The mating calls of frogs are starting earlier than usual near Ithaca, New York, researchers say. But this usually welcome sign of spring is no cause for celebration. Rather, according to a report in the August issue of the journal Conservation Biology, it's the strongest indication yet that climate warming in eastern North America is affecting the resident species.

James Gibbs of the State University of New York at Syracuse and Alvin Breisch of the New York State Department of Environmental Conservation consulted historical temperature records to determine climate change in the Ithaca area over the past century. Specifically, they looked at the average daily maximum temperatures from November through Junethose months most important to the timing of frog reproduction. Their analysis revealed temperature increases of about 2 to 4 degrees Fahrenheit during five out of the six months. Gibbs and Breisch then used studies from the first and last decades of the 20th century to assess the earliest calling dates of six local amphibian species. They found that four of these speciesthe spring peeper, bullfrog, gray treefrog and wood frogare calling 10 to 13 days earlier than they did before. (The other two, the American toad and the green frog, are singing on schedule.)

Because Ithaca is in the middle of their breeding ranges, these frogs will most likely be able to conduct business as usual despite the warmer environs. Species at the edges of their ranges, however, may not fare so well. In the case of the mink frog, whose current southern limit lies about 150 kilometers north of Ithaca, the warmer climate could quickly push that limit northward, shrinking the creature's range. As a result, the authors warn, "mink frogs would be expected to show predictable local declines if local climate warming continues."