For the Birds – Northern Finch Irruption

For the past two months, a flock of Purple Finches has been frequenting our feeders in South China. This species seems to be pretty common throughout Maine so far this fall. Over the weekend, the Purple Finches were joined by a voracious flock of Pine Siskins. Many other Maine birders have recently reported siskins as well.

Purple Finches and Pine Siskins are two species in a group of finches commonly called the northern finches. Others in the group are Common Redpolls, Hoary Redpolls (quite uncommon in Maine), White-winged Crossbills, Red Crossbills, Evening Grosbeaks and Pine Grosbeaks.

These species nest at higher latitudes. The two redpolls and Pine Grosbeak nest to the north of Maine. Although the other five species are known nesters in our state, most of their nesting occurs to the north of us.

The northern finches are well known for their winter irruptions. An irruption is the opposite of an eruption. Birds erupt or leave from one area and irrupt or arrive in a different region. In good irruption years, we may be thrilled by large numbers of Common Redpolls or Pine Grosbeaks at our feeders and crossbills in the coniferous woods.

The cause of this irruptive behavior is food. All of these northern finches are primarily seed-eaters and many feed primarily on the cones of spruces, firs, tamaracks, hemlocks and even pines. Common Redpolls depend heavily on birch seeds for their winter sustenance.

We know that cone or seed production by trees in a local area varies greatly from year to year. A year of heavy seed production, called a mast year, may be followed by several years of low seed production. This annual variability seems to be an adaptation by trees to reduce the effect of insect seed predators.

By producing relatively few seeds for several years in a row, the trees keep the insect seed predators at a fairly low population size although most of the seeds produced may be eaten by the insects. Then a mast year occurs and seed production is so high that the insect population cannot consume all of the plenty. At least some of the seeds will successfully disperse and germinate. Of course, the insect populations will rise but are sure to plummet the following year when seed production is low.

The northern finches travel broadly in search of appropriate trees having a mast year. For us, two conditions must occur for an irruption to occur. First, our trees must be having a mast year to attract and hold any irrupting birds. Second, the trees on the nesting grounds of the northern finches must have a year of low seed production, forcing the finches to erupt elsewhere in search of sufficient seeds. So, a mast year in Maine may not always be accompanied by a northern finch irruption.

When I was growing up in North Carolina, we would often be delighted by hordes of Evening Grosbeaks and Pine Siskins during the winter. Those irruptions are events of the past. Big flights of Evening Grosbeaks are now uncommon even in Maine.

It occurred to me a few years ago that the rise of bird-feeding over the past 25 years might be influencing the irruptions of northern finches. Perhaps, Pine Siskins and other finches do not need to go so far south now to find natural food because of all of the sunflower and niger seeds we put out for them. In other words, I wondered if northern finch irruptions were being short-stopped.

To test for a short-stopping effect of bird feeding, I analyzed all of the recapture data of banded birds for six northern finches (Common Redpoll, Purple Finch, Pine Siskin, Pine Grosbeak, Evening Grosbeak and White-winged Crossbill) in eastern North America from 1975-1994. I also used Christmas Bird Count data for the same years as a measure of how the magnitude of irruptions of these birds might have changed.

I was able to show that larger irruptions occurred earlier in the study (pre-1984). However, recapture of banded birds showed that there was no significant increase in finches wintering at northern latitudes over time. The data suggest that the less dramatic irruptions in recent years are due to a decline in the populations of these birds rather than a short-stopping of irruptive movements.

What will this winter bring? Perhaps Pine Siskins will stage a large irruption this year. Common Redpolls in Maine have shown a strong pattern of alternating an irruption year with a year of few individuals. We should be in for a Common Redpoll irruption this winter.

[Originally published on November 3, 2007]