Hybrid Chickadees Reveal How Species Boundaries Can Shift and Blur

Chickadees, those adorable bandit-masked visitors to bird feeders, are among the most familiar and beloved backyard birds in North America. They’re so good at spotting and scolding predators that other birds keep an ear out for their alarm calls, and they’re so fearless that they can be coaxed to snatch seeds from people’s hands.

Some of the same characteristics that make chickadees appealing to backyard birders—their ubiquity, their boldness, the ease with which their behavior can be observed—also make them ideal study subjects for ornithologists. And where the ranges of two of North America’s best-known chickadee species meet, they’ve created a surprising natural experiment in how the boundary lines between species can shift and even blur.

In the past several decades a small group of scientists have devoted their careers to studying this zone of overlap between Black-capped Chickadees and Carolina Chickadees. Their research has highlighted how human activities are muddling the relationships between species as climate change and habitat alteration change how and where organisms interact. It’s also revealed why the hybrids that result—with genes separated by perhaps millions of years of natural selection now suddenly recombined in unexpected ways—sometimes fail to thrive.

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It’s not all downside, however. Hybridization can also sometimes be a way for a species to adapt more quickly than natural selection would typically allow, letting populations borrow ready-made genetic variations from neighbors adapted for different conditions. The humble chickadee is helping scientists understand how the offspring of novel mixed-species pairings may fare—for better or worse—in our changing world.

Even for serious bird-watchers, Black-capped Chickadees and Carolina Chickadees are tough to tell apart. Both are members of the genus Poecile, with the same black cap and chin, white cheeks, and gray-buff bodies, along with the same love of seed-filled bird feeders and similar cheeky “chick-a-dee-dee!” calls.

It is possible for an experienced observer to tell a Black-­cap from a Carolina most of the time, though. For one thing, they typically sing different songs, with Black-caps whistling a two- or three-note tune and Carolinas favoring one with four syllables. But usually the easiest way to tell which of these two chickadees you’re looking at is to consult a map. Carolina Chickadees live in the eastern and southeastern U.S., whereas Black-capped Chickadees inhabit most of the rest of North America, replacing Carolinas in the U.S. Northeast, parts of the West, and Canada.

Yet along the extensive, meandering line where the two species overlap, cutting through the Midwest and into Ohio, Pennsylvania and New Jersey, things can get fuzzy. Going back to at least the 1940s, ornithologists working in areas where both species lived occasionally observed Carolina and Black-capped Chickadees that appeared to have paired off to nest and raise young together. Eventually it became clear that the two chickadees don’t just cross paths. They share a “hybrid zone” where they regularly interbreed and produce young in which their genomes are combined.

Many of us learned in a high school or college biology class that a species is a group of organisms that can reproduce together and produce fertile offspring. This is the biological concept of “species,” one of multiple definitions in use today. Although biologists still struggle to agree on what a species is, typically they think of species as being reproductively isolated from one another. Something keeps them from interbreeding—a difference in behavior, for instance, or a geographical barrier.

Still, hybrids between species are surprisingly common. Some hybrids, such as mules—crosses between horses and donkeys—are unable to have offspring of their own. These sterile hybrids are essentially evolutionary dead ends. Other species pairs, however, can interbreed and produce fertile young, blurring the lines between species as they pass on their jumbled genomes. One unique population of brown bears in a remote part of Alaska, for example, appears to be made up of the descendants of a mix of brown bear and polar bear ancestors.

Hybridization is especially common in birds. About 16 percent of all birds have been documented to hybridize with another species at least occasionally in the wild; ducks are particularly profligate hybridizers, with the familiar Mallard Duck on record as having interbred with more than 40 other duck species. Often the placement of the boundary lines between species is little more than a judgment call. Some closely related bird groups have been repeatedly split, lumped and split again over the past century, with the ultimate decisions made by a committee of ornithologists voting on proposals from their peers.

Sites that had once been home only to Black-caps now hosted Carolinas as well. The hybrid zone was moving.

Hybridization often occurs between species that share a common ancestor and are each other’s closest relatives. This is not the case with Carolina and Black-capped Chickadees. Genetic research has determined that the true “sister” species of the Black-capped Chickadee is actually western North America’s Mountain Chickadee. But after evolving separately for what was probably millions of years, the Black-caps and Carolinas came back into contact with each other as the glaciers receded after the last ice age, and they have been intermingling ever since in the hybrid zone.

In the 1990s, based on repeated surveys carried out by bird-watchers, ornithologists began to realize something particularly odd was happening with these chickadees. Sites that had once been home only to Black-caps now hosted Carolinas as well. The hybrid zone was moving.

It was around this time that Robert Curry, an ornithologist and behavioral ecologist at Villanova University, embarked on a study of hybrid chickadees that would come to define his career. Curry established three field sites at private nature preserves and state parks along a north-to-south gradient in eastern Pennsylvania: a southern site with Carolina Chickadees, a middle site full of hybrid birds and a northern site that was mostly Black-caps. Across the sites, Curry and his students erected around 500 “nest tubes,” cylindrical birdhouses that can be placed in a wider variety of spots than traditional nest boxes. Over the years their work fell into a predictable annual rhythm: they cleaned out the nest tubes in February to prepare for breeding season, then spent April through June tracking nest building and egg laying and eventually captured the adults at each nest to band them and collect blood samples.

In 2007 Curry and Matthew Reudink, then a student at Villanova, published a paper showing that the hybrid zone had been creeping northward over more than a decade. By that time Curry had amassed years’ worth of chickadee blood samples from his field sites, and he had access to tissue samples collected previously by other researchers as well. He, Reudink and their collaborators used genetic analysis to verify the composition of the chickadee population (Carolina, Black-cap or hybrid) at each site and looked at how that composition had shifted over time. (Eventually, as the proportion of hybrids at the northern site increased, Curry added a fourth site still farther north.)The researchers’ findings provided confirmation of what bird-watchers had already observed: in a decade and a half the northern edge of the hybrid zone had moved about 20 kilometers north. But why?

In the years following that publication, people asked Curry whether the movement he and his colleagues reported was connected to climate change. “My answer was always, yes, probably, but I don’t know how to study that,” Curry says. The solution came through a collaboration with researchers at Cornell University. Scott Taylor, then a postdoctoral researcher at Cornell, led an analysis using data from eBird, an online platform where bird-watchers upload their observations. The study showed that the northern limit of Carolina Chickadees’ range is roughly the point on the map where the average minimum winter temperature hits minus seven degrees Celsius—and that the rate of their northward expansion in Pennsylvania has been consistent with warming winters. The hybrid zone does indeed appear to be moving because of climate change.

Climate change is really only half of the story behind the movement of the hybrid zone. It explains why Carolina Chickadees have been able to gradually move north, but it doesn’t explain why, when Carolinas expand into a new area, female Black-capped Chickadees sometimes choose to mate with Carolina males instead of males of their own species.

It’s not a case of mistaken identity. Although humans may struggle to tell the two species apart, the birds probably know who’s who. Fascinatingly, in laboratory experiments, Black-capped and Carolina Chickadees can distinguish between the smell of a member of their own species and that of a member of the other species. But whereas female Carolina Chickadees have a strong preference for the scent of males of their own species, female Black-caps are less particular.

It’s impossible to know what’s going on inside the mind of a female Black-capped Chickadee when she selects a Carolina as her mate. Curry suspects it has something to do with social dominance. Female Black-caps may be attracted to male Carolinas because they’re sometimes higher in the flock’s dominance hierarchy, but this idea is hard to test. What scientists can study is what happens next. When the genes of two species separated by up to millions of years of independent evolution intermingle in a clutch of eggs, what will the hybrid hatchlings be like?

For one thing, not every hybrid egg will hatch. Reproductive isolation between species—the force keeping two species separate—can operate at multiple levels. Animals from two species may choose not to mate with each other in the first place; that’s premating isolation, which doesn’t seem to always be in play between Black-capped and Carolina Chickadees. But if individuals from different species do pair up, reproductive isolation is still happening if their offspring aren’t likely to survive, thrive and produce offspring of their own.

Almost as soon as Curry began collecting chickadee data, he noticed something amiss at the field site with the most interspecies pairs. “We had some nests that just had terrible hatching success,” he says. Sometimes only one egg out of a nest of eight hatched.

Working with a Villanova student, Curry set about documenting hatching success rates across the hybrid zone. The results, published in 2022, showed that as the zone moved north, a trough in hatching success swept across the landscape with it. When Carolina Chickadees moved into an area inhabited by Black-caps, the percentage of eggs laid in local nests that hatched successfully would fall; in a different area, as Carolinas became dominant and the proportion of mixed pairs fell, the hatching rate increased.

For those hybrid birds that do hatch, their mixed-up genomes lead to problems. A study from another area of Black-­cap-Carolina overlap, in Ohio, found that hybrids had higher basal metabolic rates than either parent species—even when sitting still, they need to expend more energy just to keep their bodies functioning.

Hybrids are also, to put it bluntly, a bit dim-­witted. Chickadees as a group are famously clever. In preparation for the harsh winter, chickadees hide tens of thousands of seeds to retrieve and eat later. They need to be able to remember where to find them. To accomplish this recall, chickadees grow new neurons in their hippocampus, one brain region that is responsible for spatial memory. Some studies have suggested the hippocampus swells in size every autumn to store the information necessary for winter survival.

Research on Black-capped Chickadee cognition has shown that their spatial-cognition abilities are correlated with their environment. Chickadees living in places with the coldest winters have the best memories. Based on this finding, Amber Rice of Lehigh University hypothesized that Black-capped Chickadees would perform the best at tests of learning and memory, Carolina Chickadees (which live, on average, in milder climates) would do the worst, and hybrids would be somewhere in between.

Rice and her collaborators tested captive Carolinas, Black-caps and hybrids on tasks that assessed how well they could remember the location of a hidden treat or solve a simple puzzle. To her surprise, hybrids performed worse than their parents on both tasks. “We looked at our results, and we were like, huh,” she says. The findings led Rice’s team to start thinking about genetic incompatibilities. All these problems—poor hatching success, inefficient metabolism, inferior cognitive abilities—probably come down to the fact that some sections of Carolina and Black-capped Chickadees’ genomes simply don’t combine well.

Perhaps no one has spent more time thinking about the intermingling of genes between chickadee species than Scott Taylor, the then postdoc who led the study linking the movement of the hybrid zone to climate change and now a faculty member at the University of Colorado Boulder. Taylor has been interested in hybrids since he was a kid. He recalls really liking Pegasus and unicorns and the idea that they could hybridize to make a “pegacorn.”

Taylor has studied genetic patterns in the chickadee hybrid zone across both time and space. “We’re particularly interested in regions of the genome that don’t move between species when they interbreed,” he says, “because they could be particularly important for reproductive isolation.”

This movement of chunks of genes across the hybrid zone is called introgression, and when it doesn’t happen at a certain spot in the genome, that may indicate a specific set of genes does not mix well between species. More recently, preliminary work by Taylor’s team has helped reveal the genetics underlying some of the deficits other scientists have observed in hybrids: genes related to metabolism and cognition show especially low rates of introgression.

“I think the chickadee work has clarified one of the most important mysteries of avian hybrid-zone research: What is the actual source of selection against hybrids?” says David Toews of Pennsylvania State University, an expert in hybridization in wild birds. “In many other hybrid zones, we have some inkling about what makes hybrids ‘crappy,’ but this large body of work studying Carolinas and Black-caps actually tests these ideas.”

Hybrid chickadees can be fertile. Unlike famously sterile hybrids such as the aforementioned mules, they can breed with birds of either parent species. But the problems created by their mixed-up genomes mean they presumably leave fewer descendants, on average, than nonhybrid chickadees. This ongoing natural selection against hybrid individuals is what ultimately prevents two species from collapsing into one through interbreeding.

Black-caps and Carolinas are just two of North America’s seven chickadee species—and they aren’t the only pair that hybridizes. Where the Black-capped Chickadee overlaps with its closest genetic relative, the Mountain Chickadee, in the West, these two species can also interbreed. Taylor has begun a study of these hybrids. There’s no clear hybrid zone cutting across the landscape, however; instead hybrid birds pop up sporadically throughout a wide area.

Working with then graduate student Kathryn Grabenstein, Taylor mapped locations where hybrids had been reported on eBird. (Mountain Chickadees have a distinctive white eyebrow that both Black-capped and Carolina Chickadees lack, and hybrids are relatively easy to identify by sight.) The birds’ distribution was oddly patchy with a few clear clusters. They wondered what was driving this peculiar pattern.

Eventually Taylor and Grabenstein uncovered a strong correlation between the presence of hybrids and the degree to which habitat in an area had been altered by humans. “I think what we’ve done in these disturbed areas is we’ve planted trees that favor Black-capped Chickadees,” Taylor says, “[which has] increased their populations and then increased the frequency of hybridization between the two species in an artificial way.” To study this link further, he has put up more than 400 chickadee nest boxes, from the city of Boulder to the tree line in the mountains above.

Climate change in the East, habitat disturbance in the West: in both cases, human activities are redrawing the boundaries between species. This kind of disruption is most likely only going to increase in the future. So what will happen to these species as their genomes continue to mix? “I often get asked, Is hybridization good or bad? And the answer is, it’s neither of those things,” Taylor says. “The outcomes are always context-dependent.”

The advent of genomics revealed that hybridization is everywhere—even in our own evolutionary history. Neandertals may be long extinct, but some of their genes live on in today’s humans thanks to long-ago hybridization with Homo sapiens. Scientists have linked genetic variants from Neandertals to fertility, diabetes risk and even our susceptibility to COVID. Neandertals themselves might not have died out so much as simply been absorbed into H. sapiens populations in a process known as genetic swamping, through which a common species can hybridize a rare one out of existence.

Genetic swamping is just one of many possible outcomes of hybridization. “You can have situations where hybrids have low fitness, [which] can actually make the boundaries between species clearer,” Rice explains, or “you can have this merging so that you lose that species boundaries, or you can even have cases where the hybrids form their own species.” Hybrid speciation, in which a new species originates from a cross-­species pairing, has been documented in butterflies, fish, toads and dolphins.

Hybridization can also help a species flourish, Rice adds, by acting as “a bridge for new genes to enter another species [and] provide fitness benefits in certain environments.” As Earth’s climate continues to warm and change, the ranges of more species will shift, potentially bringing them into contact with evolutionary cousins from whom they’d previously been isolated. Inevitably, there will be winners and losers. But in some cases, a new set of genes borrowed from a relative could be the difference between extinction and adaptation.

Black-capped Chickadees, Carolina Chickadees and Mountain Chickadees are all considered “least concern” species by the International Union for Conservation of Nature, meaning they’re currently plentiful and not in need of focused conservation efforts. But Carolina and Mountain Chickadee numbers appear to be decreasing overall, and Black-caps are declining in the western parts of their range. Some Mountain Chickadee biologists are worried about how these birds will cope with the extreme weather patterns that are forming in their high-elevation homes as the planet heats up. Could a hybrid bird introduce new genes into a population that, someday, could help that group adapt?

“What will happen? We don’t know yet,” Rice says. “But it will be interesting to see.”