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Farming activities were not a major source of nest failure except for northern pintails; few other species nested in Cropland and there were few nests in Hayland. We did not observe nest destruction in Right-of-way and dry Wetland by mowing that occurred on most study areas in late June. Had our study been conducted during wetter years more favorable to renesting, we might have found mowing to be a greater cause of nest failure. Mowing has been shown to cause much nest destruction and female mortality in other studies (Ordal 1964, Evans and Wolfe 1967, Cowardin et al. 1985).

Weather events, likewise, were not a major cause of nest failure, although storms were important locally. Snowstorms caused embryo mortality and nest abandonment, especially among mallards and northern pintails. Although embryos can survive limited exposure to subfreezing temperatures (Greenwood 1969, Batt and Cornwell 1972), Dzubin and Gollop (1972) reported that chilling of embryos during cold spring weather was partly responsible for failure of up to 9% of eggs in early mallard nests. Johnson et al. (1986) suggested that nest failure due to spring snowstorms may be compensated by renesting due to improved wetland conditions. Storms also may benefit nesting ducks by delaying cultivation, but Milonski (1958) and Krapu (1977) speculated that ducks might not benefit if such delay only postpones nest destruction until a later date when conditions may be less favorable for renesting. Reduced availability of aquatic invertebrates that are consumed by laying females also may occur after snowstorms and influence nesting (Krapu 1979, Swanson et al. 1979). Dane and Pearson (1971) reported that mallards and northern pintails ceased laying during a severe spring snowstorm.

Flooding destroyed duck nests on the Hay Lakes and Holden study areas in 1983 but was of little consequence elsewhere. Flooding on these study areas was aggravated by drainage; ditches that connected wetlands appeared to accelerate movement of run-off water among basins and cause low-lying areas to flood rapidly. Johnson et al. (1986) suggested that flooding in mid-May should have only minor influence on mallards because renesting would compensate for losses. We found that flooding in June, however, caused the failure of nearly 50% of the nests of mallards that were probably renesting, and also numerous nests of late-nesting species such as the lesser scaup and ruddy duck, which are not prone to renest (Bellrose 1980).