Just as other birds are leaving our region in the fall, those harbingers of winter, the dark-eyed juncos (Junco hyemalis), arrive. They have long been called snowbirds because their autumn arrival signals the approach of winter. These songbirds in the sparrow family begin trickling into Massachusetts from the north in October, with increasing numbers in November. Most return to their northern breeding grounds in the spring.
The dark-eyed junco is solid gray above and white below, sometimes described as “leaden skies above, snow below,” due to its winter presence. Since it has such distinctive plumage, why would scientists name this bird based on its eye color?
For one thing, there is another North American junco with distinctive yellow irises, called the yellow-eyed junco (Junco phaeonotus). Found in Central America and Mexico, New Mexico and Arizona, it is the common ancestor of the dark-eyed junco.
Ten thousand or so years ago, as the glaciers receded, the yellow-eyed junco from southern Mexico expanded its range to what is now northern Mexico, the United States and Canada. Its eye color changed from yellow to dark brown. These dark-eyed populations became isolated in different regions of North America, where they evolved different feather colors and patterns.
Before 1973, scientists considered these differently colored birds to be five distinct species. They had black heads or pink sides or red backs or white wings; our eastern bird was mostly gray. Although these juncos had different breeding ranges, they could interbreed where their populations overlapped.
The red-backed junco, found in the Southwest, resembles the Mexican yellow-eyed junco in plumage but has dark eyes. Scientists think this bird represents an intermediate evolutionary stage between the yellow-eyed birds to the south and the colorful variations to the north.
In 1973, based on genetic information, the American Ornithologists’ Union (now the American Ornithological Society) combined these five colorful species into one, called the dark-eyed junco, because eye color was their shared characteristic.
The word junco itself refers to a marshy reed, but juncos reside primarily in forests, brushy areas and fields, so the name is really a misnomer. Juncos eat small weed seeds: chickweed, pigweed, knotweed and grass seeds.
Why do juncos spend the winter in our region? Wouldn’t it make more sense to travel farther south, where the temperature is more agreeable, and food easier to find? Some juncos do just that. These juncos are mostly female.
In 1976, ornithologists studying dark-eyed juncos uncovered a geographical pattern to their migration. In northern wintering areas, 80% of birds were male, while in southern areas, 70% were female. The proportions of males decreased the farther south you went. This phenomenon, called differential migration, is not fully understood, but it is likely influenced by climate.
The Earth’s average temperature has increased more than 2 ° Fahrenheit since the late 1800s. The year 2024 was the warmest since global records began in 1850; the year 2025 was the second warmest. The 11 years from 2015 to 2025 have been the 11 warmest years in the 176-year record.
As winters warm, both male and female juncos are overwintering farther north. Over the past 60 years, fewer and fewer overwinter in the southernmost areas, while the proportion of females at northern locations has increased. These changes correlate with warmer winter temperatures and less snow.
Overwintering farther north means that juncos can reach their nesting grounds earlier in the spring. Males, which are larger and better able to withstand the cold, historically arrive first to stake out and defend a territory. The farther north they overwinter, the sooner they can return. Females have less to gain by returning early and have traditionally wintered farther south, where milder conditions make it easier to find seeds to eat. Females need to be well-fed and fit to produce healthy youngsters in the spring.
Because juncos hunt for seeds on the ground, snow cover matters. Less snow exposes seeds better, potentially making northern areas more and more suitable for females as winters warm and snow cover lessens.
New Englanders have long considered the fall arrival of dark-eyed juncos to be a harbinger of winter. However, today their changing migration patterns may be a harbinger of something else — a warming planet.
A stronger
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