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.

There are two species of crossbills, the red and the white-winged. They are both Holarctic birds whose range extends across the higher latitudes of both hemispheres. They are equally common in Siberia and Canada. The white-winged is generally the more northerly of the two and the one we are less likely to see here. The range of the red crossbill extends south in high mountains all the way to Mexico. Both species exhibit some of the tameness typical of northern species that have little contact with humans. Generally, the white-winged is the tamer of the two.

Both species are about the size of house sparrows. Male red crossbills in breeding plumage are brick red with dusky wings and dark tails. The females show a similar pattern but are buffy yellow rather than red. White-winged crossbills have two broad white wing bars, but are otherwise similar to the red.

Like many other northern birds, ranging from goshawks and snowy owls to evening grosbeaks and common redpolls, crossbills are irruptive. In many winters we see none at all, but in some years we see a whole lot of them.

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.

Seedeaters like the crossbills run into similar problems when their favorite food plants have a poor year and don’t set many seeds. However, one of the things I like about crossbills is that they have gone further than any other bird in adapting to the difficulties of life in the north.

To start with, consider those crossed bills. Young crossbills just about to leave the nest have beaks that resemble the standard-issue seedeaters’ bill. Their mandibles are short, stout, and more or less cone-shaped. But as the birds mature, the tips of the mandibles bend–one to the left, one to the right–and grow past each other, producing a sort of X shape. If you had never heard of crossbills before seeing your first one, you would think the bird was either the victim of some sort of genetic disorder or a poster bird for the fight against pollution, like the cross-billed cormorants produced in the poisoned environment around Green Bay.

But the crossed mandibles are beautifully fitted to the job of prying apart the scales on the cones of pines, spruces, tamaracks, and firs. Lifting the scales reveals the seeds, which the birds then pluck with their tongues. Crossbills feeding look like boreal parrots. They clamber over the cones, often using a foot to hold the cone down so they can dig out the seeds.

However, in the north country it doesn’t pay to get too specialized. I used to know a man in northern Wisconsin who ran a gas station, a restaurant, and a taxidermy business, while simultaneously working as a buyer of cowhides from local dairy farmers and a plower of snow on contract with the township to clear local roads. It took the total proceeds from all of those jobs to make an adequate living.

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.

They are also very fond of salt. There are recorded instances of birds being run down by cars as they went after salt spread on snowy roads. Because of their crossed bills, birds drinking salty meltwater must turn their heads to one side, holding their beaks in a horizontal position parallel to the ground as they lick up the water with their tongues.

Adapting to the north also means learning to move around in search of food. A typical north-woods conifer may take four or five years to store up enough energy to produce a big seed crop. If any crossbills decided to try to put down roots and establish themselves in one place, they would have some long hungry times between meals.

So the crossbills wander in search of groves of conifers that have just had a very good year. To stationary observers in the north woods, crossbill populations seem to fluctuate wildly as the birds follow the crops from place to place.

Like parrots, crossbills are very gregarious, living in flocks most of the time. This kind of behavior is common among birds whose food sources are likely to be either abundant or absent. Moving in flocks provides a measure of protection from predators and more eyes to seek out seed-laden trees. Birds that depend on foods spread thinly but evenly through the habitat are more likely to live in small family groups or as solitary individuals.

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.

Their nests, according to the literature, are the simple cups favored by most songbirds, but carefully lined with feathers, fur, or very fine plant fibers to provide insulation.

It is common for mammals to give birth in winter, but their embryos develop inside the mother. For a bird, keeping the developing embryos warm requires not only a well-insulated nest, but constant incubating as well. Female crossbills essentially live on the nest during incubation. In a familial arrangement that would please Jerry Falwell, the incubating female becomes totally dependent on the male to bring her food during the two-week incubation period. She adopts a submissive, begging posture at his approach, and he regurgitates food directly into her mouth.

Once the eggs hatch, the downy young are still in need of warmth, so the male serves as breadwinner for the whole group, feeding both the female and the nestlings. When the young are able to withstand cold for short periods, the female begins to leave the nest, and from that point on the two adults work together providing food. They often come to the nest together, with first the male and then the female taking a turn in feeding the young regurgitated food.

Crossbills provide yet another compelling argument for an idea that has become a sort of theme in these columns: the need to preserve large amounts of wild land. The need for large preserves to sustain grizzly bears and caribou herds seems obvious enough, but small songbirds and even butterflies require space too. The sparrow-size crossbills routinely range over hundreds of square miles of northern forests, but periodically even that is not enough, and they must fly south to Illinois to find food. Protecting all our species will require both large blocks of land and connecting strips between them, where wandering crossbills can at least gain a temporary refuge.