THE EFFECT OF VARIATION IN THE LAYING INTERVAL ON PROXIMATE DETERMINATION OF CLUTCH SIZE IN THE EUROPEAN KESTREL

I propose that if laying interval varies depending on daily energy intake, then laying interval should also play a basic role in the determination of clutch size. A supplemental feeding experiment with European Kestrels Falco tinnunculus was performed to test this hypothesis. Supplemented females laid larger clutches with shorter intervals between eggs than control females, but did not take longer to complete their clutches, Clutch size was negatively correlated with mean laying interval. Clutch size also declined seasonally, but the effect of laying date disappeared when laying interval was controlled for statistically. Therefore, food supply affected clutch size through laying interval, not through laying date.