15 December 2025

A long term study of dark-eyed juncos at UCLA campus has discovered that the beak sizes of campus juncos changed in just one year in response to the COVID shutdown and changed back again after the shutdown ended. Researchers concluded that human presence and absence made the difference. Authors Diamant and Yeh wrote:

The COVID-19 pandemic provided a natural experiment to test the impacts of human activity on urban-dwelling wildlife. Urban dark-eyed juncos differ in bill shape and size in Los Angeles in comparison to local wildlands. We measured juncos that hatched before, during, and after COVID-19 restrictions at a Los Angeles college campus [UCLA]. Birds that hatched during and soon after COVID-19 restrictions had bills that resembled those of local wildland birds. Yet, bills rapidly returned to pre-COVID-19 morphology in birds hatched in the years following pandemic restrictions. Thus, human activity (and lack thereof) underlies rapid morphological change in an urban bird.

— doi.org: Rapid morphological change in an urban bird due to COVID-19 restrictions

Ever since the study began in 2017, researchers have known that UCLA juncos have shorter and less deep bills than those in wild areas. The bird at top, whose beak is long and conical, was photographed in the Los Angeles National Forest. The banded bird below at UCLA has food in his beak. However it’s possible to see his beak is smaller and less conical. (photo taken at UCLA by Alexander Yan)

During the COVID lockdown (the anthropause) human activity dropped around the world though it varied by jurisdiction. In California the lockdown ran from March 2020 to June 2021. There was very little human food waste at UCLA during that time.

Juncos that hatched in 2020 were conceived before the shutdown when their parents had access to plentiful food waste; these had typical small beaks. When UCLA’s adult juncos experienced the anthropause, young that hatched in 2021 and 2022 had large wild-lands beaks. And now, ever since the shutdown ended, UCLA junco chicks again have small beaks. (photo taken in 2025 at UCLA by Alex Fu)

Two graphs from the Open Access study show how rapidly hatch-year beak sizes changed. (Note that the beak size stays with the bird its entire life.)

Not only is it amazing that the absence of humans can cause a bird’s physical appearance to change, but that it changes rapidly.

How fast can birds evolve? When it comes to beaks, they can evolve in just one year.

Read more about the study in the New York Times: How the Pandemic Lockdowns Changed a Songbird’s Beak and in the study itself at doi.org: Rapid morphological change in an urban bird due to COVID-19 restrictions.