In this episode, you’ll hear about:

• The evolutionary arms race between brood parasites and their hosts, from mimetic eggs to alien-looking chick mouth patterns

• How the "password hypothesis" explains how brown-headed cowbirds avoid imprinting on the wrong species

• Why host-switching in African parasitic finches can drive the rapid formation of new species

• 00:03 - The Dark Side of Motherhood in Nature

• 03:56 - The Intricacies of Brood Parasitism

• 06:19 - The Fascinating World of Brood Parasitism

• 16:42 - The Evolution of Brood Parasitism in Birds

• 24:16 - The Fascinating World of Brood Parasitism

• 27:59 - Debunking Myths about Brood Parasites

Timestamp Disclosure
These timestamps were generated using AI and may contain errors or omissions. They are provided for accessibility and reference purposes only and may not perfectly reflect the original audio.

Dr. Scott Taylor (Excerpt)

Female coots. If they don't think it's their baby, they'll murder it. Which is pretty gnarly, I feel like.

Dr. Chris Balakrishnan

Yeah, I feel like coots are just crazy. And there's a lot of killing of babies.

Dr. Scott Taylor

Previously on as the Nest Turns. She arrived at dawn, quiet, unassuming, her feathers a mousy, forgettable brown, the kind of bird you'd walk right past at a party.

Dr. Scott Taylor

But don't let the looks fool you, because while the prothonotary warbler, a golden jewel of the southern swamps, was away gathering breakfast, she made her move. One egg slipped into the nest like a forged check into a bank deposit. And then she vanished. No forwarding address, no note, just gone.

Dr. Scott Taylor

Okay, that was dramatic. But honestly, that's basically what happens.

Dr. Scott Taylor

Years ago, I was working as a naturalist at Rondeau Provincial Park, this beautiful spit of Carolinian forest on the north shore of Lake Erie in Ontario.

Dr. Scott Taylor

Rondeau is home to one of the last breeding populations of prothonotary warblers in all of Canada, endangered maybe a couple dozen birds clinging on in these flooded swamp forests. Part of my job was to help with monitoring their nest boxes.

Dr. Scott Taylor

And I will never forget the first time I opened a box expecting to see a clutch of perfect small, creamy white and densely speckled eggs, and found a cowbird egg sitting right there among them. Bigger, speckled as well. Obvious to me, anyways. Not to the warbler. And I remember this wave of indignation. How dare you.

Dr. Scott Taylor

These warblers are barely hanging on and you're going to hijack their nest? The gall, the audacity. And I think that's how most people feel about brood parasites. They're the con artists of the bird world, the deadbeat parents.

Dr. Scott Taylor

But are they really? Or do they get an undeserved bad rap? Which brings us to today's question. Okay, but why put eggs in another bird's basket?

Dr. Scott Taylor

To help us answer that, we're joined by Dr. Chris Balakrishnan, an evolutionary biologist and associate adjunct professor at East Carolina University.

Dr. Scott Taylor

Chris is also one of the founders of Nerd Nite, which, if you don't know, is basically the world's greatest proof that scientists are more fun at parties than anyone gives us credit for. Imagine TED talks but at a bar with a beer in your hand. It started in Boston in 2003, and it's now in over 100 cities worldwide.

Dr. Scott Taylor

And Chris, along with Matt Waskowski, literally wrote the book on making science fun. It's called How to Win Friends and Influence Fungi. If anyone can take the sneakiest strategy in the bird world and make us see it with fresh eyes, it's Chris. But first, let me make the case for the cowbird, because here's what I've learned since my days at Rondeau.

Dr. Scott Taylor

The brown-headed cowbird isn't a villain, it's an evolutionary marvel. They evolved on the Great Plains following herds of bison eating insects kicked up by their hooves.

Dr. Scott Taylor

When your whole survival strategy is to follow a herd that never stops moving, you can't sit on a nest for two weeks, so they outsourced. A single female cowbird can lay up to 40 eggs in a season across dozens of nests of dozens of species. Over 220 host species have been documented.

Dr. Scott Taylor

That's not laziness, that's a logistical masterpiece.

Dr. Scott Taylor

And the real threat to most North American songbirds isn't the brown-headed cowbird, it's habitat loss, it's forest fragmentation, which also benefits cowbirds because they search for nests along forest edges. We created the conditions that made the problem worse and then we blamed the bird. That's what I love about this topic.

Dr. Scott Taylor

Brood parasitism isn't just some weird bird trick. It's an arms race, coevolution in real time. Hosts evolving to recognize foreign eggs and chicks, parasites evolving to match them.

Dr. Scott Taylor

Some hosts bury the cowbird egg, others eject it. Others just deal with it because the cost of fighting back isn't worth it. It's messy, complicated, and morally ambiguous.

Dr. Scott Taylor

If you're the kind of person who assigns morality to birds, which, let's be honest, we all are at some point in our lives. After the break, Dr. Chris Balakrishnan helps us unpack the science of outsourced parenting. Genius villain behavior. Or something way more interesting. Stay tuned.

Dr. Scott Taylor

Well, welcome back, everyone. Thanks for joining us, Chris. I'm really excited to have you today.

Dr. Chris Balakrishnan

Thanks, Scott. I'm really excited to be here.

Dr. Scott Taylor

Yeah, what is brood parasitism and how many birds do it? Does it differ across birds?

Dr. Chris Balakrishnan

I guess I'd say a brood parasite is any bird that at least occasionally lays their egg in a nest that some other bird made. And sort of the thing I think is the most fun is really how many different varieties of brood parasite there are.

Dr. Chris Balakrishnan

And I don't mean species, I mean different flavors of parasitism. The way they do that, the way they lay their eggs in different birds' nests. Sort of a lot of options.

Dr. Chris Balakrishnan

So you could lay your egg in the nest of another bird of your species. You could lay your egg in the nest of a bird of a different species, you could do it sometimes. And then there's other birds.

Dr. Chris Balakrishnan

Probably the birds that I know the most about and are my favorite, they just always lay their eggs in the nest of a different bird species. And they never build their own nest, they never raise their own young. Those ones we call obligate brood parasites. And they're probably the most fun.

Dr. Scott Taylor

They're certainly the most fun and evolutionarily, I think the most interesting to think about, but also maybe the most broadly maligned. Like if you think about the brown-headed cowbird, which is an obligate brood parasite, before we get into kind of the obligates, what are some examples of the sometimes or the only my same species birds?

Dr. Chris Balakrishnan

An example that comes to mind of a facultative brood parasite, that's the fancy word for a brood parasite that only occasionally lays their eggs in someone else's nest, are redheaded ducks. And they sometimes parasitize other ducks like canvasbacks.

Dr. Scott Taylor

And they'll lay in canvasbacks, but I think they'll lay in other duck nests too.

Dr. Scott Taylor

And it is interesting that it's just a sometimes thing. I wonder what drives a female's decision in one year to be like, no, I'm going to do this on my own. I'm going to build a nest, lay my eggs. And other years just be like, nope. But getting to these obligate brood parasites.

Dr. Scott Taylor

So these are birds that never build their own nests. They always lay their eggs in the nests of other species. Some of them are very specific to, like, this is the other species that I'll lay my eggs in their nest of. Others are more generalist. But why do they do this? What's the advantage? Is there a biggest advantage or the biggest risk? Tell us a little bit about that.

Dr. Chris Balakrishnan

It's sort of a strategy. And so you could either invest a whole lot of energy in raising your own young or foist off that parental care, all of that effort onto a different species. And so what you see a lot in these obligate brood parasites is rather than spending all that energy, they just lay many more eggs.

Dr. Chris Balakrishnan

So the brown-headed cowbird, which you mentioned before, is one of these maligned species, might lay like five times more eggs in a year than what a normal bird would that's raising its own young. And so it's kind of an amazing feat on the one hand to do that. And it's literally like not putting all your eggs in one basket.

Dr. Scott Taylor

Yeah. It's putting your eggs in many, many different baskets in the hopes that they don't get recognized and ejected, but get raised by whoever has them.

Dr. Scott Taylor

So what do we know about how you evolve into an obligate brood parasite? Is it that the ancestral state is facultative brood parasitism that evolves to obligate brood parasitism?

Dr. Chris Balakrishnan

Yeah, there's a few different ideas, hypotheses about how this has happened.

Dr. Chris Balakrishnan

And this is one of those questions that's really been difficult to test conclusively because we don't have a time machine and we can't watch it happen. But yes, that's one of the ideas, that facultative brood parasitism provides like a stepping stone to become obligate brood parasitism.

Dr. Chris Balakrishnan

Another thing is, like colonial nesters, where you have a bunch of birds nesting all close together, you occasionally drop your egg in the nest of a neighbor, and that could provide a pathway to being a parasite all the time. The neat thing is that this behavior, this obligate brood parasitism, has evolved seven times independently in birds.

Dr. Chris Balakrishnan

So there is one origin of obligate brood parasitism in ducks. There are actually three origins of brood parasitism within the cuckoos. The cuckoos are probably the most famous brood parasites, the cuckoo clock, et cetera. They've been studied for a long time in Europe. Then there are the honeyguides in Africa, which are relatives of woodpeckers.

Dr. Chris Balakrishnan

And then there are two origins of brood parasitism among the passerines, the brown-headed cowbird and other cowbirds, and the African parasitic finches. The greatest of all the parasites.

Dr. Scott Taylor

The greatest of all the parasites. Tell us why you think they're the greatest of all the parasites, other than you dedicated many years of your life to thinking about them.

Dr. Chris Balakrishnan

They're so cool. One thing that happens in some of these cases of parasitism, particularly in the parasites that are host specific, meaning they really only lay their eggs in the nest of a particular species, is that you sometimes evolve mimicry. And so you might see in some species, like some cuckoos, you see mimetic eggs, eggs that look like those of their hosts.

Dr. Chris Balakrishnan

And the idea here is that you want to prevent the host species from detecting your eggs and kicking them out of the nest. And so there's a benefit to having eggs that look ever more closely matched to those of your host.

Dr. Chris Balakrishnan

But in the brood parasitic finches, the ones that I studied, there isn't really mimicry at the egg stage. The eggs of these birds are just plain white. So there's nothing like fancy and cool going on there.

Dr. Chris Balakrishnan

But there's mimicry of the chicks after they hatch. And that doesn't sound maybe cool at first blush, but they have these patterns in their mouth that look like aliens. They're colorful.

Dr. Scott Taylor

They truly look like aliens.

Dr. Chris Balakrishnan

Yeah, it's like bananas. Some species are like red with black spots, others are like blue with like a red background.

Dr. Chris Balakrishnan

And so the extent of mimicry between the parasites and their hosts, it's just crazy. And some of the craziest patterns actually are in non-parasitized birds that make their nests in cavities. So where it's super dark in there, that's where you get like these neon blue and yellow looking things to make sure the parents can see the mouth that they're feeding.

Dr. Chris Balakrishnan

And so now you add a parasite into the mix, and if the parasite looks totally different than the hosts, the parents are going to know which mouths to feed. These chicks are actually typically raised alongside each other.

Dr. Chris Balakrishnan

It is not like what you mentioned before, these maligned parasites famous for killing their nest mates and things like that. These chicks are most often just raised together. One parasitic chick and maybe two or three host chicks, they're all raised together.

Dr. Chris Balakrishnan

The cost to the parent is just an extra mouth to feed, if you will. And so even that, just the selective pressure caused by that extra mouth to feed is enough to result in spectacular mimicry. It's just amazing to me.

Dr. Chris Balakrishnan

But that's only one part of why these are the best of the brood parasites. The other part is that they also, in a sense, mimic the song of their host. Songbirds, as they're growing up, they imprint on their parents and they learn their song. So this is true of all songbirds.

Dr. Chris Balakrishnan

But these songbird parasites also learn the song of their foster parent, if you will, and they incorporate that host song into their repertoire. So when you hear one of these parasitic finches singing, you can actually identify which host raised them just by their song.

Dr. Chris Balakrishnan

And so they sing this mixed song repertoire that incorporates that of their host, and that is their own parasite-specific song. And that's really cool. They're also pretty.

Dr. Scott Taylor

They're also pretty. That's true. We'll make sure we show folks what they look like. There's a lot of interesting things in what you just said.

Dr. Scott Taylor

One, yes, there's not strong selection, but it's strong enough for this incredible mimicry of the inside of a chick's mouth, which I bet most people listening have never thought that inside of a chick's mouth looks different. But we're going to show you that it does. So make sure you look at the video. It's crazy, just like Chris said.

Dr. Scott Taylor

And yeah, in other cases, like with cuckoos, the chicks either shove the other eggs out of the nest. That's much stronger selection to recognize that, oh crap, there's a cuckoo egg in my nest, I gotta get it out of there.

Dr. Scott Taylor

But even then, there are some hosts that are really good at recognizing those cuckoo eggs and there are others that aren't.

Dr. Scott Taylor

And you know, earlier in the episode I talked about me noticing that there was a brown-headed cowbird egg in a prothonotary warbler nest, which I could visually recognize. But yeah, in some of these cases the cuckoos have gotten really good at painting the egg the same way, which is just also very fascinating.

Dr. Scott Taylor

When we think about birds, I don't know if anyone's ever raised ducks or geese, but if they hatch and they see you, they imprint on you. And so this question is one that I've always been fascinated by.

Dr. Scott Taylor

You've talked a little bit about with respect to the brood parasitic finches, where they do incorporate the song of their host into their own song repertoire. But it's important for those parasitic chicks to not think they should ultimately breed with their host species. So what do we know about how they have evolved away from this classic avian behavior of imprinting on whoever raises you?

Dr. Chris Balakrishnan

It's sort of the raised by wolves sort of scenario. If you're raised by a different species, that could quite seriously mess up your whole behavior, who you want to mate with, things like that.

Dr. Chris Balakrishnan

And so we've done a little bit of work on this in cowbirds to try to understand how this works. But one of the sort of model systems, the lab rat of the bird world, is the zebra finch.

Dr. Chris Balakrishnan

And the zebra finch has this well-defined critical period for learning. So they start learning when they're young birds and then after three months they're done, they don't learn any new songs.

Dr. Chris Balakrishnan

And so if you're a zebra finch and you're put into a different species' nest, you are going to learn what you hear. What cowbirds seem to be doing well is not that. So one of my colleagues, Mark Hauber, he came up with this clever name for what they're doing.

Dr. Chris Balakrishnan

And he called it the password hypothesis. And the idea is that cowbirds kind of have to hear a specific trigger made by other cowbirds in order to start learning cowbird song.

Dr. Chris Balakrishnan

So as they're being raised by a different species, like a prothonotary warbler, they're not exposed to the password. When they run into cowbirds, they are. And as soon as they hear that song, that instinctively triggers them to start learning cowbird stuff.

Dr. Chris Balakrishnan

And so we've done some experiments to show that this actually works. So you take a cowbird, you grow it in the lab, you can control what it's exposed to acoustically, and they don't learn as well until they hear another cowbird song, specifically the chatter call.

Dr. Scott Taylor

That's amazing. I mean, I can understand why that would work, but definitely the evolution of that more complicated approach, like, okay, delay the critical period of song learning until you hear the right trigger and then learn what a cowbird says, is really fascinating. And I mean, it makes intuitive sense. But prior to that description, it's hard to think of how they could get around this common factor of being a song learning bird.

Dr. Chris Balakrishnan

I think it really is a challenge for becoming a brood parasite, is not getting confused about who you are.

Dr. Scott Taylor

Yeah, certainly. We've talked about this a little bit. But what are the main defenses that hosts evolve and what are the best counters that the parasites then evolve in response?

Dr. Chris Balakrishnan

The ability to recognize the parasitic eggs and eject them. An alternative that a host bird can take is to just abandon their nest. There's recognition of the eggs, as we talked about.

Dr. Chris Balakrishnan

There are hosts that recognize the female parasites as they're meandering around their nest, and they will definitely go after them to try to chase them away. The parasitic chicks, again, as I think you mentioned earlier, they also can be quite vicious to their nest mates.

Dr. Chris Balakrishnan

But there are some parasitic species, like the common cuckoo, known for ejecting its nest mates. Honeyguides are probably the worst of all and definitely the stuff of nightmares.

Dr. Chris Balakrishnan

Honeyguide chicks are born with hooks on their bills and they basically stab all their nest mates or their nest eggs to kill them.

Dr. Chris Balakrishnan

There's a remarkable number of people that have bird phobia and presumably they're not listening to this. But don't go to the video if you are slightly afraid of birds.

Dr. Scott Taylor

Yeah, honeyguides are cool for a lot of other reasons. I know that some warblers will build a new nest bottom. If they detect an egg, they'll build a new nest bottom. So they're not abandoning the nest entirely, but they're kind of like, nope, not going to incubate those ones. Which is kind of an interesting defense.

Dr. Chris Balakrishnan

Yeah, absolutely.

Dr. Scott Taylor

And I think that recognizing the cowbirds is also just really fascinating, that they come to understand, like, oh no, that's one of the ones that's going to get in here and add an egg to my nest and I gotta get her out of here.

Dr. Scott Taylor

One thing I think about in the context of cowbirds and other nest parasites is that they have to be very well timed. Like you don't want to show up and lay an egg in a nest before the female who actually built the nest lays an egg, because she's going to know she didn't lay that egg. So you have to be monitoring.

Dr. Scott Taylor

Well, first they have to find the nests, which is not easy for anyone who's ever gone out looking for bird nests in a forest. So they've got to find the nests, know the right timing to add their egg to the nest, and then keep track of it all.

Dr. Scott Taylor

Like you said, they lay five times the number of eggs of other birds. I think about chickadees where their spatial memory is really, really good because it's directly linked to survival. And you wouldn't expect a difference between male and female spatial memory because everybody's got to eat.

Dr. Scott Taylor

They all have to hide seeds and recall those seeds. There's not a sex difference or an expectation. I think in brown-headed cowbirds, females have good spatial memory as you'd predict, because they're the ones that are like, where are all these other nests and what's the timing in all of this? But that the male spatial memory is not as developed. And I think that's fascinating, like when selection would favor these differences.

Dr. Scott Taylor

I mean, we make jokes about in many partnerships of humans, like one knows where they're going and the other one knows what food's in the fridge. Like we specialize on different tasks. But it's neat to think about in the context of these female birds that have much more on their plate in terms of finding and then depositing eggs.

Dr. Chris Balakrishnan

In some species of birds, males contribute to parental care and other species they don't. In brood parasites, neither do. But in some species, like the indigo birds, the African brood parasitic finches that I talked about earlier, neither the males nor females, of course, are providing parental care, but the males are at least expending some of that saved energy into courtship displays.

Dr. Chris Balakrishnan

So they're spending all their time, all day, every day, singing. They're the easiest birds to work with because they sit at the top of a tree and they just sing the whole day long. Sing, sing, sing.

Dr. Chris Balakrishnan

So while the female is trying to maybe figure out where all the possible host nests are, the male has only one job, which is singing and competing for females.

Dr. Scott Taylor

The indigo birds, like you said, they incorporate some of their host song into their own song. They still recognize their own species and breed with them. But there's this really interesting intersection between indigo birds and their variation and the variation in the different hosts they use.

Dr. Chris Balakrishnan

But what if one of those parasitic females lays their egg in the nest of a different host? So again, this parasite chick is laid in the nest of a host that has never been parasitized before, but it's going to automatically learn that song.

Dr. Chris Balakrishnan

And as an adult, if it's a male, it'll sing that song. And if it's a female, it will prefer to mate with males that are singing that song.

Dr. Chris Balakrishnan

And so in how we think new species form in these weird indigo birds is that you can imagine a scenario where one host becomes more rare and there are few opportunities to lay your eggs in the correct host. And maybe a few individuals therefore have to lay their eggs in a novel host nest.

Dr. Chris Balakrishnan

And so the males will automatically start singing a new song, and the females will automatically start preferring to mate with males that sing that new song.

Dr. Chris Balakrishnan

And so those birds that have been raised by a novel host will preferentially mate with each other and not the birds that were raised by the original host. And so that's what we call reproductive isolation by song.

Dr. Chris Balakrishnan

And so it results in this potential for really rapid changes in song driven purely by what host nest you're raised in.

Dr. Scott Taylor

How many times has it happened in the indigo birds?

Dr. Chris Balakrishnan

Currently, there are something on the order of 20 different hosts parasitized by indigo birds.

Dr. Chris Balakrishnan

So you can imagine going back in time, there's just perpetually this process of shifting hosts, forming new species, and maybe those hosts go extinct, maybe those parasites go extinct. And that what we're observing is kind of the current crop of host parasite pairs.

Dr. Scott Taylor

That's really cool. And indigo birds are the only example where a parasite learns a little bit of the host song. Right?

Dr. Chris Balakrishnan

Most of the obligate parasites are not songbirds, which sort of takes the song learning thing out of it. And so cowbirds are not known to do this, but the indigo birds are.

Dr. Scott Taylor

That's really, really cool. It's such an interesting different way that new species can arise in this host parasite context. I understand why you wanted to do your PhD on that topic, because I would too.

Dr. Chris Balakrishnan

What I really like is a really weird bird.

Dr. Scott Taylor

Yeah, well, don't we all? Come on. I mean, in general, birds are weird if you think about it long enough. Then there are even weirder weird birds, which is pretty awesome.

Dr. Chris Balakrishnan

Yeah, I don't know. This is a bird podcast, so maybe this is heresy to talk about not birds.

Dr. Scott Taylor

No, it's not at all. Go for it.

Dr. Chris Balakrishnan

But there actually are brood parasites that are not birds. So basically anywhere where you see parental care in any species, in any groups of animals, you also see brood parasitism evolving. So there's brood parasitic catfish. And brood parasitism has evolved a bunch of times in insects. So it's not just birds. I just wanted to say that.

Dr. Scott Taylor

No, it's a good point. I mean, lots of ants are brood parasites. There's mouth-brooding cichlids, which are fascinating. I love fish a lot. And these cichlids will brood their eggs in their mouth.

Dr. Scott Taylor

And these catfish have evolved to steal that out of the mouth of the female cichlid and then give them their own fertilized eggs. And so then that female cichlid raises a whole mouthful of catfish instead of her own babies. It's fascinating.

Dr. Scott Taylor

Which is definitely like, if you think about Alien, the movie, that's really good inspo for some sci-fi. Kind of steal your baby, raise someone else inside of your own body. But yeah, there are so many cool examples of brood parasitism across the tree of life.

Dr. Scott Taylor

But okay to talk about the others. How common is brood parasitism in birds?

Dr. Chris Balakrishnan

About 100 species total. And that translates to about 1% of all bird species are obligate brood parasites.

Dr. Scott Taylor

Obligate brood parasites. So then facultative is a little bit, there's even a little bit more going on.

Dr. Chris Balakrishnan

Yes. And I have no idea how many of those there are.

Dr. Scott Taylor

I bet we don't even know. I mean, we probably don't. I can't look at a chickadee nest and say like, oh, that one's from next door or whatever.

Dr. Chris Balakrishnan

It would take a lot of genetic work.

Dr. Scott Taylor

So I guess there are some species where we can be confident that that's not happening.

Dr. Scott Taylor

And I think, yeah, if you're doing like a long-term genetic study and you're genetically identifying every chick, then you might have a good idea.

Dr. Scott Taylor

But I bet it's much more common than we realize, because why not just dump an egg somewhere else if you know where the nest is and you're synced up on breeding anyways? Because it could overall increase your fitness.

Dr. Chris Balakrishnan

After hatching is where you see some of the most bizarre components of brood parasitism. Like a tiny reed warbler feeding like a ginormous cuckoo chick.

Dr. Chris Balakrishnan

And so this screaming, begging cuckoo that is like three times the size. I've never worked on cuckoos. So this is just, it looks like it's like 10 times. They're much larger. And the reed warbler is just feeding it.

Dr. Scott Taylor

Coots have their chicks with these weird little red feathers on their heads. And coots also will lay their eggs in the nests of other coots.

Dr. Scott Taylor

And I think there's something about kin recognition there where female coots, if they don't think it's their baby, they'll murder it. Which is pretty gnarly, I feel like.

Dr. Chris Balakrishnan

Yeah, I feel like coots are just crazy and there's a lot of killing of babies. I have to look it up again, you know. Cause I have heard talks on this but the details are super fuzzy, but I believe there's a lot of killing in coots. I think that's correct.

Dr. Scott Taylor

All right, we've reached the part of the show that we call That's BS, or That's Bird Stuff, where we give our guests an opportunity to debunk a myth that ruffles their feathers. So Chris, what do you want to call BS on?

Dr. Chris Balakrishnan

I want to call BS on this whole anti-brood parasite scene, this animosity towards brood parasites. They are so maligned, but they are amazing. A really good example is our local brown-headed cowbird.

Dr. Chris Balakrishnan

These birds are so hated, they're blamed for things like driving other birds extinct. They're implicated in the declines of other bird species, but they themselves, they're a native species.

Dr. Chris Balakrishnan

They're also declining in populations where there are cases where conservation managers have killed cowbirds in order to promote the success of rare native species like Kirtland's Warbler. And even after they stopped doing that, Kirtland's Warblers continued to recover.

Dr. Chris Balakrishnan

And so really, a lot of the success of brown-headed cowbirds is due to what humans do. We're degrading our environments. Sometimes that's going to make species like cowbirds do really well and cause other species to decline.

Dr. Chris Balakrishnan

But right now, so many of our species are declining. And brown-headed cowbirds are one of the coolest species that we have. If you look at them, they're really actually quite beautiful and iridescent. They can lay a boatload of eggs. That's very impressive.

Dr. Scott Taylor

Yeah, they have this whole password system for figuring out to recognize their own species. I agree. They're fascinating birds. And they themselves are declining and also protected under the Migratory Bird Treaty Act. So you should not remove their eggs from nests. That's actually completely illegal.

Dr. Chris Balakrishnan

Yeah. Love your brood parasites, they're amazing.

Dr. Scott Taylor

Hashtag justice for cowbirds and others. But yes, brood parasites are fascinating birds. Well, thanks so much for taking the time to chat with us today. It's been really, really amazing to learn more about brood parasites.

Dr. Chris Balakrishnan

A lot of fun for me. Thank you so much, Scott.

Dr. Scott Taylor

Birds are dinosaurs, and around here, we like our snacks. So we end each episode with a Dinosaur Nugget. Today's nugget, brood parasitism has evolved seven times independently in birds.

Dr. Scott Taylor

In ducks, cuckoos three separate times, honeyguides, cowbirds, and the African parasitic finches, the indigo birds. That's about 100 species, roughly 1% of all birds that have figured out how to outsource parenting entirely. And it's not just birds.

Dr. Scott Taylor

Brood parasitism has also evolved in catfish and multiple times in insects. Anywhere in nature where there's parental care, something has evolved to exploit it.

Dr. Scott Taylor

So the next time someone calls the cowbird lazy, remember, this strategy is so effective that it has evolved over and over again across much of the animal kingdom. That's a wrap on this week's episode. Okay, but why put eggs in another bird's basket?

Dr. Scott Taylor

It's that moment in every episode where I ask you to show our podcast some love. Leave us a rating. Give us a follow, and we'll catch you next time. Byeeee.

Transcript Disclosure
This transcript was generated using AI-assisted transcription and may contain errors or omissions. Please refer to the audio or video episode for the most accurate representation.

All audio, video, and images in this episode are either original to Okay, But... Birds (© Okay Media, LLC) or used under license/permission from the respective rights holders. Bird media from the Macaulay Library is used courtesy of the Cornell Lab of Ornithology as follows:

• Brown-headed Cowbird audio contributed by Wil Hershberger, ML94262

• Brown-headed Cowbird audio contributed by Wil Hershberger, ML516718

• Redhead audio contributed by Jessie Berry, ML139672

• Canvasback audio contributed by Arthur A. Allen, ML3537

• Greater Honeyguide audio contributed by Mike Andersen, ML140981

• Pin-tailed Whydah audio contributed by Myles E. W. North, ML14489

• Village Indigobird audio contributed by Myles E. W. North, ML14484

• Zebra Finch (Australian) audio contributed by Vicki Powys, ML226233

• Prothonotary Warbler audio contributed by Wil Hershberger, ML85158

• Kirtland's Warbler audio contributed by Rudolph Little, ML13982