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It appears this winter will be good irruptive season for winter finches: Pine Siskin, Common Redpoll, Hoary Redpoll, Purple Finch, Evening Grosbeak, Pine Grosbeak, Red Crossbill, and White-winged Crossbill. Already there are reports of Pine Siskins. Crossbills have been seen in Connecticut and at Jones Beach. At least two customers have seen Evening Grosbeaks in their back yards.

These birds reside in boreal forests, the taiga, and even the tundra. Redpolls nest as far north as northern Baffin Island. Other irruptive migrants reside in the vast spruce-fir and birch forests that stretch across Canada and Alaska.

All are members of the finch family Fringillidae and subfamily Carduelinae. Because most of us see them only in the colder months, we’ve come to know them collectively as winter finches.

They are considered partial migrants because in most years, some individuals remain within the nesting range while others fly south of it. In addition, younger birds and females tend to migrate farther than adult males.

Based on many decades of Christmas Bird Count data, we know that irruptions usually occur every other year, with a few exceptions. In one year, only a handful of redpolls or siskins will be present in southern Canada and the northern United States. The next winter, birders will report large numbers of the birds. This may be a big year.

In addition, the movements seem to be synchronous over thousands of square miles, suggesting a common cause for the irruption. It’s somewhat bizarre that the numbers of individuals migrating in each irruption vary dramatically and that the wintering range changes.

The key to understanding the movements of winter finches is their food. Seeds are usually present year-round, meaning irruptive species often don’t have to leave their nesting areas in winter to find food. Seed availability, however, varies greatly from year to year, and in some years crops all but fail. When this happens, it is believed that irruptive species must leave.

It’s worth noting that the migration behaviors of northern winter finches are similar to movements of closely related species from southern latitudes. Pine Siskin, for example, is in the same genus as the American goldfinches.

Like their irruptive northern relatives, the goldfinches are also primarily seed-eaters, and they’re social birds, spending much of their lives in flocks with others of their species. The winter finches share a tendency to undertake relatively unpredictable migrations. They are irruptive to varying degrees and nomadic, choosing new wintering and breeding areas year after year.

The difference between the goldfinches and the northern species seems to be scale. The northern birds have immense irruptions that birders can’t help but notice, whereas irruptions of goldfinches occur in smaller numbers.

Recoveries of banded individuals have shown how nomadic finches can be. One Common Redpoll was found in Siberia and Michigan in successive winters. Another Common Redpoll was banded in Belgium during one winter and found the next winter in China.

Similarly, goldfinches don’t appear to have a fixed migration direction or distance. An American Goldfinch banded in the winter in Ontario turned up the next winter in Louisiana. And in Arizona, the abundance of Lawrence’s Goldfinches changes year to year, signaling that they have irruptions.

Because irruptions don’t happen every year and vary significantly when they do happen, we don’t typically think of them as a type of migration. But in a sense, they are. Viewed more simply, irruptions are adaptations to a constantly changing source of food.