During the COVID-19 pandemic, biologists at UCLA documented a natural experiment as it played out among a population of dark-eyed juncos in Los Angeles. The pandemic lockdowns offered a rare chance to study the impacts of human activity on urban wildlife, and one impact on local juncos was a rapid change in beak morphology. In a recent paper in PNAS, SFI External Professor Pamela Yeh and co-author Eleanor Diamant describe how, in the first two years of the pandemic, juncos’ beaks began to grow longer and more slender — more like the beaks of their forest-dwelling kin whose beaks are adapted to foraging for seeds. As the UCLA campus reopened, the urban juncos’ beaks returned to their pre-pandemic, urban-adapted shape — shorter, stouter, and suited to scavenging for human food scraps.
During this anthropause, which saw a seven-fold decrease in human activity across the UCLA campus in Fall 2021 compared with the campus reopening in Fall 2022, aligned with minimal dining options and a reduction in food waste across campus. “All birds that hatched in 2020 and 2021 experienced low human activity in their early life,” the authors write. “As such, we can isolate immediate and cumulative impacts of lower human activity — including food waste — on the junco population’s morphological response.” The surprising findings highlight the substantial and inadvertent impact humans can have on other species around us.
Read the paper “Rapid morphological change in an urban bird due to COVID-19 restrictions” in PNAS (December 15, 2025). DOI: 10.1073/pnas.2520996122
MORE
- “How the Pandemic Lockdowns Changed a Songbird’s Beak,” The New York Times (December 15, 2025)
- “College Life Can Really Change a Bird,” Sierra Magazine (January 27, 2026)
- "Transmission T-027: Pamela Yeh and Ian MacGregor-Fors on studying wildlife in empty cities; COVID-19 lockdowns provide a once-in-a-lifetime opportunity to study wildlife in empty cities." SFI Transmission Series (