Dark-eyed junco songbirds have been serenading the University of California, Los Angeles campus for decades as they forage for food.
The species from the sparrow family is not usually found in cities. But as the climate crisis affected their mountain forest habitats, they started taking up residence in Southern California’s more urban settings, including colleges such as UCLA.
The city-dwelling birds have shorter and stubbier beaks, a stark contrast from the long ones their mountain counterparts use to eat seeds and insects.
However, as UCLA researchers looked over data on the birds that have resided on their campus in recent years, they noticed something odd: Juncos that hatched in 2021 and 2022, after the start of the Covid-19 pandemic, had longer beaks similar to those seen on the mountain birds. But as pandemic restrictions eased at UCLA and students returned to classes, the city bird traits returned, and the beaks of the birds hatched in 2023 and 2024 were shortened once more, researchers Pamela Yeh and Eleanor Diamant reported in December in the journal Proceedings of the National Academy of Sciences.
“We often think about evolution as a slow process,” said Diamant, the study’s lead author who was a doctoral student at UCLA during the research and is now a visiting assistant professor at Bard College in Annandale-on-Hudson, New York.
“We thought that this would probably be too short of time to see a change in the population. … So we were really surprised when we ran the data,” Diamant added. “When I did the year-by-year comparison, it was remarkable how stark the change was.”
The authors hypothesize that, compared with the longer-beaked mountain-dwelling birds, the shorter beaks on campus birds might be better suited for a wider range diet of human food scraps. But in 2020, when classes first went online and dining halls closed, the beaks of newborn birds evolved back to their longer state fitted for their natural diets.
The study highlights the effects humans can have on wildlife and shows how fast evolution can occur for animals needing to adapt to their surroundings, the researchers said.
Humans and nature are “really quite interconnected, and we’re very much a part of the ecosystem. I think that’s the first takeaway,” said Yeh, the study coauthor and a professor of ecology and evolutionary biology at UCLA. Yeh has researched the juncos for about three decades.
“I think the second takeaway is how much there is right in our backyards,” Yeh added. “I bet that there’s a lot of evolution happening, rapid evolution, that we can see right before our eyes that we don’t even know about because we’re not looking carefully enough.”
Charles Darwin famously encountered evidence for his theory of evolution in 1835 when he noticed that finches in the Galápagos off the coast of Ecuador had varying beak shapes suited to their food sources on the differing islands. Later, in the 1970s, the finches on these islands were observed experiencing rapid evolution brought about by drought. During a drought, birds with larger beaks were more successful at survival, as they were able to eat the harder seeds found on the dry ground. Natural selection, the process by which advantageous traits are passed down, resulted in the next generation having larger beaks.
Yeh and Diamant also pointed to more recent research in which birds have been observed to change beak shapes, such as a 2017 study on great tit birds and a 2025 report on hummingbirds. Both studies found that the birds evolved longer beaks in response to bird feeders.
“It’s exciting and surprising to see such rapid changes in a familiar bird like the Dark-eyed Junco. Even though modern evolutionary biologists know rapid change can happen in wild populations, I certainly wouldn’t have expected to see changes in just a couple of generations,” Ian Owens, a professor in the department of ecology and evolutionary biology at Cornell University and executive director of the Cornell Lab of Ornithology, said in an email.
“And it’s particularly fascinating to see it happen as a response to a direct human-induced change like the pandemic,” added Owens, who was not involved with the study.
Evolution is the primary hypothesis, but the authors note there are other explanations that need to be ruled out, such as gene flow, when individuals from elsewhere, such as the mountain birds, enter the population. However, the authors said they don’t think this scenario is likely since the birds did not show any behavioral changes.
The authors are working on obtaining genetic information from the birds’ blood so they can pinpoint whether the beak changes are a result of genetic changes.
“When we think about human impacts on wildlife, we tend to consider the big players including urbanization, pollution, habitat loss, ocean acidification, and the introduction of invasive species. But this study focuses on a much more subtle and transient factor — whether or not a college campus was in session. Who would’ve thought?” Jeff Podos, a behavioral ecologist and professor at the University of Massachusetts Amherst, said via email.
“The current study is the only one I know of that has documented actual adaptive evolution associated with the pandemic — and again it’s really notable that there were two pulses of evolution here, both into and then out of pandemic conditions, and that those pulses were really quick,” added Podos, who was not involved with the research.
He noted the importance of having long-term datasets such as the authors’ records on the juncos; without them, changes in features such as beak shape might go undetected. It is especially important for species that have been declining in population such as the dark-eyed junco, so that scientists can learn to protect them better, Podos said.
In the case of the campus juncos, their success in an urban setting and ability to evolve to live with humans might bring a more hopeful message.
“We’re causing a lot of harm on a global level,” Diamant said, referring to humans’ interactions with wildlife. “The juncos are declining as a whole, but they’re doing quite well in cities. And seeing how populations manage to keep up, despite what we view as these bad odds, I think brings a lot of hope that if they can do it, and it can surprise us, maybe other organisms can, too.”
Taylor Nicioli is a freelance journalist based in New York.
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