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HOATZIN - the vegetarian ruminant bird
Opisthocomus hoazin
La Selva Lodge, Ecuador
©Christopher Plummer post by jujuba

The Hoatzin, also known as the Hoactzin, Stinkbird, or Canje Pheasant, is a 24” long pheasant-sized species of tropical bird found in swamps, riverine forest and mangrove of the Amazon and the Orinoco delta in South America. It is notable for having chicks that possess claws on two of their wing digits.

The Hoatzin is arguably the most enigmatic living bird in regard to its phylogenetic relationships. No satisfying evolutionary hypothesis has been proposed, and the situation has actually become worse with the availability of DNA sequence data.

Moving clumsily, the Hoatzin uses a leathery bump on the bottom of its crop to help balance itself on the branches. It was once thought that the species could only eat the leaves of arums and mangroves, but the species is now known to consume the leaves of over fifty species. One study undertaken in Venezuela found that the Hoatzins diet was 82% leaves, 10% flowers and 8% fruit. Any feeding of insects or other animal matter is purely accidental. 

One of this species’ many peculiarities is that it has a digestive system unique amongst birds. Solely vegetarian, hoatzins use bacterial fermentation in the front part of the gut to break down the vegetable material they consume, much like cattle and other ruminants. Unlike ruminants, however, which possess the rumen (a specialized stomach for bacterial fermentation) in the Hoatzin this is the function of the crop (an enlargement of the esophagus). 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 giving way to their nickname “Flying cow.”

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.