In South America, in the swamps of the Amazon and Orinoco rivers, lives a very unusual bird.

The hoatzin is a pheasant-sized enigma. The official national bird of Guyana, the hoatzin has defied attempts of ornithologists to place it in its proper place among the families of birds. No matter where it is placed, the hoatzin simply does not appear to fit. The hoatzin was given its own family (Opisthocomidae), but since the original designation it has been moved around from being grouped with the game birds (the source of its other name, the Canje pheasant), to grouping it with the cuckoos, to its current, though still speculative placement with the seriema family (most closely related to rails and bustards).

The difficulty is the hoatzin itself. While bearing superficial resemblance to all of these other species in some way, it has many peculiarities that sets it apart from them all. These oddities Include some very primitive traits not seen in most birds since the Jurassic period, coexisting with characteristics which are otherwise unheard of among birds.

The first distinctive peculiarity of the hoatzin can be seen most easily on its chicks. They have two claws on each wing, which they use for clambering about the trees. Adults retain the claws, though they do not use them. Few other modern birds have such a thing. However the fossil record shows that several prehistoric birds had such claws, including the earliest and most famous of the ancient birds – Archaeopteryx.

The second peculiarity of the hoatzin has earned it one of its nicknames – the stinkbird. The bird has an unpleasant manure-like smell, which serves to drive off predators, including humans. Despite their resemblance to game birds and their slow, awkward flight, the hoatzin remains largely unmolested because it makes extremely unappetizing eating. Only during times of famine will locals consider hunting it for food, and even then reluctantly. It has a reputation for being toxic, but there is little or no evidence either way – apparently because no one has actually eaten more than a bite or two of hoatzin for some time.

The Guayanans attribute the hoatzin’s smell to the leaves that make up a large portion of its diet. As it turns out, they are right, but possibly not in the way that they thought. While the older information pegs the culprit as the accumulation of toxins from a particular kind of leaf, a more recent study by A. Grajal et al has shown that the hoatzin’s odor comes not from the kinds of leaves it eats, but rather from how it digests them. Uniquely among birds, the hoatzin has developed foregut fermentation.

Foregut fermentation is seen frequently in larger mammals. Cows, sloths, kangaroos, and others have all developed foregut fermentation as a way to extract maximum nutrition from the relatively poor food value of leaves. Bacteria in the upper part of the digestive tract help to break down cell walls, and the resultant cud is then reprocessed to extract more nutrients. How the hoatzin came to develop it is a mystery. Despite the abundance of leaves in the tropics, no other bird has any kind of foregut fermentation. Indeed, the scientists studying the hoatzin seem amazed that the bird manages its digestive feat with such a small space to work in. Cows stomachs, after all, are huge.

Whether it’s the most primitive of birds or the most modern, the hoatzin has little to worry about as it feeds its young on cud and merrily emanates its perfume of fermenting foliage. Why should it change, when what it has works so effectively? The odd collection of traits makes the stinkbird a highly efficient eating machine and very unappetizing prey, which is all that matters when the game is survival.