1. Institute of Arctic Biology and Department of Biology and Wildlife, University of Alaska, Fairbanks, Alaska 99775, U.S.A.
2. U.S. Geological Service, Patuxent Wildlife Research Center, 11510 American Holly Drive, Laurel, Maryland 20708, U.S.A.
3. Institute for Wetland and Waterfowl Research, Ducks Unlimited Canada, Stonewall, P.O. Box 1160, Oak Hammock Marsh, Manitoba ROC 2Z0, Canada