Variation in Incubation Period within a Population of the European Starling

Abstract

Abstract We switched eggs among nests of European Starlings soon after the eggs were laid, and we measured incubation periods to within 2 h in most cases. Data from two years were analyzed to determine how much the natural parent of the egg and the incubating parent contributed to variation in the length of the incubation period. In one year, the relative size and composition of the yolk of one egg from each clutch were measured in order to assess the additional influence of egg composition on the length of the incubation period. In both sets of experiments, nest (incubating parent) had a significant effect, whereas clutch (natural parent) had either no effect or a marginally significant effect on the length of the incubation period. The standard deviation of incubation periods within nests was 2.6 and 3.5 h in the two experiments. Incubation periods increased significantly both with the percentage of yolk in eggs laid by the natural parent and with the percentage of yolk in eggs laid by the incubating parent. Our results suggest that the length of the incubation period is determined primarily by the incubating behavior of the adult, thermal properties of the nest, or both. Genetic differences in the growth rates of embryos and maternal effects expressed through egg composition were small and generally insignificant. Incubation period was, however, weakly related to egg composition of both the natural parent and the incubating parent, suggesting a phenotypic correlation between egg formation and incubation behavior.