Without campus leftovers to pick through, the beaks of this bird changed shape during the pandemic

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Dark-eyed juncos, a bird that typically live in mountain forests, have established thriving populations in Southern California cities, where they eat food people leave behind. A UCLA biologist studying the biological adaptations that help them survive an urban environment has found that the bill shape of this species became more like that of their non-urban counterparts in the absence of people during the pandemic closures at UCLA. After campus re-opened, the bills gradually returned to their previous shape, suggesting that the presence of people and their trash is driving the evolution of bill shape. Juncos on the UCLA campus evolved a different bill shape during the pandemic closure, before returning to their previous shape once university life returned to normal, UCLA biologists who have been studying this population of birds for a decade reported in a new study. The finding, published in Proceedings of the National Academy of Sciences, shows how quickly traits can evolve when an important selective pressure is altered — in this case, humans making food easier to find.

“We have this idea of evolution as slow, because in general, over evolutionary time, it is slow,” said corresponding author and UCLA professor of ecology and evolutionary biology Pamela Yeh. “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.”

Dark-eyed juncos are a small member of the sparrow family that usually live in mountain forests. But as climate change shrinks their preferred habitat, Southern California’s juncos have surprised biologists by establishing themselves in cities and suburban neighborhoods. Non-urban juncos forage for seeds, but urban juncos have learned to exploit various crumbs, scraps and food waste that people leave lying around. Yeh’s lab has been banding, observing, measuring and taking blood samples from UCLA’s junco population to understand the adaptations that are making it possible for the birds to flourish in their city digs.

The new study found that during the pandemic closure, UCLA’s junco bills evolved to more closely resemble the bills of their forest relatives. Previously, Yeh and her students had seen relatively short, stubby bills. Over nearly two years of minimal human activity on campus, junco parents raised babies with gradually longer, more slender bills, which are typical of non-urban juncos.

“We were quite shocked, to be honest, when we saw just how strong that change was,” said first author Eleanor Diamant, who did the research as a doctoral student at UCLA and is now a visiting assistant professor at Bard College. “We think that that likely happened because when humans are not around, they’re not leaving their trash around and they’re not leaving their food around.”

At UCLA, juncos love to forage on plazas where students gather, patios around dining venues and walkways. When the food they used to find in these places disappeared in the absence of people, the birds were forced to forage in the more challenging landscape of bushes, grass and leaves like their non-urban counterparts. Leaving campus for easier pickings around supermarkets and the handful of other businesses that remained open during the pandemic closure was not a good option for UCLA’s juncos because these birds do not usually venture far from a territory they defend.

“We think that juncos with different bill shapes were more successful when campus was closed. Those with bills that improved their success at foraging for seeds probably got more food and raised more babies,” said Diamant.

But the change didn’t last. Once campus life returned to normal, the bills returned to their previous shape, which drove home the idea that the presence of people is having a strong effect on the evolution of bill shape in urban juncos.

“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,” said Yeh. 

Although no one expected to see this change in juncos, the evolution of shorter snouts in other urban wildlife, such as rats and raccoons, has been documented. Given that an estimated 3 billion birds have vanished over the past 50 years — around 168 million of them juncos — their resilience offers hope for their future survival.

Yeh said that other urban birds like house sparrows and pigeons are in some ways pre-adapted to live with people because of their generalist diet, tendency to live in large flocks  and ability to nest off the ground in human structures. But juncos are territorial, ground-nesting birds that don’t usually thrive in cities.

“The fact that juncos are doing very well, popping up in cities all over Southern California, and that we are unintentionally changing them in a way that helps them survive around us is encouraging,” said Yeh. “I don’t feel like we have a lot of success stories when we think about how human behavior affects wildlife. 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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