Mountain Chickadees Change Their Tune When They Live Beside Black-capped Chickadees

From the Summer 2025 issue of Living Bird magazine. Subscribe now.

Black-capped Chickadees and Mountain Chickadees are close evolutionary cousins that look a lot alike (Mountain Chickadees have a white eyebrow stripe that sets them apart), and they sound a lot alike, too—both sing sweet, multinote fee-bee whistles, with the Mountain Chickadee usually articulating a few more notes than the Black-capped.

But where these two species come together, their songs may be diverging. Recent research in a range overlap zone in Colorado shows that when Black-capped and Mountain Chickadees live near each other, the Mountain Chickadees are changing their songs.

A study published in the Journal of Evolutionary Biology in October 2024 found that Mountain Chickadees that live in and around the city of Boulder—where they visit the same backyard bird feeders and might nest in the same neighborhood as Black-cappeds—are singing songs with even more notes, and smaller pitch changes, than Mountain Chickadees that don’t live near Black-cappeds.

Olivia Taylor, the lead author of the new study, discovered this phenomenon after spending three months analyzing more than 2,000 chickadee song recordings from Colorado, New York, and California, from both Mountain and Black-capped Chickadees. The recordings included populations where both species occur together (sympatric), and those that live in isolation (allopatric). Taylor says that in the field, the differences between the songs of the two species is subtle, but usually noticeable.

“A Black-capped Chickadee is usually going to sing a song that has two to three notes with a relatively large frequency shift [drop in pitch] between the first and the second notes … so you can hear that it’s two different notes,” says Taylor, an independent researcher who was an undergraduate at the University of Colorado Boulder at the time of the study. “Mountain Chickadees are more variable, but usually don’t produce as big of a shift between that first and second note, so it’s more monotone.”

Listen to the Songs:

Taylor says Mountain Chickadee songs often include very soft, high-pitched introductory notes that are visible when audio recordings are plotted on a spectrogram, but can’t always be heard by humans in the field. She says the number of introductory notes is one of the song elements that set the two populations of Mountain Chickadees in the study apart.

“The main differences were in number of notes per song,” says Taylor, “and a lot of the variation that we were seeing in note number probably came from the inclusion or exclusion of an introductory note,” with the results indicating that the Mountain Chickadees that were living with Black-cappeds, on average, would sing two of those quiet intro notes, while the isolated Mountain Chickadees sang one. “Also they have a smaller glissando after their first note,” says Taylor, meaning they make even less of a discernible drop-down in pitch than the isolated Mountain Chickadees.

These may seem like minor changes. But according to Kathryn Grabenstein, a postdoctoral researcher at the Cornell Lab of Ornithology and one of the study’s coauthors, the Mountain Chickadees around Boulder “sing weird. And it’s interesting that they sing weird, because it matches what you would expect from character displacement, from competitive interactions.”

Character displacement is an evolutionary process in which natural selection acts to alter some aspect of an animal’s behavior or physical structure to reduce costly competition from other, usually similar, species. An often-cited example of character displacement occurs in Darwin’s finches, a group of closely related songbirds in the Galapagos Islands that evolved bills of differing shapes and sizes to specialize on different food sources. An example of character displacement involving vocalization comes from the amphibian world: female New Mexican spadefoot toads usually prefer males with faster calls, but in areas where the New Mexican species shares space with the Plains spadefoot toad (which has a similar call), those females have evolved a preference for slower calls by males to avoid mating with the wrong species.

Grabenstein says that something similar could be happening with Mountain Chickadees. She has been studying the chickadees around Boulder for seven years and is the founder of the Boulder Chickadee Study—a participatory-science network of nest boxes around Boulder County that enables scientists to study breeding chickadees in the region.

When it comes to dominance hierarchies, “Mountain Chickadees are subordinate to Black-capped Chickadees,” says Grabenstein. “So if you imagine that a song is a cue, then [the song changes are] the way that [Mountain Chickadees] are potentially reducing aggression.”

Chickadee aggression could be in the form of direct conflicts over a feeder or a nest site—Mountain Chickadees are more likely to be displaced in a head-to-head interaction in those cases. Or she says, like the spadefoot toads, it could be a way of minimizing hybridization.

Grabenstein estimates that hybrid individuals make up about one out of every 300 chickadees in the Boulder area—and over the course of the past decade, more than 50 records of intermediate chickadees have been reported in Boulder County using eBird. Her current research is in part trying to determine if the landscape changes in Boulder over the past 200 years, such as deciduous trees and bird feeders replacing the prairie, have caused an increase in interactions and hybridization between the two species.

Boulder County stretches out along the Front Range of the Rocky Mountains, from greened-up suburbs around 5,000 feet above sea level, all the way up to 14,000 feet elevation at the Continental Divide. Mountain Chickadees live from the treeline in the mountains down to the plains and lower elevation forests, where they interact with Black-capped Chickadees. According to Grabenstein, before development of the land in the mid-19th century, the Black-capped Chickadees in the Boulder area would most likely have been restricted to the cottonwood-lined creeks that cut through the prairie.

“As people began planting deciduous trees like the apples and the sugar maples and cherries, now all of a sudden you have a city full of Black-cappeds. They’re breeding in the Target parking lot in Boulder,” she says. “I have never been in a place with so many chickadees.”

And then there are the bird feeders.

“The Mountain Chickadees come down [from higher elevations] and are like, ‘Whoa, there’s all this food down here …. I’m just going to hang out here all winter. Why would I go up there where it’s really rough and awful?’” says Grabenstein. “So now you have these social dynamics playing out where Black-capped, the more dominant and more aggressive [species], is being preferred by females of both species. When hybridization occurs, it’s almost always female Mountains mating with male Black-cappeds.”

Whether or not the changing landscape around Boulder is truly resulting in increased hybridization is uncertain, but one tidbit from her latest research, currently in revision at the journal Evolution, points to a good reason why Black-capped and Mountain Chickadees may find ways to avoid mating with each other. Grabenstein documented a failed nesting attempt by a female hybrid chickadee in which the bird made an incomplete nest, then sat on the empty, half-built nest for two weeks before abandoning it.

“If hybridization results in these unfit individuals, then there should be some pressure to avoid hybridization,” she says, “and potentially song is one way that happens.”