Mechanisms and fitness consequences of laying decisions in a migratory raptor

Seasonal decline in breeding performance is a commonly observed pattern in birds, but disentangling the contributions of environmental conditions ("timing" hypothesis) and individual quality ("quality" hypothesis) to such a pattern is challenging. Moreover, despite the strong selection for early breeding, the individual optimization model predicts that each individual has an optimal breeding window. We investigated the causes and consequences of laying decisions in the lesser kestrel (Falco naumanni) by combining a long-term dataset on reproductive traits with information on food availability. A marked seasonal decline was found in breeding success, mostly mediated by a decline in clutch size. The "timing" hypothesis, supported by the decline in consumption of mole crickets, key prey for prelaying females, seems to explain the seasonal trend in clutch size, as this pattern was recorded in both higher (adults) and lower (yearlings) quality individuals. Contrarily, the higher proportion of yearlings breeding late in the season, rather than a decay in food availability during chick rearing, seems to drive the decline in fledging success, giving support to the "quality" hypothesis. Advanced breeding and increased clutch size, as proxies of reproductive effort, were not offset by lower survival. Low repeatability in both these traits suggests that individual quality is a dynamic attribute and reproduction costs are minimized by individual optimization. Understanding the mechanisms driving individual breeding decisions is critical to anticipate species' ability to cope with environmental changes. Here, we show that lesser kestrels failing the prelaying food window opportunity compromise reproductive performance, mostly regardless of their individual quality.