Food supply and the annual timing of avian reproduction

Abstract

Avian breeding generally coincides with seasonal peaks in food supply, but detailed studies suggest that birds may breed on either the rising or the declining slopes of food availability. Nonetheless, a seasonal decline in clutch size appears general in single-brooded altricial species, except those laying only one or two eggs. Surplus-feeding experiments suggest that food in spring affects laying date and thereby clutch size in those species in which there is a decline. Survival indices for offspring, both in the nest and after fledging, generally decline with the progress of season. These effects of date of birth can be summarized in the reproductive value of eggs as a function of date of laying. It is shown that with constraints on parental investment, optimal clutch sizes should decline with season when egg reproductive value declines, independent on the nature of the constraining and proximate control mechanisms. Experimental approaches outlined to evaluate the theory, include brood-size reduction, selection experiments, and release of birds reared in captivity on different dates.