SEASONAL AND SEXUAL VARIATION IN THE DIET OF THE COMMON BUZZARD IN NORTHEASTERN SPAIN

--We examined the diet of Common Buzzards (Buteo buteo) from a Mediterranean area (Catalonia, NE Spain), by analyzing prey remains and pellets found in the nest, and stomach contents. The diet was seasonal. Relatively large items, such as young rabbits and Ocellated Lizards (Lacerta lepida), predominated in the breeding season, orthopterans and mantodeans in autumn and insects, rodents and soricidans in winter. Males presented an empty stomach more often than females, but only small differences were found in the diet of males and females. RESUMEN. --Se analiza la variaci6n estacional en la dieta del ratonero comfin (Buteo buteo) en una zona mediterrfinea (Catalufia, NE Espafia) a base de restos de presas y egagr6pilas encontrados en los nidos y al analisis de contenidos estomacales. La dieta vari6 estacionalmente. Durante el periodo reproductivo, el ratonero consumi6 presas relativamente grandes, tales como gazapos y lxgartos, mientras queen otofio consumi6 preferentemente ort6pteros y mfintidos. En invierno los insectos, roedores y musarafias constituyeron la base de la dieta. Los machos presentaron elest6mago vaclo con mayor frecuencia que las hembras pero s61o se detectaron pequefias diferencias en la dieta de ambos sexos. The Common Buzzard (Buteo buteo) feeds on a wide range of prey, mainly rodents, but also on other vertebrates and invertebrates of appropriate size (Cramp and Simmons 1980). Its diet has been studied in most parts of its geographical range, and it reflects underlying differences in prey availability (Bustamante 1985). However, seasonal variation has received much less attention, probably because of difficulty in studying the diet outside the breeding season. In this paper we investigate the seasonal variation of the diet of the Common Buzzard in a Mediterranean area, where the species is present throughout the year. STUDY AREA AND METHODS Diet outside the breeding season was studied by analyzing the stomach contents of 69 Common Buzzards confiscated from hunters in the Mediterranean area of Catalonia (NE Spain) between October and February of 198287 The sex of 39 individuals was identified. Diet during the breeding season was studied in La Segarra county of Catalonia by collecting prey remains and pellets from 20 nests, during and after reproduction in 1985-89. Pellets were especially useful to identify small prey, which are rarely found as items in the nest (Mafiosa 1991, Real 1991). The importance of each prey was expressed as the percentage of appearance of that prey among all prey items in nests, pellets or stomachs. RESULTS AND DISCUSSION Diet During the Breeding Season (SpringSummer). European Rabbit (Oryctolagus cuniculus) was the most frequent prey species (Appendix 1). This has been found in other Mediterranean areas (Veiga 1982, Real 1987) but is unlike the deciduous forest region of Northern Spain where invertebrates form the bulk of the diet (Bustamante 1985). The Common Buzzard captured mainly young rabbits, very abundant in spring and summer (Soriguer 1981). The mean tarsus length of the rabbits taken was 37.5 mm (SD = 5.7, range = 26-64, N = 122), which corresponds toa mean weight of less than 550 g (Mafiosa 1991). The second most consumed prey were reptiles, especially Ocellated Lizard (Lacerta lepida), also very common in spring and summer (Castilia 1989). Several species of birds formed an important percentage of the diet. Invertebrates, amphibians, rodents and shrews were taken only occasionally (Table 1). Diet Outside the Breeding Season. Only 45 (65%) of the 69 stomachs analyzed contained at least one prey. A total of 240 prey items were found (Appendix 1). Insects were the most frequent prey both in autumn and winter (Table 1). Rabbits were