Every winter Maine is invaded by birds. But I don’t mean the malevolent gulls from “The Birds.” I’m talking about flocks of innocent songbirds from Canada’s boreal forests. Their unexpected visits, called “irruptions,” are all about food, not climate. They go where food is, and thus their travel patterns vary from year to year, and even week to week. Winter birding and feeder watching in Maine can be unexpectedly interesting because we never know who’s coming for dinner.

It helps to look globally to understand what’s happening locally with these birds. Most winter irruptives, including the redpoll, evening grosbeak and crossbills, rely heavily on seeds and cones. Others, including the pine grosbeak and Bohemian waxwing, depend on wild fruit crops, primarily mountain ash berries. If these food sources are poor in their particular neck of the woods, birds are going to head for other parts of Canada and northern New England in hopes of finding better eats.

Every fall, Canadian ornithologist Ron Pittaway analyzes the seed and cone crops in Ontario to determine what the general patterns of bird irruption might be. To my delight, his forecast is favorable for Bohemian waxwings showing up in Maine this winter. Because of their sleek beauty and gregarious nature, waxwings are among my favorite birds, so I’ve been thrilled to see so many “Bohos” in recent years. The 2007 Christmas Bird Count tallied hundreds in the Thomaston – Rockland count area. One crabapple tree on Old County Road swarmed with at least 100, with a few of the more common cedar waxwings mixed in, all greedily eating the frozen fruit. We even had a visitation at the Coastal Mountains Land Trust office last year. They travel in numbers and are quite vocal, so a flock is usually hard to miss.

Some winters common redpolls are everywhere. While watching ducks from the shore of partially frozen Tolman Pond in West Rockport a few winters ago, I was surprised to come across a handful of these sparrow-like finches feeding in a corridor of alders and small birches. These little birds with raspberry caps are very fond of birch seeds. I also recall a snowshoe hike on conserved land in Greene that was made memorable by a tree decorated with redpolls like living ornaments. And during Maine Audubon’s winter ecology weekend up at Claybrook Mountain Lodge in Highland Plantation one January, the feeders were inundated with redpolls visible from the breakfast table — a real treat. We didn’t even have to stop eating to enjoy them.

The redpolls were joined on the feeders by several evening grosbeaks, the striking black and gold males in particular drawing lots of “oohs” and “aahs.” Evening grosbeak numbers have diminished throughout its range due to the decline of its primary summer food, spruce budworm. So any sighting is cause for excitement. When I was a child they were a regular visitor to our tray feeder, but in recent years I’ve seen fewer than a dozen total. According to Pittaway’s report, “A few common redpolls should move south into southern Ontario and farther east and south. However, most redpolls may be drawn to good birch crops in northwestern Ontario and westward in the boreal forest into Saskatchewan.” So we might see them, or we might not. Sadly, his expectations are low for sightings of the beautiful evening grosbeak.

Driving around on snowy inland roads, we’ve often come upon pine grosbeaks in small groups on the pavement, apparently drawn to the road salt — not a safe habit. National Geographic’s “Field Guide to the Birds of Eastern North America” describes them as “usually unwary and approachable.” They certainly don’t fly from an oncoming car as quickly as they probably should. These large finches are attractive backyard visitors. The deep pink male is especially noticeable; my mother always calls to tell me when she sees them at her feeder. But the female’s more subtle hues — gray body with olive-orange head — give her a rather exotic appeal, as well. Again, our healthy mountain ash crop — boosted by backyard ornamental berry bushes and crabapples — may draw them our way this winter. We can hope, but only the birds know for sure.

The white-winged crossbill is one of our more interesting invaders. Last winter flocks of crossbills seemingly topped every clump of spruce, greedily pulling apart the small, soft cones that are their food of choice. Several years ago I helped with the Christmas Bird Count on Matinicus Island in early January, and crossbills were the bird of the day. They flew overhead constantly, bouncing from spruce to spruce in small, sweetly chirping flocks. On a pilgrimage to see a northern hawk-owl lingering in South Bristol last winter, we observed the owl flying to the top of a spruce that also harbored several crossbills. I later saw photographs of the owl eating a crossbill. Given that both birds hang out at the tops of spruces, it makes sense that crossbills would be natural prey for the owl. (Several northern owl species, including the northern hawk-owl, snowy owl and great gray owl, are also irruptive, depending on sub-Arctic rodent population swings.)

Crossbills are worth a close look because their bill is indeed crossed. And interestingly, just as people can be right- or left-handed, some crossbills’ bills cross to the left and some to the right. The unique bill structure gives them a specialized edge, providing the proper torque for prying out spruce cone seeds. Pittaway’s forecast for crossbills this winter? Well, if you’ve paid attention while hiking in the woods lately, you might have noticed that this fall’s cone crop was abundant. The hope is that spruce tops laden with cone clusters will beckon the birds southward.

It seems insane, but if the crop is good enough, crossbills will even nest in midwinter. Sure, there may be a few feet of snow on the ground. But with a safe and warm nest nestled in evergreen boughs and plenty of cones near at hand, what more does a bird need to raise a family? So keep your eyes and ears open in the woods this winter, vigilant for these potential invasions. You never know what might fly in, and unless you’re Tippi Hedren, it should be cause for delight.

Kristen Lindquist is a published poet who works as the development director for Coastal Mountains Land Trust in Camden.