Birdwatchers can help document winter bird irruptions.
Typically, the predictable seasonal movements of birds that are called migration don’t happen during January in Minnesota. But during some winters, birds normally uncommon in the state arrive unpredictably in large numbers – a surprising movement called an irruption.
Watchers of backyard bird feeders can see and help document irruptions. Some winters, feeders that normally attract only the usual chickadees, juncos and blue jays suddenly are visited by colorful yellow and black evening grosbeaks, purple finches (that look like little brown birds that have been dunked headfirst into a raspberry sauce), redpolls, pine siskins or pine grosbeaks (that resemble immense purple finches). These winter finches are hard to overlook, although their visits may be brief and sporadic; they are wanderers seeking seeds to sustain them through the winter.
Crossbills are perhaps the most unusual of these winter visitors. Crossbills are appropriately named: unlike other winter finches, the mandibles of their bills don’t touch at their ends but actually cross, giving a crossbill an odd scissor-like look to its beak. Crossbills use their unusual bills to extract seeds from conifer cones. The bill is inserted between the scales of a cone then the crossed mandibles are used as a wedge, opening the cone enough for a bird to extract the seed at the base of the scale with its tongue. Minnesota is visited by both red and white-winged crossbills. Red crossbills have thick bills, making it possible for them to attack the big cones of white and red pines. White-winged crossbills use their thinner bills to deftly open the smaller cones of spruce and tamarack, a fascinating process captured on this video.
Relying on conifers for seed can make for a nomadic life – cone crops vary greatly from year to year, forcing crossbills and other winter finches with specialized diets to travel far and wide to find food. When seed crops fail, finches may move in great numbers even beyond their usual range, causing an irruption that can bring surprising visitors to backyard feeders. Biologists try to predict these irruptions by observing the cyclical nature of wild seed crops. One forecast suggests that this winter could be a good one for watching finches south of their boreal forest home.
It could also be a good year for watching other dramatic winter visitors. Some northern owls, like finches, are known for irruptive movements. Owls, of course, don’t eat seed; their movements are driven by the abundance of their rodent prey. Snowy owls, a bird of the very far north (nesting on the tundra above 60 degrees North Latitude) hunt lemmings, a rodent famous for its boom-and-bust population cycles. Every winter, snowy owls find their way south to Minnesota and last winter the owls arrived in unusually high numbers. An excellent summary of the snowy owl irruption (with detailed maps showing sighting locations) is available from the online resource eBird. Scarce lemmings may have forced these owls further south – or it could be that abundant lemmings during the summer nesting season made it possible for the owls to raise more young that moved south in greater numbers than usual.
Great gray owls are the largest owls in North America and are denizens of Canada’s boreal forests, although a few nest in Minnesota. Those nesting birds are joined by their northern brethren when vole numbers plummet and hungry owls move south during the winter. A spectacular irruption brought thousands of great gray owls into Minnesota during the 2004-05 winter when birdwatchers could easily find dozens of these normally uncommon owls in a day. The big birds are not shy and could be watched during the day hunting by a roadside, leaning forward from a perch to listen for rodents tunneling beneath the snow surface, then plunging into the snow with talons outstretched to seize their prey. Hawk owls and boreal owls also were numerous that winter, making the irruption even more extraordinary.
Will this winter bring another irruption of northern owls to Minnesota? Irruptions can be difficult to predict, but the usual cycle of vole populations suggests that great gray owls could be numerous again (although not as numerous as during 2004-05). A good place to look for northern owls (and northern finches) is the Sax-Zim Bog, 200 square-miles of ideal habitat crisscrossed by county roads approximately 35 miles north of Duluth near the town of Cotton. The Sax-Zim Bog Winter Birding Festival in February is a good opportunity for anyone eager to watch these winter visitors.