RELATIVE SUCCESS OF OVERWATER AND UPLAND MALLARD NESTS IN SOUTHWESTERN MANITOBA

Studies of nest success in mallards (Anas platyrhynchos) typically foc us on upland nesting habitats, but mallards also nest over water where their success rates may differ from upland habitats. We monitored ove rwater and upland mallard nests in southwestern Manitoba as part of ot her 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 mor e likely (P = 0.0002) to be parasitized (primarily by redheads; Aythya americana) than were upland nests (11% vs. 0% parasitized, respective ly). Parasitized nests contained an average of 1.2 fewer (P = 0.03) ma llard eggs and had 23% lower (P = 0.006) egg success than unparasitize d nests, but these costs were not sufficient to offset the large advan tage in nest success that was associated with overwater nesting. Our d ata indicated that a large fraction of the local mallard population wa s nesting successfully in a habitat that is not searched during tradit ional nesting studies. If this pattern is widespread in other mallard populations, then nesting studies should include measurements of the f requency and success of overwater nesting to obtain unbiased estimates of overall nest success.