Northward bound

Natural History, March, 2007 by Stephan Reebs

Many bird species in the United States are shifting their breeding ranges northward, a new study shows. Similar northward shifts have been observed in Great Britain, so the cause of both is probably the same. Global warming is the likeliest suspect.

The new study comes from data gathered by the Breeding Bird Survey (BBS), a program run by the U.S. Geological Survey and the Canadian Wildlife Service. Each year, skilled birders spend a summer clay driving or walking along more than 4,100 road segments of the U.S. and Canada, identifying the bird species they hear or see along the way. Because the survey has continued uninterrupted since 1966, its data enable ornithologists to analyze long-term population trends.

Alan T. Hitch and his graduate adviser, Paul L. Leberg, a conservation biologist at the University of Louisiana at Lafayette, report that out of twenty-six southern--U.S. species they studied in the BBS records, nine have significantly pushed the northern limits of their breeding ranges northward since the late 1960s. The northward shifts vary from twenty-six miles for summer tanagers to more than 200 miles for great-tailed grackles. At the same time, the northern limits of only two of the twenty-six species, Bachman's sparrows and Bewick's wrens, have retreated southward.

Hitch and Leberg say the diversity of the nine southern species shows they are not being drawn northward by some specific factor in the midlatitudes--more bird feeders, for instance. Nor are their ranges simply expanding. If that were the case, northern species should have expanded their ranges southward, too, and there was no systematic indication of that. Instead, rising temperatures seem to be nudging southern ecosystems northward. (Conservation Biology)