What will the winter bring? Birders frequently ask this question each fall. We know we can count on seeing our resident birds like black-capped chickadees, American crows and hairy woodpeckers. We also know that most species of migratory breeding birds are gone now but will be back next summer. You can count on seeing eastern phoebes, house wrens and yellow warblers come the summer. Passage migrants (birds that breed to our north and winter to our south) seldom linger in Maine for the winter. Snow geese and semipalmated sandpipers are two examples of Maine passage migrants.

The last category of birds, winter migrants, inspires excitement in birders. These birds breed to our north, some as far north as the Arctic tundra. Some of our winter migrants like American tree sparrows are expected every year. But many winter migrants are unpredictable; in some years they may be common and in other years scarcer than hen’s teeth. These birds include snowy owls, Bohemian waxwings and a suite of finches commonly called the northern finches.

Why the variability? The answer is quite simply food availability. Snowy owls are perfectly capable of making it through a winter on the Arctic tundra if the lemming population is sufficient to provide food. Similarly, common redpolls can survive an Arctic winter given sufficient birch seeds.

However, lemming abundance, birch and conifer seed production, and soft fruit production vary from year to year. When the requisite food is scarce, birds must migrate south to find food. The result is an influx of northern birds. Who is not thrilled by flocks of common redpolls at our feeders or Bohemian waxwings in our fruit trees?

Ecologists refer to these incursions of birds as irruptions. An irruption is movement into a particular place, just the opposite of movement out in an eruption. We can think of common redpolls erupting from northerly areas when food is not available and irrupting into Maine where birch seeds may be more plentiful.

This winter is shaping up to be an irruption year for snowy owls. More than 20 of these magnificent raptors have been sighted in Maine already this winter, mostly along the coast. Last Sunday two birders in Newfoundland saw 138 on an all-day birding trip. Keep those eyes peeled.

A couple of flocks of Bohemian waxwing were seen in the past week. Look for these fruit-eaters at apple orchards or in stands of fruit-bearing trees or shrubs.

The irruptive finches show weak correlations in their abundances because they rely on different types of tree seeds for their sustenance. Ron Pittaway prepares predictions of irruptions each fall based on the production of various species of trees in the vast stretches of boreal forest to our north.

Pittaway reports that mountain ash produced an abundant berry crop to the north of us this fall. Therefore, we are not likely to see very many pine grosbeaks (fruit-eating finches) this year.

Birch and alder seeds are abundant in boreal forests to our north. We should not expect a major irruption of common redpolls.

Red crossbills prefer to extract seeds from the cones of red pine and white pine. Red pinecone production is fair to good this year and white pine production is poor. Look for occasional red crossbills in Maine, where ornamental conifers or pines are laden with cones.

White-winged crossbills prefer spruce cones. Good to excellent spruce production is seen in the boreal forest, extending down into northern New England and the Adirondacks. These crossbills are likely to be broadly dispersed so we should not expect high populations this winter.

Pine siskins with their small bills rely heavily on hemlock and spruce cones. Fair numbers of siskins are expected in northern New England this winter.

Herb Wilson teaches ornithology and other biology courses at Colby College. He welcomes reader comments and questions at whwilson@colby.edu Previous columns and other information on Maine birding can be found at his blog:

http://www.mainebirds.blogspot.com