Abstract
Many species of birds show distinctive seasonal breeding and nonbreeding plumages. A number of hypotheses have been proposed for the evolution of this seasonal dichromatism, specifically related to the idea that birds may experience variable levels of sexual selection relative to natural selection throughout the year. However, these hypotheses have not addressed the selective forces that have shaped molt, the underlying mechanism of plumage change. Here, we examined relationships between life‐history variation, the evolution of a seasonal molt, and seasonal plumage dichromatism in the New World warblers (Aves: Parulidae), a family with a remarkable diversity of plumage, molt, and life‐history strategies. We used phylogenetic comparative methods and path analysis to understand how and why distinctive breeding and nonbreeding plumages evolve in this family. We found that color change alone poorly explains the evolution of patterns of biannual molt evolution in warblers. Instead, molt evolution is better explained by a combination of other life‐history factors, especially migration distance and foraging stratum. We found that the evolution of biannual molt and seasonal dichromatism is decoupled, with a biannual molt appearing earlier on the tree, more dispersed across taxa and body regions, and correlating with separate life‐history factors than seasonal dichromatism. This result helps explain the apparent paradox of birds that molt biannually but show breeding plumages that are identical to the nonbreeding plumage. We find support for a two‐step process for the evolution of distinctive breeding and nonbreeding plumages: That prealternate molt evolves primarily under selection for feather renewal, with seasonal color change sometimes following later. These results reveal how life‐history strategies and a birds' environment act upon multiple and separate feather functions to drive the evolution of feather replacement patterns and bird coloration.
Highlights
When subject to dissimilar selective forces, traits that arose for one function often diversify to serve another (Barve & Wagner, 2013)
Selective forces vary throughout a birds' annual cycle, and this variability has been hypothesized to lead to the distinctive breeding and nonbreeding plumages shown by many species, that is, seasonal dichromatism (Mulder & Magrath, 1994)
We evaluated the number of transitions and the probability that rates of gains and losses were significantly different for presence of molts and dichromatism by reconstructing ancestral states under equal rates (ER) and all rates different (ARD) models; we compared the log-likelihoods of each model using a likelihood ratio test to obtain a p-value for rejection of the ER model in favor of the more complex ARD model
Summary
When subject to dissimilar selective forces, traits that arose for one function often diversify to serve another (Barve & Wagner, 2013). The first hypothesis, which we term the variable pressures hypothesis, concentrates on feather color and states that prealternate molt evolved in response to differential relative levels of social and natural selection throughout the year (McQueen et al, 2019; Simpson et al, 2015; Tökölyi et al, 2008). The feather wear hypothesis does not rule out variable pressures on feather colors, but instead proposes a different (mean hours) Seasonal Dichromatism Prealternate molt We examined these two hypotheses using the ecologically diverse New World warbler (Parulidae) family, which exhibit remarkable variation in plumage characteristics and migratory behaviors. We implemented a phylogenetic comparative approach and quantified the extent of prealternate molt and seasonal dichromatism in the New World warblers, as well as 31 life-history and environmental characteristics that may affect the evolution of prealternate molts and plumage coloration through natural selection
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