Irruption Year!
Another type of migration is not necessarily tied to the seasons, although there is some correlation. Irruptive migration occurs in species that respond to irregular changes in food supplies. For example, the availability of seeds and buds consumed by species such as redpolls and Pine Grosbeaks can vary substantially from year-to-year and from location-to-location. In some years the food supplies in the north may be adequate and the birds remain in the northern forests during the winter. In other years the food supplies are low and the birds move south or to other locations outside of their typical range. These movements outside their normal range are called irruptions.
Raptors
While not in the seed and berry category, some
raptors also exhibit irruptive behavior. Northern
owls such as the Great Gray Owl and Snowy Owl sometimes exhibit this
behavior. Most remain in their Canadian homes throughout the winter. In
irruptive years, presumable as the result of poor food supplies, large
numbers of these species may move into the northern United States.
Evening Grosbeaks
Evening Grosbeaks are another species that demonstrate irruptive behavior patterns. The following maps were created from Great Backyard Bird Count data for the years noted. While general patterns are consistent from year to year, in certain areas Evening Grosbeaks can be common one year and completely missing the next.
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Cornell's eBird project collects sighting records from all over North America. Peruse the data on eBird to study irruptive species, including a prediction of winter finch irruptions.