1. Montana Cooperative Wildlife Research Unit, University of Montana, Missoula, MT 59812, U.S.A.
2. U.S. Fish and Wildlife Service, 4401 N. Fairfax Drive, Arlington, VA 22203, U.S.A.
3. Smithsonian Migratory Bird Center, National Zoological Park, Washington, D.C. 20008, U.S.A.
4. U.S. Environmental Protection Agency, Environmental Monitoring Systems Lab, Lab, P.O. Box 93478,
Las Vegas, NV 89193–3478, U.S.A.