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JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Animal Ecology. Summary 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 earlylaying 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.

SUMMARYWe investigated experimentally the seasonal role of food supply in brood survival of the European coot Fulica atra. For two breeding seasons, individual pairs were offered supplemental food the first ten days after their young hatched. Under natural conditions this period was largely responsible for the seasonal variation in brood survival. Our experiment tested three hypotheses : (1) food supply is not involved in breeding success at any date (other factors h pothesis), (2) food supply limits success independently of date (ele ation h pothesis), and (3) food supply affects success seasonally and is responsible for the natural seasonal trend in success (date h pothesis). Experimental pairs with supplemental food raised heavier, larger and more chicks than control pairs. Consistent with the date h pothesis, food supplementation abolished seasonal variation in chick survival. Chick growth under supplemental food was in agreement with the ele ationhypothesis. This discrepancy was probably due to the limited supply of additional food. We conclude that seasonal variation in offspring production was in essence the result of seasonal variation in food availability.

Life-history theory suggests that the variation in the seasonal timing of reproduction within populations may be explained on the basis of individual optimization. Optimal breeding times would vary between individuals as a result of trade-offs between fitness components. The existence of such trade-offs has seldom been tested empirically. We experimentally investigated the consequences of altered timing of current reproduction for future reproductive output in the European coot (Fulica atra). First clutches of different laying date were cross-fostered between nests, and parents thereby experienced a delay or an advance in the hatching date. The probability and success of a second brood, adult survival until and reproduction in the next season were then compared to the natural variation among control pairs. Among control pairs the probability of a second brood declined with the progress of season. Delayed pairs were less likely and advanced pairs were more likely to produce a second brood. These changes were quantitatively as predicted from the natural seasonal decline. The number of eggs in the second clutch was positively related to egg number in the first clutch and negatively related to laying date. Compared to the natural variation, delayed females had more and advanced females had fewer eggs in their second clutch. The size of the second brood declined with season, but there was no significant effect of delay or advance. Local adult survival was higher following a delay and reduced following an advance. The effect of the experiment on adult survival was independent of sex. Laying date and clutch size of females breeding in the next year were not affected by treatment. The study demonstrates the existence of a trade-off between increased probability of a second brood and decreased parental survival for early breeding. Timing-dependent effects of current reproduction on future reproductive output may thus play an important role in the evolution of the seasonal timing of reproduction.