Dark-eyed juncos

Known as the snowbird, we see dark-eyed juncos (junco hyemalis) most often from October to April (but some stay here all year). These small gray sparrows eat seeds and insects in the wild. But some urban birds also eat food waste. This has led to a difference in the bill structure of urban and wildland birds in some parts of the country.

One interesting 2025 study showed junco beak shape changed during the COVID-19 lockdowns. During this period, urban landscapes changed. Humans no longer left behind food waste for the birds to devour, and the birds reverted to the habits of their forest-dwelling peers, eating mostly seeds. Urban pandemic offspring inherited beaks that were longer and slimmer—a shape similar to the beaks of juncos in forests and nonurban areas. 

Until the pandemic there had not been a good way to isolate the effects of human food waste on beak length. Scientists had been banding, measuring, and releasing juncos on the UCLA campus and in nearby mountainous forests (their natural habitat) since 2018. They measured the birds’ beaks and tarsus bone in their legs. They fed their results into statistical tools to analyze how these traits differed between urban and rural birds. They found that urban birds had shorter, stubbier beaks than their wild counterparts.

In March 2020, the UCLA campus was locked down. A few days later, the first campus juncos hatched. Most dining halls were closed from March 2020 to September 2021 (about 1.5 years), leading to less food waste. (If my dog could talk, she’d tell you that on any given day on a busy college campus, she might sniff out pizza crusts or chicken wings, cupcakes or cookies, ice cream cones or trays of Chinese takeout.) When all this campus food disappeared during the pandemic lockdown, juncos no longer ate a college-student diet. They had to find seeds and insects.

The beaks of birds that hatched in March 2020 didn’t differ from birds that hatched in 2019. However, birds that hatched in 2021 and 2022 had longer, thinner beaks, similar to the beaks of wild mountainous birds. Bill length returned to the shorter, thicker length in 2023 when food waste was again widely available.

What caused the change in beak shape? People often think that evolution takes thousands of years, but sometimes changes can occur more quickly. Scientists speculate that the UCLA birds with the longest, slimmest beaks were better adapted to finding food during the pandemic. Their thinner beaks helped them open seeds or spear insects. They survived better and had more chicks, so they passed their beak shape on to more offspring.

When the campus reopened, and human food waste returned, the birds with shorter, stubbier beaks were better adapted to eating pizza crusts. They survived better and reproduced more. Thus, junco beaks reverted to their earlier stubbier shape.

It is also possible that juncos from wild areas moved onto the college campus during the pandemic. Wildland birds had slimmer beaks so they would produce offspring with slimmer beaks. However, scientists think this is unlikely because the birds on the campus were not fearful of humans the way wildland birds would be.

Does this UCLA study apply to our urban Massachusetts juncos? Although studies have shown that animals living in cities can undergo rapid changes in their traits in response to urban habitats, there are some differences between Massachusetts juncos and UCLA juncos.

Over the past decades, urban California juncos have become year-round residents. However, most of our Massachusetts juncos breed and raise families in Canada in the spring and summer. They migrate to Massachusetts in the fall and return to the Canadian wilds when the weather warms. In Canada, they eat their natural diet of seeds and insects. It is only in the winter that some juncos may be foraging on pizza crusts or chicken wings. However, this is not even certain, as there are plenty of winter bird feeders in our region. So, although it has not been studied, I would imagine that Massachusetts birds have the long, slender beak shape of wild juncos.

Do you have questions about birds, bugs, bees, butterflies or other wildlife? Send your questions to Wild Things and we will try to find the answers.

Ben P. writes: I am so excited to be emailing you. I love the Wild Things column so much—it’s by far my favorite thing I read every week.

I have a question for you. A few weeks ago, my girlfriend and I were walking along Oxford Street in Cambridge, and a squirrel was making a high-pitched screaming/squealing noise. Most surprisingly, the squirrel had its tail entirely flat to its body. I’ve never seen this before. What is this phenomenon? Why would a squirrel do this?

Thanks, Ben, for this interesting question and observation. Despite the fact that eastern gray squirrels are common, scientists have not much studied their communication patterns. Squirrels produce quaas (which I’m guessing is what you heard) and rapid kuks when there is a ground predator, such as a cat, nearby. They make a moaning noise in response to an aerial predator, such as a hawk. The fact that this squirrel also flattened its tail against its body—making it appear smaller and less conspicuous—suggests that the squirrel was responding to a threat. Of course, I can’t know for certain why this squirrel behaved the way it did, but this is my best guess.

Have you taken photos of our urban wild things? Send your images to Cambridge Day, and we may use them as part of a future feature. Include the photographer’s name and the general location where the photo was taken.

Jeanine Farley is an educational writer who has lived in the Boston area for more than 30 years. She enjoys taking photos of our urban wild things.

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