Crossbills visit the territory regularly and even breed here, but the migrations of these small birds remain an enigma. They are the nomads of the boreal forest, and you never quite know where they're going to turn up next.

Like other finches, Crossbills are seed-eaters. When they run out of seeds in one area, they just move on, traveling thousands of kilometers if necessary. Sometimes, in what are called invasions or irruptions, they show up in huge flocks in unexpected places.

Curtis Adkisson, an American authority on Crossbills, describes these birds as almost unique. "A few other birds feed on ephemeral seed supplies and invade, but Crossbills are the absolute extreme in terms of the distances they go and their dependence on one seed supply," he says.

Crossbills, the only birds with crossed beaks, can pack away as much as 3,000 conifer seeds a day. They use their unusual bills to wedge open cone scales and then lift the seeds out with their tongues. As agile as parrots, Crossbills use their claws and bills to clamber about and get in just the right position for prying open seed cones.

The two Crossbill species seen in the Yukon are close relatives. White-winged Crossbills, the northern species, are found across the boreal forest from Alaska to Newfoundland, dipping south as far as the central Rocky Mountains. They specialize in small soft spruce cones.

The larger Red Crossbills can break open tougher cones such as those found on pine trees. This species varies more in body and bill type, and can be found as far south as Central America. It is at the northern end of its range in the southern Yukon.

The two species behave similarly. More is known about Red Crossbills since they range across more populated areas where they are easier to study. But easy is a relative term when it comes to learning about Crossbills.

Adkisson wanted to do his graduate work on Red Crossbills. His advisor talked him out of it, saying that he would never get enough data to write a thesis. He studied Pine Grosbeaks instead.

He says that much of what we know about Crossbills today is due to the efforts of three graduate students who started studying these birds in the 1980s. Working at different universities and studying different aspects of the species, each of them took about 10 years to get a degree.

Irruptions of Crossbills always attract attention. Last year, winter finches, including Crossbills, moved south in massive invasions. White-winged Crossbills showed up in places like Tennessee where they are normally never seen.

In Prairie towns, huge flocks of Crossbills have descended upon the conifer trees planted in people's yards. "You might have thousands in one area," says Adkisson. "The trees will be dripping with Crossbills and then they move on. It's really spectacular."

When Crossbills find an abundant food supply, they will usually start to breed, even in the dead of winter. Give them enough food and they can produce two or three broods of young in quick succession.

"That is extremely unusual," says Adkisson. "These birds are out on the edge of the curve all the time."

But why would birds develop such a demanding lifestyle? "If you can pursue this strategy successfully, you can live in Fat City all the time. These birds are strong flyers. If they migrate in the conifer belt, sooner or later they are going to find food," says Adkisson.

Theoreticians can only speculate why conifer trees produce cones so erratically. Sometimes large areas of forest will have a poor cone crop, and after a really bumper crop they will not produce any seeds at all the next year.

Possibly this is an evolutionary strategy to control the number of seed predators like squirrels and rodents. According to this theory, the trees just cut off the food supply once in awhile to make sure that there are not too many predators out there harvesting their seeds.

In winter Red Crossbills have occasionally been spotted at bird-feeders in Whitehorse, but usually they leave the territory in the late fall, returning to the southern Yukon in the spring. The elaborate and melodious songs of White-winged Crossbills can be heard year-round in the southern Yukon, but do not expect to see this species at your local bird feeder; they stick to cone seeds.

In the Yukon huge fluctuations in Crossbill numbers have not often been reported, but then again, in the Yukon, these birds could invade a remote section of boreal forest and no one would ever know it. Much remains to be learned about these mysterious birds.

For more information on Crossbills contact the Canadian Wildlife Service at 667-3400.