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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. While bearing superficial resemblance to several other bird species, the hoatzin has many peculiarities that set it apart from the rest. 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 a strong, 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. The source of the off-putting aroma is a feature unique among birds: the hoatzin has evolved foregut fermentation to extract nourishment from leaves.

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 and grasses. 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.

Today, the ornithological consensus is that the hoatzin is the last surviving species of a genetic line that branched off from the rest of the birds 64 million years ago. 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.