It looks like it’s finally going to be a grosbeak winter. Back in early November I read forecasts that evening grosbeaks would head south in impressive numbers this winter.

If you’re not familiar with evening grosbeaks, you’re in for a treat. These robust irruptive finches are totally unpredictable here in the mid-latitudes. I haven’t seen any at my feeders since 1996.

So far this fall I’ve received grosbeak photos from neighbors and readers as far as 300 miles away. For most, it was a bird they had never seen before. Because I now live in North Carolina, I haven’t seen any yet, but I’m hopeful.

Even when grosbeaks visit, they can be frustratingly unreliable. They might hang around the feeders for a few days, then disappear for a week or two, then return. Only their unpredictability is predictable.

I observed the same erratic pattern of winter occurrence when I lived in Oklahoma in the early 1980s. Among the membership of the local Audubon society, it was always big news when grosbeaks descended on Stillwater. Maybe they would stay for the Christmas bird count, we’d hope.

In years when evening grosbeaks visit, they’re hard to miss and easy to recognize. They are husky, starling-sized finches with a huge, lightly colored bill and a short, dark tail. Yellow is the dominant color. To some, grosbeaks suggest an oversized goldfinch on steroids.

Males sport a bright yellow forehead and eyebrow stripe. The rest of the head is dark, and the belly and rump are variably yellow. In flight, which is distinctly undulating, black wings are set off by large white wing patches.

Females are grayish with just a whisper of yellow on the belly. The wing pattern is similar to the male’s, but contrast between the black and white is muted. The loud, clear call note grosbeaks give while feeding is also easy to recognize after a few encounters.

But why do grosbeaks show up some years and not others? To understand why these birds don’t always spend their winters with us, we must understand a little about their natural history.

Evening grosbeaks live “permanently” in the coniferous forests of the Rocky Mountains, southern Canada, and the northern tier of states in the midwest and northeast. Their favorite winter foods are the seeds of conifers and maples. Herein lies the answer to their wandering ways.

The number of seeds trees produce from year to year varies greatly. Some winters trees hang heavily with seeds; other years they are virtually bare.

Such an unreliable winter food supply compels evening grosbeaks (and other migrants such as red-breasted nuthatches, pine siskins, and purple finches, which depend on similarly unpredictable winter foods) to wander in search of winter food during lean years. They move south until they find seed-laden boxelders (a type of maple and a grosbeak favorite) or bird feeders stocked with sunflower seeds. Dozens of grosbeaks may spend all day on well stocked feeders.

And there they’ll stay, roaming from yard to yard, neighborhood to neighborhood or even town to town, gorging themselves on the easy pickings provided by their human benefactors.Ornithologists call these erratic southern movements “invasions” or “irruptions” and refer to these species as “irruptive migrants.”

Though grosbeak invasions are natural events, they are probably exaggerated by our penchant for feeding birds and by boxelders planted as backyard shade trees.

After one good look at a grosbeak, it’s easy to understand why they like sunflower seeds. As the name suggests, a grosbeak’s bill is enormous — perhaps even a bit larger than a cardinal’s. It’s designed to crack seeds.

At feeders grosbeaks are fun to watch because they can be domineering. They seem to prefer open trays or seed that is simply cast upon the ground. Grosbeaks hold their place at the feeder by opening their bills and lunging at other birds that violate their “personal space.” Sometimes they spend more time jousting than eating.

For a video preview of what you may see at your feeders this winter, google “evening grosbeaks at feeder cams.

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Questions and comments to Dr. Shalaway at sshalaway@aol.com or 229 Cider Mill Dr., Apt. 102, Hendersonville, N.C., 28792.