The pandemic had a noticeable impact on the environment, though not always on the same scale. While the rare absence of humans reduced some pollution to nature, that sudden change also encouraged more aggressive behavior from invasive species. Then there are cases, like the one involving the dark-eyed juncos in California, that don’t quite fit in either category.
In a recent paper published in the Proceedings of the National Academy of Sciences, scientists reported that during and after the pandemic, dark-eyed juncos experienced two quick evolutionary changes. Specifically, the small songbirds’ beaks grew longer during the pandemic and then became stubbier once more as human activity resumed, just like in the movie, Pinocchio. But in this case, there wasn’t any magic or morals about honesty involved—just the consequences of human influence on nature.
“We have this idea of evolution as slow because, in general, over evolutionary time, it is slow,” Pamela Yeh, one of the study’s lead authors and an evolutionary biologist at the University of California, Los Angeles (UCLA), said in a statement. “But it’s amazing to be able to see evolution happening before your eyes and to see a clear human effect changing a living population.”
Easier means shorter
Dark-eyed juncos generally reside in mountain forests, but in southern California, climate change drove a sizable population of the birds into cities, where they learned to pick off crumbs and scraps from human food waste. Compared to their mountainous relatives, the beaks of Californian juncos evolved to become short and stubby.
“Wild animals have to work hard to find and get their food. When humans make it that much easier, the parts of their bodies, such as their mouths, that animals use for foraging adapt,” Yeh explained.
So when the juncos settled nicely onto UCLA’s campus, they caught the attention of Yeh and her colleagues, who began a long-term study of the songbirds in 2018. Surprisingly, the birds had gradually developed a diet “closer to the average college student,” Ellie Diamant, the study’s other lead author and an evolutionary biologist at Bard College, told The New York Times. So that included “things like cookies, bread… [and] pizza,” she recalled.
Harder means longer
Then the pandemic struck. As classes shifted online, the campus became mostly abandoned and scrap-free—much to the detriment of juncos. It was around 2021, roughly a year after the start of the pandemic, that Yeh and Diamant noticed a slight change in newborn juncos: a longer, slimmer beak.
“We were quite shocked, to be honest, when we saw just how strong that change was,” Diamant recalled. In such a short period of time, California juncos had essentially “evolved” so that their bills were back to the shape held by their counterparts in the wild. That change likely increased the success rate of foraging for the birds, Diamant added.
But as pandemic restrictions loosened, UCLA students, faculty, and staff returned to campus. Remarkably, as people returned, so did the shape of the juncos’ beaks. As quickly as the beaks had grown, they shrank back again in junco chicks born between 2023 and 2024.
“It is remarkable evidence of these birds’ rapid ability to adapt to changes in their environment and food resources,” noted Graciela Gómez Nicola, a biologist at Complutense University of Madrid who was not involved in the study, to Science Media Centre Spain.
A gray area
There have been other recent studies on how exposure to human activity has changed the morphology of wild animals. But juncos are somewhat different from other urban birds like house sparrows or pigeons, the researchers explained. House sparrows and pigeons are “in some ways pre-adapted to live with people” due to their generalist diet, tendency to flock, and capability to nest in human structures.
Juncos, by contrast, are territorial and typically nest on the ground. So the dark-eyed juncos of UCLA, as common as they may be on campus, represent an ongoing evolutionary mystery, the researchers concluded.
“I don’t feel like we have a lot of success stories when we think about how human behavior affects wildlife,” Yeh said. “I wouldn’t fully call it a success story yet, but it’s not a disaster story, and that’s no small thing.”
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