Hoatzin

Opisthocomus hoazin. Also Canje pheasant, stinkbird. Rio Parnaíba, Piauí, Brazil. 1903.

The diorama illustrates one behavior typical of hoatzins: chicks use their wing claws to climb back into the nest after dropping into the water as a means of escape.

CLIMBING WITH WINGS
The hoatzin, which lives along riverbanks and lake shores in the tropical rain forests of South America, is a strange bird for several reasons: in no other species living today do the chicks have wing claws, just as Archaeopteryx did. This is why the hoatzin was previously even regarded as the missing link between modern and extinct birds. Today we know that the wing claws are an adaptation from an earlier time and that hoatzins are not directly related to Archaeopteryx.
The digestive tract of the hoatzin is also highly unusual, being more typical of that of a sheep rather than a bird. Rather unnutritious leaf material is digested primarily in the greatly enlarged, very muscular foregut, lined with cornified ridges. As with ruminant mammals, digestion takes up to 48 hours.
The greatest mystery revolves around the question of the hoatzin’s relatives. Is it a pheasant, chicken, rail, bustard or perhaps a dove? Currently the most popular theory is that it related to the galliformes or cuculiformes. Not even its parasites help solve the classification problem, because they too are unique and occur on no other species of bird. For the time being, the hoatzin will remain its own order.
The birds’ skins were a gift to the NHM after the Imperial Academy of Sciences’ expedition to Brazil. They have been used to display the hoatzin in its natural habitat. When climbing, the young chicks use the claws on their second and third wing-fingers, which they lose again when they are about three months old.