I first heard about this bird in wildlife nutrition, the same course in which I did the mongoose presentation. Our professor told us a story about the hoatzin (pronounced kinda like Watson… you have no idea how many puns that created in my nutrition class). Hoatzins are quite beautiful birds from South America, and at some point (I have no idea when) someone decided to put some in a zoo. Now, with a lot of foreign wild animals (and especially a while ago, when I believe this happened), zookeepers don’t have a clear idea of what the animals eat in the wild. So what the keepers do is look for a similar animal species that we do know the diet for, and feed the animal that diet. For example, if you didn’t know what a species of deer ate, you might try and feed it a cow’s diet.

For birds, the go-to diet is usually chicken feed, a mixture of animal protein and grains. So when the hoatzins arrived at the zoo, that’s exactly what they were fed. Quite quickly the birds became very sick and stopped eating the food they were given. The zoo had no idea what was wrong with the birds, but someone noticed the sick animals were eating the decorative foliage in the exhibit. So the zookeepers put the hoatzins on a leafy diet and lo and behold they got better.

Hoatzins perching on a tree branch. Notice the enlarged neck (where its fermentation chamber is) and the striking coloration. Image by Cláudio Dias Timm from Rio Grande do Sul, CC BY-SA 2.0, via Wikimedia Commons

After a bit of research the zoo discovered why. Hoatzins have a digestive tract unlike that of any other bird. I’m going to give a quick anatomy lesson here on digestive tracts, so bear with me. There are lots of different systems of digesting food in the animal world, the one we are most familiar with being the monogastric human one. In this, there is one stomach filled with acid that breaks up food before it enters the small intestine for further digestion. Most nutrients are absorbed in the latter half of the small intestine, while the large intestine is where most of the water is reabsorbed. This system is quite efficient but lacks a major component of more complex digestive tracts: it can’t digest fibre. In fact, no animals can digest fibre.

Fibre is found in plants and is a key energy source for all herbivores. So how do they get energy from it if they are unable to digest it? The answer lies with bacteria. Bacteria live everywhere in our bodies, but certain animals have developed special relationships with particular bacteria that can digest fibre. There are two systems for doing this: foregut fermentation and hindgut fermentation. Ruminants are examples of foregut fermentors. They have a system of three forestomachs and one true stomach that allows bacteria to successfully digest fibre, which is then sent to the small intestine where all the released nutrients can be absorbed. Examples of foregut fermentors are cows, sheep and goats.

Hindgut fermentors are a bit different. The bacteria are prominent in the large intestine, so all the digestion takes place here. This presents a problem for hindgut fermentors, because of course most absorption of nutrients takes place in the small intestine, which the food has already passed through. It isn’t such a problem though, because the bacteria turns the fibre into volatile fatty acids, which are so small and light that they are easily taken up in the large intestine. Hindgut fermentors include horses, guinea pigs and rabbits.

A comparison of the digestive tracts of six different animals. From top left to bottom right: a human, with a simple monogastric tract; a sheep with a complex foregut used for fermentation; a horse with a simple foregut but a greatly enlarged hindgut; a chicken, the crop is the topmost chamber, in most birds simply used for storage; a rhea, with an enlarged hindgut; and the hoatzin, with a very large crop where leaves are fermented. From: Stevens and Hume 1995.

Alright, I think that’s enough anatomy for now. Hopefully it wasn’t too painful. Let’s go back to the hoatzin. The hoatzin is unique among birds in that it is the only foregut fermenting bird in the world. There are a few hindgut fermentors (ostriches, rheas and emus for example), but no foregut fermentors. The thing about fermentation is it takes time and it takes space. That’s why those birds that are hindgut fermentors are large and flightless. So what about the hoatzin? Its not quite flightless, but it certainly has a difficult time of it. The chamber in which fermentation occurs is so large in the hoatzin that there’s hardly any room for flight muscles, so the best this bird can do is flutter clumsily from tree to tree. It even has trouble balancing on branches, and uses a special leathery bump on the bottom of its crop (the fermentation chamber) to help it do so.

The peculiarities of the hoatzin don’t end there, either. The products of fermentation are so smelly that another name for the hoatzin is the stinkbird. And even more interesting is that hoatzin chicks are born with two claws attached to their wing joints, which is another unique feature of these birds. They use these to escape predators, clawing their way up trees while their parents make a racket and try and distract the predator. If all else fails, the baby hoatzins have a backup plan. Hoatzins build their nests over streams and flooded areas of forest, so if the chicks can’t escape the predator by climbing, they simply let go of the tree and drop into the water below. There they claw their way along the bottom of the water until it’s safe to climb back to the nest.

A young hoatzin climbing a tree branch. Notice the two claws on the tip of its outstretched wing. Image by J. Arthur Thomson., Public domain, via Wikimedia Commons

The hoatzin is an enigma to scientists; the debate about its relationship to other birds is ongoing. Despite its clumsiness and sedentary ways, the hoatzin manages to survive, and indeed does quite well in its jungle home. I just wish I could be there when baby hoatzins leap in droves from their trees to the safety of the water (that’s how I picture it happening, anyway). Even if that isn’t the case, I still think hoatzins are pretty awesome birds. Also sorry about the lengthly digestion lesson, though I hope you learned something! If you’re interested in learning more about animal digestion check out this site:


Its pretty thorough, and you can look up individual species by clicking on the heading (birds, reptiles, mammals, etc) and it will bring you to a species list. There are some pretty interesting ones (kangaroos and guinea pigs, for example), so enjoy!

Cover image by Kate from UK, CC BY-SA 2.0, via Wikimedia Commons