Evolutionary and ecological determinants of complete moult duration and timing in European passerines

Identifying the determinants that shape the duration and timing of avian moult is essential for understanding the evolution of moulting strategies and predicting life-cycle responses to ongoing environmental change. Although body size, migration distance, and breeding latitude have been proposed to explain interspecific variation in the annual complete moult among songbirds, breeding duration remains largely unexplored, and these factors have rarely been evaluated jointly within a phylogenetic comparative framework. We combined these four predictors in phylogenetically informed models to assess their effects on complete moult duration and timing across European passerines. Phylogenetic relationships did not explain variation in moult duration. Instead, migration distance, body mass and breeding latitude had independent effects on summer moult duration. After accounting for these variables, species with longer breeding periods exhibited shorter moults. In contrast, moult timing was partially explained by phylogenetic structure, with no detectable effects of continuous predictors. These contrasting patterns suggest that the drivers of variation in moult duration and timing might differ. Contrary to previous parsimony-based reconstructions, maximum likelihood ancestral state reconstruction inferred winter moult as the ancestral state and detected relatively few transitions to summer moult, likely limiting statistical power to detect ecological effects on moult timing. Overall, our results provide a comparative assessment of the determinants of summer moult duration in European passerines and preliminary support for winter moult as the ancestral state. Broader phylogenetic sampling is needed to further test this latter possibility, which, if confirmed, could prompt a reassessment of prevailing interpretations of moult timing evolution.