A wheezy alarm call, like a smoker's cough, is the first clue. Then, as your dugout noses further into the Amazonian backwater, a flurry of wingbeats has you peering upwards. A big, chestnut-coloured bird lurches out of the tangle and flaps clumsily away, like some deranged, prehistoric pheasant.

Striking, perhaps, but not especially weird, you might think. Until you learn that this bird has claws on its wings and a stomach in its throat. What's more, it stinks.

This bizarre creature is widespread in swamp forest habitat across northern South America and the Amazon Basin as far south as Bolivia – and there's nowhere better to see one than Guyana, especially as it's the country's national bird. Guyana itself qualifies for the description "weird", since it is the only British fragment of the South American mainland, and the only place on the continent to host Test cricket.

Scientists once thought the hoatzin – pronounced "ho-AT-zin" – traced its lineage to the Jurassic. And with its stubby wings, and spiky crest, it is certainly a dead ringer for that precursor of all feathered fowl: Archaeopteryx.

The claws – two on each wing – are found only on the nestlings. Should a marauding snake or raptor approach the nest, the youngsters flop straight overboard and into the water below. Once the coast is clear they use these tiny grappling hooks to haul themselves out and clamber back up.

And the stomach? Hoatzins feed on arum leaves, and use a system called "foregut fermentation" to digest this cellulose-heavy diet. Put simply, the food breaks down in their enlarged crop before it passes to the real gut.

Ingenious, perhaps, but with drawbacks. First, the flight muscles have shrunk to make room for this supercrop – hence the feeble flying. Second, the bird reeks of manure.

Small wonder that scientists still puzzle over where the hoatzin belongs in the great avian filing cabinet. Placed, at various times, alongside game birds, turacos and pigeons, today it sits all alone: sole member of the sub-order Opisthocomi. Perhaps it's the smell.

Local guides will know where the hoatzins hang out; just take a small boat into a quiet backwater and listen for that smoker's cough.

* Naturetrek (01962 733 051; naturetrek.co.uk) operates an expert-escorted 17-day wildlife tour of Guyana, from £4,695 per person, including flights. Hoatzins feature among a range of exciting wildlife, including giant river otters and giant anteaters. The next available departure is 7 November 2011.

Mike Unwin is the author of 100 Bizarre Animals, published by Bradt Travel Guides (£16.99). Independent readers can purchase the book for only £11.50 including UK p&p via www.bradt-guides.com. Enter coupon code 'mudskipper' at the checkout.