Decomposing the seasonal fitness decline
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Seasonal fitness declines are common, but the relative contribution of different reproductive components to the seasonal change in the production of reproductive young, and the component-specific drivers of this change is generally poorly known. We used long-term data (17 years) on breeding time (i.e. date of first egg laid) in northern wheatears (Oenanthe oenanthe) to investigate seasonal reproductive patterns and estimate the relative contributions of reproductive components to the overall decline in reproduction, while accounting for factors potentially linked to seasonal declines, i.e. individual and habitat quality. All reproductive components—nest success (reflecting nest predation rate), clutch size, fledging success and recruitment success—showed a clear decline with breeding time whereas subsequent adult survival did not. A non-linear increase in nest predation rate caused nest success to decline rapidly early in the season and level off at ~80 % success late in the breeding season. The combined seasonal decline in all reproductive components caused the mean production of recruits per nest to drop from around 0.7–0.2; with the relative contribution greatest for recruitment success which accounted for ~50 % of the decline. Our data suggest that changing environmental conditions together with effects of nest predation have strong effects on the seasonal decline in fitness. Our demonstration of the combined effects of all reproductive components and their relative contribution shows that omitting data from later stages of breeding (recruitment) can greatly underestimate seasonal fitness declines.
KeywordsDate hypothesis Farmland bird Quality hypothesis Phenology Reproductive performance
We thank all the field assistants for their invaluable help with collecting data on wheatears and all land owners allowing us to work on their land. We also thank the editor (Toni Laaksonen) and four anonymous reviewers for their valuable comments on previous versions. The study was funded by The Swedish Research Council VR (grants to T. P., D. A. and M. L.), FORMAS (grant to T. P. and M. L.) and the Swedish Royal Academy of Sciences (D. A.).
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