Prey and prey-size selection by the short-toed eagle (Circaetus gallicus) during the breeding season in Granada (south-eastern Spain)

The diet of the short-toed eagle (Circaetus gallicus) was studied during th e breeding season by analysing pellets and remains generated by adults and nestlings. The raptor proved to be a specialist feeder, as snake prey compr ised almost 95% of the diet, in both frequency and biomass. We gathered inf ormation on prey availability and prey size availability (1499 specimens of nine different species) by searching for snakes in the study area. Regress ions of vertebra centrum length and dorsal-scale length on snout-vent lengt h (SVL) of the snakes were used to calculate prey size. The taxonomic diet composition differed depending on the sample analysed - remains or pellets - but we failed to find between-year diet differences. Most of the snakes i dentified (140 out of 141) belonged to only three species, Malpolon monspes sulanus, Elaphe scalaris, and Coluber hippocrepis. Other prey included Lace rta lepida, Natrix maura, Bufo bufo, and Alectoris rufa, and many secondary prey (prey from stomach of the prey) were also detected in the pellets. Th e three main prey species were consumed according to their availability in the study area, but the eagles selected on the basis of prey size. Large sn akes within 700-1000 mm SVL were positively selected. whereas snakes under 600 mm SVL were negatively selected. Adult eagles consumed the same prey sp ecies as those carried to the nest to feed their single nestling, although prey given to nestlings proved larger in size and biomass, suggesting that adults consumed the smallest prey, reserving the largest for nestlings.