What Is Irruptive Migration?

Ornithologists say that an irruptive migration occurs when a large number of birds of one species moves into an region where these birds do not normally occur. These irruptive migrations can greatly expand the winter distribution of a species.

These two maps show the difference in winter range of Common Redpolls during an irruption year (left) and during a typical winter (right). The gray shading indicates locations where FeederWatchers supplied data. The red points are feeders at which redpolls were reported on at least 2 different occasions (and probably spending the entire winter).

Irruptive migrations have been recorded for centuries:

"...there was greate plenty of strang birds, that shewed themselves at the time the apples were in full rype, who fedde upon the kernells onely of these apples, and haveinge a bill with one beake wrythinge over the other... The oldest man living had never heard or reade of any such like bird... They were very good meate."

- quote from 1640 in Ian Newton's book Finches

However, irruptive migrants arrive at irregular intervals and often come from areas with few human inhabitants. As a result, to some extent, scientists still know little more about irruptive migrants than was known in 1640. Data collected by participants in Project FeederWatch have allowed researchers at the Cornell Lab of Ornithology to answer the following basic questions about one irruptive migrant, the Common Redpoll:

Copyright 1998, Cornell Laboratory of Ornithology