RELATIVE SUCCESS OF OVERWATER AND UPLAND MALLARD NESTS IN SOUTHWESTERN MANITOBA

Studies of nest success in mallards (Anas platyrhynchos) typically focus on upland nesting habitats, but mallards also nest over water where their success rates may differ from upland habitats. We monitored overwater and upland mallard nests in southwestern Manitoba as part of other ongoing research projects during 1985-91. Nest success estimates (Mayfield method) were higher (P = 0.005) for overwater nests (43.9%; n = 47) than for upland nests (12.2%; n = 49). Overwater nests were more likely (P = 0.0002) to be parasitized (primarily by redheads; Aythya americana) than were upland nests (11% vs. 0% parasitized, respectively). Parasitized nests contained an average of 1.2 fewer (P = 0.03) mallard eggs and had 23% lower (P = 0.006) egg success than unparasitized nests, but these costs were not sufficient to offset the large advantage in nest success that was associated with overwater nesting. Our data indicated that a large fraction of the local mallard population was nesting successfully in a habitat that is not searched during traditional nesting studies. If this pattern is widespread in other mallard populations, then nesting studies should include measurements of the frequency and success of overwater nesting to obtain unbiased estimates of overall nest success.