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Fidelity to Roost Sites and Diet Composition of Wintering Male Urban Common Kestrels Falco tinnunculus

Abstract. During harsh winters, partially migratory raptors face to making a decision whether leave their breeding sites or not. However, decision to leave brings advantages for maintaining own body condition, but also disadvantages for further reproduction. We studied individual variability in fidelity to roost sites during two winters. The kestrel males in České Budějovice (Southern Bohemia) roost near their breeding sites for most of the winter. Individuals occupy the same roost site continuously, unless they temporarily leave the city. We collected pellets at the males' roost sites from November to April in 1996/1997 and 1997/1998. We analysed the factors that affect males' presence (proportion of days with snow cover — PSC, snow depth and mean temperature). There were fewer absences, overall, in 1997/1998 than in 1996/1997, probably due to milder weather conditions, and better prey availability. Individuals responded differently to periods of snow cover; some males left the city but others remained. Males that remained had a higher proportion of non-vole prey (birds, insectivors and insects) in their diet compared to those that left. Moreover, the proportion of birds in diet was positively correlated with PSC in males that stayed at roost sites during the period with snow cover. We suggest that males able to feed on birds during snow cover could remain in the city in harsh winters, and this would give them an advantage during competition for breeding sites in the following spring.