Early this winter flocks of white-winged crossbills invaded northern Illinois. Sightings were reported from various locations in Lake County, including Illinois Beach State Park, where the pine groves at the park’s southern end are very attractive to crossbills.
Irruptive behavior is a response to the exigencies of life in the far north. Northern ecosystems–cold and dark for much of the year–are not very productive, and they also tend to support small numbers of species of both plants and animals. Great horned owls in an Illinois woodlot can prey on a variety of small mammals: tree squirrels, ground squirrels, woodchucks, rabbits, chipmunks, and assorted rats, mice, and voles. Snowy owls on the tundra depend to a considerable extent on a single species, the collared lemming. When the lemmings suffer one of their famous periodic population crashes, the owls must move south in large numbers in search of something to eat.
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Crossbills, despite their specialized bills, are equally diversified. In addition to conifer seeds, they eat the seeds and buds of willows, alders, birches, sunflowers, and other plants, and a variety of insects as well.
The most amazing adaptation of crossbills to life in the colder climates is their ability to nest in midwinter. If a flock of crossbills discovers an abundant supply of spruce seeds, the birds will proceed directly to courtship and nest building, even in January in northern Ontario. Late-winter nesting– February and March–seems to be the usual practice of both species.
