Evolution of winter moulting strategies in European and North American migratory passerines

Moult is critical for birds as it replaces damaged feathers and worn plumage, enhancing flight performance, thermoregulation, and communication. In passerines, moult generally occurs on the breeding grounds during the post-breeding period once a year. However, some species of migrant passerines that breed in the Nearctic and western Palearctic regions have evolved different moulting strategies that involve moulting on the overwintering grounds. Some species forego moult on the breeding grounds and instead complete their prebasic moult on the overwintering grounds. Other species moult some or all feathers a second time (prealternate moult) during the overwintering period. Using phylogenetic analyses, we explored the potential drivers of the evolution of winter moults in Nearctic and western Palearctic breeding passerines. Our results indicate an association between longer photoperiods and the presence of prebasic and prealternate moults on the overwintering grounds for both Nearctic and western Palearctic species. We also found a relationship between prealternate moult and generalist and water habitats for western Palearctic species. Finally, the complete prealternate moult in western Palearctic passerines was linked to longer days on the overwintering grounds and longer migration distance. Longer days may favour the evolution of winter prebasic moult by increasing the time window when birds can absorb essential nutrients for moult. Alternatively, for birds undertaking a prealternate moult at the end of the overwintering period, longer days may increase exposure to feather-degrading ultra-violet radiation, necessitating the replacement of feathers. Our study underlines the importance of the overwintering grounds in the critical process of moult for many passerines that breed in the Nearctic and western Palearctic regions.