Feather wear and structural needs
Photodegradation is a primary source of feather structure atrophy in feathers (Ito et al. 2017, Pearlstein et al. 2014). The main variables that predict extent of prealternate molt are migration distance, day length, and foraging stratum. Migration distance likely affects feather degradation through increased overall day length (Fig. 1a). Long-distance migrants experience longer days overall because they experience long summer days in the temperate zone, but escape short winter days. For example, the longest-distance migrant in our dataset, and one of the most seasonally dichromatic species with one of the most extensive prealternate molts, the Blackpoll Warbler (Setophaga striata) experiences an average of 1.7 more hours of daylight each day, or 621 more hours of ultraviolet exposure each year, when compared to the species exposed to the least amount of ultraviolet radiation, the Masked Yellowthroat (Geothlypis aequinoctialis ), which also shows no seasonal dichromatism and no prealternate molt. Additionally, many warbler species exhibit prealternate molts that do not result in seasonal dichromatism, this phenomenon may seem paradoxical from the standpoint of hypotheses focused on coloration as the evolutionary catalyst for the prealternate molt, but makes sense within the context of the feather wear hypothesis.
Evidence from other taxa outside the New World Warblers provides additional context for the relationship between prealternate molt and seasonal dichromatism. The most extensive prealternate molts in birds occur in three species of long-distance migrants that breed, winter, and migrate in open, solar-exposed environments: Franklin’s Gull (Leucophaeus pipixcan ; Howell 2010) , Bobolink (Dolichonyx oryzivorus ; Renfew et al. 2011) and Willow Warbler (Phylloscopus trochilus; Underhill et al. 1992) . Bobolink shows seasonal change in feather color, but Willow Warbler does not, and Franklin’s Gull only shows a partial plumage color change. In these species, anecdotally, migration distance and habitat better predict prealternate molt than color change. The Willow Warbler is an extreme example: this species completely replaces all feathers twice a year, but the basic and alternate plumages are indistinguishable. Further research into this phenomenon should expand beyond the new world warblers to other groups of birds, as well as attempt to measure relative feather degradation rates in association with life history, habitat, and environment in birds, and study groups with more variable social systems. Other resident species of birds with strong variable selection on feather color, such as Ptarmigans (Beltran et al. 2018) may indeed have molts that evolve solely for variables needs of feather color. Overall, our results demonstrate the importance of molt strategies in the functional diversification of feathers and illuminates the value of considering interactions between different functional requirements for birds in the evolution of feather function.