By Bill Chaisson
Like ski bums showing up in a resort town before the snow flies, as winter approaches, boreal birds descend upon the northern tier of the United States. There are three different groups of winter residents.
One includes the birds of the tundra and the taiga (coniferous forest) that simply shift their activities southward a bit. This is not migration as one usually thinks of it: flying south to somewhere warm. Instead these birds, because where they breed becomes quite cold and dark in the winter, move somewhere less cold and dark. Species in this contingent include the rough-legged hawk (which will get its own column), tree sparrow, snow bunting, and Lapland longspur.
Another group could be called “the irruptives.” These are species that live alongside the first group way up north, but they don’t always move southward in the winter. It is a phenomenon like the “squirrel-mageddon” we experienced last year. There were two banner years for tree mast (seeds), which gave squirrels all they could eat. So, they raised large broods and an unusual number of them survived predation and caused a population boom. When this was followed by a poor mast year, these large populations had to roam to find food. Something similar seems to drive the movements of many finches of the taiga. Redpolls, pine siskins, crossbills, and pine grosbeaks may abruptly descend after being absent or rare for years.
The final group doesn’t so much move latitudinally as altitudinally. In the northern United States spruce and fir species are largely restricted to higher elevations. Birds that prefer predominantly or wholly coniferous surroundings in which to breed, like dark-eyed juncos and red-breasted nuthatches, simply wander away (and often downward) from these evergreen assemblages to forage at lower elevations through the fall and winter.
The tree sparrow even has a nickname, “winter chippie,” because the actual chipping sparrow (Spizella passerina) disappears from our region during the winter and is apparently replaced by Spizella arborea, which in spite of its name ranges out of the spruce/fir forest and up into the tundra. These attractive sparrows have gray heads surmounted by a rufous crown and divided by a rufous eye line. Because they are often seen at feeders, it is possible to get a close and long look at them.This is how you can see their bi-colored bill, which is dark on the upper mandible and yellow on the lower one.
Perhaps the field mark you will see first is the dark spot in the middle of its breast. Many sparrows have this spot in the middle of their chests, but its usually accompanied by a lot of streaking (e.g. song sparrows), and it is sometimes present in juveniles of a species and then disappears in the adult plumage (e.g. white-throated sparrows). The latter suggests that, like the spotting on the breasts of thrushes, it is a juvenile character that persists into adulthood, i.e. neoteny.
Tree sparrows have relatively small bills for a sparrow, and they eat a wide variety of insects, fruit, and seeds on their breeding grounds. They extend their summer range northward into the tundra by following the clumps of alder and willow that extend along protected streams and valleys from the treeline. In the winter they drop insects from their diet and forage on the ground in cut-over agricultural fields, meadows, and hedgerows for seeds and any berries that have remained into winter. At feeders they prefer to feed on the ground, where they, along with their junco cousins, are a species that will eat all the millet some folks might think of as filler in the sunflower seed bag.
About a decade ago (winter 2008-2009) there was a huge irruption of pine siskins (Spinus pinus). I was living in central New York at the time and we were positively inundated with these tiny, conservative-looking finches. Siskins have quite narrow beaks for a finch, and they are an entirely brown bird covered with dark streaks. Their only bright color is yellow streaking on the outside edges of some of their primaries and secondaries. It is also found on either side of the base of their tails but is less visible there.
A banding project conducted during the 2008-2009 irruption event revealed a fascinating pattern to the movement of populations. According to Project Feederwatch, birds banded in the northeastern United States were recovered the following winter in southwestern Canada and northwestern Canada. In contrast, birds banded in the southern central United States were recovered the following year in central Canada.
Pine siskins will sometimes remain in the region that they have irrupted into and breed. The evening grosbeak, another irruptive finch, did something similar on a longer timeline. The grosbeaks extended their range from west to east in a series of irruptions between the late 19th and early 20th century and reached a population peak in the 1980s. Since then the species has been in steep decline, particularly in the eastern part of its range colonized over the previous century.
After breeding well south of their ordinary distribution in 2009, however, pine siskins disappeared from most of the northeast. (The southern edge of their breeding range extends from Maine over to the High Peaks region of the Adirondacks.) It is, however, always worth watching out for them each winter. As their small bills suggest, they like smaller seeds and they will nudge goldfinches to one side in order to dominate the thistle feeder.
Last winter I didn’t see any irruptive species at all at my feeder, aside from a single evening grosbeak. This was my misfortune, as there was an irruption of pine grosbeaks into New England, although most of the reports came from northern Vermont and New Hampshire. As they say in baseball, “Maybe next year.”