Evolution of breeding plumage in birds: the relative influence
of life history, color change, and environment on the evolution of
biannual molt in the New World warblers (Aves: Parulidae).
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 are 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
non-breeding 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.