The role of prealternate moult in nonbreeding period carry-over effects in neotropical migratory songbirds

Feathers serve myriad functions, including flight, thermoregulation, and communication. Because feathers are keratin structures, they cannot be repaired once damaged so birds must shed and regrow new feathers each year through a process called moult. Despite the importance of moult for feather function, it is one of the least understood events in the annual cycle of migratory birds. For Nearctic-Neotropical migratory songbirds that moult twice annually, little is known about the ecology of their prealternate moult, which typically occurs on the stationary nonbreeding grounds prior to pre-breeding (northward) migration. Improving our understanding of prealternate moult is fundamental for identifying how it interacts with other life history stages, breeding and migration, across the annual cycle, knowledge that is critical for determining periods of the year that limit declining bird populations. Here, I provide a detailed quantification of the timing, patterns, and intensity of prealternate moult for six warbler species (Family: Parulidae) on their stationary nonbreeding grounds in Jamaica. I demonstrate that prealternate moult is common for Northern Waterthrush (Parkesia noveboracensis), Black-and-white Warbler (Mniotilta varia), American Redstart (Setophaga ruticilla), Northern Parula (Setophaga americana), and Prairie Warbler (Setophaga discolor), and for most species increases in frequency and intensity across the nonbreeding period. I confirm the occurrence of prealternate moult in some Ovenbirds (Seiurus aurocapilla). I then assess the role of prealternate moult in carry-over effects from the nonbreeding period to the timing of migration departure for the Black-and-white Warbler, Northern Waterthrush, and American Redstart. First, I assessed the influence of body condition on the timing and intensity of prealternate moult in distinct habitats that differ in moisture regime: dry second-growth scrub forest and wet mangrove. I analyzed stable-carbon isotope ratios (δ13C) from birds’ red blood cells to support my understanding of relative habitat quality for the study species. Using the Motus Wildlife Tracking System, I tracked precise dates of migration departure so I could evaluate the influence of habitat quality, body condition, and prealternate moult on departure timing. Moult intensity was generally higher in young birds but its relationship with body condition varied among species, indicating that body condition may not be the most important driver of prealternate moult phenology. For the American Redstart, the effect of age on moult varied with habitat type, suggesting that birds with the poorest quality feathers, associated with poorer quality, open habitat, require a more intense moult than older birds and those in higher quality habitat. I found the first potential evidence of a carry-over effect from prealternate moult to departure timing in a parulid warbler, the American Redstart. Male American Redstarts with later, high-intensity moult departed on migration later, suggesting that prealternate moult can represent an important seasonal interaction in some birds. Ultimately, this study is an important step in beginning to understand the role of prealternate moult in nonbreeding period carry-over effects. To that end, it will be crucial for researchers in the global north to establish equitable partnerships with local neotropical researchers to aid in furthering prealternate moult research on the nonbreeding grounds of many migratory birds.