What is an "Irruption?"

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 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.

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.

Scientists at the Cornell Lab of Ornithology would like to better understand how and why these irruptions occur. You can help us by monitoring birds in your backyard or your favorite birding areas and reporting irrupting species over the Internet to the Irruptive Bird Survey at BirdSource.

By doing this you can help us answer these questions:

  • What bird species and how many individuals are irrupting?

  • When does the irruption begin and how long does it last?

  • To what geographical extent is the irruption?

  • What influence does food availability have in areas where birds have irrupted?

    So please be on the look-out for irrupting species (and note wild food availability in your locality) and fill out an Irrupting Bird Survey form. As always, thanks for your help!

    Follow this link to read more about irruptions.

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