Results of the IWRB international waterfowl census 1967-1986 : population estimates, trends and distribution in selected species of ducks, swans and Coot Fulica atra wintering in the western palearctic and West Africa /
Monval, J- Y.
Pirot, J.-Y.(Jean-Yves)
International Waterfowl and Wetlands Research Bureau.
Slimbridge, Glos. : International Waterfowl and Wetlands Research Bureau,
1989.