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COVID-19 restrictions coincided with a rapid change in the bill shape of dark-eyed juncos living on the UCLA campus. Birds hatched in 2021 and 2022 had longer, more slender bills than earlier and later UCLA birds, but the urban-associated shape returned after campus activity resumed.
COVID-19 restrictions coincided with a rapid change in the bill shape of dark-eyed juncos living on the UCLA campus. Birds hatched in 2021 and 2022—when the campus was unusually quiet and food waste had fallen—had longer, more slender bills than earlier and later UCLA birds. Their bills more closely resembled those of juncos from nearby mountain forests. After campus life resumed, the urban-associated bill shape returned in birds hatched in 2023 and 2024.
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That is the finding of a December 2025 study in the Proceedings of the National Academy of Sciences. It is an intriguing example of how quickly wildlife can respond to changes in human environments—but it does not prove that lockdowns genetically rewrote birds’ beaks, or that every bird changed its bill.
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The study focused on dark-eyed juncos (Junco hyemalis), small songbirds that traditionally occupy mountain and forest habitats. Southern California juncos have also established year-round urban populations, including a long-monitored population on the University of California, Los Angeles campus.
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Before the pandemic, UCLA’s juncos tended to have relatively short, deep or “stubby” bills compared with juncos captured at nearby nonurban, forested montane sites. The comparison birds came from locations roughly 10 to 100 miles from campus.
Researchers measured bill length, bill depth and tarsus length, while also recording age and banding information. Their analysis included 302 UCLA juncos of known hatch year and 132 adult wildland juncos. The UCLA birds represented hatch years from 2017 through 2024, with captures and measurements conducted from 2018 through 2025.
What happened during the campus shutdown?
UCLA restrictions began in March 2020, shortly before the first expected campus junco nest hatchings. Restrictions remained largely in place until fall 2021. The study reports that human activity in 2021 was approximately seven times lower than during unrestricted 2022 conditions, and most campus dining options were closed until fall 2021.
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That change also altered the birds’ surroundings. UCLA juncos commonly forage around plazas, patios, walkways and dining areas, where students and staff leave food scraps. With much of the campus closed, those human-associated resources and the organic waste they generated fell sharply. The birds presumably had to spend more time searching vegetation and natural substrates for seeds, as their mountain relatives do.
The clearest morphological response appeared with a delay:
- 2017–2019 hatch-year birds: The pre-pandemic urban-associated bill form was present.
- 2020 hatch-year birds: No clear difference from the pre- or post-restriction groups was detected.
- 2021 and 2022 hatch-year birds: Bills were significantly longer, and the bill-length-to-depth ratio was greater. In other words, the bills were more elongated and slender.
- 2023 and 2024 hatch-year birds: Bill traits returned to the earlier UCLA pattern as normal campus activity resumed.
The 2021 and 2022 birds were not significantly different from the wildland-type comparison in the measured bill traits. This does not mean they became mountain birds in every respect; it means that their bill measurements more closely matched the wildland juncos used in the study.
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Why did the bill change lag behind the lockdown?
The timing matters. It would be misleading to say that the study found a clear bill change in birds hatched in 2020. Those birds were produced around the beginning of the shutdown, when the environmental change was still new. Birds hatched in 2021 experienced a longer period of reduced human activity during the conditions leading into the breeding season.
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Why might bill shape matter?
A bird’s bill is a feeding tool. Its shape can affect how the bird handles, opens or gathers different foods. The researchers propose that UCLA’s shorter, deeper bills may have helped juncos exploit food waste around people, while the longer, more slender bills seen during the restrictions may have been better suited to seed foraging in vegetation and natural areas.
That explanation fits two observations: the urban population already differed from nearby wildland birds before COVID-19, and the UCLA birds temporarily shifted toward the wildland bill form when the campus food environment changed. But the study did not record the complete diet of each individual bird. Campus-level reductions in human activity and organic waste were environmental indicators, not measurements of exactly what each junco ate.
Food availability is therefore a plausible selective pressure, not a proven individual-level cause.
Did lockdown cause genetic evolution?
Possibly, but the study cannot establish that conclusively. The authors describe the result as rapid morphological change and interpret it as likely adaptive evolution. If birds with bills better suited to the available foods found more nourishment, survived at higher rates or raised more young, those traits could become more common in later cohorts.
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However, a difference between hatch-year groups does not automatically demonstrate a change in gene frequencies. Several processes could produce the same pattern:
- Genetic selection: Birds with advantageous bill shapes may have survived or reproduced more successfully.
- Phenotypic plasticity: Developmental conditions, including nutrition, could have influenced bill growth without a lasting genetic change.
- Demographic filtering: Different adults may have survived the shutdown or contributed offspring afterward.
- Movement: Juncos from surrounding green spaces could have moved into or out of the campus population.
- Mate choice or reproduction: Changes in which birds paired or bred could have shifted the traits seen in hatchlings.
The observational design does not separate these possibilities. Stronger evidence for a genetic explanation would require information such as parentage and pedigree data, common-garden or reciprocal-rearing experiments, genomic analysis, and direct estimates of survival and reproductive success.
The most accurate summary is: the study documented rapid, likely adaptive morphological change in an urban junco population associated with pandemic restrictions.
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What the study shows—and what it does not
| Supported by the study | Not established conclusively |
|---|---|
| UCLA’s urban juncos differed in bill shape from nearby wildland juncos before the pandemic. | That reduced food waste was the only cause of the change. |
| Campus human activity and dining-related organic waste fell sharply during restrictions. | The exact diet of each bird during each period. |
| Birds hatched in 2021 and 2022 had longer, more slender bills. | That the change was entirely genetic evolution rather than plasticity, demographic change or movement. |
| The pandemic-era bill traits more closely resembled those of wildland juncos. | That all juncos, or birds generally, changed their beaks during lockdown. |
| The urban-associated bill shape returned in birds hatched in 2023 and 2024. | That the change was permanent. |
Why this is an unusual urban-wildlife study
The COVID-19 “anthropause”—the temporary reduction in human movement and activity—created an unusual natural comparison. Researchers could examine a population with measurements from before, during and after a sudden change in the human environment.
The UCLA juncos were especially useful because the population had been monitored for years, and nearby wildland juncos provided a comparison for the urban birds’ earlier and temporary bill forms. Many wildlife studies document changes in movement or behavior when people disappear from a landscape. This one reports a change in morphology, the physical form of the animals themselves.
It also shows why urban wildlife should not be treated as biologically static. A campus is not simply a patch of concrete occupied by birds from the surrounding countryside. Human schedules, dining patterns, landscaping and waste can create a distinct ecological environment, and animals living there may respond when that environment changes.
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Does this mean birds’ beaks changed because people stayed home?
Not in the simple, universal sense suggested by the headline. The evidence concerns one population of dark-eyed juncos at one Los Angeles campus. The strongest comparison is between groups of birds born in different years, not a demonstration that every adult bird’s bill physically lengthened when the lockdown began.
The result also reversed after people returned. That reversal is important: it supports a close relationship between the birds’ morphology and changing campus conditions, but it argues against describing the lockdown form as a permanent transformation.
For now, the study is best understood as evidence that human activity was associated with a fast shift in the average bill shape of young UCLA juncos. The likely ecological story involves food, but the exact balance among natural selection, development, reproduction and movement remains unresolved.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to observe dark-eyed juncos responsibly
Dark-eyed juncos can be identified by their compact shape, conical bill and regionally variable plumage. Bill proportions alone are not a reliable way for a casual observer to determine whether a junco is from an urban or wildland population, and the study’s differences were population-level measurements rather than a field identification rule.
If you want to observe local juncos, use a bird identification field guide to check plumage, range and behavior, and record the date, location and habitat. Birdwatching binoculars can help you watch from a distance without approaching nests or flushing birds.
A camera-equipped feeder can document visiting birds, but it cannot reproduce the UCLA study. Feeding wildlife changes local food availability and may concentrate birds, so follow local wildlife guidance, keep feeding stations clean and do not use supplementary food to draw birds into unsafe or crowded areas. A simple birding notebook can be just as useful for recording observations.
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The takeaway
COVID-19 restrictions were associated with a rapid, temporary shift in the bill shape of UCLA’s urban dark-eyed juncos. Birds hatched during the quietest, least food-waste-intensive period had longer, more slender bills, closer to those of nearby wildland birds; the earlier urban-associated shape returned after campus activity resumed.
The finding is compelling because it links a sudden change in human behavior with a measurable change in urban wildlife. It is not proof that lockdowns genetically altered birds’ beaks, nor evidence that birds everywhere underwent the same transformation. The careful conclusion is narrower—and more scientifically useful: when the human-made food environment changed, the average morphology of this urban junco population changed with it.
Primary study: Eleanor S. Diamant and Pamela J. Yeh, “Rapid morphological change in an urban bird due to COVID-19 restrictions,” PNAS. The associated underlying data are available through Figshare.
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What changed in the birds during the COVID-19 lockdown?
The study found a temporary shift in bill shape among dark-eyed juncos born at UCLA during 2021 and 2022, when campus activity and food waste were sharply reduced. Their bills were longer and more slender, resembling nearby wildland juncos.
Did lockdown change birds’ beaks everywhere?
No. The research examined one long-monitored dark-eyed junco population at UCLA and did not show that birds everywhere changed their beaks.
Was the bill change genetic evolution?
Not definitively. The pattern may reflect natural selection, but developmental plasticity, different breeding adults, bird movement and other demographic processes could also contribute.
Did birds change their bills because they could no longer eat food scraps?
The study did not directly track each bird’s complete diet. Reduced access to campus food waste is a plausible explanation, but the precise causal mechanism remains uncertain.
Was the change permanent?
No. The urban-associated bill form returned in birds hatched in 2023 and 2024 after normal campus activity resumed.
The Bottom Line
Bottom line: UCLA dark-eyed juncos born during COVID-19 restrictions had temporarily longer, more slender bills, probably in response to altered urban foraging conditions. The study shows rapid morphological change associated with the lockdown—not proven permanent genetic evolution, and not a change documented in birds generally.
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