Every winter, bird watchers across North America anxiously await the possible incursion of birds that don't normally winter in their areas. These periodic bird irruptions add a dramatic level of excitement to winter birding. The birds most commonly associated with these winter irruptions are the winter finches (Pine Grosbeak, Red Crossbill, White-winged Crossbill, Purple Finch, Pine Siskin, Common Redpoll, and Evening Grosbeak), but other species will also shift from their typical wintering grounds into other areas. For example, Red-breasted Nuthatch, Clark's Nutcracker, Bohemian Waxwing, Black-capped Chickadee, and Varied Thrush will also stage periodic winter irruptions.

The arrival of winter finches to your backyard does not necessarily indicate a harsh winter ahead. It is generally believed that irruptions are driven by a lack of food on the normal wintering grounds. For example, Common Redpolls feed primarily on the catkins produced by birch and alder trees. When catkin production is low, Common Redpolls leave these areas and irrupt into areas where food is more plentiful. Common Redpoll irruptions can be extensive, ranging as far south as the middle Atlantic states or central Kansas. Participants in the Great Backyard Bird Count have documented just such an incursion in the winter of 2000 (See maps below).

Sites Reporting Common Redpolls GBBC 1999	Sites Reporting Common Redpolls GBBC 2000

It is a rare winter when no species of bird is irrupting somewhere in North America. An event could be geographically limited; for example, Varied Thrushes occasionally undergo greater dispersion on their typical winter range in the Pacific Northwest. On the other hand, an event could be as dramatic as the simultaneous irruption of several species into one area. This occurred in the Northeast during the winter of 1997-1998, when Red and White-winged Crossbills, Common Redpolls, Evening Grosbeaks, and Pine Grosbeaks staged a massive "superflight" into this region.