The Unicolored Blackbird is well named, at least as it applies to the male’s plumage, which is entirely black with dark eyes. Females are another situation entirely; through much of the range the females are yellowish below, with a dark mask, brown above and rusty on the wings. Interestingly there are isolated populations, one near the mouth of the Amazon River (xenicus) which is blackish throughout with a paler belly, and another in coastal southeastern Brazil (atroolivaceus) that is dark above, with a yellowish throat and lacking the rusty wing edgings of the main form. These isolated populations may in fact be good species, similar to the closely related Pale-eyed Blackbird (Agelasticus xanthophthalmus), although the latter also differs in the male’s eye color from the male Unicolored. In terms of plumage the males of these two closely related species are very similar. Unicolored Blackbirds are found in large expanses of marsh and are not as gregarious as many other marsh-loving blackbirds. Their vocalizations are often squeaky and unpleasant, but the clearly whistled song is more attractive and often compared to that of the Black-capped Donacobius (Donacobius atricapilla).