Here's what the chickadees can tell you about human activity

Cities, suburbs, roads, buildings, and landscaping shape the world in which we live. This development can harm vulnerable species or break up migration routes. But not every species is impacted the same way. New research finds that a common, adorable songbird is a subtle barometer of human activity.

A University of Colorado study finds that human cities and towns cause chickadee species to hybridize. If humans build towns where ranges of mountain chickadees and black-capped chickadees overlap, those species produce hybrids.

Previously, neither species was thought to mate with the other. But birdwatchers kept reporting weird chickadees.

“We looked at the distribution of where people had seen these hybrids on eBird and plotted that on a map,” said Kathryn Grabenstein, the postdoctoral researcher who wrote the study. “And we were like, hmm, people are seeing these wonky-looking chickadees almost exclusively in cities.”

Black-capped chickadees are the familiar bird-feeder visitors we see here in Connecticut. They have black heads with white edging on their wings with cinnamon-brown on their sides. Mountain chickadees are grayer and lack white edging on their wings but have white eyebrow markings. Early generation hybrids have a bit of both, thin eyebrow markings, cinnamon coloring on their sides and some white on their wings. Over time hybrids can become indistinguishable from black-capped chickadees at a glance.

Hybridization can occur in nature between closely related species that happen to have overlapping habitats. Sometimes two members of different species meet and mate. Sometimes, but rarely, these offspring are healthy and fertile.

“It isn’t necessarily a good thing or a bad,” Grabenstein said. “The biggest thing is that it’s happening.”

One of the better-known cases is in salamanders in the mountains of California, where closely related species sometimes share valleys and steadily produce hybrids.

Sometimes hybridization can occur with invasive species. For example, in Montana, the human-introduced non-native rainbow trout can reproduce with vulnerable species like the threatened, native west-slope cutthroat trout. The introduced rainbow trout produces hybrids with the cutthroat trout, threatening to breed it out of existence.

“If you get a lot of hybridization, you can get the breakdown of species barriers,” said Amber Rice, an evolutionary biologist at Lehigh University, Pennsylvania. Rice also studies chickadees. “What you used to consider two species becomes one.”

Hybridization can also create entirely new, stable species. The Italian sparrow, previously thought only to be a hybrid of the Spanish sparrow and the house sparrow, was determined to be its own stable species.

“We see it more often in plants, but sometimes hybridization leads to completely separate hybrid species,” Rice said. “It has been found in animals too. It’s called hybrid speciation.”

The Italian sparrow prefers not to mate with Spanish sparrows even though they are almost identical and sometimes share the same range. The Italian sparrow seems to prefer urban habitats over others, which might indicate that humans may have helped cause the species to evolve, even if we weren’t breeding it ourselves.

The chickadees were a little different. Nobody had proven those two species hybridized before, so it wasn’t clear what was going on.

“Is it that there’s more people in cities looking for birds so they’re more likely to find them?” Grabenstein said. “Is there actually a real biological effect?”

If there was a real biological effect, that would be a big deal. Scientists had previously documented hybridization occurring in other species in areas of human interference, but having an example of a stable, natural, ongoing hybridization would make studying human-induced hybridization much easier.

Black-capped chickadees and mountain chickadees are estimated to have diverged from a common ancestor over 2 million years ago, but their ranges overlap across many areas in western North America, including the Rocky Mountains.

The team gathered sighting reports from the eBird, an online birding site, and DNA samples from roughly 200 black-capped and mountain chickadees from across 81 sites between the US and Canada. The genetics work was conducted by long-time songbird researchers from Canada over decades.

They found that the two chickadee species were substantially more likely to hybridize in cities and towns. But this particular hybridization is not likely to produce a new, city-specific chickadee species. Female hybrids, for unknown reasons, are likely to be sterile. Male hybrids can reproduce and tend to breed back into the black-capped population.

Grabenstein said that while they don’t know why the chickadees are hybridizing, it could be because humans create forest canopies in cities. Mountain chickadees tend to live in open meadow habitats or at the edges of mountain pine stands. Black-capped chickadees prefer broadleaf forests like we have in New England. Cities and suburbs tend to plant trees as part of landscaping, creating forest pockets on the mountain plains in higher elevations than would normally occur.

“Those trees are also the preferred habitat for black-capped chickadees,” Grabenstein said. “So you get these artificially high populations of black-caps in environments that otherwise wouldn’t support them.”

This isn’t the first time chickadees have been found hybridizing. Rice’s research found that Carolina chickadees from the American Southeast and black-capped chickadees in the Northeast hybridize in a zone between their two preferred habitats. Those species are divided not by tree preference or elevation but by cold tolerance.

Rice’s research found that by 2050, the range of Carolina chickadees would expand north, blending further with the black-capped chickadee range. Consequentially the hybridization zone would expand and move northwards.

“One of the things that we found that’s mirrored in this study is that you could look at a bird and think, 'Oh, the parent is this species,'" Rice said. “But when you do the genetic work, you find out actually the bird has a fair bit of hybrid ancestry.”

What all this means for chickadees isn’t totally clear. Carolina/black-capped hybrids have been found to have some memory and spatial orientation problems. But it’s not clear if that’s the case for the new hybrids identified by the Colorado study. But the implications for humans are clear: We are causing evolution, hybridization and speciation in species all over the world, even our backyard birds.

“We still know almost nothing about the birds in our backyards,” Grabenstein said. “So, there’s still a lot of value in studying common, backyard birds.”