Like other cowbirds and some other birds, the Giant Cowbird is a brood parasite, and lays its eggs in the nest of other large-bodied birds, principally oropendolas and caciques.
Unlike other cowbirds, though, the Giant Cowbird's offspring doesn't destroy the hosts' eggs or chicks.
The male Giant Cowbird has a conspicuous ruff of feathers around the neck (clearly visible on the first two photos) which may be puffed up during courtship; this ruff may contribute to the Giant Cowbird's peculiar small-headed appearance. 
The male Giant Cowbird is 36 cm long, and is iridescent black.
The female is smaller, averaging 28 cm long, is less iridescent than the male and the absence of the neck ruff makes her look less small-headed.
Juvenile males are similar to the adult male, but browner, and with a pale, not black, bill.

El Triunfo Biosphere Reserve, Chiapas, Mexico, 23 & 24 April 2011