We map behavioral characters related to mating system onto a phylogeny of the New World blackbirds (family Icteridae) in order to test hypotheses on the evolution of polygyny in Red-winged Blackbirds (Agelaius phoeniceus). The two hypotheses we test are 'long-term models' in the sense that, unlike most polygyny hypotheses, they allow the ancestral mating system to differ from the present one in characters other than female preferences for mated versus unmated males. In one model, polygyny evolves from the typical territorial monogamy system of most terrestrial passerines; in the second model, polygyny evolves from a system resembling that of monogamous Agelaius species, with marsh breeding and without male territoriality. Both hypotheses assume that female-biased parental care coevolves with polygyny. Our reconstruction suggests that the closest non-polygynous ancestor of Red-winged Blackbirds was characterized by monogamy, male territoriality, equal sharing of parental care between the sexes, and terrestrial breeding. Further, polygyny and female-biased care are suggested to have evolved on the same branch as marsh nesting. These results refute our second hypothesis in which polygyny evolves from 'Agelaius monogamy,' while providing provisional support for the first model in which polygyny evolves from territorial monogamy.