Flocks of common redpolls are among the lists of irruptive migrants being reported throughout Maine this season.

Redpolls are finches that breed and winter in portions of the Canadian Arctic and Alaska. When the trees that they depend on -such as spruce and birch – fail to produce sufficient crops of seed, the birds migrate south in waves in search of food.

A close relative, the hoary redpoll, looks almost identical, and close observation is needed to tell the species apart.

Basically, both finches have the bright red crowns they are named for. Both have conical yellow bills and black chins. Males of each species have a pink wash on their chests; however, this feature is very pale on the hoary.

The upper parts of both types are brown-streaked, but the hoary appears lighter, overall, than the common. The common redpoll has much darker, heavier streaking on its flanks and underparts; the hoary has much finer streaking, or none at all. The undertail coverts and the rump of the hoary are unstreaked or very finely streaked, while the common’s is heavily streaked.

Another characteristic described to distinguish the two is the apparent bulkiness of the hoary compared to the common redpoll. This makes the bird appear as a large, fluffy snowball. This feature is due to the hoary’s greater feather mass and fluffier plumage, which provides greater insulation for the bird during the long, freezing Arctic winters. This difference in plumage thickness is important, since the hoary generally has a much more northerly range than the common.

The adaptations that enable the birds to survive the severe temperatures and decreased daylight of the Arctic are simple, yet very effective. First, the birds select high-energy foods such as birch seed.

These seeds are procured quickly in large quantities and stored in laterally expandable pouches of the esophagus.

Later, once the bird has found dense cover to shelter in, it regurgitates the seeds, husks them, and then swallows them. This practice, along with increased digestive efficiency, highly insulative plumage, and continued activity during low light levels, enables redpolls to survive where other species cannot.

Look for the redpolls in open willow and birch thickets, and in weedy fields, which can provide good foraging habitat for them. They will also come to backyard bird feeders for sunflower and thistle seed. If you see a redpoll, it is more likely to be a common, rather than a hoary, because of its more southerly range. But you never know – a hoary could make an appearance. One word of caution: They are very hard to tell apart in the field. Very careful study and observation is necessary.

Redpolls seem to move south, in response to seed-crop failure, every two years. Other irruptive migrants also respond to such patterns, as was evident during the winter of 1997-98. Researchers dubbed this year a “superflight” year, which saw record numbers of common and hoary redpolls, red-breasted nuthatches, red crossbills, white-winged crossbills, and pine grosbeaks reported far out of their normal ranges.

Will this year be a superflight year? Only time will tell. Report any sightings to the Fields Pond Nature Center at 989-2591.

Chris Corio’s column on birds is published each Saturday. Corio, a volunteer at Fields Pond Nature Center in Holden, can be reached at fieldspond@juno.com