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The hoatzin (Opisthocomus hoazin) is one of the world's most peculiar bird species. It is the sole member of its family, Opisthocomidae. It is peculiar enough to have defied taxonomists' best efforts for years.

This bird lives only in the rainforests of northern South America. Its feathers are dark brown on the back and lighter below, and chestnut-colored on its sides. The skin around its red eyes is a startling electric blue. Its head is topped by a crest of long chestnut-colored feathers.

The hoatzin builds its next of sticks in trees or large bushes, usually on boughs that overhang the water. Into this nest the female lays two or three (though sometimes as many as five) buff-colored eggs speckled with blue or brown. After an incubation period of 28 days, out of these eggs hatch some of most remarkable chicks in the order Aves.

Hoatzin chicks are naked, and proof of the old aphorism "a face only a mother could love." But beauty does not count: what does count is the tiny claws on the chick's wings. The claws help the chick to hold on as it moves through the branches. But even if it should tumble off the branch and fall into the water below, the hoatzin chick can swim to the nearest branch or tree trunk and climb back up the tree into the nest.

Some people have called the hoatzin a living fossil and equated these claws with those of Archaeopteryx, the ancestor of modern birds that lived 150 million years ago. However, the claws are not unique among birds: some species of geese retain spurs on their wings into adulthood, and young European coots have a single claw on each wing that helps them climb back to the nest as well. It is more likely that the hoatzin's claws are not a relic, but a recent adaptation to its rather precarious nesting site.

Hoatzins are remarkable for still another reason. Their diet consists strictly of vegetable matter—leaves, flowers, and fruits. It is the only tree-dwelling bird that feeds its young on leaves. To handle this fibrous diet, the hoatzin has evolved a very large crop, or gizzard, in which it grinds up the tough cellulose fibers of the leaves. The crop is so large that it accounts for about one-third of the adult bird's 28 oz (793 g) body weight.

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