Evolutionary changes in color patches of blackbirds are associated with marsh nesting

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Abstract
Fully unraveling the mechanisms of sexual selection requires an understanding of the variation in secondary sexual traits across species in a monophyletic assemblage and an understanding of the evolutionary relationships between those species. The role of red and yellow male plumage coloration in territory defense and sexual selection has been well studied in the red-winged blackbird (Agelaius phoeniceus), and males of many other close relatives of this species also have what appear to be carotenoid-pigmented patches in their plumage. We explored variation in male plumage coloration across species of New World blackbirds (family Icteridae): traits known to be involved in sexual selection in this group. We document that blackbird lineages in which extant species breed in marshes tend to have evolved from an all-black ancestral plumage to one exhibiting carotenoid plumage patches. The two most likely hypotheses to explain this pattern are (1) increased sexual selection intensity in marshes because of increased variance in territory quality and (2) increased frequency of male-male territorial interactions because of an increased density of territories in marshes, but other hypotheses cannot be ruled out. This pattern is consistent with either intersexual or intrasexual selection and warrants further investigation.