When the COVID-19 pandemic shut down the UCLA campus, students weren’t the only ones affected.
A small brown bird called the dark-eyed junco also felt the change — and responded in a surprising way.
According to a new study by UCLA biologists, the beaks of these birds evolved during the campus closure and then shifted back again once people returned.
The findings, published in Proceedings of the National Academy of Sciences, offer a rare, real-time glimpse of evolution in action.
They show just how quickly animals can adapt when a major influence on their environment suddenly disappears.
Dark-eyed juncos are small birds related to sparrows and are usually found in mountain forests, where they forage for seeds.
But in Southern California, some juncos have moved into cities and suburbs, including the UCLA campus.
There, they have learned to take advantage of food scraps left behind by people — crumbs on walkways, leftovers near cafés, and spills in busy plazas.
For over a decade, researchers in UCLA professor Pamela Yeh’s lab have been closely monitoring the campus junco population.
They’ve tracked individual birds, measured their bodies and beaks, and studied how urban living is changing them.
Before the pandemic, the campus juncos tended to have shorter, stubbier beaks than their forest relatives. These bill shapes work well for picking up easy food left by humans.
But when the campus emptied during lockdowns, that food supply vanished almost overnight.
With far fewer people around, juncos could no longer rely on scraps. Instead, they had to forage more like wild juncos, digging through grass, leaves, and shrubs to find seeds.
Over nearly two years of reduced human activity, the researchers observed a steady change: young birds began developing longer, slimmer beaks, much closer to the shape seen in non-urban juncos.
The change was strong enough to surprise the scientists. Birds with beak shapes better suited to natural foraging likely found more food and raised more offspring, causing those traits to spread quickly through the population.
Just as striking was what happened next. When students, staff, and campus life returned, so did easy access to human food. Not long after, the birds’ beaks shifted back toward their earlier, shorter shape.
The rapid back-and-forth change made it clear that human presence — and the food we leave behind — is a powerful force shaping these birds’ evolution.
While evolution is often thought of as a slow process that takes thousands of years, this study shows that meaningful physical changes can happen within just a few generations when conditions change sharply.
The findings also offer a cautious note of optimism. Although bird populations worldwide have declined dramatically over the past 50 years, juncos appear to be adapting to city life in ways that help them survive alongside humans.
“We don’t often see clear examples of human activity benefiting wildlife,” Yeh said. “This isn’t a perfect success story, but it shows that some species can adjust — and that gives us hope.”
