Conclusions:
Based on the observation that prealternate molt often involves feathers that appear to be identical between basic and alternate plumage, Pyle and Kayhart (2010) proposed that prealternate molt may evolve to replace sun-exposed feathers, then later be co-opted for seasonal dichromatism. We examined this hypothesis across warblers and found that it better explains patterns of evolution of prealternate molt than color change alone. Here, we present evidence that selection on coloration and structure interact in complex ways to influence the evolution of molts and plumages in warblers. Namely, we find that color change poorly explains the evolution of the molts that produce these changes. This suggests that biannual molt acts as a preadapted platform for color change, instead of evolving in direct response to needs for color change. These results provide a more nuanced understanding of plumage evolution in birds by incorporating the mechanism for plumage generation. Rohwer and Butcher (1988) made a novel contribution to our understanding of delayed plumage maturation in birds by arguing that molt must be understood first in order to understand plumage maturation in birds. They found that the breeding-season driven hypotheses lose support when molt is studied, and that the limitations of preexisting molts explain delayed plumage maturation in birds better that social selection on the breeding grounds. Our results largely agree with this study, in that we find needs for plumage change do not appear to be able to influence the evolution of molt strategies, instead they only influence the phenotypes of feathers produced within molt strategies that have evolved for other reasons. Similarly, needs for seasonal color change may be present in species, but the translation of that need into phenotype may be limited by the extent of prealternate molt. Following our results and those of Rohwer and Butcher (1988) that hypotheses about the role of social selection on feather color may look different when viewed through the lens of molt, we encourage other authors studying the evolution of plumage to consider molt strategies when attempting to understand mechanisms of feather evolution.
Feather color has attracted much attention, especially into selective processes that may have produced the diversity of coloration present in birds’ feathers (Darwin 1871, Wallace 1891, Chaine and Lyon 2008, Li et al. 2010, Payne 1984). Both natural and sexual selection play roles in the colors of feathers as well as diversification of birds (Barraclough et al. 1995, Møller and Cuervo 1998, Stoddard and Prum 2007, Marcondes and Brumfield 2019). However, the molts that produce these feathers have been largely ignored in these studies. Birds show a diverse array of molt strategies (Stresemann and Stresemann 1966), but how and why different species of birds have different strategies for the timing and patterns with which they replace their feathers remains poorly known. How selection interacts with molt, and not just feather phenotype, is an essential question because molt is the underlying mechanism of feather production. How molts provide limitations and opportunities for seasonal change may be of widespread importance for understanding evolution of avian color at a broader scale. For example, some juvenile birds are brightly colored, and then lose this bright coloration in the highly conserved preformative molt that occurs shortly after fledging (Pyle 2009). It may be that the preformative molt provides an opportunity for these chicks to respond to selection from parental choice in the nest (Lyon et al. 1994) without being “stuck” in a bright plumage for their entire first year of life.
The needs for seasonal dichromatism likely vary by latitude and social system in birds (Friedman et al. 2009, Simpson et al 2015), but these results suggest that they may not be the primary factor influencing the evolution of prealternate molt in the New World warblers. A major study into global variation in seasonal plumage coloration in birds found that seasonal color change is more uncommon than predicted by social systems and predation risk (McQueen et al. 2019). We believe our study sheds some light on this conundrum. From the viewpoint of the feather wear hypothesis , the answer to this problem is that needs for seasonal color change alone are not strong enough to influence molt patterns in many species, and so seasonal color change can only evolve within the context of pre-existing molts. This is similar to how pre-existing molts limit the phenotypic realization of plumage maturation (Rohwer and Butcher 1988). A two-step relationship between a birds’ needs for feather color change and the response of phenotypic evolution to those needs may not necessarily be unexpected. Selection for color and structure on feathers likely interact in complex ways. For example, sexual selection on feathers may act as a “bridge” between peaks on natural selective landscapes for feather structure (Persons and Currie 2019). Our results provide evidence for similar “bridges” across adaptive landscapes, where naturally selected molts may provide bridges between spaces on a social selection landscape, in this case between year-round monochromatism, and seasonal dichromatism. Further research into the interplay between different types of selection on the evolution of molts and plumages in birds could consider groups with disparate social systems, as well as quantification of feather degradation. We suggest that molt should be considered when attempting to understand the evolution of plumages in birds.