The hoatzin ( ;also )　(Opisthocomus hoazin) is a species of tropical bird found in swamps, riparian forests, and mangroves of the Amazon and Orinoco Basins in South America. It is the only extant species in the genus Opisthocomus Despite being the subject of intense debate by specialists, the taxonomic position of this family is still far from clear.

The hoatzin is notable for its chicks having primitive claws on two of their wing digits. It is unique among birds in possessing a digestive system that significantly supports the fermentation and the effective breakdown of plant matter, a trait more commonly known from herbivorous ungulate-ruminant mammals and some primates. This bird is also the national bird of Guyana, where the local name for this bird is Canje pheasant.

Description

The hoatzin is pheasant-sized, with a total length of , and a long neck and small head. It has an unfeathered, blue face with maroon eyes, and its head is topped by a spiky, rufous crest. The long, sooty-brown tail is bronze-green tipped with a broad, whitish or buff band at the end. The upper parts are dark, sooty brown-edged buff on the wing coverts and streaked buff on the mantle and nape. The underparts are buff, while the crissum (the undertail coverts surrounding the cloaca), primaries, underwing coverts, and flanks are rich rufous-chestnut, but this is mainly visible when the hoatzin opens its wings.

It is a noisy bird, and makes a variety of hoarse calls, including groans, croaks, hisses, and grunts. These calls are often associated with body movements, such as wing spreading.

Young wing claws

Hoatzin chicks have two claws on each wing. Immediately after hatching, they can use these claws and their oversized feet to scramble around tree branches without falling into the water. Since Archaeopteryx had three functional claws on each wing, some earlier systematists speculated that the hoatzin was descended from it, because nestling hoatzins have two functional claws on each wing. Modern researchers, however, hypothesize that the young hoatzin's claws are of more recent origin, and may be a secondary adaptation from its frequent need to leave the nest and climb about in dense vines and trees well before it can fly.

Taxonomy, systematics, and evolution

The generic name Opisthocomus comes from Ancient Greek derived from ópisthe ( ópisthen before a consonant) "behind" and kómē "hair" altogether meaning "long hair behind" referring to its large crest.

The hoatzin was originally described in 1776 by German zoologist Statius Müller. Much debate has occurred about the hoatzin's relationships with other birds. Because of its distinctness, it has been given its own family, the Opisthocomidae, and its own order, the Opisthocomiformes. At various times, it has been allied with such taxa as the tinamous, the Galliformes (gamebirds), the rails, the bustards, seriemas, sandgrouse, doves, turacos and other Cuculiformes, and mousebirds. Another genomic study in 2024 instead places it as the sister group to the Phaethoquornithes (containing numerous aquatic bird orders). The combined group was found to be sister to the Mirandornithes (flamingos and grebes).

In 2015, genetic research indicated that the hoatzin is the last surviving member of a bird line that branched off in its own direction 64 million years ago, shortly after the extinction event that killed the nonavian dinosaurs. Another genetic study from 2024 instead suggested a Late Cretaceous origin (around 70 million years ago), but found that this early divergence is shared with a majority of extant bird orders, making it no more basal than they are. These claws disappear by the time the bird reaches adulthood.]]

With respect to other material evidence, an undisputed fossil record of a close hoatzin relative is specimen UCMP 42823, a single cranium backside. It is of Miocene origin and was recovered in the upper Magdalena River Valley, Colombia, in the well-known fauna of La Venta. In addition to being the earliest fossil record of an opisthocomiform, Protoazin was also the earliest find of one (1912), but it was forgotten for more than a century, being described only in 2014.

Hoazinavis is an extinct genus of early opisthocomiforms from Late Oligocene and Early Miocene (about 24–22 Mya) deposits of Brazil. It was collected in 2008 from the Tremembé Formation of São Paulo, Brazil. It was first named by Gerald Mayr, Herculano Alvarenga and Cécile Mourer-Chauviré in 2011 and the type species is Hoazinavis lacustris.

Namibiavis is another extinct genus of early opisthocomiforms from early Middle Miocene (around 16 Mya) deposits of Namibia. It was collected from Arrisdrift, southern Namibia. It was first named by Cécile Mourer-Chauviré in 2003, and the type species is Namibiavis senutae.

One of this species' many peculiarities is its unique digestive system, which contains specialized bacteria in the front part of the gut that break down and ferment the foliar material they consume (much like cattle and other ruminants do). This process is more efficient than what has been measured in many other species of birds, with up to 70% of the plant fiber being digested. Unlike ruminants, however, which possess a specialized, chambered stomach (rumen, reticulum, omasum, and abomasum for microbial fermentation), the hoatzin has an unusually large crop that is folded into two chambers, with a large, multichambered lower esophagus.

Serrations on the beak help cut leaves into smaller pieces before they are swallowed. Because they lack the teeth of mammals, hoatzins do not chew the cud; instead, a combination of muscular pressure and abrasion by a "cornified" lining of the crop is used as an equivalent to remastication, allowing fermentation and trituration to occur at the same site. The fermented foliage produces methane, which the bird expels through burping. Its stomach chamber and gizzard are much smaller than in other birds. Its crop is so large as to displace the flight muscles and keel of the sternum, much to the detriment of its flight capacity. The crop is supported by a thickened skin callus on the tip of the sternum, which helps the bird support the crop on a branch during rest and while digesting its food. A hoatzin's meal takes up to 45 hours to pass through its body. With a body weight as low as , the adult hoatzin is the smallest known animal with foregut fermentation (the lower limit for mammals is about ).

Because of aromatic compounds in the leaves they consume, and the bacterial fermentation required to digest them, the birds have a disagreeable, manure-like odor and are only hunted by humans for food in times of dire need; local people also call it the "stinkbird" because of it. Its preferred habitats of forests and inland wetlands are threatened by Amazonian deforestation. The hoatzin is believed to remain fairly common in a large part of its range, but its population is likely decreasing due to habitat loss. The hoatzin is the national bird of Guyana.

See also

  • The turaco, a convergently evolved bird in the order Musophagiformes, is a large-crested, arboreal, mainly herbivorous bird whose nestlings also use wing claws for climbing.

Notes

References

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  • Cornell Lab of Ornithology: Mystery Birds: Hoatzin Adults and Young. Retrieved 2008-JUN-16.
  • Hoatzin videos, photos & sounds on the Internet Bird Collection.