24-3-2017 RANCHO NATURALISTA, COSTA RICA - MONTEZUMA OROPENDOLA (Psarocolius montezuma)

The Montezuma oropendola, Psarocolius montezuma, is a strikingly dimorphic bird, with males significantly larger than females. Adult males boast a chestnut body contrasted by a blackish head and rump, and a vivid yellow tail save for two central dark feathers. Their facial features include a bare blue cheek patch and a pink wattle, with a brown iris and a long bill that is black at the base and red at the tip. Females are similar in plumage but are smaller with a less prominent wattle. Juveniles appear duller with a paler bill. This species is known for its remarkable sexual size dimorphism, with males being twice the body mass of females.

When identifying the Montezuma oropendola, look for the male's large size, about 50 cm in length, and the bright yellow tail with dark central feathers. Females are notably smaller, measuring 38 cm. The distinctive blue cheek patch, pink wattle, and two-toned bill are key features to observe.

This tropical bird inhabits the forest canopy, edges, and old plantations, preferring the Caribbean coastal lowlands and some Pacific slopes.

The Montezuma oropendola ranges from southeastern Mexico to central Panama, with notable absences in El Salvador and southern Guatemala. It is also found in Nicaragua, Honduras, and parts of Costa Rica.

Males exhibit a female-defence mating system, where the dominant male mates with most females in a colony after an elaborate bowing display. They are known to be aggressive in defending receptive females from other males. Females forage on thinner branches, while males prefer thicker branches, likely due to their size difference.

The male's song is an "unforgettable" series of bubbling and gurgling sounds, ending in a loud gluuuuu. Both sexes emit loud cack and crrrk calls. Males at breeding sites may alter the lowest peak frequency of their song to out-compete rivals.

Montezuma oropendolas are colonial breeders, with females constructing the nests. These woven nests hang from trees and can be up to 180 cm in length. The dominant male in the colony mates with most females. Females lay two dark-spotted eggs, incubating them for 15 days, with fledging occurring in 30 days.

The chestnut-headed oropendola shares some of its range but is smaller, mainly black with a chestnut head, and lacks the colored facial patches, making confusion unlikely.

Omnivorous, these birds forage for small vertebrates, large insects, nectar, and various fruits, including bananas and Cecropia spikes. They are often seen in flocks outside the breeding season, indicating some mobility and seasonal movements.

The Montezuma oropendola is currently classified as Least Concern by the IUCN, indicating a stable population without significant threats.

25-3-2017 RANCHO NATURALISTA, COSTA RICA - MONTEZUMA OROPENDOLA (Psarocolius montezuma)

26-3-2017 RANCHO NATURALISTA, COSTA RICA - MONTEZUMA OROPENDOLA (Psarocolius montezuma)

24-3-2017 RANCHO NATURALISTA, COSTA RICA - MONTEZUMA OROPENDOLA (Psarocolius montezuma) COURTESY MRS VALERIE FISHER

The Montezuma Oropendola (Psarocolius montezuma ) is a New World tropical icterid bird. It is a resident breeder in the Caribbean coastal lowlands from southeastern Mexico to central Panama, but is absent from El Salvador and southern Guatemala. It also occurs on the Pacific slope of Nicaragua and Honduras and northwestern and southwestern Costa Rica. It is among the oropendola species sometimes separated in the genus Gymnostinops. The English and scientific names of this species commemorate the Aztec emperor Moctezuma II.

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.