The chemical, methyl anthranilate, is unpleasant enough for the birds to make them stay away, but has no long term negative effects on the animals or the environment, officials said.

"Methyl anthranilate is actually a non-toxic food flavoring that is used in grape Kool-Aid and grape bubble gum, but birds find it very aggravating," said Richard Dolbeer, national coordinator of airport wildlife services at the U.S. Department of Agriculture.

Machines that spray the mist have been installed around a 105-acre retention pond at the airport. Officials said Wednesday that it is still too early to tell whether the aversion therapy, conducted with the help of BirdTec of Hersey, Mich., has worked.

At O'Hare and Midway Airport, the Chicago Department of Aviation has also taken steps including installing spikes on light posts so birds can't land and setting off audio distress calls and firing propane cannons to try to discourage birds from hanging around.

Bird strikes are a serious and expensive problem for the nation's airports. They are especially dangerous this time of year when migrating birds compete with planes for air space, according to the Federal Aviation Administration.

Last year, the number of bird strikes at O'Hare fell to 139 from 173 in 2000, according to the city's aviation department. The latest incident happened Nov. 2 when an American Airlines jet had to make an emergency landing after sucking a bird into its engine. No one was injured.