Not many of us have probably got close enough to a bird to smell it, and although some species secrete oils to keep their feathers healthy which can have a fairly strong odour, in general birds keep themselves pretty clean and are normally odourless.
However, one bird has a particularly noxious odour, so much so that it has been nicknamed the “stink bird’ or “stinking pheasant”.
That bird is the hoatzin (Opisthocomus hoazin) and, according to the Guinness World Records, it’s the smelliest bird in the world.
It is a tropical bird found in the swamps, mangroves, and rainforests of the Amazon and Orinoco basins in South America. It is the only living member of its family, the Opisthocomidae, and the only one in the order Opisthocomiformes. The names of both the order and the family are derived from an Ancient Greek phrase meaning “long hair behind”, which refers to the large crest on the bird’s head. Genetic research has shown that the family branched off from the main bird line about 64 million years ago, shortly after the extinction of the non-avian dinosaurs, and has evolved independently since.
(In case you’re wondering, Hoatzin is pronounced HWAT-sin.)
The hoatzin is about the size of a chicken with a small head and long neck. The upperparts are buff with dark streaks on the back and nape, and sooty-brown edges on the wing coverts. The underparts are buff except for the undertail coverts and flanks which are chestnut-brown, and the tail is sooty-brown with a buff tip. The bare head is covered with scaley blue skin, and there is a spiky chestnut-coloured crest on its crown. The eyes are maroon, and the hooked bill is grey.
The hoatzin has a very specialised diet. It is the only known true avian herbivore subsisting on a diet of mainly leaves during the rainy season which they subsidise with fruits, flowers, and buds at the end of the dry season when trees have few leaves. In fact, because leaves make up over 80% of their diet they are sometimes described as folivores.
Although some other birds are herbivores in adulthood such as many large waterfowl, they feed their chicks with insects and other animal matter, so biologically they are omnivores, in the same way that a panda is an omnivore despite only eating bamboo. Hoatzin chicks are fed regurgitated food from the adults for up to two months after which they can feed independently.
During the day, the hoatzin tends to forage just 50 m from the watery environment which it inhabits, but at night will travel up to 300 m to find food.
It is this unusual diet that has caused the hoatzin to evolve a unique way of digesting food. All other birds have a stomach with a single chamber and are what are known as hindgut fermenters, where cellulose is digested with the help of symbiotic bacteria after it has passed through the stomach. Food is processed quickly and can travel through them in just a few minutes.
The hoatzin, however, is a foregut fermenter and it uses bacterial fermentation in the front part of its gut to break down the food it consumes. Other examples of foregut fermenters include cattle, deer, sheep, camels, hippos, kangaroos, and leaf-eating monkeys and sloths.
Unlike ruminants though, the hoatzin doesn’t have a second stomach – the rumen – where fermentation takes place, but instead has a very large crop divided into two chambers, a large lower oesophagus with multiple chambers, and a much smaller stomach and gizzard than other species of birds.
Vegetable matter that the hoatzin has eaten is stored in the crop and then fermented with the aid of symbiotic bacteria which may also help detoxify any poisonous plants it has eaten.
Although foregut fermentation is more efficient, it takes much longer. Hoatzins can take up to 18 hours to digest liquids and up to 48 hours to digest solids, which is similar to the length of time sheep take to process their food.
The chemicals including methane produced as bacteria break down its food, coupled with the aromatic compounds in the leaves it has eaten means the hoatzin has a very unpleasant odour that smells a bit like manure.
The terrible smell and apparently equally bad taste means that hoatzins are only rarely hunted by the indigenous people of the Amazon when food is very scarce.
It is not fully understood why hoatzins use foregut fermentation to process their food. It is one of the smallest endotherms to use this form of digestion with a lower metabolic rate than is predicted for its size.
However, leaves don’t contain much nutritional value and they are tough to digest. Animals who subsist on a diet of purely plant matter need to eat a lot of it and it is the sugars and short-chain fatty acids produced by microbes during the digestion of cellulose that is the main source of energy for ruminants.
But access to highly fermentable food sources, increased efficiency of digestion, the ability to eat and extract value from toxic plants, and the nutritional value that comes from fermentation may all have influenced the evolution of the hoatzin’s digestive system.
However, this unique characteristic comes at a cost. The outsized crop means that the anterior sternum is much smaller leaving less room for flight muscles which are also relatively small compared to similarly sized birds. This means hoatzins are poor fliers and young birds don’t start flying until they are about 65 days old.
Instead, hoatzins tend to move about by climbing, albeit rather clumsily, and will only do so when they feel threatened. Mostly, they stay fairly still which also helps the long digestion process to take place.
To aid climbing, hoatzins are born with another feature unique to birds. Chicks have functional claws on their wings, which they use to help them scramble up trees, before dropping into water and swimming away to escape predators. Hoatzins crawl by alternating the movements of their front and rear limbs – the wings and legs – on opposite sides of the body just like lizards.
Although other species of birds such as ostriches, emus, and some ducks and swans have wing claws, they do not use them.
Research suggests that hoatzins did not retain their wing claws as they evolved from dinosaurs, but lost them and regained them through atavism where an ancestral genetic trait reappears either as a mutation or because they become useful again.
Despite its strange appearance and unique place in the evolution of birds, the lack of hunting and inaccessible, watery habitat which is no good for agriculture means the population remains stable and it is classified as Least Concern on the ICUN Red List.
However, eco-tourism may be having an impact on the likelihood of chicks making it to adulthood. The disturbance from tourists can cause the chicks to be stressed making them lose their appetite which in turns means they have a lower body mass and are sometimes too weak to escape from predators.
A scientific study conducted in 2003 showed that although hatching success was similar in areas visited by tourists to those that weren’t, chick survival was much lower than at undisturbed nests.
