Individual optimization may explain differences in breeding time in the European kestrel Falco tinnunculus

Two hypotheses have been suggested to explain why individual variation in breeding time persists. The constraint hypothesis posits that the optimal laying date will be the same for every pair of a population. However, because egg formation is proximately constrained by the food supply, laying occurs after the ecologically optimal time. Females in poor condition cannot lay as early as females in better condition. By contrast, the individual optimization hypothesis proposes that individual variation in timing of breeding is adaptive since the optimal laying dale depends on parental quality. To test these hypotheses I manipulated hatching date in a European Kestrel Falco tinnunculus population by cross-fostering clutches of equal size but differing in laying dates. Experimentally advanced clutches produced fewer fledglings and fledglings of worse quality than control clutches. Moreover. control clutches were also more productive than experimentally delayed clutches despite young being of similar quality. These results are consistent with the individual optimization hypothesis.