Opisthocomiformes

Opisthocomiformes is an order of birds that currently contains a single living family, Opisthocomidae, represented by the hoatzin (Opisthocomus hoazin). The order is characterized as a small, relatively ancient lineage with a sparse fossil record, offering insight into early avian evolution. The hoatzin is native to the freshwater and forested wetlands of the northern Amazon and Orinoco basins in South America.

Taxonomy and evolution considerations remain a topic of debate. While the hoatzin is the sole extant member of Opisthocomiformes, its precise relationship to other birds has varied among analyses. Some molecular and morphological studies have placed the group as a distinct, early-diverging lineage, whereas others have recovered more distant or uncertain affiliations with other land birds. The fossil record includes several extinct relatives from Eocene to Miocene deposits, indicating a wider past distribution and diversity for the group.

Morphology and ecology: Hoatzins are relatively large, mainly folivorous birds adapted to a life in and around aquatic and riparian habitats. They forage in trees along rivers, feeding primarily on young leaves and shoots. A notable physiological feature is an enlarged crop that hosts a microbial fermentation system to help digest cellulose. Juvenile hoatzins possess small claws on their wings, which assist in climbing branches. They are generally poor fliers and prefer slow, deliberate movement through vegetation.

Reproduction and behavior: Hoatzins nest above water, typically laying two to four eggs. Both parents participate in incubation and caring for the chicks. The species is currently listed as Least Concern by the IUCN, though habitat loss in the Amazon basin can affect local populations.