The hoatzin (Opisthocomus hoazin) is a large tropical bird endemic to South America. They are unique among birds for their complex digestive system, specialized for fermenting and digesting leaves! The gut bacteria responsible for this gives them a bad smell, which also affords them the name "stinkbird".
The hoatzin (also known as the hoactzin or stinkbird) is a mid-large sized tropical bird native to South American rainforests. They prefer swamps, mangroves, riparian forests, and generally any other location with bodies of murky water. They share many distinct similarities to chickens due to their chunky appearance and clumsiness in the air, but it remains debated exactly what they are related to. In fact, they have an order all of their own known as Opisthocomidae. (named after the genus Opisthocomus)
Approximately 80% of the hoatzin's diet is comprised of leaves and foliage. They are almost entirely herbivorous! Unfortunately, foliage is not the most nutritious food option. Large ruminants like cows, horses, and goats have multi-chambered stomachs, in which they ferment the food they consume and process it with complex gut bacteria. Hoatzin take after this strategy, and are in fact the only birds which ferment their food!
Hoatzins have a very specialized crop. Crops are pouches that birds use to store food before it enters the stomach! The hoatzin's crop is massive, so much so that it rests on the brances they perch on. It contains the bacteria needed to ferment their food. Instead of having a multi-chambered stomach, they have a sort of multi-chambered esophagus instead! The bacteria that allows them to digest their food has a poor smell, and makes them taste and smell rather poor. Hoatzin also frequently burp to pass methane produced by the fermentation of their food, which does not help!
Hoatzin chicks are born with pretty well developed wing claws, which allow them to climb through trees. From a few days old, chicks will explore around the nest and the surrounding areas by climbing around with their odd little proto-arms. When threatened, chicks will sometimes drop into bodies of water. They are adept swimmers, and can easily climb their way back out and up to their nests again. When they age, these claws become vestigial (practically useless) and covered by their wings. However, adult hoatzins still often prefer climbing over flight!
Hoatzin are known for being very noisy as well. They create loud hoarse calls, similar to something you'd imagine a pterodactyl making. They are territorial, but do nest in small groups with other hoatzin. Typically these groups are family groups, in which young help the parents defend the territory and care for chicks before moving on to their own territory. Their chicks are fed with regurgitated food, (yum...) which provides them the gut bacteria they require later on in life.