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.