Foregut fermentation is a mammalian speciality, used by cattle, sloths and colobus monkeys. The hoatzin evolved its own version. Its crop and lower oesophagus are enlarged into a muscular chamber holding a dense microbial community that breaks down cellulose, and food may sit there for many hours. Leaves make up the great majority of the diet.
The cost is mechanical. The fermentation chamber occupies space at the front of the body normally taken by flight musculature, and the sternal keel is reduced where the crop rests against it. Hoatzins fly heavily and briefly, and often crash-land into vegetation. The fermenting gut also gives the bird a manure-like smell, hence the name stinkbird.
Chicks add a second oddity: they have functional claws on two digits of each wing, used to climb back into the nest after dropping into water to escape a predator. The claws are lost as the bird matures.