Both sexes are similar in coloration in that they have a chestnut colored body, a black head and bright yellow outer tail feathers. Oropendolas nest in colonies ranging in size from four to over one hundred nests, although there are usually between thirty and forty. The large baglike nests, well over three feet in length, are most often found at the edge or in a clearing of the rain forest. Females almost always out number males in colonies, sometimes as high a ration as 5 to 1. The breeding season takes place during the dry season, from the end of December to the middle of June.
Females weave the large nests and they alone incubate the eggs and feed and care for the young.
They may raise up to three broods during the season. Usually two eggs are laid, but the average number of chicks fledged is less than one half percent. The mortality rate is very high in oropendola colonies due to predation by toucans, snakes, monkeys and botfly larvae. The botfly lays its eggs directly on the young birds and if too many larvae hatch, they will kill the bird. Not all colonies of oropendolas are subject to botfly infestations.