Differences in the diets of resident and non-resident Kestrels in Spain

The Eurasian Kestrel (Falco tinnunculus) is a partial migrant in Spain, i.e . some individuals migrate in autumn, while other ones are sedentary. In th is paper the diets of resident and non-resident kestrels were compared duri ng autumn to study whether differences in the diet may account for individu al migratory behaviour. Results indicated that kestrels changed their feedi ng habits during autumn. Resident kestrels substituted grasshoppers, a typi cal summer prey, for field crickets and/or mammals (typical winter prey), w hereas non resident kestrels hunted fewer crickets and mammals, but more ma ntids and flying ants, which are not available later, during winter. Trophi c diversity (H') was larger for non-resident kestrels than for residents, w hich indicates that profitable substitution prey might be scarce in the ter ritories of nonresident kestrels. Indeed, BPP (Biomass per whole pellet), w hich may be an indicator of daily energy intake, was lower for non-resident than for resident kestrels. The negative relationship between the date of disappearance and H' on one hand, and the positive relationship between dat e of departure to BPP for non-resident kestrels on the other, indicates tha t these kestrels remained at the breeding sites for as long as possible.