Abstract Understanding how animals respond to environmental variability is crucial for predicting which species are most vulnerable to global environmental change. In birds, the timing of egg-laying is a key behavioural trait influencing reproductive outcomes. Whilst factors affecting breeding phenology are well studied, their impact on reproductive outcomes in human-altered landscapes—where environmental cues may be unreliable—remains less explored. Here, we investigated the interplay between land-use transformation and local climate on laying dates of a long-lived generalist raptor over 8 years (219 breeding attempts) in an agricultural area in the Ebro valley. We also assessed how laying dates, influenced by these environmental factors, affected reproductive performance, measured through breeding success and number of fledglings. We accounted for inter-individual variation using plumage colouration as a proxy for differences in age and personality. Our results showed variation in laying date in relation to spring precipitation and plumage colouration. No effect of land-use transformation was found. Laying date significantly influenced breeding success in a non-linear form: breeding success decreased with laying date, reaching its minimum value at intermediate values of laying date, and slightly increased for late breeders. Crucially, this pattern was modulated by spring precipitation. A significant interaction between precipitation and laying date revealed that delays in laying date were particularly associated with reduced breeding success when precipitation was higher at the beginning of the breeding season. These findings highlight the critical role of local climatic conditions, particularly spring precipitation, in shaping laying date and breeding success in this species.
Cacciarelli et al. (Thu,) studied this question.