Birds
Hoatzin
Opisthocomus hoazin · also called stinkbird, reptile bird, canje pheasant
The hoatzin is one of the oddest birds in the world. It lives like a cow, smells like a barnyard, flies badly, and its chicks are born with claws on their wings and a habit of diving into rivers to escape danger. It is the only species in its entire family, and nobody is quite sure what it is related to.
Overview
Hoatzins live along the wooded margins of slow rivers, oxbow lakes, and swamp forest throughout the Amazon and Orinoco basins. They are sedentary and social, living in noisy groups of a dozen or so that clamber about in the streamside vegetation and rarely fly more than a short, laboured distance. They are most easily found by the smell and by their harsh calls, and by their striking appearance — a spiky rufous crest, bright blue bare facial skin, and red eyes.
A bird that ferments its food
The hoatzin is a folivore — it eats mostly leaves, a food few birds can use because leaves are low in energy and hard to digest. It solves the problem in a way otherwise seen only in mammals like cattle and kangaroos: foregut fermentation. Its crop and lower oesophagus are enormously enlarged and muscular, and lined with bacteria that break down leaf fibre over many hours before it even reaches the stomach. The trade-off is that the fermentation vat pushes the flight muscles aside (making the hoatzin a poor flyer) and produces the pungent, manure-like odour that gives the bird its nickname, “stinkbird.”
Clawed, diving chicks
Hoatzin nests are flimsy platforms of sticks on branches over water. The chicks hatch with two clawed fingers on each wing, which they use to climb through the branches. If a predator approaches, a chick drops into the water, swims — often submerged — to safety, and then climbs back up the vegetation to the nest using its wing claws and feet. Adults lose the claws as their wings develop for flight.
Reproduction
Hoatzins breed cooperatively: a dominant pair does most of the reproducing, while their grown offspring from previous years act as helpers, defending the territory and feeding the chicks (regurgitated fermented leaf paste). Breeding is often timed to the rainy season, when leaf growth is at its best.
Status
The hoatzin is Least Concern and common across its wide range. It is little hunted, because both the meat and the bird itself smell strongly unpleasant, and it tolerates disturbed and edge habitat reasonably well. Localised declines follow the clearing and draining of riverside and swamp forest, and the species is a popular subject for wildlife tourism in the Amazon and the Llanos, where its strange biology makes it a highlight.
Hoatzin: common questions
Why does the hoatzin smell so bad?
It is the only bird known to digest its food by foregut fermentation, the way cattle, sheep, and kangaroos do. Instead of grinding food in a gizzard, the hoatzin has a huge, muscular crop and enlarged lower oesophagus lined with bacteria that break down tough leaf fibre over many hours. The by-products of that fermentation are smelly compounds, so the bird — and its droppings — reek of fresh manure. Local people call it the stinkbird and generally do not hunt it because the meat tastes as bad as the bird smells.
Do baby hoatzins really have claws on their wings?
Yes. Hoatzin chicks hatch with two functional clawed digits on the leading edge of each wing, which they use to grip branches and haul themselves through the tangle of vegetation around the nest. The claws shrink and disappear as the bird matures and its wings become fully feathered for flight. This has made the hoatzin a favourite talking point in discussions of bird evolution and Archaeopteryx, though the wing claws are a modern adaptation, not a leftover from a dinosaur ancestor.
What do hoatzin chicks do when a predator comes near?
They dive. Hoatzins nest on branches overhanging water, and if a snake, monkey, or bird of prey threatens the nest, the chicks scramble out and drop straight into the river or lake below. They can swim, including underwater, and once the danger has passed they use their wing claws and feet to climb back up the vegetation to the nest. Adults will also splash and lunge to distract predators from the young.
Where does the hoatzin fit in the bird family tree?
Nowhere comfortably. Despite decades of study, the hoatzin's relationships are still debated — it has been linked at various times to cuckoos, turacos, gamebirds, and pigeons, and current genetic work places it on its own ancient branch with no close living relatives. It is the only member of its family and order. Its odd digestion, poor flight, and specialised body all reflect a lineage that has been evolving separately for a very long time.