LAYING DATE IN THE COOT - EFFECTS OF AGE AND MATE CHOICE

1. Laying date in coots advanced with the age of both the male and the female. 2. Individual females had different but rather constant laying dates. After controlling for these differences, the effect of female age on laying date remained, but that of male age disappeared. This implies that both early and late females advanced their laying date when becoming older. It also implies that male age has no direct effect on laying date, but that older males are better able to compete for early-laying females. 3. Selective loss of late-breeding females from a cohort is a potential additional cause for a correlation between age and laying date. However, female survival rate was independent of laying date, thus refuting the selection hypothesis. 4. The data supported neither increased reproductive effort with age (residual reproductive value hypothesis), nor differential delay of reproduction between early and late females (delayed reproduction hypothesis) as additional explanations for the advancement of laying date with age. This advancement in the individual female is therefore considered to be due to increasing quality (maturation hypothesis). 5. The repeatability of laying date in females was 0.32, but the heritability, estimated with daughter-mother regression, was only 0.02. This indicates that the variation in laying date between individuals is predominantly environmental.