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As far as birds go, the hoatzin is really doing its own thing. It's a picky eater, an evolutionary mystery, and its babies have a super bizarre adaptation that might not be as uncommon as you'd think.

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Sometimes, in researching things that seem weird, we find out by accident that stuff that we thought was totally normal is actually also weird.

So in this episode, I’m going to introduce you to a bird that is, don’t get me wrong, weird as heck. It’s bad at doing regular bird-stuff, like flying and having the kinda-useless babies that many other birds do.

And I knew a particularly odd fact about its babies, starting out. Or so I thought! Turns out, a lot of other birds, birds that I thought I knew, like, for example, ducks, are hiding a bizarre secret. [ ♪ INTRO].

Meet the hoatzin, a South American bird that lives in the swamps and flooded forests of the Amazon and Orinoco basins. As far as birds go, it’s really doing its own thing. It’s got wings, sure, but it’s not very good at flying because its breastbone doesn’t have a ton of space for the big flying muscles to attach, instead, that space is taken up by what’s basically the bird version of a cow’s stomach.

We’ll get back to that. It also has Bear Grylls nestlings. Your average baby songbird is toast if it falls out of the nest, but the hoatzin chick?

When they get freaked out, they intentionally dive out of the nest, which, by the way, is almost always built over water, they swim to safety, and then climb back into the nest when the threat is gone. I don’t look at this bird and think: swimmer. And, yes, I said they climb.

But there are other birds that do that, right? Like, woodpeckers hang out on vertical tree trunks and get around, no problem. They’ve got feet and toes that are specifically adapted for it.

Hoatzin nestlings don’t. Instead, they climb using the pair of claws they have on each wing. Wing claws!

Two of them, on the tips of their two longest digits. They eventually lose these claws once they get to be between 70 and 100 days old. But, here’s another weird thing.

I just got done telling you that hoatzin nestlings are bizarre because they have wing claws, but it turns out that a lot of other birds also have wing claws. Even birds that I tend to think of as pretty familiar, like geese and swans and CHICKENS, have at least a single claw on each wing. And yes, they are nightmare fuel, thanks for asking.

I don’t want an ostrich coming at me with one of those. So, okay, a whole bunch of supposedly regular birds are also very strange and that’s cool. But what does this all mean for the hoatzin?

Does it make it less bizarre? No, no it does not. The hoatzin has managed to do a thing that, actually, for real this time, no other bird has evolved to do.

It has become a foregut-fermenting folivore. That means leaves make up the majority of its diet and it has a highly specialized digestive system to deal with them. Here’s the thing about leaves, they tend to be pretty low in nutritional value, so you’ve got to eat a lot of them to get the calories and nutrients you need.

And they’re bulky, they take up a lot of space. These are the opposite of the things you want in your diet as a flying bird. Flying takes a lot of energy and it’s usually easier if you’re also lightweight.

The hoatzin has effectively traded off its ability to be a good flyer for the ability to be a good digester. Because, in order to break down the cellulose in all those leaves, the hoatzin has to ferment them in its crop, which is basically an enlarged section of its esophagus that forms a two-chambered pouch. This is called foregut fermentation because it’s happening in the front half of the digestive tract, and it’s the same thing ruminants like deer, cows, and goats do.

And the crop in the hoatzin is much larger than in other birds and is full of all kinds of microbes that do the work of fermenting those leaves for it. The product of all of that microbial fermentation is a delightful odor of manure. Yep, the hoatzin is also called the “stinkbird” for good reason: because it smells like fresh cow poop.

Awesome. Now, if you’ve made it this far and you’re wondering how this very weird bird evolved, well, so am I. And so is everyone.

The hoatzin is a bit of an evolutionary mystery. It’s not just the only living species in its genus and family, it’s the only living species in its entire order. Picture one lonely bird at the tip of a single long branch of the whole bird family tree.

Scientists don’t even agree about what it’s most closely related to. Some analyses have suggested fowl, like turkeys, quail, and chickens, are on the next branch over. But others have said it’s related to cuckoos and turacos, and still others have suggested shorebirds!

Or, maybe it’s the bird that’s mostly closely related to the group that includes all other landbirds. The problem is that birds seem to have diversified really fast after the Cretaceous-Paleogene extinction around 66 million years ago, and the hoatzin probably split from its closest relatives sometime during that radiation. That makes it hard to figure out these deep time relationships.

From its solo place in the bird family tree to its unique diet, the hoatzin is definitely a strange bird. And, while finding out that its wing claws actually aren’t that weird after all was pretty unexpected, that’s part of the beauty of Bizarre Beasts. Sometimes, the things that seem weird can push us into making new discoveries about the familiar things.

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And, as always, profits from the pin club go to support our community’s efforts to decrease maternal mortality in Sierra Leone. [ ♪ Outro ].