Thanks to Skillshare for supporting this episode, and this whole week, of SciShow. [ ♪ Intro ].

In the tropical rainforests of South America, there’s a flying animal that lives in colonies in caves, emerges at night in search for food, and navigates using echolocation. And I’m not talking about a bat.

Believe it or not, I’m actually talking about a bird, the bizarre oilbird, known to locals as the guácharo. Oilbirds diverged from their closest living relatives 50 million years ago, and in a lot of ways, they’ve become more like bats than other birds. They roost high up in caves, for example.

One oilbird colony can include as many as 20,000 crow-sized birds. And since there’s not a lot of nesting material available in a cave, they build their funnel-shaped nests out of a mixture of regurgitated fruit and their own feces. Sounds cozy, yeah?

And like a lot of nocturnal animals, including many bats, for the record, they have excellent night vision. They accomplish that by packing their retinas with rods, the light receptors responsible for vision in dim lighting. In fact, oilbird retinas have the highest density of rods of any known vertebrate: one million of them per square millimeter.

Your retina has a max of about 150,000 rods per square millimeter. These birds have so many rods that there’s almost no room leftover for cones, the other light receptors, which handle visual acuity and color. That means their view of the world is probably fuzzy and dull.

Even the world record holder for rod density needs some light to see, though, so the birds eyes are no help in pitch black caverns. Which might be why they’re the only birds that have figured out how to echolocate. To keep from getting confused in a densely populated cave, each bird clicks at a slightly different frequency.

And unlike bats, oilbirds’ clicks are audible to human ears, so if you were standing in one of these caves when the birds return to rest, you’d hear quite the cacophony. Oilbirds’ resemblance to bats is a classic example of convergent evolution, where different animals facing similar pressures from natural selection end up with similar traits. They’re so bat-like, that you would think we’d call them batbirds.

But if you’ve been wondering where that name came from, yes, there’s a story there. Oilbird comes from the fact that their favorite food is the fatty fruit of the oil palm. Baby oilbirds in particular become so plump from their palm-rich diet that indigenous people in Venezuela used to collect chicks so they could render their fat in pots to use as fuel.

You know, the more I think about it, the more I like batbirds instead. Maybe it’s time for, like, a re-branding. Let’s try it out:.

Thanks, batbirds, for the reminder of just how weird nature can be! So I know that you know bats and birds are different classes of life, but there’s still a lot of differences just between species of birds. All this week we’ve been sharing Skillshare classes we enjoyed, and thinking about oilbirds, made me curious if Skillshare offered a class on bird identification.

I found this one called The Casual Birder taught by Sue Pulsipher, and it’s kind of a hidden gem on Skillshare. It’s like if David Attenborough was your sweet aunt and you took a trip to the Galapagos islands together. At first it seems pretty basic, like a flamingo and a hawk are both pretty easy to identify, but she goes into really interesting details about all the birds she talks about, like that bill of flamingos and how they use it to eat upside down!

And she shares footage of other animals you’ll see while birding, like, in the Galapagos, iguanas. I was surprised by how charming and fun this class was, and it just goes to show that you no matter what you’re looking for, you can probably find a class about it on Skillshare. Thanks to Skillshare for supporting SciShow and for offering all SciShow viewers two months of unlimited free classes.

Click the link in the description to take advantage of this offer, and support SciShow at the same time. And if you go birding or take Sue’s class, let me know what you saw and learned in the comments. [ ♪ Outro ].