Adult males are mainly chestnut with a blackish head and rump, and a tail which is bright yellow apart from two dark central feathers. There is a bare blue cheek patch and a pink wattle, the iris is brown, and the long bill is black at the base with a red tip. Females are similar, but smaller than males with a smaller wattle. Young birds are duller than adults and have a paler and less demarcated bill. No subspecies are currently recognized.
The sexes are very different in size; the male is 50 cm (20 in) long and weighs 520 g (18 oz); the smaller female is 38 cm (15 in) long and weighs 230 g (8.1 oz). In total body mass, the males are 100% bigger than the females, which is a 2:1 body-to-mass ratio and makes the Montezuma oropendola one of the most sexually-dimorphic birds in the world.
Webster et al. says that the size difference in males and females is probably directly related to differences in foraging habits. The females often forage on thin branches, eating insects out of curled up leaves, while the males often perch on thick branches and forage in epiphytes and bromeliads.[2]
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