Species: Opisthocomus hoazin
Range: South American rainforests
Weird Feature(s): Wing claws, cow stomach
The hoatzin looks like a harmless, even regal bird until you see its clawed babies and smell its distinctive cow manure aroma.
Aside from a very odd diet (which we’ll get to in a minute) the hoatzin’s juvenile form has an adaptation which makes it stand out in the avian world–claws on their wings. When a predator approaches a hoatzin’s nest, which usually hang over bodies of water, the babies will launch themselves out of the nest and drop down into the river, lake, or pond below. Once the threat is gone, the hoatzin babies use these claws to climb back up the tree and into their nests.
A bird with claws makes it very tempting to posit that the hoatzin is a close relative of bird/dinosaur “missing link” Archaeopteryx. But instead, the hoatzin belongs to neoaves, the group that includes nearly all modern birds. Within that group, however, hoatzins are something of an evolutionary enigma, being the only representative of the order Opisthocomidae.
Despite the fact that many birds nest, roost, and spend plenty of their waking hours in trees, very few species eat leaves. Hoatzins are an exception to this rule. And in order to accommodate this unusual habit, these folivores have evolved foregut fermentation systems like cows have.
Despite what your vegan friends might tell you, many plants are actually pretty difficult to draw any meaningful nutrition out of. To get the nutrients trapped in plant cells, foregut fermenters like sheep, cows, and hippos have pouches in their stomachs containing symbiotic microbes. These microbes are able to synthesize enzymes that breakdown the plant matter and unlock the fatty acids and sugars. Hoatzins are one of the only birds known to have developed foregut fermentation. This awesome example of convergent evolution aside, the plant-based diet does give the bird a smell similar to cow manure, gaining them the alternate name “the stink bird”.
Foregut fermentation is a costly process in that it requires a lot of digestive space to pull off. The hoatzin’s esophagus and double-chambered crop–where the foregut fermentation takes place–make up 70% of the animal’s digestive tract and 17% of its entire body mass. The massive digestive anatomy of this bird means that its sternum is reduced in size, allowing it smaller flying muscles. As a result, hoatzins are poor long distance fliers and spend much of their time at rest admiring each other’s sweet mohawks.