Following their tight victory in our recent poll, we went and searched for the best possible avian candidate to feature here on Life Gone Wild. We think we’ve outdone ourselves if we do say so ourselves!
The Hoatzin is a unique denizen of the tropical rainforests of the Amazon in South America. They are fairly widespread across northern and central South America within their rainforest homes, and you can find them fairly frequently beside bodies of water (particularly oxbow lakes and rivers). They spend most of their lives living in the riparian (riverside) vegetation. In fact, they nest in trees and branches that hang over the water! The Hoatzin is about the size of a turkey, with a large rufous coloured crest on top of its head, and blue scaly skin around its large red eye (Hoyo et al 1996). So they look… pretty strange. Strap yourselves in, and hold onto your hats because we’re only just getting started! Physically, behaviourally, and even historically, the Hoatzin is pretty damn weird. Let’s find out what this means!
The Hoatzin is very particular about what it eats. So particular in fact, that it is one of the worlds only primarily herbivorous birds. They favour young leaves, buds, and fruits and very rarely have they been seen to eat anything else. They rarely forage more than 50m from the watercourse that they call home during the day, but at night they have been known to increase that to closer to 300m (Hoyo et al 1996). This rare avian diet has led to evolution doing some pretty peculiar stuff to the Hoatzin.
The digestive process of the Hoatzin is entirely unique to birds. Quite a few animals around the world have what is known as a hindgut, where cellulose is digested thanks to symbiotic bacteria after passing through the stomach. The Hoatzin has a stomach that is actually more similar to cows, kangaroos, and sloths. They have an enlarged crop (more or less the bottom of the oesophagus) that stores herbaceous material that is has fed on (Hoyo et al 1996). It is then fermented with the aid of almost Hoatzin-specific symbiotic bacteria (potentially aiding to detoxify some plant species in the process). Fermentation in a foregut digestive system takes a long time to break everything down. This, in turn, has consequences too!
- In the majority of bird species, food passes through them in only a matter of minutes. However, because of the Hoatzin’s diet and specialised digestive system, digestion takes just a liiiiiittle bit longer… in fact, they hold the record for the longest digestive period of any bird species(Hoyo et al 1996). In experiments, the Hoatzin retained liquids for 18 hours and solids for an incredible 24-48 hours(Hoyo et al 1996)! These sorts of times are similar to those of sheep. This long period of time helps to cultivate the special bacteria populations within the Hoatzin’s guts that allow it to break down foods successfully.
- So fermentation. The breaking down of substances using bacteria. Inside the Hoatzin for up to 48 hours. Can you see where we’re going with this? The process of fermentation gives off plenty of pretty damn smelly chemicals as bacteria breaks down the material. This chemical reaction is going on within Hoatzin’s almost permanently and has led to them receiving a very particular nickname. In parts of their range (particularly in Venezuela and Guyana), the Hoatzin is also known as ‘the stinkbird’ or the ‘stink pheasant’ because they smell like, well frankly, s**t.
The enlarged crop within the Hoatzin means that the bird has had to make some sacrifices internally to properly digest all its food, such as a much smaller stomach and gizzard compared to other birds. But that’s not all! The crop is so large that other parts of the bird have had to give way as well. The breast muscles (aka the muscles birds use to fly) of the Hoatzin are much smaller comparatively than those of other birds. This means that flying is a difficult prospect for the Hoatzin, as it takes MUCH more work. In fact, anything over about 300m is a real struggle for these birds (Hoyo et al 1996). Because of the amount of effort that has to go into flying, they prefer to move around by other means. Instead, the Hoatzin prefers climbing, utilising any part of its body it can to move through the trees. However, even climbing is still done clumsily. When disturbed they’ll ‘quickly’ climb higher into the trees before stopping, and look back, watching the offender suspiciously (Hoyo et al 1996). Because moving around is just a real slog for the Hoatzin, coupled with slow digestion to allow everything to ferment properly, Hoatzin stay pretty still. Think something like the bird version of a sloth (though by sloth standards the Hoatzin must look like a very active bird!)
The Hoatzin spend a lot of their time resting in trees, giving their body time to digest. Because they are such relatively inactive birds, the Hoatzin has become a highly social bird. Sometimes Hoatzin’s will gather in groups of nearly 50 birds within a small area (Hoyo et al 1996). During the breeding season, they get a bit more territorial. This tends to drop down to rarely more than 5 individuals, who are generally all of the same family. Speaking of Hoatzin breeding, let’s discuss their ugly-cute little babies.
Hoatzin chicks are small, blue-grey skinned little things with two little claws on their wings. They use these to help clamber around before they are able to fly (which takes around 50-60 days, and only short distances at that stage) (Hoyo et al 1996). The claws drop off when the chicks are 70-100 days old, but it’s a behaviour that the claws allow that is particularly interesting! When threatened, from as young as two or three days old, chicks will drop from the nest into the water below (a height of between 2-5m) and then swim for their lives until they reach low-hanging twigs that they can grasp onto and use to clamber to safety. While the chicks swim, adults climb higher into the trees, attracting the attention of whatever predator may be threatening them (Hoyo et al 1996, BBC 2018). It’s just such a bizarre behavioural adaptation, but one that clearly works for them! Chicks can swim for up to 6m, but you’ll very rarely ever see an adult Hoatzin in or by the water (even to drink! 70% of their food is made up of water in the plants they eat) (Hoyo et al 1996). They try to stay well clear. For an adult Hoatzin, the water poses much greater dangers than life in the trees!
Let’s backtrack for a moment. You may have noticed that we mentioned that Hoatzin chicks have claws on their wings. While wing-claws in baby birds are not unique to the Hoatzin, the uncertainty around the evolutionary history of the Hoatzin has led some to believe that these birds are direct descendants of the very first bird, Archaeopteryx. Archaeopteryx had three claws on their wings, which is what gave rise to this theory. However, it is now thought that the claws of the Hoatzin are a more recent adaptation due to the amount that chicks have to climb around before they can fly (they normally leave the next within 3 weeks, but as mentioned earlier can’t fly for up to 60 days!) (Hoyo et al 1996).
So what is their closest relative though? To be blunt, nobody is entirely sure. The Hoatzin is an entirely unique bird, with no other species particularly close relatives. While they are generally unsure, some fossil finds in the last 20 years have perhaps provided some insight.
Fossilised remains of other species related to the Hoatzin have been discovered in Colombia, Brazil, Argentina, and North America in the past (Naish 2011). In 2003 a skeleton was uncovered of a bird labelled Namibiavis senutae, but it was only in 2011 that it was recognised as having distinctly Hoatzin-like features. As you may have guessed from the name, this bird was not found in South America, but in Namibia! This makes N.senutae the first fossil linked to the Hoatzin to be found outside of South America. The leading theories currently suggest that Hoatzin ancestors must have travelled across the Atlantic Ocean millions of years ago. Given their weak flying ability, it is thought that the most likely method for this dispersal is on a vegetation raft (similar to how it is thought lemurs first made it to Madagascar!). It is very rare that species made it from South America to Africa across the Atlantic (despite the oldest fossil records for Hoatzin relatives currently being found in South America). As a result, it is now thought that the Hoatzin must originate from Africa, opening up new possibilities as to their closest living relatives (leading candidates are the Turacos). The Hoatzin is just such an enigma!
The Hoatzin, though, has managed to be fairly lucky on the conservation front. Thanks to their reputation as the stinkbird, they are not regarded as being particularly good eating. That, and that the smell has been known to make it into the meat… As a result, they have not been hunted unless under desperate circumstances. Additionally, their riverside habitat is generally a difficult one to be recruited for agriculture as it is just too close to the water's edge (Billerman 2012). This has allowed for the Hoatzin to be listed as ‘Least Concern’ on the IUCN Red List (Birdlife International 2016). However, this doesn’t mean that these bizarre birds aren’t being impacted by humans. Destruction of suitable habitat is threatening their numbers, as it is all animals over the globe. Eco-tourism is also hurting Hoatzin chicks. The disturbances that come with eco-tourism stress out the young chicks, making them less likely to eat and giving them a lower body mass. This makes it much harder for young chicks to develop, and the accompanying weakness makes them easier targets for predators. Areas less disturbed by tourists had a higher rate of juvenile survival (Müllner et al. 2004).
Hoatzin are a truly bizarre animal. These unique, stinky birds are unlike anything else on the planet. They are a magnificent example of the wonders of evolution, and a reminder of how weird some of the inhabitants of our planet are. If you think you know of a stranger bird, get in contact and let us know. Here at Life Gone Wild, we can’t even imagine what that might be like. The Hoatzin is one of a kind, and truly a remarkable bird.
Billerman, S. (2012). Hoatzin (Opisthocomus hoazin), version 1.0. In Neotropical Birds Online (T. S. Schulenberg, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA.
BirdLife International. 2016. Opisthocomus hoazin. The IUCN Red List of Threatened Species 2016: e.T22684428A93028795
"Hoatzin - Natural History, Best Places To See And Photograph". 2018. Peru Rainforest Expeditions. http://www.perunature.com/the_wildlife/tambopata-wildlife-hoatzin-bird-html/.
Hoyo, Josep del, Andrew Elliott, Jordi Sargatal, and Jose Cabot. 1996. Handbook Of The Birds Of The World: Hoatzin To Auks. 1st ed. Barcelona: Lynx Editions.
Müllner, A., K.E. Linsenmair, and M. Wikelski. 2004. Exposure to ecotourism reduces survival and affects stress response in hoatzin chicks (Opisthocomus hoazin). Biological Conservation 118: 549-558.
Naish, Darren. 2011. "Hoatzins Are No Longer Exclusively South American And Once Crossed An Ocean". Blog. Scientific American. https://blogs.scientificamerican.com/tetrapod-zoology/hoatzins-in-africa/.
"The Hoatzin as a bird that resembles a reptile". 2018. BBC. http://www.bbc.com/earth/story/20170518-the-hoatzin-is-a-bird-that-resembles-a-reptile.