Here's a bird so unusual that it has no living relatives. It's the sole species in its genus, family and order. Even with DNA evidence scientists have found nothing quite like it. The best they can say is that Opisthocomus hoazin is ... a bird.
The hoatzin (pronounced "watson") is a noisy, colorful, pheasant-sized bird that lives in the swamps and river forests of the Amazon and Orinoco basins in South America. Like herons it nests communally in trees or shrubs over water. Unlike any other bird (except the owl parrot of New Zealand) it eats leaves as more than 80% of its diet.
Leaves are not very nutritious and require bacterial fermentation to digest them. Leaf-eating mammals (cattle, sheep, goats) have a rumen for that purpose. Hoatzins have a huge crop that does the job, but it's so large and heavy that the birds are clumsy in flight.
Fortunately, they don't need to fly to escape. Few animals want to eat them. The fermentation in their crops is so foul that adult hoatzins smell like manure. Ewww!
Hoatzin eggs and young are preyed upon but the young have unique escape tools. Before they grow flight feathers they have claws on the thumb and forefinger of each wing -- like dinosaurs! When danger threatens they drop from the nest into the water below, swim away and then climb back to the nest using their finger and toe claws.
Since people rarely hunt them, hoatzins seem tame and are easy to find.
An odd, stinky bird in the Amazon jungle gloom?
It's a hoatzin, I presume.
(photo from Wikimedia Commons. Click on the image to see the original)