As this season progresses, birders are spotting increasing numbers of returning migrants. What they are also commenting about is the continued presence of one of our winter visitors, which doesn’t seem to want to leave.

I’ve experienced that myself. Each morning for the past four months, a flock of roughly 30 common redpolls – sometimes more, sometimes less – has descended upon my well-stocked feeders and proceeded to devour seed at a rate seemingly out of proportion to their body size. If I forget to refill the feeders before bedtime, the next morning their peevish, squabbling twitters would wake me up as they fought with each other over the dregs at the bottom.

When they first showed up back in January, I was thrilled. First, because in the past I had not often seen redpolls, and second, I had never had such good, closeup views of them. One had even landed on me as I stood by the feeders.

Now, however, I have to admit I’ll be glad when they finally take off for their Arctic breeding grounds. I’ve become a slave to their appetites. But I should consider myself lucky; one birder reported a flock of more than 200 at his feeders. I’d hate to have his bird seed bill.

As many of you may know, common redpolls (and hoary redpolls) both breed and winter in the far North; they “irrupt” south of this range every one to two years. They do this in response to widespread seed crop failure – specifically of birch, spruce, and alder trees. I always thought of them as visitors of deep winter, so it seemed unusual to continue seeing them here into April. After consulting my trusty “Birds of North America” species account, I found that April is the month they usually begin departing.

Ahem. Did you hear that, redpolls? Begin departing.

The monitoring of irruptive bird species is a fascinating science. The last big irruption year for redpolls appears to have been 2000. Prior to that, the winter of 1997-98 proved to be even bigger, when irruptive birds of many species (pine grosbeaks, red crossbills, white-winged crossbills, evening grosbeaks, and common redpolls) staged what is known as a “superflight,” into the Northeast.

This year, the Cornell Lab of Ornithology reported February’s Great Backyard Bird Count saw an increase in redpolls over last year, which was considered a “nonirruption” year. During the four days of the GBBC, more than 40,000 people across North America counted more than four million birds (redpolls and otherwise).

In a press release from the Cornell Lab, National Audubon chief science director Frank Gill said, “We got a hint of the irruption to come based on reports from birders back in the fall…looking at the irruption based on the GBBC maps is very exciting – there’s basically a wall of redpolls reaching across much of the northern United States and southern Canada.”

A wall of redpolls – it is fascinating. Just as long as they don’t all show up at my feeders.

Chris Corio, a volunteer at Fields Pond Audubon Center in Holden, can be reached at fieldspond@juno.com