A Rare Hybrid Bird Sheds Light On Evolution's "Mistakes" : Short Wave When Steve Gosser heard the song of a scarlet tanager in the woods, he knew to look for a bright-red bird with black wings. But when he laid eyes on the singer, he saw instead a dark-colored head, black-and-white body, with a splash of red on its chest. "Well, that sort of looks like a first-year male rose-breasted grosbeak," he said. The song of one bird coming out of the body of another suggested this little guy could be a rare hybrid. Gosser enlisted the help of some pros, including biologist David Toews, who conducted a genetic analysis to see if this was truly the offspring of two species that diverged 10 million years ago, and today run in very different circles. On today's episode, Gosser and Toews fill Aaron in on this avian mystery, and what hybrid animals can teach us about evolution.

He Had His Father's Voice: Tracking A Rare Bird Hybrid

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EMILY KWONG, BYLINE: You're listening to SHORT WAVE from NPR.

(SOUNDBITE OF MUSIC)

STEVE GOSSER: Well, this happened just about 30 minutes north of where I live at a place called...

AARON SCOTT, HOST:

Steve Gosser is a wildlife photographer and a self-proclaimed die-hard birder. He lives outside Pittsburgh. And he didn't know it yet, but he was in for something unexpected.

GOSSER: You know, I got there kind of early in the morning and get out of my car. And, you know, usually the first thing that I start noticing are, you know, the birdsongs.

(SOUNDBITE OF BIRDS SINGING)

SCOTT: There are warblers.

GOSSER: And those are probably my favorite birds to try to find and get pictures of.

SCOTT: There is a scarlet tanager.

(SOUNDBITE OF BIRD SINGING)

GOSSER: I thought, oh, you know, I've already got plenty of pictures of that just recently, so I'm not going to even bother with that bird.

SCOTT: And there was a northern parula.

(SOUNDBITE OF BIRD SINGING)

GOSSER: And that was the bird that I was really hoping to spot. So I went over to the spot where I heard it singing. And as soon as I get over there, it, like, completely goes quiet. So it basically just seemed like it vanished.

(SOUNDBITE OF BIRD SINGING)

GOSSER: So I was, like, well, that scarlet tanager is still up there singing away.

SCOTT: So he starts looking for it up in the tree canopy.

GOSSER: And I remember seeing it, with my naked eye, fly out from this one spot. And scarlet tanagers are bright red birds with black wings, and usually you can spot that bright red like a mile away. And I remember when I saw this bird come out, it was, like - looked real dark to me. So I can, like - immediately knew that it wasn't a scarlet tanager. So I get my binoculars on it finally, and I look at it, and I think, well, that sort of looks like a first-year male rose-breasted grosbeak.

SCOTT: Rose-breasted grosbeaks have black heads, white bodies and a Valentine's Day red patch on their chest.

GOSSER: So I'm looking at it through my binoculars, and all of a sudden, it starts singing just like a scarlet tanager.

SCOTT: So Steve thought, maybe this grosbeak had somehow learned a tanager song. But then he noticed something funny. Rose-breasted grosbeaks have white bars on their black wings, but this bird just had black wings, like a scarlet tanager. And instead of having a black throat, the red on its chest went all the way up to the beak. And that beak was too thin for a grosbeak. It was more like a tanager's.

GOSSER: The first thoughts of - were coming into my head, like, could this possibly be, you know, a mix between - like, a hybrid?

SCOTT: Steve knew that some closely related species can reproduce together. But he couldn't find anything online mentioning these two. So he fired off a photo to an ornithologist friend of his, Bob Mulvihill.

GOSSER: I was expecting him to have, like, some explanation. But he just replied back, and he said, that's one weird bird you got there, Steve.

(LAUGHTER)

SCOTT: So the next day, Bob and another ornithologist with the National Aviary came out. They caught the bird, drew a blood sample and sent it to someone they thought could solve their mystery.

DAVID TOEWS: So I have cornered the very lucrative market of detective of weird avian hybrids.

SCOTT: David Toews is a biology professor at Pennsylvania State University.

TOEWS: I get a lot of folks reaching out about certain individuals that they've identified that they just can't fit into our nice, neat square boxes of bird taxonomy.

SCOTT: And so it sounds like in birder circles, hybrids are something that is talked about? I mean, is it almost more exciting to see a very rare hybrid than a normal bird on your list?

TOEWS: I mean, it really is, in many ways, a sport. And the most elite players are able to identify, you know, every species that they're able to see. But then the next level of the most elite is not only being able to identify rare species but potentially rare hybrids between species. And so, yeah, absolutely, they are sought after.

SCOTT: And this hybrid turned out to be a wonder. That's because its parents are not close cousins. In fact, they haven't shared a common ancestor in over 10 million years. For perspective, humans and chimpanzees hadn't even diverged yet by then.

So today on the show, solving the mystery of a bird that shouldn't exist and what it can teach us about evolution. I'm Aaron Scott, and you're listening to SHORT WAVE, the daily science podcast from NPR.

(SOUNDBITE OF MUSIC)

SCOTT: David Toews' whole career is dedicated to studying hybrid birds. But according to him...

TOEWS: These are mistakes. This is a situation where birds don't read our field guides.

SCOTT: (Laughter).

TOEWS: And so - you know? They- you know, they're probably better at identifying bird species than we are. But even still, they make mistakes.

SCOTT: About 1 in 10,000 birds are hybrids. And usually they pop up between species that are closely related, which is why Steve's discovery of a cross between a rose-breasted grosbeak and a scarlet tanager is especially surprising. Not only did they diverge millions of years ago, but even in the present day, they're not really hanging around the same circles.

TOEWS: So rose-breasted grosbeaks prefer, I would say, more fragmented habitat. So they'll show up in what we would call, like, younger forests, like, secondary-growth forests that are less mature, whereas, scarlet tanagers, they prefer more sort of unfragmented, mature forests. So you kind of have to be a little bit deeper onto, say, a hiking trail in your local woods to see one.

SCOTT: That said, just because Steve's bird looks like a grosbeak but sings like a tanager doesn't mean it's a hybrid. A more likely scenario is that somewhere in its infancy, it picked up the tune of a male tanager neighbor.

TOEWS: You can have situations where birds will learn the wrong song, that just didn't cue in on the right species during that learning process - doesn't mean it was a hybrid.

SCOTT: So Step 1 of David's work was to go through the bird's song with a fine-toothed comb. That means taking the recording and pulling apart the sonic bits and pieces that belong to different species. It's called bioacoustic analysis.

TOEWS: And then we can actually go in and with - even with a ruler, you know, measure how high the frequency, how long the song was, how many modulations there were throughout the song.

SCOTT: When David did this for the recording of the bird Steve found, something jumped out at him - a little component of a larger song that he recognized immediately.

TOEWS: It was actually called a chick-burr call, and that little - it was almost like a little calling card for the scarlet tanager. This is a vocalization that really only a scarlet tanager makes and a rose-breasted grosbeak does not.

(SOUNDBITE OF BIRD SINGING)

SCOTT: For David, this was pretty clear evidence that somewhere in this rose-breasted body was tanager blood.

TOEWS: But, really, it's the genetics that tells us who's mom and who's dad.

SCOTT: So next, he sequenced the mitochondrial DNA. This is DNA that's only passed from the female to her offspring, which lets you know who the mother species is.

TOEWS: And it came up with a near-perfect match for a rose-breasted grosbeak, which already sort of lit off the - OK, something's interesting here because, you know, this bird clearly sang like a scarlet tanager, and its mom was a rose-breasted grosbeak.

SCOTT: The last piece of the detective process is to move to the nuclear genome, the DNA that's in the cell nucleus, which comes down from both parents, and to compare the genetic markers.

TOEWS: Looking at the nuclear genome allowed us to confirm mom was a rose-breasted grosbeak - check, she was - and that the father in this case was a scarlet tanager.

SCOTT: Steve had discovered a never-before-found hybrid, a bird whose very existence is one big surprise. So why study hybrids in the first place? After all, many hybrids don't reproduce and turn out to be evolutionary dead ends. But the ones that do, they can actually play a special role in adaptation. David says to understand how this works, you can look at our own hominin lineage. Hybridization between Neanderthals and humans, which diverged more than 500,000 years ago, still shows up in our DNA today.

TOEWS: But even cooler is another lineage - they're called the Denisovans. And they found the genetic remnants of that lineage still being maintained in human populations across the world. And in some cases, they think it's actually been beneficial for those human populations that are still living at high elevations. Yeah, I think that type of hybridization and sharing of the answers to evolution's problems is probably pretty common.

SCOTT: So you can think of the bird that Steve Gosser found, that grosbeak-tanager hybrid, as a little evolutionary experiment. Just maybe it could give the hybrid a survival advantage, which in turn might shore up an avian lineage that's in peril.

TOEWS: The main groups of birds that I study, those species in particular have been showing significantly steep declines over the past several decades.

SCOTT: But there might be hope. David says it's part of the reason he studies hybrids in the first place.

TOEWS: Hybridization is actually a - is a generator of novelty. It allows, you know, independently evolved groups to share, that they're, you know, sort of trading information back and forth on solving problems that the environment presents to them. So this might actually be important for adaptation to climate change, for example.

SCOTT: What do you think the future holds for this little hybrid? And, I mean, maybe he's going to sing like a scarlet tanager but only court rose-breasted grosbeaks. The question I'm left with is, what are the chances he's going to find love?

TOEWS: (Laughter) Yeah. So unclear. We don't know, unfortunately. The little mystery of who his parents were was easily solved. But because the folks from the aviary went back to the same site in '21 and '22, and he wasn't there - now, that doesn't mean he didn't disperse and find some other patch with some other, you know, female scarlet tanagers that thought they'd give this bird with a beautiful song but looking a little funny a shot. We can hold out hope, I guess (laughter).

SCOTT: For all of us who look a little funny but still like to sing, I'm crossing my fingers for him.

TOEWS: (Laughter) Yeah, same.

SCOTT: David, it was so great to talk to you. Thank you so much for sharing your time with us.

TOEWS: Yeah, absolutely. Thanks. This was great.

(SOUNDBITE OF MUSIC)

SCOTT: The recordings you heard in this episode were done by Wil Hershberger, Jeff Ellerbusch and Tom Johnson. They're from the Macaulay Library of the Cornell Lab of Ornithology. You can find links to these birdsongs and more in our show notes.

This episode was produced by me and Margaret Cirino. Gabriel Spitzer was our editor. Brit Hanson checked the facts. The audio engineer was Stu Rushfield. Gisele Grayson is our senior supervising editor. Brendan Crump is our podcast coordinator. Our senior director of programming is Beth Donovan. And the senior vice president of programming is Anya Grundmann. I'm Aaron Scott. Thanks, as always, for listening to SHORT WAVE from NPR.

(SOUNDBITE OF MUSIC)

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