Molt and coloration across feather regions
Selective pressures on plumage may vary across birds’ bodies (Marcondes and Brumfield 2018, Dale et al 2015). We know that molt in different species of birds varies in which feathers are molted and when (Stresemann and Stresemann 1966), but, despite some hypotheses being put forward (Howell 2010), little work has investigated the interplay between feather function and molt patterns across feather regions. Among species of warblers, certain feather regions were repeatedly more or less likely to be involved in prealternate molt. Despite variation in prealternate molt extent, ancestral state reconstruction suggested that the prealternate molt evolved in a stereotyped manner (Fig. 2g; Fig. 4). The head is involved in prealternate molt in all species, amnd then most frequently followed by the back, breast, belly feathers and wing coverts. Prealternate molt rarely replaces other parts of the body, including wing and tail feathers, which are often shaded from the sun by covert feathers and each other. The feather regions more involved in prealternate molt appear to be those more exposed to the sun on a perched bird (Fig 2f). Although the wing and tails are prominent features on birds, when folded, each individual remex is almost entirely shaded by coverts and other remiges. (Fig. 2). It may also be the case that the larger and stronger remiges are costlier to replace than body feathers, but we interpret this evidence as at least suggestive that feathers that are more exposed to the sun are more likely to be replaced in prealternate molt. This pattern was confirmed by both ANOVA analysis and ancestral state reconstruction, where feather regions most strongly associated with prealternate molt were also correlated with long-distance migration (Fig 2g, Fig. 4), and showed increased rates of evolution in seasonal dichromatism and prealternate molt (Fig. 4). In each feather group, we recovered the same positive relationship between migratory distance and likelihood of replacement in prealternate molt (Fig. 2g). We found gains and losses of long-distance migration and prealternate molt; and, importantly, found no gains of prealternate molt in birds without long-distance migration, but a high transition rate to prealternate molt in lineages with long-distance migration (Fig. 1g). The predictable evolution of prealternate molt in regions of the body more exposed to the sun, coupled with a lack of seasonal dichromatism in lineages which recently evolved prealternate molt, lends support to the feather wear hypothesis for the evolution of prealternate molt.