Ecology of the Folivorous Hoatzin (Opisthocomus Hoazin) on the Venezuelan Plains

Abstract

-The Hoatzin (Opisthocomus hoazin) is the only avian folivore known to have fermentative digestion in the crop. We compare results on nesting, growth, and feeding in Piniero Ranch, Cojedes State, Venezuela, with results from another location in the Venezuelan plains. The Hoatzin nests in trees abundant in the habitat. Nests containing two eggs were predominant. Growth rate of chicks was linear over the nesting period (5.68 g/day). The diet of the Hoatzin consisted of young leaves and twigs of some of the most abundant trees in its habitat. Although there are no reports on the phytochemistry of dietary plants, families to which these plants belong include plants known to contain secondary compounds. The crop, which functions as a mediator in plant-Hoatzin interactions and as a detoxification chamber, deserves further study. Received 4 November 1992, accepted 13 November 1993. THE HOATZIN (Opisthocomus hoazin), a folivorous bird that inhabits riverain forests in South America, is peculiar in many respects. A cooperatively breeding bird, with young having special swimming and climbing abilities (Grimmer 1962), the Hoatzin is the only avian folivore with pre-gastric fermentation (Grajal et al. 1989). The first reports on the Hoatzin date from the 17th century (Hernandez 1651, cited by Strahl 1985), but only in the late 19th and early 20th centuries did more precise anatomical descriptions appear (Young 1888, Boker 1929). The claws on the chick wings (Fig. 1), unlike those in other birds, are functional and of high adaptive value for the young. The Hoatzin is the only species in the family Opisthocomidae (Cuculiformes; Sibley et al. 1988). The Hoatzin nests on branches of trees over water courses, and the incubation of eggs for 30 to 31 days is performed by male and female breeders, and sometimes by nonbreeder helpers of the group (Strahl 1988, Vander Werf and Strahl 1990). We present results that provide comparative data on ecological aspects (nesting, growth, and diet) relevant to the study of the nutritional strategy of the Hoatzin. Such information provides a basis for physiological and microbiological studies that might clarify the evolution of the Hoatzin's unusual fermentative digestion.