Hoatzin: the strangest bird in the world has bad breath

Okay, the hoatzin may not have won the title of “world’s strangest bird” in a championship, but it should be in the final three, despite its halitosis.

The hoatzin (Opisthocomus hoazin) inhabits swamps and rivers of the Orinoco-Amazon region, and accumulates unique and surprising characteristics, both for observers and for scientists. The hoatzin is a real headache for experts in avian phylogeny. Until now, it has not been possible to accurately determine their kinship relationships with other birds.

The hoatzin (Opisthocomus hoazin).
Rodrigo Mariaca. CC BY-SA 4.0, Author provided

The hoatzin is a very unique looking bird. It is the size of a pheasant, (60 – 89 cm the adult male, 50 – 63 cm the adult female). Its varied colors, its long tail, a rough crest and a blue face, in which two red eyes stand out, offer a visual spectacle.

His diet is no less striking. The hoatzin feeds on young leaves and shoots. This is not exceptional, but it does ferment its food in the crop before entering it in the stomach. The anaerobic bacteria in the crop digest the cellulose and transform it into carbohydrates. This is just what cows and other ruminants do, which also ferment their food in compartments of the esophagus.

The “stinking turkey”

The hoatzin’s crop occupies a large volume of its chest cavity. This hypertrophy is done at the expense of the large sternum characteristic of birds. His is small, with a greatly reduced keel, and is fused to other bones of the pectoral girdle. The sternal keel is essential for the attachment of the pectoral muscles that power the wings, allowing a bird to fly.

file 20220825 20 syugpm.JPG?ixlib=rb 1.1
Relative sizes of sternum and crop in birds in general (top) and in the hoatzin (bottom)
Author provided

A large crop like that of the hoatzin, with the consequent reduction of the sternum, greatly impairs its flying abilities, which are very scarce, and gives it another unpleasant property, permanent halitosis. This bird’s breath stinks! It is not in vain that it is known in Colombia as “stinky turkey”.

Brothers take care of each other

Hoatzin reproduction is also peculiar, as they form cooperative colonies to raise their chicks. This is also done by some species of American cuckoos, which lay eggs in communal nests, but it is very rare.

The reproductive colony is made up of a couple and up to six “helpers” that are usually derived from previous clutches of the couple. The helpers collaborate in the incubation, the feeding of the chickens (their little brothers) with the fermented product of their crops and the defense of the colony. The females tend to stay less time than the males in the “family”, since they leave soon to establish another colony.

Claws like those of Archeopteryx

Hoatzin nests are located above water for an important reason. If the chicks are threatened, they jump from the nest into the water, swim with great skill and climb trees again thanks to the claws on their wings. In birds in general these claws were lost in the course of evolution, but hoatzin chicks keep them on the end of two well-developed toes. This toe pattern in young hoatzines is reminiscent of that of Archeopteryxan extinct genus of primitive birds, with characters intermediate between feathered dinosaurs and modern birds.

file 20220825 25 5qh8yi.JPG?ixlib=rb 1.1
Skeleton of the wing of the young hoatzin (left). Toes 1 and 2 are well developed and have claws at their ends. This arrangement can be compared to that typical of birds (above right) and to that of Archeopteryx (right, bottom)
image taken from Parker, 1891, Author provided

The mechanism they use to climb is typically quadrupedal, which is also exceptional in birds. In fact, in birds there is a decoupling of locomotor coordination so that the wings move synchronously while the legs act independently. Hoatzin chickens, on the other hand, move their four limbs in a coordinated manner, like quadrupeds.

file 20220829 17 i60s6d.jpg?ixlib=rb 1.1
Movements of the young hoatzin climbing up the trunk of a tree. The coordination pattern of the forelimbs and hindlimbs is typically quadrupedal.
Abourachid et al. (2019) CC BY-SA 4.0, Author provided

All of these features support the hoatzin’s candidacy for the world’s strangest bird. But there is more, especially its enigmatic position in the phylogeny of birds.

The great unknown: the relatives of the hoatzin

There are strikingly contradictory studies about the relationship between the order to which this species belongs (Opisthocomidae) and the other orders of birds.

Based on studies, especially molecular ones, the hoatzin has been related, among other groups, to nightjars and to waders, plovers, and the like. A recently published study acknowledges the extreme difficulty of placing the hoatzin in the phylogeny of birds, as different types of genomic data result in different hypotheses of relatedness.

The study tentatively proposes proximity to waders and cranes, but with many doubts. What seems more or less clear is that it is a very old lineage, with possible origin in the Paleocene, at the beginning of the Tertiary era, about 60 million years ago.

The fossil record supports this, as fossils similar to the hoatzin (Onychopteryx) in the Lower Eocene of South America (about 50 million years before present).

What we can rule out today is the idea that circulated at the end of the 19th century that the hoatzin was nothing less than the “missing link” between Archeopteryx and modern birds. In any case, the enigmas of the hoaztin remain at the forefront of scientific debate. Of course, it has my vote as the strangest bird in the world.

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