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They come out at night, emerging from their caves to fly into the dark rainforest. Their colonies can number in the hundreds or even thousands. And the weird, raspy sounds they make have earned them names that translate to ‘one who laments’ and ‘little devil.’ But these spooky guys aren’t bats.

And they’re not owls, either. They’re an ancient, mysterious branch of the bird family tree, one that has traveled a lonely evolutionary road into darkness. [♪♪ INTRO ♪♪] Before we tell you more about these spooky birds, we wanted to tell you that the BizarreBeasts pin club is open for subscriptions from now through October 16th! Sign up now and the first pin you get will be THIS Bizarre Beast!

Oilbirds, or guácharos, as they’re known locally, live on the island of Trinidad and are endemic to seven South American countries: Guyana, Venezuela, Colombia, Ecuador, Peru, Bolivia, and Brazil. They’re found in mountain forests and the lowlands that surround them, because these environments provide them with the two key things they need, that also make them pretty weird birds. The first thing is caves.

Oilbirds roost and nest on ledges within caves and tunnels, sometimes so deep within the cave that no light makes it in. And they usually hang out in those caves during the daytime, though they have also been reported roosting in the forest canopy nearby. When night falls, they leave their roosts to feed.

Now, you may have noticed that the eyes of oilbirds look pretty uniformly dark, which is maybe a little unnerving for us humans. If you’re a birder, they might remind you of the eyes of barn owls, who are fairly spooky in their own right. And one study from 2018 found that, in owls, having dark eyes was a pretty good indicator that the species hunted at night, so there might be a good evolutionary reason for nocturnal birds to have dark eyes.

Oilbirds even take things a step further than owls in how incredibly sensitive their eyes are. They have the highest photoreceptor density in their eyes ever recorded for a vertebrate. Their pupils are also relatively large, giving them the highest light-gathering capacity calculated for any bird, including owls that are active at night.

And if that wasn’t enough for them to thrive as nocturnal, cave-roosting birds, they can also do another thing that makes them even more bat-like, they can echolocate. The echolocation of oilbirds isn’t ultrasonic, the way some bat echolocation is, though. They echolocate at a low enough frequency that we can hear the clicks they make.

And they seem to use their echolocation abilities to navigate the dark caves and tunnels they roost in, and maybe for communication, too. But researchers disagree about whether they use bio-sonar to find their preferred foods. And that’s the second weird thing I was alluding to before.

The only other birds that echolocate, some of the swiftlets from the Indo-Pacific, eat bugs, like echolocating bats do. But the oilbirds don’t use this incredible ability to find bugs to eat. Because they eat only fruit, instead.

Which they consume whole and barf back up the seeds. Sounds unpleasant, considering the seeds can be up to 6 centimeters long by 3 centimeters wide, which would be like swallowing a date fruit whole! They even build their nests out of regurgitated fruit fiber, seeds, and their own poop, which works for them, I guess.

And the fruits they specialize on, like palms and wild avocados, are super high in fat, which allows their hatchlings to grow really quickly. But it also made them a target for people to catch to render down for oil before they became legally protected, which is how they got their common name. Presumably before anyone realized they should just be calling them bat-birds.

And all of this makes me ask, why would a bird evolve to be so bat-like in the first place? Even though bats are the only mammals to evolve powered flight, they are incredibly speciose. There are over 1400 species of bats, approximately one in five mammals is a bat.

But there is only one flying, nocturnal, fruit-eating bird. And there are certainly bats that eat fruit and live in the same places as oilbirds, so it seems like bats should have the upper hand in this ecological niche. And yet, the oilbird exists, somehow.

One researcher, David Snow, hypothesized that their evolutionary journey toward bat-ness looked like this: The ancient ancestors of oilbirds were probably nocturnal or crepuscular, which means active at twilight. He thought this because the closest living relatives of oilbirds, things like potoos, frogmouths, and nightjars, are all nocturnal or crepuscular, too. And in evolutionary biology, we think that when a whole group shares a feature like this, it’s likely to have come from a shared ancestor, rather than having evolved independently multiple times.

So oilbirds probably started off sharing the night air with bats. Those oilbird relatives also snatch their prey out of the air with their beaks and swallow it whole, rather than using their feet to grasp and hold food… Which is how the oilbird eats too, snagging fruit with its beak and gulping it down whole. They also feed socially, like many other tropical fruit-eating birds.

Oilbirds probably adapted to feeding like this because fruits tend to grow in patches throughout the forest that change with the seasons, so they’re easier to locate, especially at night, if you have a big group of birds searching for them. Eating fruits that were high in fat allowed the oilbirds to get bigger and, like many fruit-eating birds, to have large, slow-developing nestlings. And having babies that take a long time to develop means you need a very safe nest for them.

Snow thought nesting on rocky cliff-sides was probably the transitional step towards roosting in caves, because many cliff-nesting birds are also social. This also could’ve been when the birds started echolocating, potentially to help them find their own nests among the crowd. And, eventually, predation pressure on their big, fatty hatchlings could’ve pushed them into caves, seeking more and more protected nesting sites.

While this whole hypothesis is difficult to test, there is some evidence that supports part of it. There are two fossil skeletons of an oilbird ancestor from Wyoming that date back about 50 million years. They don’t seem to have the adaptations that living oilbirds do to hovering and living in caves.

But the same families and genera of plants that oilbirds today eat were found in the same rock formation as the fossils, so they might’ve already been eating fruit back then. And, funny enough, that formation is also where we find the earliest complete bat skeletons, so who really copied who here? Oilbirds remind us that, while we might be very familiar with bats, things get a little bit bizarre when we see a lot of what we think of as their defining traits in a different kind of animal.

If you’ve been hearing about these nocturnal birds and started to think that you too might want to start sleeping more during the day, you should check out this episode’s sponsor Manta Sleep. Manta Sleep is decidedly pro-nap and thinks that it’s impossible to unlock your full potential if you’re not getting an afternoon nap every day. Manta Sleep’s focus is giving you the energy to create your best life.

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Visit the Manta Sleep website at mantasleep.com and use code bizarrebeasts or click the link in our description for 10% off your order! And then, take a nap! And thank you for watching Bizarre Beasts! [♪♪ OUTRO ♪♪] Oh hey, you’re still here, so you get BONUS FACTS!

The scientific name of the oilbird is Steatornis caripensis. It’s the only species in its genus and the only genus in its family! Its genus name translates to ‘fat bird,’ a reference to its incredibly chunky nestlings, and its species name comes from the town of Caripe in Venezuela, which is close to the cave where this bird was first described by Western science.

And, according to a study published in 2015, the oilbird lineage split off from its closest evolutionary relatives, the potoos, around 55 million years ago. But another study from 2014 said the split was even older, at least 60 million years, and that the oilbird holds the honor of being the most evolutionarily distinct living bird in their analysis, which would mean it beats out our season 1 weirdo, the hoatzin.