Daniel and Kelly’s Extraordinary Universe - What is interspecific brood parasitism?

Episode Date: January 29, 2026

Daniel and Kelly talk about interspecific brood parasites - which are species that sneak their eggs into the nests of other, unsuspecting species. How do they get away with this?See omnystudio.com/listener for privacy information.

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Crook and Chase Nashville chats with Chase Matthew. Listen and subscribe on the Iheart radio app, Apple Podcasts, or wherever you listen to podcasts. Approximately 1% of all bird species are total jerks. No, approximately 1% of all bird species are not seagulls. They're what are called interspecific brood parasites. And fine, they're not really juries. jerks, but evolution has endowed them with a profoundly jerk-like strategy.

You see, interspecific brood parasites put their eggs into the nests of other species, often without them knowing it. And to add insult to injury, the parasitic birds often damage the host nest as well. For example, just in case the parents of the parasitized nest can count, the brood parasite takes one of the host eggs out of the nest and yeats it overboard. Now the nest has the same number of eggs as it did before the doting parents went out to grab a quick snack, but one of those eggs is not their own. And then the brood parasite mom typically scrams, leaving the host parents to do the difficult job of raising the kids.

How did this deadbeat parenting strategy evolve? And why can't the host species seem to figure out what's happening and fight back? On today's episode, we're going to dig in to interspecific brood parasitism. And I'm going to be absolutely thrilled to be combining some of my favorite topics, animal behavior and parasitism. Welcome to Daniel and Kelly's parasitic universe. Hi, I'm Daniel. I'm a particle physicist and I have zero tolerance for parasites. Hello, I'm Kelly Weeter Smith. I study parasites and space. And I have all the time in the world for parasites.

All right, Kelly, when then, my question for you today is, how many non-human species, are there in your brood? Oh, you're asking me about pets. You've already asked me about pets. You just found a different way to ask. I guess, you know, you need an update, right? Because you asked me this a couple weeks ago, and the number changes regularly. But now I'm framing them as parasites on Kelly's Goodwill.

Oh, probably just my dog biscuit, because she's been a real handful lately. But no, she's a good girl. I've got a goose that's been trying to bite me lately. I don't appreciate that. That wasn't in the terms of service you got when you got the Goose? No, yeah, we used to call him Jacques Gusto, and we thought that was really cute. But now we call him Schmaltz because we're thinking he might become Christmas dinner

because he has become very aggressive. So just Schmaltz. Wow. What lesson do you think your kids take from that sort of change in status based on behavior? Well, hmm, that's interesting. Based on, I don't necessarily want them to take the message that based on behavior, these things happen. But I am okay with the fact that they live on a farm and they need to understand where their food comes from, although they've insisted that they won't eat it.

But will, I was going to make a joke about them going hungry on Christmas, but I wouldn't do that to them. Well, I think that's a pretty powerful incentive, you know, behave well or you're not going to be here for dinner. You're going to be part of dinner. Yeah, but you know, this isn't an episode on cannibalism, so it really doesn't extend to the family. T-ting, we already got there. Pull out your bingo cards. But I am under the impression that all of your pets are cherished family members and that none of them you would consider parasites. No, none of our pets are parasites.

Currently, we have a dog who lives in our house and we have a horse who doesn't live in our house. But my daughter Hazel considers part of the family. Oh. Yeah. No, I do. I shouldn't. Well, I'm not going to go down that. I shouldn't have said anything bad about Biscuit.

She's been chewing on her feet a lot lately. but we got to get her to the vet. That's my fault. They're doing their best. Yeah, yeah, yeah, yeah. She's a good girl. She's the best girl, yeah. Yeah, yeah.

But today we're not talking about pets we love who we've welcomed intentionally into our family, even if they do cross the species barrier. We are talking about something much more sinister, aren't we? We are. Actually, I should have talked about my livestock guardian dog who chews on our goats because that shouldn't happen. How do the goats feel about that?

They don't love it, although we've got one goat who kind of spaces out when it's happening. And it doesn't really seem to mind so much because she really likes food and she's just kind of chewing on the grass. And he's just playing. But anyway, okay. So, yes, interspecific brood parasitism. Interspecific is a complicated word.

It just means cross species, right? Yes, it does. It means, yeah, between species. And actually, so when, you know, when you say interspecific brood parasitism, it's fun to watch people try to break the different pieces down to try to figure. out what it means. And so this is a perfect transition, actually, to the questions that we asked the listeners. So let's go ahead and see how they broke it down. So I asked our listeners who are willing to volunteer to answer questions for us, what is interspecific brood parasitism? And I'd like to mention here that you're probably not going to hear as many voices from women as I would like for us to be hearing. And so I would like to make yet another plea for women to please go ahead and answer these questions. It's so, okay if we don't know the answers. Or, you know, some of us probably do know the answers. But even if you don't know, add your voice to the chorus of, uh-uh, or, you know, just go ahead

and guess. I would like some more women voices. So anyway, let's go ahead and hear what answers we got. It is when a animal family builds up a herd immunity to a certain parasite. I had no idea. You guys got me. I'm not sure where it is, but it sounds like a parasite that invades a nest. Zomber 4s, the young ones in there. I think the bird lays an egg or moves its egg into a different bird's nest and gets that other bird to raise its young or something like that, I think. I think interspecific brood parasitism is when a parasitic animal fools a different animal species into taking care of its eggs or offspring.

And I only know this because my daughters and I watched a PBS Nature documentary on this subject a couple years ago. I think of broods like egg clutches of species. relying on the egg clutches of other species in order to exist and thrive. Probably an interspecies-specific parasite or a parasite that it goes after a specific species. But the word brood makes me feel like the parasites are brooding within their host. Interspecific parasitism brutism is a tongue twister and something to do with. I don't even know what a brood is, like an IPA.

I think we're talking about animals like cuckoos. I remember hearing them a lot when I was a child and thinking there was something a bit icky about their nesting behavior. However, I expect the science here is really interesting. Maybe it's how parasites select for specific characteristics in a host population, or maybe even that the parasites doing that guide the evolution of the host population. Either way, I'm sure Kelly's going to tell us really excitedly. These are great answers, and I have to say, I don't think I would do very well either here

because the phrase interspecific to me hits on the specific instead of species. And so I'm thinking, like, interspecific as opposed to intergeneral. Like, what does that even mean? Interspecific is hard for my brain. And brood is not a word I typically use to describe, like, people in my species who I'm taking care of. Oh, well, you know, I went... Is that an Ohio thing? No, I guess not. But like, so, you know, when I was an undergrad, I thought I was going to go to med school. And then I was like, you know what, I could never deliver to someone the news that, like,

their kid didn't make it. That's not for me. Yeah. And then I decided I was going to go to vet school. And then I was like, you know what? I could never deliver to someone the news that their dog didn't make it. So, like, that's not for me either. And then I took an animal behavior class. And I was like, this is what going to do with the rest of my life. And then I ended up going to grad school for behavioral ecology, which is trying to understand, you know, why animals do what they do in the wild. And so that's where I came across the word brood. And so I do think of, you know, my children as my brood. But I probably would not have thought of that had I not gone to grad school for behavioral ecology. Because brood to me implies some sort of like dark gothic mood. Like he stands at

the parapet of his castle brooding over his future or something. Yes. As a god. As a kid I had heard the word used in that way. Yeah. So this is an especially tricky phrase because each word on its own has several meanings with very different contexts. And so putting them all together in the right order, it's tricky stuff. So tell us, Kelly, what the heck is this? Break it down for us in basic English. Okay. So first of all, I want to note the reason that we are enjoying this amazing topic today is because a listener named Daniel saw a New York Times article on brown-headed cowbirds that have this strategy and he wanted to know more. So that's what we're going to talk about today.

So interspecific brood parasitism is when one organism tricks or coerces a parent of a different species into caring for their offspring. So this is not biscuit being taken care of by you because that's something you did intentionally. Add biscuit to your brood. That's right. I knew that I was adding biscuit to my brood. I love biscuit very much. I wish she didn't have allergies. We're working on it.

She's great. What about the fact that dogs are like selected for to be cute and to like, you know, pull on your heartstrings and effectively emotionally manipulate their way into your brood? Does that count? No, it doesn't. Like you're still choosing to join. And like kids are, well, I was going to say kids are cute, but not all kids are cute. but we still love them. I can't believe you just said that on the record.

I mean, I totally agree with you. Some kids look like Winston Churchill or Richard Nixon, but I would never have said that. Oh, my God, I just said it also. Oh, no, the truth is out. But we love them anyway, right? You don't have to be cute to be absolutely amazing and loved. And so the point with parasitism is you're essentially being tricked or coerced. And okay, so we're going to start with my favorite example.

And almost all examples involve birds. And so the bird people out there are going to be like, how dare you start? start with an example that's not birds. But look at people, the fact of the matter is, like, number one, I'm a parasite person. But after being a parasite person, I'm a fish person. And so it's going to be a fish example. So we're going to talk about the cuckoo catfish. All right.

So down in Africa, there are these cichlids, and these are the host species. So this... What's a cichlet? A cichlid is a fish, and, like, Africa is sort of famous for their cichlids. These are species that sort of, like, radiated. There's loads of different species. A bunch of evolutionary biologists go down to Africa to try to figure out why there's so many different species of this kind of fish in Africa and how they've managed to, like, fill all of these different niches. So it's just a species that you find in lakes in Africa.

Okay, what does it look like roughly? I don't know, it looks like a fish. You may have gone to a pet store and you've seen convict sicklids. Like, there's, syclyds are a very popular, freshwater fish species that you can find at pet stores. And this is like a different species than what you've seen at pet stores, but it's got like the same body shape. Okay. One of them does this amazing thing where they lay eggs and then they stick them in their mouths. On purpose.

On purpose. Hold them in their mouth to like, you know, keep them safe or something? Yes. They're called mouth brooders. Whoa. And so like these mouth brooders, they pick up the eggs. Not the same as mouth breathers, right? That's a totally different kind of thing.

That's me when I have a cold. These are mouth brooders. Okay. And so they pop the eggs in their mouth, the eggs hatch, and the fish can swim in and out of the mouth. And so when the fish want to go, like, get some food, they'll swim out and they'll, like, eat little things that are in the water. And then when a predator comes, they swim into mom's mouth for safety. And sometimes mom will, like, open her mouth and create an inflow of water to, like, you know, help them get in when she sees a predator. And so, like, the mouth is a safe place for these little babies.

And how do they avoid being eaten? or how does mom eat with her babies in her mouth? I believe mom ejects the babies when it's time for her to eat. I see. And so, yeah, like, so the babies, you know, when they're in the egg stage, she might sort of like spit them under a rock for a little while. Wow. And then go and get a snack and then suck them all back up again.

And I'm not an expert on the mouth brooders. It's amazing to me, though, that, like, things other species do sound gross to me, like spitting your babies under a rock from your mouth. But things that our species do, like, you know, creating milk from your body. body and feeding it to your baby, that's not gross to me at all. So it's amazing how your instincts are just like totally parochial. Yes, I will note that Zach stayed on the side of the blanket near my head when labor was happening. And so human birth is pretty gross too. So. Well, I guess it depends on the husband because when Hazel was being born and Katrina was having a C-section,

they were like, please stay on that side. And I was like, no, you're opening up my wife to pull out my daughter like I'm going to watch and I did and it was fascinating I would have been watching yeah yeah I would have been watching and then they went to hand Zach the umbilical cord and they're like do you want to cut and he was like that's what I pay you guys for yeah no wipe that thing off and give it to me when it's clean but but he was very supportive through the whole thing interesting for a man whose office is a disaster that he only wants clean things handed to it yes yes well we're all still trying to understand the emotional support Doritos bag but anyway Yes, exactly. All right. So we have this mom who has babies and keeps them in her mouth.

Yes. And then that's the host. That's the host. Yes. And so the parasite is called the cuckoo catfish. And, you know, I think a lot of us have seen catfishes. They've got those like whiskers. They move around on the ground. This one. They're good in sandwiches. Sure. Sure. This one's kind of tiny. It's like white. It's got some speckles. That doesn't really matter. The point is it's like a tiny catfish. Okay. And so these catfish try to eat the host eggs. Okay, so like when the hosts are first laying the eggs, they have to lay them on the sand before the mom can get them in her mouth. Okay. And so while the eggs are being laid on the sand and getting fertilized, the catfish are going around and trying to eat them. And the parents are freaking out and they're trying to get the catfish to leave.

And they're like, go away, go away. And while they're distracted and trying to chase the catfish away, the catfish lay some of their own eggs. in amongst the host eggs. Wow. And so then the catfish go away. And so the mom is like, okay, fine, they're gone. Thank goodness. And of course, I'm being super anthropomorphic here.

Who knows what the mom is thinking. It might just be like, bloop, bloop. Yeah, right. I don't know how smart fish are. So anyway, the mom eventually goes to acquire all of the eggs, and she sucks them all up into her mouth. And she has accumulated her eggs and the eggs of the parasitic catfish. I see.

So they're not replaced. They're just in addition. They're in addition, right. But the catfish eggs hatch first. So they're like set to hatch faster. And so the catfish eggs hatch before the host eggs hatch. But when the host eggs hatch, the catfish are there ready for a snack. Yeah.

And so as all the host eggs are hatching, the catfish are eating them one by one. Oh, my gosh. And so in a lot of cases, none of the host babies will survive. they will all become catfish snacks. What? And the parents will spend a significant amount of their energy protecting the catfish babies. So the catfish babies will like leave the mouth. But then when a predator comes, they'll swim towards the, you know, the parasitized mom.

And the mom will open her mouth to give them a place to hide because the mom, you know, recognizes anything that swims out of her mouth as her offspring. And so she has been tricked. Wow. Crazy. That is crazy. Can I ask you how I'm supposed to feel about this? Because I have all of these emotions when I watch like nature documentaries.

I always end up like rooting for one animal. But I find that like if I'm watching the documentary about cheetahs, I'm rooting for the cheetah as it chases the antelope. If I'm watching the antelope documentary, I'm rooting for the antelope. And I'm like, you dang cheetah. What are you doing? So what am I supposed to feel here? Because it feels like one of these is getting tricked. It feels unfair.

Yeah. In all cases, you should be rooting for the tapeworm that lives in all of those animals. But no. You didn't see it coming, but no, yeah, I'm, nature is just, it's complicated. There's no clear answer. Like, you know, I feel that way also. Like, when, you know, when I'm watching the antelope documentary and the antelope gets away, I'm like, oh, you know, I'm happy with that antelope.

But that cheetah maybe had babies that aren't getting fed. And nature is, you know, it's not, life isn't fair, especially if you are, you know, living out in the wild. But I feel like parasitism. is especially emotionally fraught. Was that what attracted you to this field? Or are you totally impervious to all of that emotion? Oh, interesting.

No, I'm not impervious to the emotion. And like, you know, especially when it's humans who are getting infected. Like, I'm totally on board with trying to eradicate the parasites that are hurting people. And not just trying to eradicate them, but like feeling angry at them. Yes. Yeah, feeling angry at them. Judgmental. Yes.

But I also, but I still have like a begrudging respect for, for example, a species. like, what was the species? John Hodden was telling us about a species of parasite that is in our body and is able to essentially like rip proteins off of our cells and stick it on itself. And essentially we're like a, you know,

host coat to escape from us. And in our zombie fish episode, we were talking about trematodes that go through snails and fish and birds. And then they also have a phase where they're swimming through the water. Like imagine being able to succeed in environments that are that different.

It's impressive. And you've got immune systems that are trying to attack you in those different kinds of hosts. Like, that's kind of incredible that evolution has come up with solutions in all of those different environments. And so I am begrudgingly impressed with their abilities. And so anyway, I'm amazed with the solutions evolution has come up with for complicated problems. But, you know, I don't think that the suffering is funny or wonderful.

and especially when it's people, but it still is kind of amazing. It is amazing. Unless it's happening to people, then it stinks. Yeah. And I think it's probably misguided to put yourself on one side of it or the other or to assert like moral superiority

or inferiority to one side of these battles. Because, you know, who knows what these animals are thinking that they really have choices to make any way. In the end, we're like really imposing a lot of human values and emotions on these things. Yeah. Which is something weirdly we do, right? Like I impose human emotions on my dog.

I interpret my dog's emotions in the context of humanity. And so here I'm like really feeling for the mom. Like not only has she lost her babies, she ended up taking care of the babies that killed her babies. Yeah, I know. And you know, what's kind of weird is that, you know, we have all of this human baggage and we sort of carry it differently depending on what species we're talking about.

So when I was talking about the fish, I didn't feel that bad for the sicklet. They've kind of got like empty. vapid looks in their eyes. Your impersonation of the sickled mom was not especially empathetic, I have to say. Yeah, I'm sorry, but we're about to talk about birds. And when I see the bird babies getting yeated out of the nest, then I feel bad. And I don't know why. And it's kind of arbitrary and humans are arbitrary, I guess. Well, then let's take a break. And when we come back, we'll hear about more examples of interspecific brute parasitism, ones that make even Kelly feel bad.

Even Kelly. The social media trend that's landing some Gen Z years in jail. The progressive media darling whose public meltdown got her fired. I'm going to take Francesco off the network entirely. The massive TikTok boycott against Target that makes no actual sense. I will continue getting stuff from Target, and I will continue to not pay for it. And the MAGA influencers, whose trip to the White House ended in embarrassment. So refreshing to have the press secretary after the last few years who's both intelligent and articulate.

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Played poker with Harry Truman and had a long affair with a congresswoman. And then he took his talents to Hollywood, where he worked alongside Walt Disney and Alfred Hitchcock before writing a hit James Bond film. How did this secret agent wind up as the most of the secret agent? most successful children's author ever, and what darkness from his covert past seeped into the stories we read as kids. The true story is stranger than anything he ever wrote. Listen to the secret world of Roll Dahl on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. Okay, we're back and we're talking about species that trick other species moms into taking care of

their youth for them. Why can't they take care of their own babies, Kelly? What's wrong with them? Well, that's a great question. So, you know, evolution has now, them down this path where they can't take care of them anymore because they don't need to. There's this other strategy where they can get somebody else to do it. And so to me, the question isn't why can't they take care of their own babies anymore? The question is, why are the host parents allowing this to happen to themselves? Which sounds very much like victim blaming. It does. It does. Why were you wearing that? Oh, yeah. All right. I'm feeling uncomfortable about the phrasing already. But so let's walk through. So a big part of the way scientists think

about this question is how much pressure does it put on the host in terms of costs? Like how much, how many babies essentially are they losing in a season because this is happening to them? Okay. And it turns out the answer is it depends a lot. What a surprise. What a surprise. Yeah, right, on the details. So let's talk about the common cuckoo first, because this is one of the most well-studied examples. And for a common kuku, you lose a lot if a common kuku lays eggs in your nest. Some folks may have seen videos of this because these videos kind of really make the rounds when they start getting some traction. But so a common kuku will lay an egg in the nest of a bird that's much smaller than it is when that bird is like off collecting food or something. Okay. And when the

egg hatches, the egg tends to hatch again before the host egg hatches. And while that chick is still like completely naked, no feathers. It's at the stage where it's still like sort of trembling, trying to get its body in order, like its eyes are still closed. Feels like a helpless babe or something. Yeah, right. You'd imagine like, well, this couldn't do any harm at all. Someone needs to take care of this poor little baby.

It fumbles around the nest. And when it feels another egg, it puts its back up to the egg, lifts its butt up. Uh-oh. The music just got dark. Yeah, gets the egg up on its back. lifts it up over the side and then pushes its neck back to throw it over the side of the nest. What? And it does that over and over again until it is the only thing in that nest.

I'm imagining like security footage of some hospital where they have the babies in those little trays and like one baby is crawling around like pushing all the other babies out of the trays. That's the moral equivalent of what's happening here, right? Oh, yeah, that sounds, somehow that sounds way worse. Again, we're back to the humans having different sets of values depending on the species you're talking about. But yes, that would be the equivalent. So here, the cuckoo doesn't just want to be taken care of by the mom. It wants, like, exclusive access. Is that because of the food limitation?

It's like, want a bigger share of breakfast? Yeah. And it is going to grow to be much bigger than its host parent. And so it needs a lot of food. And if it does happen that some of the eggs do hatch before the baby gets a chance to push the eggs out, then it will just go ahead and push the babies out of the nest. It's kind of horrible. Is there no limit to what these cuckus will do?

I know. I know. Darn you, cuckus. And so sometimes these babies will grow to be like three times bigger than the parents that are frantically trying to feed it. Wow. Which is crazy. Like you'll see it. It's like, you know, the baby cuckoo is like spilling out of the nest.

And the parents are still frantically trying to shove food in its mouth as it screams at them. Like feed me, feed me, feed me. And there's nothing in the parents' brain that's like, hmm, something is weird here. or trigger some sort of response so it recognizes that it's not its own baby? Yeah, all right. So here is where evolutionary biologists get super excited about this system, right? Because that should be happening. Yeah.

And so this has become a really sort of exciting system for studying co-evolution. And so that is when you get evolution happening with both the host and the parasite. So what should be happening here is that the host should be figuring out some way to detect the cuckoo egg so they can throw it out of the nest. And then the cuckoo should end up with some way to make it harder for the host to detect the cuckoo egg. And then you should end up with this arms race where you keep getting the host getting better at evicting the cuckoo and the cuckoo getting better at tricking the host and back and forth. Yeah, that makes perfect sense. And so is that what happened? Is the cuckoo, like, developing techniques that it knows will take advantage of weaknesses in the host bird brain?

Okay. So I sent a picture to you in the outline. Oh. And I want you to scroll down. Okay. Scroll in, scrolling, scrolling. I've shared with Daniel a photo where the top row are all eggs that were made by cuckoos. And it's like a bunch of different eggs.

It's like green and orange and ones with squiggles on them and spots. Very, very different kinds of eggs. Very different kinds of eggs. And then the bottom row are all the host eggs that they're trying to match. Wait, those are not from the same species? That's crazy. They match exactly. They look like detail for detail the same as the original eggs.

Are you just being nice to me or do you mean it? Because I think they're pretty good matches. They're incredible matches. Yeah, I mean, maybe they're a little bigger or a little smaller or whatever. And I don't know what the variation is. But the difference between the host and the cuckoo is much smaller than the difference between the different species. So they're pretty good matches, yeah. Yeah, they're pretty good matches.

And there are stories about, you know, evolutionary ecology. going out into the field and looking in a nest and being like, okay, I just saw a cuckoo lay an egg in here. Which one is the cuckoo egg and which one is the one that was laid by the hosts? All right. So the cuckus know that the hosts are looking out for this and they're doing their best to camouflage their babies. Yeah. And by doing their best, essentially what we mean is that, you know, there's a lot of cuckus laying a lot of eggs. Randomly a cuckoo will lay an egg that happens to be a shade or two that's a little bit closer.

And that egg is less likely to be rejected. So that female makes a lot more babies that make it to the next generation. And their egg also tends to match a little bit more. Right. There's no intentionality or design here. It's just natural selection, right? Yes, exactly. And so over time, these cuckoos have ended up with eggs that do an amazing job of matching

these increasingly complex eggs that their hosts are making. Incredible. And so they have essentially made it very costly for the, moms to eject an egg from the nest. And so there have been cases where, you know, scientists have been watching a nest and a mom has looked at the eggs in the nest and, you know, presumably, you know, we don't know what the moms are thinking, but they've ejected an egg. And we've, you know, seen that a cuckoo has laid an egg. We know which one was the cuckoo egg. And she ejected her own egg and not the cuckoo egg.

That's even more tragic. I know, right. So mistakes get made. Mistakes get made. Wow. So when the dad comes back and the mom is like, by the way, I killed one of our babies, mistakes were made. Do you think he's going to accept that passive voice? Well, you know, the secret at Daniel is they don't actually talk to each other like that. Oh, I see. So you're saying that's the secret to marital harmony is just stop talking to each other. That's right. Low communication ability. That's the key.

This is not a marriage advice podcast, by the way. That's right. That's right. Yes. Please go somewhere else for that. We're going to get you in trouble. But parents will start using other information in the environment. So, you know, scientists will go out there and they will, like, put a mimic cuckoo egg in a bird's nest. And they'll note that, like, the birds won't respond to it. They'll, like, look at their nest and they won't eject any eggs. But if you put a stuffed cuckoo near their nest, they'll be like, oh, those jerks are around.

They might be, and then they'll start ejecting. eggs. And so they're, you know, they're trying to collect extra information, like how high is the risk? Are there cuckoos in the area? If so, maybe I'm going to have to take a closer look at my eggs. Oh, boy. And so, you know, it's not like they're just sort of passively accepting things. They're constantly looking, collecting information about risk. Sometimes they're making mistakes. It's getting increasingly complicated. Wow. But what about the scenario we talked about originally where now you have this cuckoo and it's growing and it's like much bigger than your own babies, isn't that a clear signal? How can the cuckus continue to be fed by the mom when they're so

outrageously big? Yeah, so there are examples in some systems or with some of the 200 plus hosts of common cuckus where the hatchling will get rejected at that stage where like a mom or dad or whatever will be like, ah, darn it. You know, we thought we had this figured out, but you are clearly not our own and they'll just abandon the nest. But there's also selection for cuckoo offspring to have the features that the parents are looking for. So, for example, some offspring have very particular markings inside of their mouths that the parents queue in on when they're bringing food. And they can be, you know, like orange splotches.

And so sometimes cuckoos will have like these very species-specific markings on the inside of their mouths. And I forget if this is common cuckus in particular or other species of cuckus because there's a lot of cuckoo species with a lot of different strategies. but there is also an arms race at various other stages. So yes, there's the egg recognition stage, but then there's also the hatchling recognition stage. Yeah, there's a lot going on. And so not only does it vary depending on like how well the eggs match

or how well the hatchling matches, but there's also pressure depending on how bad the situation is for the parents. So like if you've got a common cuckoo hatchling that's going to throw all your eggs out, there's a lot of pressure to get that right. But sometimes there are some other species of birds that will like lay one egg in your nest. They won't throw your eggs out. And their offspring will take about as much energy as the rest.

And so the parents still get all of their babies that will hatch out. Maybe they all get a little bit less food than they would have otherwise. But in that case, there's a lot less like selective pressure to figure out which egg doesn't belong and kick out the intruder because, you know, your babies are still going to survive. So we see a lot of variability depending on what the parasite species is and what it does to the host, on whether or not hosts are just like they tolerate what's going to happen to them or if they're like actively trying to fight back against, you know, the presence of parasites in the area. And for birds, is it both moms and dads that are feeding the babies? Do they both need to evolve this? Are the dads involved? How does that work?

Depends on the species. Sometimes the dads are involved. Sometimes the dads aren't involved. I think in general the pressure is more on the moms. And so there's a big question in the field, how do the parents figure out what their eggs should look like? Yeah. So initially there was this idea that maybe moms figure out what their eggs should look like by, like, the very first egg they see in their nest, they will, like, imprint on that and use that as a template for the rest of their lives. But if they make the nest and the very first thing that shows up in their nest is a cuckoo egg, is that going to mess things up for the rest of their lives?

Yeah. And it turns out that's probably not how it works. It looks like maybe at the beginning of every season for like this, for some of the species that we've looked at, the very first egg that they lay, they turn around and they're like, that's what I do. That just came out of me. So that's a, that's a me. That's a me. And it's not like it's monogrammed or something. But like, and the problem is, you know, like you mentioned, there's variability.

And so, you know, you hope that's a pretty good representation of what you're going to be doing that season. but that seems to be kind of how they do it. And the pressures must be complicated also because if the cuckoo is too successful, then it's essentially going to kill off the host. Then there'll be no host babies to grow up to be tricked parents to raise the next generation of cuckoos. So isn't there pressure for the cuckus to not push all the other babies out of the nest? So in situations like that, you can get cycling.

So if you do end up with a whole lot of cuckus one year, then the next year there's just not going to be a lot of parents. There's not going to be a lot of host nests that you can lay eggs in. So a lot of cuckoos are just not going to be successful. So there won't be a lot of cuckoos the year after that or, you know, five years after that or something, depending on how long the cycling takes. So it's self-regulating that way. Yeah, it's self-regulating that way. Interesting.

I'm sure there are some cases where cuckoos, for example, have driven local species locally to, you know, extinction or extirpation. Gr, now I'm mad at the cuckus again. That's right. But usually, my sense is that usually you would get. something like cycling. And I don't even know what a cuckoo looks like. Is it cute? Because weirdly, that affects my emotional reaction here.

It's pretty cute. Yeah, I mean, yeah, it's pretty cute. It's got like bars on its chest. Some think that it's kind of meant to mimic some raptors so that warblers will like, we'll leave it alone a little bit rather than mobbing it because they're scared of it, at least initially. And then they realize, oh, you're a cuckoo. You're not going to eat me.

I hate you. And then they'll try to chase it away. All right. Let's take a break. And when we come back, we'll hear about why sometimes birds might even tolerate these invaders in their nest, even when they do discover them. The social media trend that's landing some Gen Z years in jail. The progressive media darling whose public meltdown got her fired.

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But did you know he was also a spy? Was this before he wrote his stories? It must have been. Our new podcast series, The Secret World of Roll Doll, is a wild journey through the hidden of his extraordinary, controversial life. His job was literally to seduce the wives of powerful Americans. And he was really good at it. You probably won't believe it either.

Okay, I don't think that's true. I'm telling you, the guy was a spy. Did you know Dahl got cozy with the Roosevelt's? Played poker with Harry Truman and had a long affair with a congresswoman. And then he took his talents to Hollywood, where he worked alongside Walt Disney and Alfred Hitchcock before writing a hit James Bond film. How did this secret agent wind up as the most

successful children's author ever, and what darkness from his covert past seeped into the stories we read as kids. The true story is stranger than anything he ever wrote. Listen to the secret world of Roll Doll on the IHeart Radio app, Apple Podcasts, wherever you get your podcasts. Okay, we're back and we're talking about interspecific brute parasitism when birds or fish are tricked into raising birds of another species. And we've heard all about the clever tricks cuckus use and other birds used to avoid being spotted. But you were also telling me, Kelly, about the mafia hypothesis that there's an idea for why these birds might actually tolerate these invaders in their nest. What is that all about? Yeah. So one person noted, like, you know, wow, there's a lot of instances where it really

seems like a host bird should have been able to notice that there was a, like, parasitic egg in its nest. It should have thrown it out, but it didn't. What other reason could we come up with for why a host bird might tolerate it? And one idea that I believe it was a Matsahavi came up with was that maybe they're being coerced. Maybe it's sort of like a mafia sort of thing where if they were to kick it out, if they were to destroy the parasite eggs, then the parents would come back and would like take retribution in some way. And there have actually been a couple tests of this. So for example, brown-headed cowbirds, which you can find in the United States, they parasitize. prethinottery warblers.

I'm just going to call them warblers because nobody wants to hear me stumble over that again. And there was this experiment where they had these greased poles with nest boxes up top. And the reason it's important to know they had greased poles is because snakes and small mammals couldn't get up the greased poles

to eat the eggs. And so now you can sort of control predation and know what got the eggs because the only thing that could steal the eggs from the nest would be things that could fly up there. Okay. All right.

So they had three different treatments. They knew that cowbirds had laid their eggs in each of the warbler nests. In the first treatment, they removed the cowbird egg. So the parasite egg was taken out. Yeah. In the second treatment, they left the cowbird egg in there. In the third treatment, they took out the cowbird egg, and then they narrowed the hole a little bit. So the warblers could still get in.

But the cowbirds, which are big. bigger couldn't get in anymore. All right. And then they looked to see what happened to the host eggs, which had been left in all of the nest boxes. And what they found was that the nests where they had removed the cowbird eggs. The invading eggs. The invading eggs.

The host eggs had been destroyed. Destroyed. When a cowbird egg was removed. But that didn't happen when the cowbird egg was removed, but the cowbirds couldn't get into the nest because the hole had been destroyed. narrowed. And it didn't happen when the cowbird egg was left in the nest. And so it really looked like the cowbirds were coming back and taking retribution on the warblers. You know, presume they thought the warblers had removed the eggs and they were taking retribution. And why would they do that? Well,

when the warbler nest fails, they'll remove everything from that nest and start over again. And when they start over again, they're probably going to accept the cowbird egg the next time. And if they don't, they'll get attacked again. And so this is the cowbirds way of saying, hey, you've got another chance, make the right decision the next time. Awfully nice nest you've got here. It'd be a shame if something were to happen to it. That's right. Exactly. I see. There have been some other experiments that people have done that have gotten similar results. There's also some evidence of what we call farming. And the idea here is that if a parasitic bird arrives too late in a season and the eggs are about to hatch, like the host eggs are about to hatch, that would be bad for them because

their babies need that head start. And so they'll go through and they'll destroy a bunch of nests. So the parents have to start over again. And when the parents start over again, they'll go and lay their eggs in those nests. And so they call that farming because they're essentially like creating resources for themselves that they'll be able to use because now they're at the right stage at the right time. And this implies somehow that the host bird, the one that destroyed the invading eggs the first time and not the second time, has like learned this. You know, because like in the mafia example, you're very clear. You're like, look, I'm going to bust up your shop if you don't pay the protection money or whatever. But there's no like conversation

between these birds. So the host has to effectively learn this behavior. It's like negative reinforcement of this behavior. That's something that these birds are capable of. It seems that they are. I would bet that not all species catch on and I would bet that some couples need to learn the lesson more than once. And I should note that we don't always see this happening. Not all bird species

that do interspecific brood parasitism use this mafia strategy. And we don't always even see the species that do this using the mafia strategy all the time. And so I think a lot of these parasitic birds will lay their eggs in a bunch of different nests.

And so some years, maybe they're like, oh, you know what, I've laid a hundred eggs and a hundred different nests. I don't need to watch them all. This was a great year for me. But in some years where maybe the stakes are higher, they might be like watching a little bit more closely, you know. So yeah, I'm guessing it doesn't always go according to plan, but there are some years where they give it a shot and hope that the message sticks and other years where it doesn't work

as well. And so just so that I understand, this is not a selection thing where you're just choosing the nests where you don't get destroyed by cutting off the reproduction path of those that have destroyed it. You're really teaching these birds that if you destroy the invading eggs, we will also destroy your eggs. So this is like within the life cycle of an individual bird, not like within generations of that host. That's right. Yeah. This is within the, so for some bird species, they're able to have multiple nests in a year. And if their nest gets destroyed earlier in the year, then they're able to start. a new nest sooner and maybe they can go through more cycles or maybe they can just do one cycle

if their nest goes through the normal timing. But maybe they can get two in there if the first one is destroyed and they restart. And to really convince yourself with this, wouldn't you want to do something like analyze the behavior of the birds who had their nest destroyed relative to those who didn't to see if they're like less likely to kick out the invading eggs? Yes. Right. So you are wanting to know how well this strategy works. Yeah. Yeah, it would be great to know how well the strategy works, as opposed to just saying that, like, brownheaded cowbirds are real jerks. Yes.

Yeah, it would be great to have those data. I'm under the impression that we don't have those data and that the people who have done the follow-up work have been like, oh, well, we didn't actually see it happen at all this time. And so maybe it's sort of a rare thing. Interesting. Yeah, so we're at the cutting edge of understanding mafia behavior in birds. Yes. And maybe we will end up discovering, actually, this is the thing. that happens pretty rarely, and maybe it happens pretty rarely because birds aren't brilliant

and they don't pick it up real fast. Are you whispering in case birds are listening and you don't hurt their feelings? Yeah, well, I don't want them to attack me, you know, like maybe they'll go after my kids. Oh my gosh, yeah. Yeah, it'd be bad. And so how rare is this? Like what fraction of bird species or fish species do this? So it has been most thoroughly studied in birds.

Like it's been a little bit studied in fish, some insects do it. It's like scattered around the animal kingdom, but in birds it's gotten the most attention. And about 1% of bird species seem to do it. Wow. It looks like it's popped up about seven times. Seven times. What do you mean? So if you look at like the tree of life and you've sort of started at the branch where birds pop up,

or along seven of the branches, this strategy of laying your egg in the nest of a different species has developed. And then there's been like speciation where you get more species that do it and they keep this strategy with them. Oh, I see. So you're saying it's not just like one branch of birds do this and all the branches of birds that do it today come from that same branch. You're saying it evolved independently seven different places on the tree. That's incredible. Yeah, right. And so then one of the things that people have been trying to figure out is what were the birds doing before they started this strategy. And so one of the ideas is that maybe they were doing what we call conspecific brood parasitism, which is to say they were laying their eggs in the nest of other members of the same. species. Oh. So for example, wood ducks will nest in tree cavities. So, like, if there's a hole in a tree, wood ducks will lay their eggs in there.

But if there's not enough wood cavities that season and there's too many ducks, sometimes two females will lay their eggs in the same tree, and one female will evict the other. But maybe she can't tell that, I don't know if it says she can't tell the difference between all of the eggs or what, but she will sit on and raise all of the eggs and the chicks that were in that tree cavity. Okay. And so the idea is that maybe over time, you know, females started laying eggs in nest cavities. And instead of fighting to be the winner, they were just like, oh, no, I lost.

You can raise all of these. And then maybe they just started like laying eggs in other places and letting other, maybe even other duck species raise their young. So you could imagine it's starting that way. Another example are American coots. They're duck-like things, but they're extra kind of unattractive. sorry coots. But females that can't get nest sites tend to lay their eggs in the nest of other females.

What do you mean unattractive? They look cute. Oh, yeah. Okay, now I'm zooming it a little bit. Yeah. It's a little bit of a vultrifice. Yeah, yeah. I still like them.

I still like them. But anyway, so females who couldn't get their own nest will lay their eggs in the nests of other females or females who have gotten their own nest, but they, like, have made a bunch of eggs. And they're like, I can't really raise any more kids. But it can make more. eggs and they'll go ahead and lay their excess eggs in the nests of other females.

Like you, you know, you can imagine that maybe you could go from that strategy where you fill up your nest and then maybe you see if you can get some of your eggs in the nest of another species and, you know, you just try some stuff out and maybe that's the path through which interspecific brood parasitism arose. But for that to happen, for it to be passed down, for that behavior to exist also in your children, it has to be genetic, right? So there's something in these genes that have been tweaked that changes the behavior of the birds to accept this kind of scenario or to attempt this kind of approach. Yeah, exactly. But you can imagine it being less of a jump if you are genetically encoded to lay your eggs in the nest of another individual of the same species.

And then you lay your eggs in the nest of an individual of a different species than to go from I'm laying my eggs in my own nest and then jumping to, I'm laying my eggs in a totally different species nest. But actually, when you look at the genomes of all of these different species and you look at where they differ and you try to make a tree of life based on what those codes say, it looks like most of the time, maybe every time but once, that jump went through like normal parental care. So like they were sitting on their own nest, their own babies. and then the next step was they were laying eggs in the nest of another species.

And we're not quite sure how that jump happened. And so there's a lot of research going into trying to understand that. But for a long time, we thought there was probably this intermediate step. But we don't have any evidence to support that hypothesis. So now we're changing the way we think about it and doing some more research. And there doesn't have to be an intermediate step, right? If you can go in one generation from just being parental to live. laying your eggs in another nest and it works, then it's going to be reinforced by selection,

right? Yeah, so it's a little hard for me to imagine that in one generation there was a jump. But as long as that behavior was somehow encoded in a way that could be passed either through learning, like maybe you saw your mom do it. And so you do it or maybe like imprinting was genetically encoded. But how can you see your mom do it if you're raised by another mom, right? Yeah, right. So maybe it wasn't that you saw your mom do it.

Maybe it was just when you were hatched in the nest of another species, you saw that species and you were like, this is where eggs go. This is where eggs go. Right. And so that was like the thing that took one generation for the, you know, the change to happen. Hold on a second. I just had a complete realignment of my emotional connection because now I'm like, maybe cuckoos think that they are this other species. And they're just going there because that's how it works. You know, that's how they grew up.

they were raised by this other kind of mom, and so that's what they want for their babies. But they're still killing their brothers and sisters, though, if they think that. Okay, that's true. That's still not okay. I can't go all the way there. Yeah, right.

We got like, I don't know, 25% of the way there or something. Anyway, I think this is fascinating, and it's really interesting as a way to study, like, evolution between parasites and hosts and, you know, sort of, like, how this arm race works and how one sort of tries to counter the effects of the other. And it's just kind of crazy the solutions that evolution has come up with. Like I will post on our Blue Sky account and our Discord channel and our Instagram account.

I'll link to like a picture of the cuckoo eggs and the host eggs that I showed to Daniel because it is incredible to me how close the match is. Yeah. And we didn't even talk about the genetics. Like it looks like genetic information you only get from females. So, you know, I'll be really quick about this. But like in humans, females have the XX, X, X, X, Croix. chromosomes and males have X, Y. But in ducks, the males are ZZ and the females are ZW. So they have different chromosomes, but the females are the ones that have two different chromosomes. So if the females are the ones that are continuously like picking a certain kind of host species to end up in like their nest over time, the genetic information about what the eggs look like could get passed on the W chromosome. And that's how you can. have one species that produces eggs that look super different and match a bunch of different

species, host species, egg coloration patterns. And you get your genetic information and your mitochondria from your mom. So that's another way egg color and pattern information could get passed through mom. And anyway, it's absolutely fascinating how they managed to color match all of these so closely. And I just think it's super neat. Evolution's weird and doesn't care about your feelings. Yeah. Well, let's go out on a limb. What do you think is the future of interspecific brood parasitism? If birds went from like, I'm going to have another mom take care of my babies in the same species to then eventually to another species in a million years, are we going to have like elephants taking care of bird babies? Hmm. You know, I don't necessarily

doesn't really work necessarily like work that way. Like it doesn't have to like become more extreme or more species don't necessarily have to do it over time. We're not going to have antelopes raising cheetah babies or something? Yeah, no, I don't think so. I don't think that'll happen. You know, I think it's possible that some introspecific brood parasites will drive the hosts that they were parasitizing, like, to extinction. Or maybe they'll switch to a different host species if their other host species

is like, you know, the population numbers are getting low. But I'm not expecting that, like, next year I will be raised. raising kookos and I won't realize they're not human babies. All right. And if any of our listeners are warblers, maybe, you know, go out there and check on those babies and make sure you're not raising kookos unless, hey, you want to, in which case, more power to you because nature tells us that there's all kind of families out there. That's right.

All right. Until next time, everyone. Daniel and Kelly's Extraordinary Universe is produced by IHeart Radio. We would love to hear from you. We really would. We want to know what questions. you have about this extraordinary universe. We want to know your thoughts on recent shows,

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And each week we dive into real conversations that help you move with more clarity and confidence. This episode, we're breaking down what really happens to your information online. and how to protect yourself with intention. Listen to therapy for black girls on the Iheart radio app, Apple Podcasts, or wherever you get your podcast. You know Roll Doll. He thought up Willie Wonka and the BFG. But did you know he was a spy?

In the new podcast, The Secret World of Roll Doll, I'll tell you that story, and much, much more. What? You probably won't believe it either. Was this before he wrote his stories? It must have been. Okay, I don't think that's true. I'm telling you.

I was a spy. Listen to the secret world of Roll Dahl on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. The social media trend is slanding some Gen Z years in jail. The progressive media darling whose public meltdown got her fired and the massive TikTok boycott against Target that actually makes no sense.

You won't hear about these online stories in the mainstream media, but you can keep up with them and all the other entertaining and outrageous things happening online in media and in politics with the Brad versus Everyone podcast. Listen to the Brad versus Everyone podcast on the Iheart radio app, Apple Podcasts, or wherever you get your podcasts. Crook and Chase Nashville chats with the coolest cat in country music. He is edgy. He's fun.

He is Chase Matthew. Keith Urban helped make him a global sensation last year, but it's his recent number one hit Darling that put Chase on the map and launched a fun and difficult conversation about lovers who cheat and lie. That's why I'm scared to get married. You give me trust issues, brother. Crook and Chase Nashville chats with Chase Matthew. Listen and subscribe on the IHeart Radio app, Apple Podcasts, or wherever you listen to podcasts. This is an IHeart podcast.

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