When students vanished during the pandemic from the University of California, Los Angeles, the beaks on its campus songbirds quietly morphed. That remarkable finding comes from a new study in PNAS, which is the first to document a pandemic-related change in the physical form of a wild animal group, per the New York Times. It describes how dark-eyed juncos who first adapted to urban life in Los Angeles soon adopted shorter wings and beaks, possibly in response to diet changes. Rather than dining on seeds and insects as usual, these urban birds instead found bread, cookies, and dropped pizza.
But chicks hatched in 2021 and 2022, after classrooms emptied and dining halls closed, developed longer, slimmer beaks more like their wildland counterparts. By 2023 and 2024, with students and their leftovers back, the stockier urban beak shape returned. "We were really shocked," co-author and UCLA evolutionary biologist Pamela Yeh tells the Times. Both the speed of the changes and the reversibility are "equally surprising," an outside expert tells Spain's Science Media Center.
Researchers say they can't prove the exact cause, but the leading explanation is rapid natural selection tied to food. With human scraps suddenly scarce, birds better adapted to eating natural fare may have been more likely to survive and breed, passing on wild-style beaks to the next generation. An alternative idea, that shy forest juncos moved onto the quiet campus and interbred with city birds, is considered less likely based on the field data.
The work is part of a broader look at the so-called "anthropause," when global lockdowns offered an unplanned experiment in animal behavior; earlier studies documented quieter bird songs and bolder movements by species like mountain lions and sea turtles, as humans were largely confined to their homes. The findings underscore how tightly human activity is woven into nearby ecosystems—and how swiftly wildlife can adjust to our presence or absence.
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