February 13, 2015
You’re out taking a winter walk along a country road and a small flock of dark gray birds flies up in front of you—they utter a few sharp chip notes and you notice that they flash white on the edges of their tails as they flit away in all directions. They are Juncos. You may know them as Slate-colored Juncos, but a decade ago taxonomists lumped several junco forms together under a new name, Dark-eyed Junco.
Some people call them “snowbirds,” and not because they fly to Florida for the winter! They are harbingers of winter, often making the first appearance at our bird feeders with the first snowfall. You may be surprised to learn that this ordinary little bird scratching for seeds under your bird feeder has had an extraordinarily important role in scholarly studies in biology for more than seventy years.
Dr. Ellen Ketterson is a researcher from Indiana University who recently produced a film called “The Ordinary Extraordinary Junco.” She describes the Junco as “a ‘rockstar’ for scientists who study animal behavior, ecology, and evolutionary biology.” Indeed, generations of researchers have addressed complex questions simply by studying the Junco.
Each fall, female Juncos leave their Canadian breeding grounds before the males and, ultimately, they fly much farther south. Because of this, the first Juncos you see in Pennsylvania in mid-October are the smaller, lighter gray and browner females. By early November, most of the females have flown south; and more of the larger, darker gray males begin arriving. Unlike the transient females, most of the males remain here.
By this time of year, they are all through sorting themselves out; almost three-quarters of the Juncos visiting feeders in Pennsylvania and points north are males, while a similar proportion of the Juncos wintering in the southeastern U.S. are females.
For one thing, by wintering farther north males have shorter distance to return to their nesting grounds in the spring. This means they can arrive earlier, which helps when there is competition to stake out breeding sites. Because males tend to battle over territories, larger size helps them to be competitive.
The larger size also gives male the ability to winter further north. It has to do partly with Bergmann’s Rule, one of those textbook ecological principles you may remember from biology class. It’s the one that states that within a species or a group of closely related species, individuals in northern areas will tend to be larger. That’s because the bigger the bird, the great its ability to conserve heat. So the larger males are better suited for cold winters.
Now it makes more sense why female Juncos are selected to winter farther south than the males. And that’s just one of many extraordinary things that scientists have learned about this ordinary little bird!
Photo Junco closeup: Peter Miller via Flickr.
Bird calls provided by the Macaulay Library at the Cornell Lab of Ornithology, and recorded by Jay W. McGowan and Gerrit Vyn.