HOATZIN Opisthocomidae
The Hoatzin (left and below) is a very odd, monotypic species of uncertain affinities which lives in backwater swamps of the Amazon and Orinoco basins of South America. It is often considered one of the most primitive of birds. While it is not difficult to find Hoatzins once their habitat is reached, I have had much difficulty in taking photos. The fine lead photo (above left) was taken by Bill Rydell in the Hato Piñero region of the Venezuelan llanos, and shows the odd pear-like shape, the bare face, and the shaggy crest of this unique bird. My own efforts (below or right) from Peru and Brazil record not much more than brief, enigmatic images of a strange bird in a strange place. One can almost feel the muggy  dank air of each swamp, and the whine of hundreds of mosquitos...
The Hoatzin is strictly vegetarian, eating the leaves, flowers and fruits of more than 50 species of marshland plants. Hoatzins are very strange among birds, though, because they ferment this vegetable matter in their foregut like cows, sheep, deer and kangaroos, and thus have a specialized digestive system. Nestlings are fed this regurgitated matter. Hoatzins forage in the early morning and early evening, and spend much of the time roosting quietly and digesting their meal.

The Hoatzin chick features a rare anatomical feature two claws on each budding wing which help it grip branches and clamber about awkwardly. This feature has been compared to Archaeopteryx, the fossil proto-bird, and lend a antediluvian background to what is already a really weird bird. Great photos are in Thomas (1996) and fantastic video of adult and young Hoatzins is in David Attenborough's "Life of Birds" and "Life on Earth" series.

Hoatzins live in family groups and small aggregations (up to 40 birds) and are social throughout the year. During breeding birds occupy densely packed exclusive territories, sometimes up to 28 nests in one tree. They are noisy and often vocalize in unison with a collection of hoarse cries, grunts, growls and hisses.

Hoatzins are limited to their lowland swamp habitat. Old cut-off river channels and backwater lagoons are appropriate, but they avoid swift-moving water. Thus in many riverine places in South America, Hoatzins will be found only in a few well-known places. Many jungle lodges will offer boat trips to see Hoatzins although in some places (like La Selva Lodge, Ecuador) they are right at the main boat dock.

Hoatzins are poor fliers, and they put a lot of noisy effort into using their broad rusty wings to cross channels or move away from perceived danger (below).

The affinities and relationship of the Hoatzin have been the subject of much debate over the years. They have variously been grouped with the cranes, rails, tinamous, sandgrouse, doves, mousebirds or seriemas. The more conservative systematists still retain the Hoatzin in the Galliformes, based on osteological and immunological data (Thomas 1996). Recent biochemical evidence (bothegg white proteins and DNA-DNA hybridization; Sibley & Ahlquist 1990) and behavioral studies have shown an affinity with the Cuculiformes (cuckoos and relatives), and Sibley & Monroe (1990) consider it within a "parvorder" in the midst of their various cuckoo/ani groupings. One objection to this arrangement, however, is that hoatzins have anisodactyl feet (three toes forward, one backwards) like most birds, rather than the zygodactyl feet (two toes forward, two backward) of all the other cuckoos. Thus, whatever their relatives may be, they are unique and odd birds, and well deserving of the attention showered on them by visiting birdwatchers to South America.

Photos: The upper photo of a Hoatzin Opisthocomus hoazin was taken by Bill Rydell in the Hato Piñero area of south-central Venezuela in Feb 2000. The "hoatzin at dawn" shot was from Tambopata Nature Reserve, Madre de Dios, Peru, in June 1987. The flight shot of a hoatzin dashing across a small gap in the foliage was along the upper Rio Cristalino River, n. Mato Grosso, Brazil, in August 1999. Upper photo © 2000 Bill Rydell (used with permission), lower photos © 2000 Don Roberson; all rights reserved.
Bibliographic note:

There is no "family book" per se of which I'm aware, but an excellent introduction to the family including some wonderful photos is in Thomas (1996).

Literature cited:

Sibley, C. G., and J. E. Ahlquist. 1990. Phylogeny and Classification of Birds: A Study in Molecular Evolution. Yale Univ. Press, New Haven, CT.

Sibley, C. G., and B. L. Monroe, Jr. 1990. Distribution and Taxonomy of Birds of the World. Yale Univ. Press, New Haven, CT.

Thomas, B. T. 1996. Family Opisthocomidae (Hoatzin) in del Hoyo, J., Elliott, A., & Sargatal, J., eds. Handbook of the Birds of the World. Vol. 3. Lynx Edicions, Barcelona.

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