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Mallard (Anas platyrhynchos) breeding densities in the prairie pothole habitat of eastern North Dakota during 1961-1980 varied from 2.28 birds/km2 in 1977 to 9.47 birds/ km2 in 1963 and were correlated with pond abundance (r = 0.543, P < 0.05). The number of basins used by pairs declined with drought, as did home-range size. Nesting activity also varied with the number of ponds holding water/km2, ranging from high (including substantial renesting) under favorable water conditions to low during extreme drought. The span between first and last nest initiations declined by 19 days from a wet to a dry year. With severe drought conditions during spring 1977 on the Medina Study Area, pairs returned to attempt nesting but were unsuccessful, and most abandoned activity centers by mid-May. Although the average clutch size declined by about 0.7 egg from a wet to a dry year on the Interstate Study Area, hatchability of eggs remained constant. We describe the adaptive strategy of Mallards for breeding under variable water conditions and food resources in the semiarid prairie environment of midcontinent North America.