Cow Bird?
Well no, not a cow bird. That's its own separate animal. This bird just has unique features very similar to that of a cow. The Hoatzin, the Canje Pheasant, or the Stinkbird; is a large, clumsy bird living in the swamps and mangroves of the Amazon.
This bird eats leaves! It is the only bird that relies on leaves as its primary food source. Breaking down plant matter is very difficult and time intensive. Cows have large, multi-chambered stomachs for the rechewing, fermentation, and digestion of their food. Hoatzins however, have very small stomachs. Their unique little trick is call foregut fermentation. Their crop and esophagus are large and folded up into a bunch of little chambers for the cultivation of bacteria to break down the food before it ever reaches the stomach. The stomach in this image is both the proventriculus and the gizzard. It takes two days for food to pass through this system.
This is found in no other birds! As a matter of fact, the hoatzin is the last remaining species of its kind, diverging from other birds at the fall of the non-avian dinosaurs. Their ancestors are all found in the old world so scientists theorize a chunk of their old swamp broke off in a storm and carried the hoatzins across the Atlantic.
They get the name "Stinkbird" because of the gasses produced when they ferment their food. They burp all the time and it smells so bad that people leave this large, heavy bird alone. Their large crop takes up space birds usually use for their flight muscles and bone for those muscles to attach to. As a result, they're really weak flyers. They mostly jump between branches within a few feet, though they can actually fly. Their crop also has a hard lump at the bottom to rest their big heavy body on branches and such. They spend a lot of the time sitting around, slowly digesting their food.
In contrast, their chicks are super mobile. No other bird crawls on all fours. These chicks have two claws on each wing they use to climb trees and riverbanks. When their parents sound the predator call, the chicks will climb out of their nest and jump into the swamp, coming home when the coast is clear. They look very much like the ancient dinosaur "Archaeopteryx," a three clawed dinosaur thought to be one of the first birds. This is cool and it can tell us about how the evolution of birds, but the hoatzin isn't more closely related to the dinosaurs than any other bird. Chickens and Turacos also have claws on their wings!
Being unique is always a big plus when it comes to ranking. It's almost poetic that this swamp bird stinks and has little goblin chicks. To it's detriment, it is horribly unathletic in adulthood and it smells worse than manure. They live in groups so I bet it smells horrible! Supposedly, they did cross the ocean on a raft and they look really cool though. I think it's looking really good for the Hoatzin.