pre-Alps (45ø55'N, 8ø50'E). Altitude ranges from 275- at eight focal nests and reference to information con- 1125 masl. The landscapeis characterizedby forested tained in Tubbs (1974), Melde (1976), and Cramp and mountain slopesinterspersed with medium-sizedcliffs Simmons (1980). Laying date was estimated by subtract- and rare patches of herbaceous and scrub vegetation, ing 34 d, the median incubation period (Cramp and Sim- caused by frequent burning. Overall, open areas were mons 1980), from hatching date. Prey remains found •n scarce,mainly due to human modifications,and concen- the nest cup during each nest visit were identified assum- trated on the valley floors. The area included 16 small ing the minimum possiblenumber of individualsper col- villages,all located on the valley floors. Seventy-oneper- lection event, and by reference to a reference collection cent of the area wascovered by woodland,13% by urban and information contained in Debtor (1982). areas,9% by water bodies,6% by natural grassland,and Terminologyfollows Steenhof (1987): a reproductive 1% by farmland. pair is one which laid -->1 egg, a successfulpair is one Dominant tree speciesin the forest included sweet which raised -->1nestling until >45 d old, and breeding chestnut ( Castaneasativa), downy oak ( Quercuspubescens), successis the percentage of successfulterritorial pairs. A sessile oak (Quercuspetraea), European hop-hornbeam nest area is an area where -->1 alternate nest is found ( Ostryacarpinifolia), and locust tree (Robiniapseudoaca- within any one year, but where only one pair nestseach cia). Forests were managed for timber production pri- year (Sergio and Boto 1999, Sergio and Bogliani 1999). marily by means of stool shootsregeneration (coppice Statistical Methods. The degree of regularity of nest system;Matthews 1989), with a rotation of 20-30 yr. Ma- dispersion was estimated by means of the G-statistic ture trees were often maintained as single individuals or (Brown 1975), calculated as the ratio between the geo- in small clumps as seed bearers (coppicewith standards; metric and arithmetic mean of the squared nearest Matthews 1989). However, most of the woodland had neighbor distances(NND) between used nestsand vary- been recently felled and consisted of a homogeneous ing between 0 and 1. Values close to 1 (>0.65) indicate cover of young secondgrowth forest. Some young wood- a regular dispersionof nestsites (Brown 1975). Statistical land patcheswere being converted to mature woodland, significance of the deviation from randomness toward but at the time of study mature
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