There are 2 basic groups of irruptive migrants – boreal finches and other songbirds that depend on fluctuating tree-fruit crops, and owls and other predatory birds that depend on cyclically fluctuating rodent populations. Both groups specialize on food supplies which may fluctuate more than 100-fold from year to year. However, seedcrops in widely separated regions may fluctuate independently of one another, as may rodent populations, so that poor food supplies in one region may coincide with good supplies in another. If these species are to have access to rich food supplies every year, they must often move hundreds or thousands of miles. In years of widespread food shortage (or high populations relative to food supplies) extending over many thousands or millions of square miles, large numbers of individuals migrate to lower latitudes as an irruptive migration.
Compared to regular migrants, these nomadic populations rarely return to the same breeding areas in successive years. Moreover, band recoveries and radio-tracking confirm that the same individuals can breed in different years in areas separated by hundreds or even thousands of miles. Similarly, irruptive species typically winter in widely separated localities in different years, sometimes on opposite sides of a continent. A few extreme examples include a Pine Siskin that was banded in Quebec in one winter and recaptured 2,455 miles away in California during a subsequent winter, and a Common Redpoll banded in Michigan and recovered in East Siberia 6,350 miles distant!
This year, there are indications that some of the boreal seed-eaters are irrupting south in good numbers. Birders and bird banding stations across the Northeast are reporting higher than normal numbers of Purple Finches, Pine Siskins, Evening Grosbeaks, and both nuthatch species. But, since the arrival time and movement patterns of irruptive species are highly variable, it is nearly impossible to predict with certainty whether irruptions will occur and/or for how long. Data from the online birding checklist, eBird, revealed that reports of Pine Siskins were high during the fall of 2004, but then dropped suddenly in mid-December to more typical levels. The following year however, the opposite pattern emerged, with very few siskins reported until mid-December when sightings increased dramatically and remained stable throughout the winter.
Locally this week, a flock of 2 to 3 dozen Pine Siskins were reported in Etna, NH, while at Dead Creek Wildlife Management Area in Addison, VT, a Clay-colored Sparrow and a Nelson’s Sharp-tailed Sparrow were sighted. Finally, a Long-tailed Jaeger was observed on Grand Isle. You can explore all the birds reported last week in Vermont and add your own sightings at Vermont eBird.