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Of 98 nests of American black ducks (Anas rubripes) which were abandoned, 52 were judged to have been abandoned as a direct result of the author's interference. However, capturing and marking of nesting females performed only during late incubation did not seem to contribute appreciably to nest losses. On Ile-aux-Pommes, the losses were largely attributed to egg predators (gull, Larus sp.) and occasionally American crows (Corvus brachyrhynchos) or to nest-site competitors (common eiders, Somateria mollissima) which occasionally take over American black duck nests. On the mainland where the red fox (Vulpes fulva) and the American crow are the chief predators, human scent and disturbed vegetation may have resulted in a greater degree of observer-induced losses than on the island.