I’m a bit under the weather this week, so here’s a Patreon episode about a weird bird!

Further reading:

Hoatzin nestling locomotion: acquisition of quadrupedal limb coordination in birds

Show transcript:

Welcome to the Patreon bonus episode of Strange Animals Podcast for mid-November, 2019!

We’re going to learn about a mystery bird today. When I say mystery bird, I don’t mean that people aren’t sure if it exists. It definitely exists. You can go to South America and look at it if you like, because fortunately it’s not rare or endangered. But scientists aren’t completely sure what it’s related to, because it’s a really weird bird.

The hoatzin [pronounced what-seen] is a large bird, over two feet long, or 65 cm. It’s shaped sort of like a pheasant, with a chunky body, long neck and small head, and a long tail made of stiff feathers like a hawk’s. Its face has no feathers and blue skin, it has red eyes, and it has a spiky feather crest on its head. It’s black and chestnut brown with some darker and lighter streaks, and is a softer brown underneath. It’s a really pretty bird, in fact, with a strong bill. But it really doesn’t resemble any other bird alive today.

The hoatzin is the only species in its genus, and the only genus in its family, and the only family in its order. It’s basically not really related to any other bird alive today, although in 2012 its genome was sequenced and found to be most closely related to cranes and plovers—but only very distantly. In fact, a 2015 study determined that the hoatzin started evolving separately from other birds 65 million years ago, right after the Cretaceous-Paleogene extinction event that killed off the non-avian dinosaurs.

We only have a few fossils of hoatzin ancestors, but they show that it was much more widespread in the past and lived in what is now North America and Europe. But these days it only survives in northern and central South America. It likes swampy areas and forests near rivers or other water.

The hoatzin eats plants—specifically leaves and buds, although it also eats some flowers and fruit. And leaves require a lot of digesting before the body can make use of the nutrients. The hoatzin’s digestive system is unlike any other living bird’s, because the hoatzin is a foregut fermenter. Its crop, which most birds only use to store extra food temporarily when the stomach is full, acts as a bacterial fermentation chamber—two chambers, in fact, since it’s divided into two sections. This acts like the rumen of a cow. Its crop is so big it doesn’t have room on its body for big flight muscles, so it’s not a strong flyer. It mostly stays in trees and bushes, eating leaves, flapping its big wings for balance and display, and hanging out with other hoatzins.

The hoatzin’s digestive system has a weird side effect. It smells bad. It’s supposed to smell like manure. It’s sometimes called the stinkbird and, fortunately for the hoatzin, almost no one wants to eat it as a result.

As you probably know, birds developed from dinosaurs. It’s easy to forget that, since birds have evolved structures like toothless beaks and front legs modified for flight and they no longer have lizard-like tails. But the hoatzin retains something from its dinosaur ancestry that is a startling reminder.

The hoatzin is a social bird that lives in small flocks. It breeds during the local rainy season and builds its nest over water when the forest floods due to rain. The female lays two or three eggs, and when the babies hatch, they can climb around in the branches near the nest right away. This means they can hide from predators instead of being helpless in the nest. And the reason a hoatzin chick can climb so well is partly because it has big feet, and partly because it has finger claws on its wings: specifically a thumb claw and one finger claw, which are fully functional and make it look a lot like a fuzzy baby dinosaur.

Not only does the baby hoatzin use these claws for climbing, a study published in May 2019 shows that the baby hoatzin uses its wings differently when climbing than it does as an adult bird. Obviously, birds fly by flapping both wings at the same time. But the baby hoatzin climbs by using its limbs in an alternating motion. You know, the way you would climb a tree. Or the way a small dinosaur would climb a tree.

But primitive and dinosaur-like as this trait is, researchers have discovered that it developed relatively recently. That is, as the hoatzin’s distant ancestors evolved from a small dinosaur into a primitive bird, it lost the claws on its front legs as they became more and more modified into wings. But at some point, the hoatzin re-developed those claws. Researchers think it’s what is called an atavistic trait, which you may remember from way back in the Patreon episode where we talked about horses with extra toes. In other words, the genes to grow claws on the front limbs are still present in birds, but are suppressed by other genes, since claws just get in the way when you’re flying. But occasionally a small mutation causes the claws to grow anyway, and in the case of the hoatzin, it proved so useful that those babies with claws survived better than those without claws, and therefore lived to pass on their genes. But the claws are no longer useful once the babies grow up and learn how to fly, so they lose them as adults.

Hoatzin chicks climb using alternating motions of the wings, but swim by moving both wings together. Oh, didn’t I mention that the babies swim? They have to, because sometimes a predator attacks and they have to get away fast. They can’t fly yet, and they can’t climb all that quickly, so they drop out of the branches and fall into the water below. That’s why the parent birds build the nests over water. The babies can swim just fine, and they swim to safety and climb back up into the branches where their parents can find them.

The hoatzin isn’t the only bird that has wing claws as a baby. Some species of turaco do too. The turaco lives in Africa and shares many traits with the hoatzin, so for a long time people thought the two were related. But now we know they’re not and that the similarities are due to convergent evolution.

Thanks for your support, and thanks for listening!