The punkish hoatzin is a misfit that ripped up the evolutionary rule book.
James Parry follows his nose to find the pungent, leaf-munching hoatzin, the Amazon’s evolutionary throwback that thinks it’s a cow.
Paddling along a tributary of Ecuador’s Napo River shortly after dawn, I felt closer to the rainforest than on any previous visit. A tangled mesh of vegetation cascaded down from the canopy above and trailed in the water next to my canoe.
Iridescent blue morpho butterflies the size of dinner plates sailed past, almost within touching distance, and squirrel monkeys cavorted in the waterside trees.
I thought I heard the yelp of a giant otter, while closer to hand the siren-like call of a screaming piha – surely the noisiest rainforest bird – reverberated through the understorey. All stirring stuff, but something was missing: the ‘stink birds’.
These zoological curiosities were what had lured me here. I didn’t want to leave without seeing – or smelling – them.
A gang of stink birds, or hoatzins, was here the previous day, but they had obviously moved on. Then I realised why: the tree in which they had been spotted was stripped bare.
Uniquely among birds, hoatzins are folivores, or leaf-eaters. They’d simply scoffed the lot and gone to find more.
Hoatzins are often said to reek of fresh cow manure or sweet-smelling hay, a by-product of their unconventional diet.
While wondering if the claimed avian pong had any basis in fact or belonged in the realm of folklore, I rounded a bend in the river and there they were: half a dozen geeky-looking birds draped over a shrub, busy tucking in.
A couple of the group flapped about clumsily to find a new sprig on which to gorge themselves; one wasn’t eating at all, instead perching almost motionless in a shaft of early morning sunshine. That’s another thing about Opisthocomus hoazin – nothing is ever done in a hurry.
Beauty in the eye of the beholder
By no stretch of the imagination could hoatzins be described as beautiful. They have a few assorted handsome features, to be sure, but the sum total of these parts almost borders on the grotesque.
Yet I’ve always found a quirky charm in the species’ eclectic, thrown-together ‘thrift-shop chic’ appearance.
A funky mohican crest and neon-blue facial skin surrounding a beady red eye are accessorised by dramatic, cape-like wings and an extravagant fan-shaped tail – used to keep balance when scrambling around in vegetation. The black, russet and cream plumage has a hint of the Georgian gentleman.
But the elegance stops there. Hoatzins are awkward birds. Their flight is laboured, with many a comical crash-landing, and their flouncing gait probably gave rise to one of their local Brazilian names: cigana, meaning gypsy.
Hoatzins have intrigued and befuddled scientists ever since they were first described by German zoologist Statius Müller in 1776. Taxonomists later spent decades bickering over their origins, originally lumping these oddities with pheasants, then moving them around the avian family tree from pigeons to cuckoos and rails to turacos.
Today, they have their own family, the Opisthocomidae (from the Greek for ‘those with long hair behind’).
Hoatzins split from other bird groups a very long time ago, though precisely how and when remains unclear since only one fossil hoatzin has ever been found.
The species has been seen as living evidence of the transition between reptiles and birds – the wing claws of young hoatzins are frequently cited as proof of ancient ancestry. But the trait probably evolved separately in hoatzins, rather than being inherited directly from Archaeopteryx and other early birds.
Whatever the truth about the hoatzins’ past, their diet makes them unique. They are the only birds known to possess a foregut fermentation system.
This highly specialised arrangement equips them to process the huge quantity of foliage needed to provide enough energy (leaves are low in nutrients).
The hoatzins’ oesophagus and enlarged crop serve as fermentation chambers. Inside are anaerobic bacteria that secrete enzymes able to break down the otherwise indigestible cellulose present in plant tissue.
The birds ‘chew’ leaves before swallowing, and ridges inside their crops help to break down the leaf bulk further so that it can be processed more easily. In digestive matters, hoatzins have more in common with cattle and sheep than with their feathered relatives.
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.