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All islands were intensively nest searched five times during one year of study. We recorded predominant vegetation on the islands and at the nest site, location of the island in the marsh, and patterns of predator visits to the island.

Predator management techniques were evaluated on control and test areas in 1985. We removed mammalian predators from islands in the 5,000 acre Pool 320, while there were no removals in Pool 326, a similar sized pool.

Depredation by mink Mustela vison, was the only significant deterrent to successful nesting except when islands were connected to uplands by cattail corridors or mudflats. In those situations red fox Vulpes fulva and raccoon Procyon lotor were devastating to waterfowl production. We were unable to protect nests in those situations.

Nest success varied from zero to 100 percent from different islands. During the only year of intensive nest searches 41 islands, totaling 65.25 acres, had 1,022 nests. There were 587 nests hatched for an observed success of 57.47 percent.

Nest success (Mayfield corrected) was lowest in Pool 326 (21%) where no predator management was conducted but was 44 percent for all nests.

Weedy forbs were the most often selected nestsite. Ducks selected shrubs if canopy cover was not too tall (less than four feet), and when shrub spacing allowed some opening above the nest. Dense nesting cover was not used as often, but mallards Anas platyrhynchos did appear to choose DNC more often than other species. Other vegetation was seldom selected.