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

Fully unraveling the mechanisms of sexual selection requires an understandi ng of the variation in secondary sexual traits across species in a monophyl etic assemblage and an understanding of the evolutionary relationships betw een those species. The role of red and yellow male plumage coloration in te rritory defense and sexual selection has been well studied in the red-winge d 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 spe cies of New World blackbirds (family Icteridae): traits known to be involve d 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-bla ck ancestral plumage to one exhibiting carotenoid plumage patches. The two most likely hypotheses to explain this pattern are (1) increased sexual sel ection intensity in marshes because of increased variance in territory qual ity and (2) increased frequency of male-male territorial interactions becau se 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.