Pine grosbeaks compete for pebbles at Jamies Pond Wildlife Management Area in Farmingdale. Andy Molloy/Kennebec Journal
Isn’t it great when our winter bird feeders are overrun with common redpolls, pine siskins and purple finches? When pine grosbeaks feed voraciously on fruit still hanging on trees? And, if we are really lucky, when evening grosbeaks and crossbills descend on us?
These northern finches periodically move from their breeding grounds to our north when local seed production of favored trees is poor. Such movements are called irruptions. Because these species specialize on different kinds of seeds, winter irruptions into New England are unpredictable. Some years we may get siskins and in others common redpolls. Rarely, we get a superflight and many of these finch species push into our part of the continent.
So far, this winter seems to be a poor one for northern finches in Maine. Don’t despair: sometimes the finches don’t push south to Maine until they have depleted the seed crops to our north later in the winter.
About 30 years ago, some birders were suggesting that the dearth of northern finches over several years was because of a phenomenon called short-stopping. The idea is the rise in popularity of bird feeding to our north was providing south-bound northern finches with sufficient food, and thus the birds could stop short of more southerly destinations.
Using Christmas Bird Count data, I published a paper testing this hypothesis. If short-stopping is occurring, one should see an increase in northern finches on northern Christmas Count circles and a corresponding decrease in southern circles. I failed to find such a pattern.
The common redpoll is one of the more predictable of the northern finches. Redpolls tend to alternate between spending their winter on their breeding grounds with moving to destinations south. Perhaps, birch seed production (their favored food) shows a biennial cycle of seed production and failure.
A Colby student of mine, Paul Dougherty, did an honor’s thesis with me in 2016. He examined the year-to-year variation in common redpoll and American goldfinch winter abundance, again using Christmas Bird Count data. He found a positive relationship between the two species. At any given latitude, expect to see lots of goldfinches if you are seeing lots of redpolls.
Food may not be the only driver of southward irruptions for winter finches. Courtenay Strong and co-authors used Project FeederWatch data for pine siskins (over 2 million records). They noted some irruptions were north to south and others west to east. The authors found that rather complex relationships between air temperature and precipitation between source and destination could explain some migrations.
Strong, Ivy Widick and colleagues recently published an article in the “Journal of Animal Ecology” on the winter movements of nine species of northern finches over the past 60 years using Christmas Bird Count data. They did detect the biennial migration of common redpolls and the tendency of pine grosbeaks to show three to five irruptions in consecutive years, followed by several years of absence.
To be sure, these irruptions are complex, but the authors were able to parse the 60 years of data into three periods. In the 1960s and 1970s, irruptions were reasonably spaced and superflights of many species occurred fairly regularly. From 1980 to 2000, the coherence of irruptions were less predictable, with superflights becoming rare. Superflights returned after 2000.
One of the most striking results of their study was a poleward shift in the distribution of these nine species. Six species showed significant northward retractions of their southern range. Three species showed a northern shift in the southern limit of their irruptions.
These changes in distribution are clear. The reasons for the changes are not, although the authors suggest climate change and changes in resource availability are likely. These two factors are likely to interact in complex ways.
From our perspective, we should expect northern finch irruptions to be less spectacular as the birds shift their ranges northward. Too bad for us.
Herb Wilson taught ornithology and other biology courses at Colby College. He welcomes reader comments and questions at whwilson@colby.edu
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