Seasonal variation in local recruitment of great tits: the importance of being early

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To understand seasonal variation in reproductive decisions it is important to quantify the fitness costs and benefits of these decisions in relation to date within breeding season. In this paper we investigate the effect of fledging date on local recruitment of great tits (Parus major). The probability that fledglings were recaptured as breeding birds was analysed in long-term data sets of two great tit populations in the Netherlands, one on the mainland (Hoge Veluwe) and one on an island (Vlieland). The results showed that young that fledged early in the breeding season were more likely to recruit into the breeding population than young that fledged late. The negative relationship between local recruitment and fledging date was also present after controlling for fledging mass, year, sex and brood type (first, replacement or second brood). Moreover, the relationship did not seem to be due to late fledged young being more likely to disperse. The decline of local recruitment rate with fledging date was further tested with two experiments. In the first experiment clutches were removed, forcing parents to lay a replacement clutch and thereby delaying the fledging date of their young. Fledglings from experimental pairs had significantly lower recruitment rates than fledglings from unmanipulated controls, showing that local recruitment was causally related to fledging date. In the second experiment fledging date was not changed, but the parents and the territory from which the young fledged were altered by swapping clutches between parents with early and late laying dates. No significant differences were found in recruitment rates of fledglings between the treatments and unmanipulated controls. There was no evidence for an indirect relationship between recruitment and fledging date through different quality fledglings produced by early and late parents. One of the reasons for the observed seasonal variation in reproductive decisions is that early and late fledglings do not contribute equally to the parents' fitness.

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