Red crossbills are nomadic finches of North America that roam the coniferous forests of southern Alaska, southern Canada, New England, the upper Great Lakes region and the mountain corridors of the West, from British Columbia and Alberta to Mexico. Monogamous, their breeding is prompted by an abundance of food and may occur during any season of the year; in good years, multiple broods are raised. Feeding primarily on the seeds of conifers, their crossed bills have evolved to pry open the cone scales, allowing them to extract the seeds with their tongue; red crossbills also feed on a variety of non-conifer seeds, berries and insects, especially aphids.
Irruptive, red crossbills often wander from their breeding range, especially during the winter months, and may turn up almost anywhere in the Continental U.S.; indeed, they have been discovered nesting as far south as the pine woods of Georgia. In recent decades, ornithologists have identified eight subspecies of red crossbills, which vary in body and bill size and seem to have adapted to feeding on specific types of conifers (pines, spruces, firs or hemlocks); smaller subspecies favor the tightly wound cones of spruces while larger birds are associated with pine forests.
Since there is no documentation of interbreeding among these subspecies and since each delivers a unique call, there is ongoing debate whether they represent distinct species. If so, the diverse population of North American red crossbills demonstrates the phenomenon of species radiation, where adaptation to the regional food supply (in this case, conifers) has, through natural selection, led to the varied species. Whatever the outcome of this scientific investigation, I will always enjoy observing red crossbills along the Front Range foothills and take special delight in the sound of crackling fire as these hardy nomads attack the pine cones.
