1. U.S. Fish and Wildlife Service, Patuxent Wildlife Research Center, Gulf Coast Research Station, P.O. Box 2506, Victoria, Texas 77902, USA
2. U.S. Fish and Wildlife Service, Buffalo Lake National Wildlife Refuge, P.O. Box 228, Umbarger, Texas 79091, USA
3. Present address: Lake Ophelia/Grand Cote National Wildlife Refuge, P.O. Box 256, Marksville, Louisiana 71351, USA
4. U.S. Fish and Wildlife Service, National Wildlife Health Research Center, 6006 Schroeder Road, Madison, Wisconsin 53711, USA
5. U.S. Fish and Wildlife Service, Ecological Services Field Office, Stadium Centre Building, 711 Stadium Drive East, Suite 252, Arlington, Texas 70611, USA