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.

Dark-eyed juncos are a small member of the sparrow family. Credit: Alex Fu / UCLA

“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.