Chestnut-headed Oropendola

Habitat
The Chestnut-headed Oropendola prefers the canopy and edges in humid, forested regions.

Range
This large bird lives between southern Mexico and northwestern Ecuador. In Costa Rica, it lives on the Caribbean slope in the lowlands, generally below 1,200 m, especially in the Coto Brus region; on the southern Pacific slope they are sometimes in the Golfo Dulce region and lower Valle del General.

National Parks
Sometimes found in La Selva, Monteverde (such as Monteverde Cloud Forest Reserve) and the Osa Peninsula (such as Corcovado National Park).

Physical Description
This large dark bird builds its nest in conspicuous colonies-one can spot the chosen trees full of long, narrow sack nests from far away, as well as hear the loud, resonant kuk call from the vocal oropendolas. This oropendola can be identified by its mostly black body followed by a bright yellow tail, with a chestnut-colored head, pale blue eyes, and long ivory bill that starts high up on the forehead. Adults of both sexes are colored similarly, while juveniles are duller and take 2 years to develop complete adult plumage. This bird can be distinguished from the Montezuma Oropendola, which has a chestnut body and dark bill. The Chestnut-headed also uses calls that are deeper and more resonant than those of the Montezuma.

Interesting Biology
The oropendola's characteristic colonies of 12 to 50 long (56 cm) sacklike nests hang from the ends of branches of large trees that are prominent in a clearing or forest edge. The females weave these nests of fibers, vines, Spanish moss, and other materials. They also incubate and raise the two young without the help of adult males. During the nonbreeding season (June-December), flocks of these birds travel, searching for fruiting trees; their colonies often stay together, and return to their nesting trees when the breeding season resumes. Within a colony there is a sex ratio of 5 females for every male, indicating that their mating system is polygamous. The males do not fight, though, and show no sort of dominance hierarchy in the colony.

The dramatic disparity in numbers of each sex is due to the energy costs of raising males: females have to spend twice as much energy to raise male chicks, and males fledge when they are twice the weight of females; so when food is scarce, more male chicks die. In addition to this difficulty, Chestnut-headed Oropendolas lose their chicks to predators like toucans, snakes, opossums, and bats, but the most severe damage comes from botfly larvae.

The oropendolas are involved in a system of intriguing interactions as a result of botfly parasitism. Some colonies are built in trees that also host wasps or stingless (but biting) bees; the wasps and bees do not harm the chicks, but they do attack botflies, and the oropendolas resultantly have higher survival rates. Similarly, the Giant Cowbird (Scaphidura oryzivora), which is smaller than a female oropendola, are brood parasites-that is, they lay their own eggs in the nests of other birds. The female oropendola will spend energy feeding the cowbird chicks, but the cowbird chicks pick off botfly eggs and larvae from the oropendola chicks, so more of the oropendola chicks survive. However, oropendolas in trees with protection from wasps or bees do not need the cowbird chicks, so they prohibit cowbirds from laying or later remove the cowbird eggs from their nests. If oropendolas are not nesting in a tree with wasps or bees, they will permit the cowbird eggs in their nests. This dynamic is important to the oropendola's survival, because few chicks survive to fledging-but once grown, the survival rate is strong.

Diet
The agile Chestnut-headed Oropendola mostly forages in the canopy, as it hops, runs, and hangs on branches, looking for fruit or the nectar of large flowers like those found on the balsa tree; they may also descend to look for small frogs and lizards.

Height/Weight
Males are 35 cm long, females smaller at 27 cm; males (212 g) weigh twice as much females (110 g).

Brief Taxonomy
Order: Passeriformes
Family: Icteridae

Sources
N. G. Smith in Janzen, Daniel H. Costa Rican Natural History. Chicago: University of Chicago Press, 1983.

Stiles, F. G. in Janzen, Daniel H. Costa Rican Natural History. Chicago: University of Chicago Press, 1983.

Skutch, Alexander F. and F. Gary Stiles. A Guide to the Birds of Costa Rica. Utica: Cornell University Press,1989.

-Amy Strieter, Wildlife Writer