Human activity appears to be causing two related species of songbirds to mate.
The research finds disturbances to the habitats of the black-capped and mountain chickadees are leading to this hybridization in the Mountain West.
The Show spoke with Kathryn Grabenstein — a post-doctoral research associate at the University of Colorado at Boulder who worked on the study — and asked what led her to look into this hybridization among these different kinds of birds.
Full conversation
KATHRYN GRABENSTEIN: Yeah, so this is work that came out of my dissertation work, so when I was a graduate student. And I noticed this trend, not just in chickadees, but kind of in a bunch of different species all over the globe, that humans seem to be doing something to the habitat that was causing species that naturally lived together but didn't hybridize to actually then hybridize.
And so that was kind of the impetus. Was like, OK, there's really kind of a broad pattern in a lot of different species. Can we basically investigate this a little bit closer? And then the first step with that was looking at basically finding a system, which, in this case, was the chickadees. So that's kind of the impetus was, we saw it in a bunch of other different species, and we're like, ‘OK, do we see it in chickadees?’
MARK BRODIE: So these are different species that are not unfamiliar to each other, but just hadn't been, basically hadn't been breeding with each other, right?
GRABENSTEIN: Exactly. Yeah. So in basically most of the mountainous kind of part of North America, mountain chickadees and black cap chickadees do, broadly co-occur. So they're both kind of living and operating in the same space at the same time. Historically, they weren't considered to hybridize, but this study actually is one of the very first that shows that they do hybridize. And they hybridize, really, over much of the range that they co-occur in.
BRODIE: So why is that happening? And what do humans have to do with that?
GRABENSTEIN: Oh, yeah, that's a great question. The why is probably much harder to get at. What we think is happening is kind of twofold. So first, in these cities in the Mountain West, people like to plant pretty trees, right? So they like to plant their ornamental, you know, sugar maples, these deciduous trees that turn great colors in the fall.
BRODIE: Yeah.
GRABENSTEIN: And those are trees that normally wouldn't be in those mountainous cities, right? Like, typically you'd have one species of pine, maybe an aspen, but you're not getting all these ornamental species from out East. So people plant those, and that creates this really great habitat for black cap chickadee, so we see this kind of artificial increase in black cap chickadee populations. That's one piece.
And then the other piece is we think there's something to do with humans kind of disrupting, something to do with the mountain chickadee distributions, where they're coming down from these high elevation habitats and the low elevation habitats, and then mixing with these really kind of artificially inflated populations of black cap chickadees. So it's like a two-fold thing that's going on, where one population is increasing and one population is now moving more than it otherwise would.
BRODIE: But it sounds as though what you're saying is that these birds are sort of changing their relationship, or like they've been together. They've coexisted, as you say. What's causing them to hybridize now?
GRABENSTEIN: yeah, so that's work. That's going to be probably a little bit stickier to get at, because it involves some sort of basic behavior, right? And the only way to really get at that is with experiments. We don't really know what's going on there, one. So basically, the black cap chickadees are more dominant than the mountain chickadee. So when they come in and interact at feeders, black capped chickadees will kind of always supersede mountain chickadees, and dominance really matters for chickadee mating decisions.
So we think it's something going on there where you're getting these two species coming in, interacting at feeders, and these kinds of basically urban, low elevation habitats. One species is more dominant, therefore more attracted to females, and then that's what's causing the hybridization, because these species aren't pairing up, right? They're not forming these social pairs. They're kind of mating on the side. We call it extra pair copulation, when they're mating outside of the social pair. So that presents this interesting component of basically, it's potentially this dominance interaction, but we really don't know.
BRODIE: So is this creating, like, a new species or a new subspecies? What's the result of this?
GRABENSTEIN: Yeah, so that's kind of where my work is continuing on. It's definitely, almost certainly, not going to create a new species. Hybridization has been really important for lots of different species' evolutionary trajectories. I mean, even humans, right? Some proportion of us have a pretty solid amount of Neanderthal DNA in our genome.
So we know that hybridization is important. It's very unlikely that it'll create a new species. But what it actually matters for as climate change, or as things become more urban, is one species better at navigating urban environments versus the other. There we might see implications, but in terms of creating a new species, it's very unlikely, especially in birds, we typically see that more in plants.
BRODIE: Do you have a sense that this might be happening among other species of birds as well?
GRABENSTEIN: Almost certainly, yeah. I think the cool thing about this study is it's pretty novel. So most people, when they think about human impacts on biodiversity or human impacts on birds, they think about, ‘OK, I've built this city, I've cut down a tree, and now a bird can't nest here’, but they don't necessarily think about, ‘OK, I've planted a new tree, and now I'm changing the way that species are interacting’, right? So I'm changing kind of the status quo, the way things that have been here are relating to one another. And so this is something that we're really just now beginning to look at, but almost certainly it's impacting other bird species.
BRODIE: Is it the kind of thing, do you think, that the more you look for it, the more you'll find it?
GRABENSTEIN: Probably. yeah, I think also, it's only just now become feasible in the past, probably, like five to 10 years, for us to be able to sequence the genomes of non-model organisms, right? So the fact that I can go out and, you know, catch a chickadee, take a blood sample, sequence its DNA, and basically look at its entire genome, all for, you know, on the order of 500 bucks, which is not that expensive, is pretty novel.
BRODIE: What are the implications for the chickadees of this? Like, is this a problem for them?
GRABENSTEIN: Oh, I don't know. It depends, like, like, all things. It's complicated. Whether or not it's problematic, I don't, I don't know. I think of it in terms of, there's not much left that humans haven't touched or meddled with or changed. So whether or not it's good or bad, I don't like ascribing, you know, or labeling things that way.
I think it's happening. I think it's something to be mindful of and something to consider. But whether or not you know one species is going to go extinct versus the other, or things are going to die out, I don't, I don't think so, but, but, yeah, it's hard to say whether it's good or bad - it’s complicated.
BRODIE: Well, and I guess to the point you were making, it doesn't seem terribly likely that people are going to stop planting certain kinds of trees and moving into certain parts of the West. So it doesn't seem as though this phenomenon is going to slow down anytime soon?
GRABENSTEIN: No, and that's again, like it's not necessarily a thing. I think something to consider is, you know, Earth and biology is constantly in flux, right? The way the earth is now is not necessarily the way it was 500 years ago or 1,000 years ago. So we're changing things faster than things typically have changed.
But it's not necessarily a bad thing, right? In terms of what's going to happen, specifically with the chickadees, right? I just think it means that things are changing, and what it means for, you know, evolution, or what it means for the persistence of these birds, I don't know. I think if birds bring you joy, and you like putting out feeders and you like planting sugar maples, then that brings you closer to nature, that, to me, is a bigger win than necessarily caring about increasing hybridization with common songbirds.
