The pheasant-like hoatzin is the only bird species that ferments its food, an unusual digestive system that also makes it the stinkiest bird in the world.

The hoatzin is a very strange bird. Native to the backwaters of the Amazon, its chicks are born with clawed wings, it is the last surviving member of an ancient bird line believed to have branched off on its own around 64 million years ago, and its diet consists only of plants, which is very odd for birds. But this living oddity is most known for its strong manure-like smell, which has earned it the nickname ‘stinkbird’. The hoatzin is the only known bird species with a foregut fermentation system similar to the one found in cows, which releases significant amounts of methane via constant burping. The smelly burps and particularly stinky droppings give the hoatzin its signature odour.