Impact of Female Condition on Egg Size in Lesser Kestrels (Falco naumanni)

Egg size is a key determinant of reproductive success in birds, influencing nestling growth, survival, and overall fitness. For Lesser Kestrels Falco naumanni, a small raptor often supported by nest-box programs, we hypothesized that female condition would significantly predict egg size, with higher-quality females producing larger eggs. Thus, over four breeding seasons, we investigated the influence of female condition, expressed as the Scaled Mass Index (SMI), and clutch size on egg dimensions (length, breadth, and volume) in a nest-box population in central Greece. We found that female condition significantly and positively affected both egg breadth and volume but did not influence egg length, and clutch size had no discernible effect on any egg dimension. Among the traits measured, breadth exhibited the highest repeatability, suggesting a stronger genetic component, whereas length and volume were more sensitive to environmental factors. Our findings suggest that conservation efforts should prioritize maintaining or improving female body condition - through, for example, ensuring high habitat quality, adequate food availability, and minimizing stress - to support optimal reproductive investment by female Lesser Kestrels in nest-box populations.