Irruptions do not apply to all species, but are known to occur with crossbills, siskins, purple finches, redpolls, bohemian waxwings, red-breasted nuthatches, and also some owls. By definition, irruptions are irregular, but studies show that they occur in cycles of approximately 2-10 years.
Species irruptions occur when food sources are poor on their wintering grounds, compelling the birds to spread out more widely than usual, and in larger numbers. Finches and other seedeaters move in as a result of poor crop production. Owl irruptions coincide with cyclical crashes of rodent populations, which happen to rely upon seeds.
Interestingly, irruptions do not always involve the same species at the same time, and are not related to cold winters. While last winter there was an irruption of Saw-whet Owls, this winter we are currently experiencing an irruption of Snowy Owls, which are being reported in unexpected areas throughout the Delaware Valley region, much to the delight of birders!
photo © adrian binns