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Fig. 11. Relation of density of blue-winged teal (Anas discors), gadwall (Anas strepera), mallard (Anas platyrhynchos). northern shoveler (Anas clypeata), and northern pintail (Anas acuta) pairs to pond size during 1987-90 in the prairie pothole region of the United States. Error bars = 1 standard error.

Fig. 12. Relation of density of blue-winged teal (Anas discors), gadwall (Anas strepera), mallard (Anas platyrhynchos). northern shoveler (Anas clypeata), and northern pintail (Anas acuta) pairs to area of ponds during 1987-90 in the prairie pothole region of the United States.

Fig. 13. Relation between average number of pairs of blue-winged teals (Anas discors), gadwalls (Anas strepera), mallards (Anas platyrhrnchos), northern shovelers (Anas clypeata), and northern pintails (Anas acuta) per pond and pond density during 1987-90 in the prairie pothole region of the United States.

Fig. 14. Relation between the average number of duck (Anatinae) pairs per km² and year by landownership class during 1987-90 in the prairie pothole region of the United States. Landownership classes include easements (easement) where the U.S. Fish and Wildlife Service has easements on wetlands to prevent draining, filling, or leveling: land owned by the U.S. Fish and Wildlife Service (service); privately owned land (private).