The seasonal decline in the first-year survival of juvenile coots: An experimental approach

1. We investigated experimentally the seasonal variation in the local first-year survival of individual European coots, and introduced a method of survival analysis that extends the use of standard logistic regression. 2. Under natural conditions, survival of young showed a decline with hatching date. This decline differed in level, but not in slope, between cohorts. 3. To test whether the seasonal decline in survival was caused by a general regress in environmental conditions (the date hypothesis) or by differences in parental or territory quality between early and late breeders (the parental quality hypothesis), we cross-fostered clutches differing in laying date between pairs, thus establishing 'delayed' and 'advanced' pairs over most of the season. 4. Survival of young fostered by experimental pairs differed significantly from control young of the same hatching date, which defied the date hypothesis. In contrast, survival of experimental young was similar to that of control young, which hatched on the foster parent's original brood hatching date. The results were therefore consistent with the parental quality hypothesis. 5. Survival was essentially independent of size (tarsus length) for early-hatched young. Late-hatched young showed an initial increase in survival with tarsus length. At large sizes, the curve flattened to the level of the early-hatched young. 6. The small size of late-hatched young therefore largely explained the natural seasonal decline in survival. Variation in tarsus length did not account for the experimentally established effect of parental or territory quality differences on the seasonal decline in juvenile survival.