Hoatzin

The hoatzin (Opisthocomus hoazin) is a very strange bird indeed. It has so many odd features that scientists have not been able to reach agreement as to what family of birds it belongs to, and the best solution reached to date has been to assign the hoatzin its own order (Opisthocomiformes) and family (Opisthocomidae). It is quite possible that the hoatzin offers a link to long-extinct ancestors of modern birds and helps to explain how they might have evolved from reptiles. The hoatzin could be to birds what the coelacanth is to fish.

The hoatzin inhabits trees alongside streams and rivers in South America, its range being from the Amazon basin into the Guianas, Venezuela and Colombia. It exists in separate colonies of up to 50 birds, its inability to fly great distances meaning that populations in adjoining valleys are unlikely to mix.


Appearance

In shape the hoatzin is not unlike the pheasant, being about two feet long with a longish neck and very small head. Its plumage is mostly brown, with some lighter colouring below and reddish-brown wings. The face is blue and the bird has a long loose crest of stiff feathers. It has strong legs and long toes. The wings and tail are long and rounded.


Breeding

The hoatzin nests anything from five to 20 feet up, usually in trees that overhang the water, although the hoatzin is not a water bird. The nest is a platform of sticks on which the female lays two or three eggs.

It is when the eggs hatch, after about four weeks, that the most peculiar feature of the hoatzin becomes apparent. The chicks are almost naked, but are born with functioning claws at the ends of their wings. They use these to creep away from the nest, using all four limbs to keep hold of the branches. These claws vanish after two or three weeks and the adult birds show no signs of ever having had such claws.

The first true bird found in the fossil record was the Archaeopteryx, which lived during the Upper Jurassic period around 130 million years ago. The fossils clearly show claws on the wings, which therefore offer interesting room for speculation as to a link between the hoatzin and a proto-bird that, if not Archaeopteryx itself, was around at the same time.

Should a hoatzin chick fail to hang on to its branch and fall into the water, all is not lost because it is able to swim. It can regain the bank and climb back up into its tree.


Feeding and behaviour

Another very strange feature of the hoatzin is its digestive system. Most bird species have a crop, which is an enlarged portion of the alimentary tract in which food is stored prior to digestion, but in the hoatzin this is so large that it occupies a third of the forward part of the body. It performs an important function by acting as both crop and gizzard, thus enabling the bird to pre-digest some of its food, which consists mainly of the rubbery leaves of the arum plant. Incidentally, the hoatzin’s diet and its digestive processes have led to it acquiring a highly unpleasant odour, and hence the alternative name of “stink bird”.

The downside of the above arrangement is that the sternum keel, to which the flight muscles are attached, is so short that flight is made very difficult. The hoatzin can therefore do little more than glide from a high point on one side of a stream to a low point on the other side. It then has to climb up the tree before being able to make the return journey. This inability to fly any distance has led to hoatzin populations becoming isolated.

This local isolation, coupled with environmental conditions that have been constant for millions of years, has meant that the hoatzin has had no need to evolve in order to survive, which has presumably not been the case with species to which it might have been related in the past. As these former species have developed into other species that exist today, the hoatzin has stayed as it always was, a fascinating relict of avian life from a far distant time.

© John Welford