E41: Okay, but griffins, and ligers, and birds, oh my!

Scott Taylor: My favorite movie is The Last Unicorn, and the only thing that could have improved it is if the unicorn had wings, which would make it a pegacorn, also known as an alicorn. It's kind of funny that I've spent most of my career studying hybridization because technically a pegacorn is a hybrid, a unicorn crossed with a pegasus carrying features of each and, of course, all the magic in the world. The fact that anyone was surprised when I came out as gay is still puzzling to me. Centaurs, ligers, mermaids, minotaurs, we've been mixing creatures together in our stories for about as long as we've been telling them. Which suggests the idea of two things becoming one has always unsettled us a little. And birds do it constantly. When two species mate and produce offspring, we call that hybridization, and the offspring are hybrids. Where that happens across a landscape, usually at the edges of where two species meet, we call those places hybrid zones.

Scott Taylor: One of the most common sentences I've written in my published papers is this, hybrid zones are windows into the evolutionary process. They're the gray areas where what is a species gets much harder to answer and where you can watch evolution work in real time instead of reconstructing it after the fact. Mixed up colors are obvious to visual creatures like us, which is why bird hybridization has been studied for a very long time. That doesn't mean we figured it out, though, which brings us to the topic of today's episode.

Scott Taylor: Okay, but griffins and ligers and birds. Oh my. To help us unpack that, I'm joined by Dr. Glaucia Del-Rio, curator of ornithology at the Florida Museum of Natural History and assistant professor at the University of Florida. After the break, Glaucia walks us through why a river can be an impossible barrier for a bird with perfectly good wings. We hear about an expedition she led into a stretch of the Amazon almost no ornithologist had worked before, and the woman she named it for.

Scott Taylor: And we get into the question sitting underneath all of this. When two species start blending together, are we gaining one or losing two? Stay tuned.

Scott Taylor: All right. Welcome back to the podcast, everyone. I'm really excited to have Glaucia here today to talk about hybrid zones. Thanks for joining us.

Glaucia Del-Rio: Thank you so much, Scott. I feel really happy to be here and be part of this podcast.

Scott Taylor: Awesome. Yeah. Well, I appreciate it. I mean, as you know, this topic is very close to my heart, I guess, and brain and career. So I'm excited to get to chat about it with you.

Scott Taylor: When you tell someone at a dinner party that you study bird hybridization, what is the first question they ask you?

Glaucia Del-Rio: I mention that I work with hybrid zones, usually people ask, wait, but if these are two different species, and they are hybridizing, isn't the point of a species that they cannot mate with each other? That's usually what people learn when they learn what a species is. And then because I don't want to go into that conversation because it's so philosophical. And it's like, oh my gosh, I'm going to get all my taxonomist friends very mad here. But because I don't want to discuss like what a species is and so on and so forth, I lead by saying, oh, I work with Amazonian birds. I go to the Amazon and I try to understand what birds are there and how different species of birds arise. That's usually how I lead the conversation. And then I throw in some genetics in there. Otherwise, if I just keep saying birds, birds, birds, people will start saying, oh, I had a cockatoo at some point. It got sick.

Scott Taylor: Why did it die? You're like, I don't know. I'm not a vet.

Glaucia Del-Rio: Yeah, exactly. Ah, my bird, I have a pet bird and it's not eating. What do I do? And then I say, I'm not that kind of doctor. I'm not sure.

Scott Taylor: Yeah. Or they're like, I saw this brown bird once in this place. What was it? And you're like, I don't, I might know, but like I need more context clues. Yeah. I guess like when people, when I tell people what I do, I often say, yeah, I studied chickadees or something like that. I don't necessarily lead with the hybrid part. And it's because of what you just said. It's a little bit. More complicated to talk about. And you have to like explain a few different parts before people can really get it. But if I was to ask you to try to do that, what would you like, how would you approach it?

Glaucia Del-Rio: Yeah, that's when I start saying, okay, so it's more complex than that. When in hybrid zones and in birds, they are quite common. It's not like they are like rare. Scenarios. In hybrid zones, like these two species can still mate, but the chicks, they probably can still mate too, but they face more challenges than what we call like the parental forms or the birds that are in populations that are not coming into the hybrid zone. And these challenges can be, oh, maybe when their eggs hatch, not as many eggs hatch. Or maybe they have trouble finding food when compared to the parentals. So here I'm trying to bring that conversation about fitness and the fact that these birds that are... hybridizing that are in the center of the hybrid zone. They are experiencing selection or selective pressures, which I'm calling challenges here, that can be things about their natural history that are not exactly as successful as it would happen in the parental forms.

Scott Taylor: Yeah, I take a similar approach. Oftentimes I just bring up a mule. I mean, I'm from a rural part of Ontario where we would go to like plowing matches and the mules always kicked the horse's ass because they were like way stronger. And I was always fascinated by like, okay, well, we generate mules by crossing horses and donkeys, which are quite divergent. And then most mules, but not all mules, but most mules are sterile. So it's like a pretty like... costly thing if you produce offspring that can't themselves make more offspring that you know you should avoid that and so I usually start with mules but then yeah the nuance especially in the birds we work on is like well they're sometimes sterile but maybe it's only one sex and then maybe neither sex is sterile and they can do most of the same things as the parents but not quite and like yeah but it is it's the gray zone that's kind of fun to talk about but more complicated too.

Glaucia Del-Rio: I think in the case of the birds that I study, because they are in the Amazon, in these super isolated areas, there's so little that we know about actually what are the consequences of hybridization for them.

Glaucia Del-Rio: Of the reasons I like these hybrid zones between Amazonian birds is that what is separating them is not like these differences in the habitat in general, but the rivers. Amazonian rivers have this only like history, they have these crazy characteristics that make them like good barriers for these birds. And then that's another question that I get in dinner parties. Okay, so you are telling me that these Amazonian rivers separate birds, but couldn't birds just fly over it? And the way they are migrating, they're not going to fly over it? No, wait. We are talking here about birds that are very different from your general conception of a bird. We are talking about these birds in this family, antbirds. They behave much more like small mammals than actually birds. They don't fly a lot. They spend their entire lives in... just these tiny territories and hopping around in the vegetation. They avoid the sunlight as much as possible. And they don't even go very close to the rivers because the forest close to the rivers is different from the upland terra firme forest where they live. And because of that, so these Amazonian rivers can... separate these different bird species that will actually only meet again and form these hybrid zones at the headwaters of the rivers where they can either circumnavigate or where the rivers are so narrow that birds are then more able to cross them. And so we don't see like these differences in temperature and precipitation across the Amazonian rivers.

Scott Taylor: It's hard to conceptualize just how wide some of these rivers actually are. They're gigantic. But even when they're not gigantic, yeah, like you're saying, these birds avoid going over rivers. There's a study, I don't remember what year it's from, where they, took birds to the middle of a river and then let them go to see how far they could fly or whether they could get across the river. And I feel like it was an antbird that just never made it. Oh, yeah.

Glaucia Del-Rio: Yeah, and they kept doing those experiments first in Barro Colorado, but then Luciano Naka and Brazilian researchers, they did some of these experiments in Amazonian rivers. And yes, not only like antbirds, but also like manakins and... some woodcreepers, they just cannot make it. They try to release the bird from a boat in the middle of the river, and they just fall in the water.

Scott Taylor: Yeah. And then they scoop them up and save them. So don't worry about that.

Glaucia Del-Rio: Yeah. So don't worry about them. It's all good. It's all approved by the ethics committees.

Scott Taylor: Okay, so walk us through what's actually happening at a hybrid zone. What does it take for two species to meet, mate, and then produce offspring that survive?

Glaucia Del-Rio: Gosh, this sounds like my general exams, my A exams.

Scott Taylor: Exactly. I know. Your A exam coming from someone who knows the answers, but whatever. Let's just talk about it.

Glaucia Del-Rio: Yeah, I think especially with Amazonian birds, we are thinking about... a time in the past when these populations of birds were isolated from each other. So we can think about like forest refugia separated by these areas that look more like savannas and the birds never go there. But then nowadays in certain areas these refugia are expanding, reaching these riverbanks and these forest now there are opportunities after I don't know, maybe hundreds of thousands or millions of years in isolation, these birds are now able to meet each other. And then it depends. It depends how many differences they accumulated along the way, right? Not only differences in how they look, but how they behave or how well adapted they are to a different thing. A local thing, right? That's why we were talking about that in chickadees, right? Like the differences that can be ecological. Or I don't know, maybe there are some differences in antbirds, thinking about the birds in the Amazon. Maybe they are super well adapted to one specific ant that is only in one side of the hybrid zone and not in the other. We know so little about the natural history of these birds that I don't know for sure what could be the very local, special things they have.

Glaucia Del-Rio: But then they can meet again in what we call secondary contact, right? So they were one day one thing, they were separated, they developed these differences, and then they can meet again in secondary contact. And that's when, like, species are tested. They can survive by the sight of each other. And do not recognize each other as like potential mates, right? So that happens all the time. So if, I don't know, let's think about a very crazy example just to make sure this point goes through. So if, let's say, a bird population is separated and then after time, one is very close to like a flamingo and the other one is... I don't know, a spoonbill. And they come back together and the flamingo doesn't recognize the spoonbill as a mate and vice versa, right? And then they can survive side by side as different species. But there are cases where the differences are not so dramatic. There are differences. There might be even like incompatibilities, like in the case of the but they are not so strong to prevent these birds from completely mating with each other. So they have the possibility to form these that we call hybrids. So for people who are not or who are seeing this on YouTube, you can see the image here. Here's my hybrid, my antbird, which is basically in the way we learned it was a hybrid. It was just basically looking at the colors of the plumage. So here are the two antbirds that I study are the White-breasted Antbird and the Harlequin Antbird. White-breasted has a white throat, white chest. And the Harlequin Antbird has these black throat and rufous brown chest. And where ranges of these two birds would meet at the headwaters of these rivers, we started finding these birds that had the rufous chest and a white throat.

Scott Taylor: So the wrong colors together. Yeah.

Glaucia Del-Rio: The wrong colors together. And this is kind of like a very recent discovery. The first person to localize this was Brett Whitney, and this was like 2004. And just because he was passing through an area in the Amazon that nobody had ever been before, where they built this highway that to this day is a dirt road. But basically he was like exploring that area and found this bird that showed this intermediate feature in plumage.

Scott Taylor: When I say I study hybrid zones and hybridization, people think that produces new species. And it usually does not. I think part of the problem is we've given at least common North American hybrids names like Lawrence's Warbler or Brewster's Warbler that then makes people think they're these distinct things.

Glaucia Del-Rio: I think an important distinction here, too, is that we are talking about hybrid zones. So these areas where hybridization is common.

Glaucia Del-Rio: Here. So hybrid zones, they are this special case because... hybridization is relatively common. So we can use these to study things that we don't know how to answer in terms of like what characteristics are important for these birds and what characteristics are selected for or selected against, what characteristics are incompatible between them.

Scott Taylor: Yeah, so tell us more about, okay, I'm going to say this wrong. Rhegmatorhina, is that the genus?

Glaucia Del-Rio: Well, that's how I say it in the U.S. I say Rhegmatorhina because that's how people understand what I'm talking about. But with my Portuguese accent, speaking Latin here, it becomes Rhegmatorhina.

Scott Taylor: Okay. All right. So that's the genus of the antbird group that you've spent years studying. So maybe tell us a little bit more about what these birds are doing. Why are they called antbirds? And then let's get into that aspect of what do you think is breaking down in these hybrids between the two species that you're working on?

Glaucia Del-Rio: Right, so Rhegmatorhina is this genus of antbird. And here we are talking about this super diverse group of birds that are only in the neotropic, so only like in tropical South America and Central America and a bit of North America. And they are called antbirds, but this name can be a little bit misleading. Because first, not all antbirds, actually only like probably 20% or less, 15% of antbirds are actually army ant followers. But Rhegmatorhina, yeah, is an army ant follower. They are an obligate army ant follower. So Rhegmatorhina, they are specialized in stealing the food from these army ants swarms.

Scott Taylor: Oh, I didn't know that.

Glaucia Del-Rio: People think about army ants, they think about that Indiana Jones movie Indiana Jones and the other main characters are running away from these ants that are trying to eat them and eating their boots. And it's not the right species of ants in that movie, but and I think what's happening with these people is the equivalent that what is going on in the head of insects and spiders. And because these ants that I'm talking about, they are predators. They are actually going like for these big arthropods and even like small vertebrates, small frogs, small lizards. When these swarms show up in the forest, birds get crazy. All the animals just try to run away except for these army ant followers, okay? So in that you have like some groups that are obligate army ant followers and this is the case of Rhegmatorhina, antbirds.

Glaucia Del-Rio: They are not interested in eating the ants. They are interested in stealing the food from the ants because in that way, you know, the vertebrates, they are vulnerable. So the birds can go for them and in that way they are able to hunt things much bigger than they would by themselves. And with this obligate army ant followers.

Glaucia Del-Rio: Don't find ants, they can starve to death and their chicks can starve to death. It's the only way they know how to find food. It's by stealing the food from the ants.

Scott Taylor: That's crazy. Ants move around a lot, right? Like they're going all over the forest. When the first time I was in Belize, I was at this eco lodge and every once in a while, they like try to get the army ants to go through their kitchen because then the army ants would like clean out all the cockroaches and all that stuff. But so these swarms are quite mobile. So what do your birds do? Like they have to nest and raise chicks, but then still be accessing mobile army ant swarms. That seems complicated.

Glaucia Del-Rio: Yes, so we think they have like much bigger territories than other antbirds and they need to keep track of three or four colonies of ants at the same time, especially during like breeding season so they are able to help the chicks. We also think that they might have helpers or like the young birds from different breeding season will help the parents find food for the new chicks. But all of that, it's very anecdotal. It's not like we have papers from the 60s and 50s describing all of that or with all this data. It's much more from our observation in the field.

Scott Taylor: Thinking about this, like in terms of what might be going wrong with your hybrid. So they have this intermediate plumage. So you can tell that they are admixed. So a mix of each parent species. But I immediately thought of if they've got to be locating ant colonies. I mean, you know that I work on spatial memory and chickadees. That's clearly very important. It'd be so cool to know if the spatial memory of your hybrids was some way impaired, right? If they have a harder time finding and tracking these ant colonies, that would be pretty strong selection potentially.

Glaucia Del-Rio: Yeah I also think And this is firsthand here because this is not published yet. So please, no one scoop me out there.

Scott Taylor: No one can scoop you on these antbirds. Can you imagine someone trying to go there and study them? Yeah.

Glaucia Del-Rio: But we think.

Glaucia Del-Rio: Exactly the areas of the bird, the plumage colors that are different across these species and then are mixed in the hybrids are really important for these displays that they do when they are competing with other antbirds, because there is a hierarchy. So, for example, there are other obligate army ants followers that occur in the same area as Rhegmatorhina. Some of them are stronger, bulkier. Some are smaller. And... The stronger ones, they occupy the center of the army ant swarm. Rhegmatorhina is a little bit more into the periphery, the outskirts, and then the smaller ones are way behind. So what defines that is all these interactions that they have with each other, including these displays that are vocal and also like puffing the feathers in the chest, the throat, and the crest. So we think it's really important for their communication. And we think that maybe the white plumage gives an advantage. To them because in other antbirds we know that showing like white patches in a mantle, showing white patches or white spots on the wings, it's important to display that power and dominance in these interspecific interactions.

Scott Taylor: Cool. And you've seen.

Scott Taylor: Right. So there's probably, and we've sort of hinted at this throughout the podcast, a question that listeners are sitting with thinking about what happens like when two species hybridize, are we gaining a new species or new kind of bird? Are we losing two old ones?

Glaucia Del-Rio: Not that simple. I think that as we start looking at their entire genomes, as we start looking at the variation in the DNA of these birds, as we start describing all these morphological variations and how they... this variation is important for birds in different manners, in different places, we can relax a little bit about thinking about species here and thinking about numbers. We can start thinking about the many... entities that are worth paying attention to and that are important for these evolutionary processes to keep happening. So I don't think I know that part of my work is showing like that the white plumage advanced over a geographic area. So it seems like. The white form, the White-breasted Antbird is going to erase the Harlequin Antbird. But we don't know that. We don't know because we don't know if there is something very specific to the Harlequin Antbird that white form will never be able to cross. And to overcome. So I think in that sense, and I think we are moving that direction already.

Glaucia Del-Rio: I think we are going to start seeing species as things that are more fluid. And that the important thing is to make sure that we are not thinking only about the numbers of species and which species we prioritize or not, but making sure that we preserve and pay attention to enough like geographic breadth that we are covering and preserving these entities that important and doing an important service where they occur.

Scott Taylor: Yeah, totally. I agree. I think if you study hybrid zones, you have to think of species kind of in this broader way and the porous barriers between species and all of this matter a lot. And that's what we focus on trying to understand. So this question of gaining one or losing two is too simplistic. And honestly, it's much more interesting just to understand what's going on in these gray zones of how species interact and exchange genes and You know, we've come to understand just over like the last 30 years that hybridization has played a role in the evolutionary history of most lineages, certainly many plants and now also many vertebrates and invertebrates. And so it's this really fascinating aspect of evolutionary history that's even influenced the evolutionary history of humans, you know, with hybridization between Homo neanderthalensis and Homo sapiens. And so thankfully you've got a lot more to learn about it, which is pretty exciting.

Scott Taylor: We've reached the part of the show we call That's B.S., or That's Bird Stuff, where we give our guests an opportunity to debunk a myth that ruffles their feathers. So, Glaucia, what do you want to call BS on?

Glaucia Del-Rio: When people know that I work with Amazonian birds, I think what immediately comes to their minds are like these colorful big birds, like parrots, macaws, toucans. And then... That's not true, right? Most of the diversity in the Amazon and tropical regions in general are made of these brown, gray birds that have very specific roles. In their environment. And that's true with the Amazonian birds, with antbirds, woodcreepers. You all go after, check these birds out. We are talking here about like hundreds of species, sometimes like 10 species in one single genus occurring in one... area and occupying different forest strata. And we don't see them that much when we are in the forest. We hear a lot. They sing. They have like very similar songs, very similar calls. So it's only when we set up a mist net in the middle of the forest and we start capturing them that, oh, we realize.

Scott Taylor: Mm-hmm. Yeah, they're quite cryptic. Yeah.

Glaucia Del-Rio: The numbers of different species that form these groups. So I think, yeah, that's the BS that gets me. When I say that, yeah, no, my birds are not super colorful, but still they are super cool. Although Rhegmatorhina has the lime green skin around the eye. The modified feathers that look like mascara. So yeah, they are interesting, beautiful features too, so.

Scott Taylor: Awesome. Well, thanks so much for joining us on the podcast. It's obviously been great for me to be able to talk more about hybridization and hybrid zones and lovely to have you as the person to talk about all of that with. So thanks for taking the time.

Glaucia Del-Rio: Thank you so much.

Scott Taylor: Amazon is. You have to maybe spend four days getting to these places that you study. And I know you organized this expedition, the Emilie Snethlage Expedition. Can you tell us a little bit about why you did that and how it went?

Glaucia Del-Rio: Yeah, so I was in the fifth year of my PhD, I guess, and I just came up with this idea and I brought it to my advisors at the time in Louisiana, Robb Brumfield and Brant Faircloth. And I'm glad they were very receptive. They just said... Okay, you should do it. And the idea was basically, I wanted to pay a tribute to an ornithologist who was very famous in Brazil, but not known in the US.

Scott Taylor: Yeah, I think I hadn't heard about her until you told me about this expedition.

Glaucia Del-Rio: Yeah. And she was like this amazing German-Brazilian ornithologist. She went to Brazil to be a curator of birds in the Goeldi Museum in Belém, in the Amazon. And she worked all over Brazil and other countries in South America. She described like 40 different taxa of birds. She basically would travel by herself, get to a place. And hire people to work for her. There is this story of her having a piranha biting the tip of her finger and getting an infection, but she was super far away from a hospital or anything like that, so she just walked around and asked people to help her. Nobody wanted to mess up with the wound. And then she just grabs a machete and she cuts the part of the finger that was getting an infection. Yes. Yeah, exactly. So that's a joke that I always tell to my parents specifically. This is the kind of role model I want for me, you know. And she cuts that part of her finger and just... stay in the field going after more birds and this is all like in the beginning of the 20th century and I think okay this is a very badass person and I want to pay tribute to her and I also want to raise awareness for the fact that we need more women in ornithology especially collections based ornithology. So I gathered this team of like nine women. We went to the Juruá River, a very crazy river, very winding, very dynamic with chunks of forest passing from one bank to the other in a matter of like few decades. And it's still a barrier for some birds. So it's, yeah, it was a great expedition. Nothing related to Rhegmatorhina, but still I learned so much about not only bird diversity, but also I think like the ecology of the Amazon and a little bit more about the history of these birds and the importance of the history of these birds for how interesting and diverse are Amazonian birds.

Scott Taylor: Whoa. That's pretty metal. Jeez.

Scott Taylor: Yeah. Amazing. Very cool.

Scott Taylor: Birds are dinosaurs, and around here we like our snacks, so we end each episode with a dinosaur nugget. Today's nugget is, draw two circles for two species. We want them side by side, separate, clean. But almost every one of these is really a Venn diagram, and the part we keep ignoring is that lens in the middle where they overlap.

Scott Taylor: Hybridization has shaped the history of a huge number of lineages, plants and animals alike, and ours too. Most people alive today carry DNA from Neanderthals, which means we are standing in the overlap. So when someone asks whether a hybrid zone means we're gaining a species or losing two, they're really asking us to erase the middle. That's a wrap on this week's episode. Okay, but griffins and ligers and birds, oh my. I've wanted a pegacorn since I was six and it hasn't worked out. So at this point, I'll settle for your five-star rating and a lovely review. We'll catch you next time. Byeee.

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.