The feather wear vs. variables needs hypotheses
The variable needs hypothesis , which proposes that prealternate molt evolves in response to birds’ needs to change colors, predicts coevolution of prealternate molt and seasonal dichromatism. We found discrepancies in the best fit models of evolution, timing, pattern and external correlates of evolution of between the prealternate molt and seasonal dichromatism. The character that we studied with the strongest evidence for selection, as interpreted by the ratio of likelihood for an Ornstein-Uhlenbeck model to a Brownian Motion model, was presence of prealternate molt. This may imply that prealternate molt itself is under stronger selection (Butler and King 2004) than the color change it produces, which fit slightly better to a model of Brownian motion. We interpret this as support for the feather wear hypothesis , because this hypothesis predicts stronger selection on molt patterns than on seasonal dichromatism. The life history characteristics that best predicted prealternate molt were migration distance, day length, and solar radiation experienced on the breeding grounds (Fig 1, table 1). Top models for seasonal dichromatism all included prealternate molt and foraging stratum on the breeding and nonbreeding ranges. Combined with the results of the path analysis, we interpret these results as evidence for prealternate molt and seasonal dichromatism evolving along separate pathways.
The feather wear hypothesis invokes preadaptation in the relationship between prealternate molt and seasonal dichromatism. Thevariable needs hypothesis may predict synchronous evolution of prealternate molt and seasonal dichromatism, whereas the feather wear hypothesis predicts that prealternate molt should precede seasonal dichromatism and correlate with separate external parameters When we investigated the evolutionary timing of these characters, prealternate molt appeared to arise before seasonal dichromatism, and in more species and feather regions (Fig. 4). The idea that a character can evolve in response to selection for one function, and then be co-opted to serve another, has been well-explored in evolutionary biology (Bock 1959). Preadaptation has been implicated in the evolution of a wide array of evolutionary novelties (Cheney and Seyfarth 2005, Ketola et al. 2013, Schiestl and Cozzolino 2008, Quiñones and Pen 2017), and is an important phenomenon to understand when investigating how traits evolve. Our ancestral state reconstruction suggests that prealternate molt is a preadaptation, rather than an adaptation for seasonal dichromatism. We do not present these results as a rebuttal to variable needs for feather color. Clearly, needs for feather colors vary with life-history and latitudinal gradients in social behavior (Friedman et al. 2009), needs for crypsis on migration (Simpson et al. 2015) and nest stratum (Martin and Badyaev 1996), though we found little support for a relationship between nest stratum or nest type and prealternate molt or seasonal dichromatism. Our findings suggest that latitudinal gradients likely do play a role in the evolution of color change in feathers once prealternate molt is present. From these results, we propose a two-step pathway for the evolution of disparate breeding and nonbreeding plumages in warblers: a biannual molt evolves in response to structural needs for feathers, and then serves as a preadapted mechanism for seasonal dichromatism.
Immediacy of needs helps explain why structure may influence molt more than needs for color change. Structural functions provided by feathers are more immediately necessary for survival of birds than colors that function for social signaling. Without feathers, chicks are poikilothermic (Whittow and Tazawa 1991) and reliant on their parents for warmth. In adult birds, worn feathers directly influence survival through decrease of important functions such as flight (Swaddle et al. 1996). Timing of molt appears to be so important that experimentally malnourished birds will undergo a molt in spite of losing up to 40% body mass in the process, instead of delaying molt (Murphy et al. 1988). Because of more immediate implications on survival, it may make sense that selection on feather structure is stronger than on color change, and that selection on feather structure may be more likely to influence the evolution of molt strategies.