Effects of intra- and interspecific brood parasitism on a precocial host, the canvasback, Aythya valisineria

Canvasback ducks (Aythya valisineria) suffer both intra- and interspecific brood parasitism. During 3 years in Manitoba, 80% of canvasback nests (n = 179 nests with completed clutches) were parasitized by redheads (A. americana), other canvasbacks, or both, with an average of 4.7 parasitic eggs per parasitized nest. Parasitism had significant negative effects on the reproductive success of nesting canvasbacks, although the proximate mechanisms involved differed from those operating in altricial species. Accidental displacement of eggs when parasitic females forced their way onto host nests was the principal negative effect of parasitism, reducing the number of host eggs that were incubated and ultimately hatched. Parasitism by redheads was relatively more costly to canvasbacks than was intraspecific parasitism, with approximately 0.31 and 0.17 host eggs displaced per parasitic redhead and canvasback egg laid, respectively. No additional negative effects of parasitism on the hatchability of host eggs occurred subsequent to parasitic laying. Posthatch survival of canvasback ducklings was lower in broods from parasitized nests but was unrelated to the presence or absence of redhead ducklings. Canvasback hosts resisted intrusions by parasitic females but showed no evidence of discrimination against parasitic eggs or ducklings. Because most costs of parasitism in this system are inflicted at the time of parasitic laying, subsequent rejection of parasitic eggs or ducklings is probably of little benefit to canvasback hosts, while the evolution of behavior that might prevent parasitic laying in the first place, such as more vigorous nest defense, may be constrained by its high costs