From the Winter 2020 issue of Living Bird magazine. Subscribe now.
Every winter carries with it a flight of snowbirds—retirees fleeing New York snow or Chicago slush for southern sunshine. The cheery Dark-eyed Junco is also known as a snowbird, because it escapes cold mountains and boreal forests to gather in gray-and-brown flocks at backyard bird feeders across the Lower 48 states.
But in Southern California, some juncos have thrown out the rulebook, stopped migrating, and are now year-round residents among the landscaped lawns and shady groves of coastal college campuses. And the longer they stay, the more their coloration and behavior seem to change with their surroundings.
Normally, such substantive changes in biology take a long time, but scientists say these sudden shifts reveal just how fast evolution can happen. It’s even possible that human habitation could be the driving force behind the shifts.
“[It’s impressive] how rapidly these vertebrate species can evolve. In a matter of a handful of years, we can find some pretty significant changes,” said Pamela Yeh, an associate professor in ecology and evolutionary biology at UCLA.
The junco story begins in the early 1980s. Juncos have always occurred in coastal California in winter, but about four decades ago bird watchers around the University of California, San Diego, started seeing them on campus during the summer months, instead of on their normal breeding grounds 50 miles away in the Laguna mountains. A few dozen pairs skipped the spring migration and just set up shop in sunny San Diego.
In 1997, ecologist Trevor Price and Yeh, then a PhD student with Price, decided to study the juncos more closely. At one point, they had banded a full 80% of the UC San Diego summer junco population. Eventually, the breeding birds spread as far as Santa Barbara and Los Angeles, where Yeh still studies the phenomenon at UCLA. Oddly enough, in both L.A. and San Diego, juncos seem to favor college campuses.
“I didn’t go into ecology and evolution wanting to stay in the city,” Yeh laughs. “I wanted to go out in the middle of nowhere, far away, somewhere exotic.”
But she became fascinated with these brave little pioneer birds in urban areas. The city is not a friendly place for most wild birds: Strange food, strange machines, and strange predators can be lethal, which is why the few city birds that can survive—House Sparrows, Rock Pigeons—become ubiquitous.
Yeh found that in just two decades, the small pioneer group of San Diego–nesting juncos exhibited a suite of changes. They were less aggressive with each other and more comfortable around people. Male juncos were singing slightly higher songs. Females were building nests in places up off the ground, in trees and on buildings (juncos are traditionally ground nesters). The birds’ plumage changed, with duller black in the head and smaller white flashes in the tail. Further research showed that male juncos were spending more time tending to their nests, and were more monogamous.
The San Diego juncos were also having more chicks. A lot more. In the mountains, juncos have to work fast within a two-month breeding season to lay one, maybe two clutches. Competition is fierce, and males have little time for much else besides mating. Meanwhile, the laid-back campus birds have up to four clutches over a breeding season that sprawls out over seven months—with males putting more effort into caring for hatchlings.
“Up in the mountains it’s all about sex. Whereas on the campus it’s all about family,” says Price, now a professor at the University of Chicago.
Price isn’t quite sure what triggered this behavioral shift away from a focus on breeding and toward better parental care in that first group of juncos 40 years ago. Perhaps the males just started singing early and the females stuck around in response. But at the heart of it seems to be lower peak levels of testosterone in males, which can affect behavior, song, and plumage coloration. Studies suggest the testosterone changes are coming from genetic differences unique to the Southern California birds.
Studying junco evolution is more than just an academic exercise. An October 2019 study in the journal Science, led by the Cornell Lab of Ornithology, found that—among 3 billion birds lost in North America over the last 50 years—the Dark-eyed Junco population is rapidly declining, down by 168 million juncos. But if juncos come equipped with an evolutionary toolbox for adaptation, perhaps their future won’t be so bleak.
Juncos do seem to be particularly good at change. The species evolved into six distinct forms—from the rich brown “Oregon” juncos in San Diego to the “Slate-colored” form in the East—in a spurt of diversification over just the last 18,000 years. Price says their flocking nature probably also helped the San Diego juncos change their lifestyles, providing an initial threshold of nests to survive that first summer some 40 years ago.
And it might be happening again, in the Midwest. At Indiana University Bloomington, evolutionary biologist Ellen Ketterson and a postdoctoral researcher in her lab, Daniel Becker, are looking at a population of juncos in Ohio that seems to have shirked off its annual migration. Rather than traveling back north to Canada in spring, some seem to be lingering and breeding on their wintering grounds. Ketterson says it’s still too early to know whether the Midwestern birds have similarly changed their hormones and behavior, but it shows that this is not just a California phenomenon.
Or even a junco phenomenon. Ketterson, who has made juncos her life’s work and won several prestigious ornithology awards for her groundbreaking research on the species, says it’s entirely possible that other birds are adapting away from migration, too.
“As much as I’d like to say, ‘My child is the best child,’ I don’t think this is unique to juncos,” she says.
Take the case of the Argentinean Barn Swallows. Typically, Barn Swallows that breed in North America spend Northern-Hemisphere winters in South America. But 35 years ago, a few swallows opted not to migrate back north at the end of the season. In succeeding years they started breeding under a few bridges near Buenos Aires, and eventually they even began a new migration pattern to spend the austral winter near the equator—reversing the pattern of their North American ancestors. Today they have flip-flopped their molt, migration, and breeding schedules by a full six months—all in just a few swallow generations.
The year-round San Diego juncos have made the switch to ditch migration, but their population isn’t thriving. It appears to have plateaued, despite the relatively high nesting success that Yeh and Price measured for campus birds. The researchers suggest there may be downsides to living in the city (they specifically mention feral and outdoor cats) that make it less likely for a young junco fledgling to survive its first year on campus.
But still, these juncos demonstrate that they have the capacity for adapting and changing their breeding biology—and doing it in a hurry.
“[It] makes us hopeful, I think, for at least some bird taxa, as we look to a rapidly changing climate,” says Ketterson.
Erik Vance is a National Geographic Explorer who writes about the brain and the environment for National Geographic, Scientific American, The Washington Post, and other national publications.
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