The punkish hoatzin is a misfit that ripped up the evolutionary rule book.
Hoatzins digest their food very, very slowly. A meal takes up to 45 hours to pass from bill to cloaca. This is why these birds loaf around for up to 80 per cent of the time – they are effectively chewing the cud.
There is a downside to having a supersized crop, however. Hoatzins only have enough space left inside their bodies for a simple, reduced sternum (breast bone) and puny flight muscles.
Small wonder then that they are such weak flyers. I have seen birds so engorged that they can’t take off and simply sit for hours, beaks gaping and wings drooping, until they have at last processed their meal.
Besides their reputed perfume, hoatzins are endowed with another typically bovine characteristic: they are highly social. The birds form family groups (or should that be herds?) of up to a dozen or so individuals. Flocks of 40 or more birds have even been recorded.
An entire family may engage in the hoatzin equivalent of cattle lowing – a cacophony of grunts, squawks and hisses, often delivered in unison. Usually the birds are set off by one particularly enthusiastic individual, who leads the rest of the ensemble in a bizarre chorus.
Very much creatures of habit, hoatzins are almost always to be found feeding in trees or shrubs, but they can still spring surprises. One of the entries in last year’s BBC Wildlife camera-trap photo competition revealed a group feeding on the ground in a dry riverbed – unusual behaviour possibly explained by the presence of fallen fruit.
The flesh of stink birds is reputed to be disgusting, so they are rarely hunted – the chief threats to the species are deforestation and disturbance, including from uncontrolled tourism.
Researchers who placed microphones in hoatzin nests at Cuyabeno in Ecuador discovered that, while the owners did not flee straight away, their heart rates and stress levels soared. Repeated visits from tourists may therefore adversely affect hoatzin populations near lodges and camps.
In contrast to jaguars and macaws, hoatzins are somewhat unconventional icons of the Amazon. But what about their smell? For the record: I couldn’t detect any (maybe I wasn’t quite close enough).
Rest assured, though, that this avian enigma will continue to defy the normal rules of the natural world.
HOOK, LINE AND SINKER: The hoatzin escape act
Hoatzin chicks have a pair of small claws on the bend of each wing, which are lost after three months. The adaptation is thought to have evolved as a predator-avoidance strategy.
A young hoatzin has a trick up its sleeve – it fools its foes by performing a wacky version of the triathlon. First, it leaps out of its nest into the water below.
Next, it flaps underwater to a safe, secluded spot further along the bank. Though scrawny-looking, hoatzin chicks are quite capable swimmers.
Finally, when the coast is clear, the nestling emerges triumphantly from the water and hauls itself up a branch using its wing claws as grappling irons.
Did you know?
Hoatzins don’t do well in zoos. The first birds to be taken abroad were brought to London Zoo in 1931, but died soon after.
More recently, Bronx Zoo housed a small group in the 1990s, hatching the first ever chicks in captivity. Sadly, all of them eventually died.
To watch a film showing how a hoatzin chick uses its wing claws to escape danger, click here
To read more from BBC Wildlife about the archaeopteryx and other discoveries, click here.