The hoatzin (Opisthocomus hoazin)

The hoatzin is a folivore - it eats the leaves (and to a lesser degree of fruits and flowers) of the plants that grow in the marshy and riverine habitats where it lives. It clambers around clumsily along the branches, and being quite tame (though they become stressed by frequent visits), often allows close approach and is reluctant to flush. The hoatzin uses a leathery bump on the bottom of its crop to help balance itself on the branches. Once it was thought that the species could eat the leaves of only arums and mangroves, but the species is now known to consume the leaves of more than fifty species. One study undertaken in Venezuela found that the hoatzin's diet was 82% leaves, 10% flowers, and 8% fruit. Any feeding of insects or other animal matter is purely accidental.


In flight, Bolivia
One of this species' many peculiarities is that it has a digestive system unique amongst birds. Hoatzins use bacterial fermentation in the front part of the gut to break down the vegetable material they consume, much as cattle and other ruminants do. Unlike ruminants, however, which possess the rumen (a specialized stomach for bacterial fermentation), the hoatzin has an unusually large crop, folded in two chambers, and a large, multi-chambered lower esophagus. Its stomach chamber and gizzard are much smaller than in other birds. The crop of the hoatzin is so large as to displace the flight muscles and keel of the sternum, much to the detriment of their flight capacity. Because of aromatic compounds in the leaves they consume and the bacterial fermentation, the bird has a disagreeable, manure-like odor and is only hunted by humans for food in times of dire need.