1. Ducks, Geese & Swans of the World;Todd,1979
2. Swans, Geese, and Ducks, in The Sibley Guide to Birds,2000
3. National Geographic Field Guide to the Birds of North America;Gray,1999
4. The Birder’s Handbook;Ehrlick,1988
5. Title 50, Code of Federal Regulations, Part 21 Migratory Bird Permits, Subpart B 21