Intraspecific nest parasitism in Red-breasted Mergansers

We studied intraspecific nest parasitism in an island nesting population of Red-breasted Mergansers (Mergus serrator) in New Brunswick. Of the nests examined, 64% contained eggs from more than one female. This estimate is probably low because the criteria to detect parasitism were conservative. Parasitized nests were initiated, on average, 3 days earlier than normal nests. Parasitic laying occurred throughout the nesting season. A larger proportion of eggs hatched from normal (89%) than from parasitized (75%) nests because 14% of the eggs in parasitized nests were laid during the host's incubation period and failed to hatch in time. A greater percentage of eggs hatched from parasitized nests during the peak nesting period. Parasitized and normal nests produced the same number of ducklings. There was no difference in the percentage of dead, infertile, and lost eggs from normal compared to parasitized nests. The number of nests containing at least one dead egg increased with clutch size. Most host females accepted parasitic eggs during incubation without deserting. Incubation was prolonged on parasitized nests and on nests with low synchronization of embryo development. Nest parasitism is a common reproductive tactic in at least some female Red-breasted Mergansers.