Most birds adhere to predictable migration patterns, wintering and summering in specific geographic ranges. However, some birds, known as irruptives, demonstrate a fluctuating winter distribution, an adaptive response to variations in the availability of food or prey.
Perhaps the most famous irruptive in the U.S. is the snowy owl. In winters when lemming populations fall, these northern owls turn up across central latitudes of North America, attracting bird watchers from across the country. Great gray owls and boreal owls also demonstrate this pattern, though their potential range is usually restricted to more northern States.
Among songbirds, snow buntings, Bohemian waxwings, redpolls, crossbills and evening grosbeaks are the best known irruptives. Their numbers in the Heartland vary dramatically from year to year; when their northern or high country food crops are deficient (or covered by heavy snow), these erratic wanderers may suddenly appear in large flocks, providing a special treat for bird watchers. Unfortunately, they often disappear just as suddenly.
