Parasitic laying in nests of gadwalls

Abstract

At Waterhen Marsh, Saskatchewan, at least 60 (17%) of 355 completed gadwall (Anas strepera) clutches were parasitized by other gadwalls (9%), lesser scaup (Aythya affinis) (8%). or both species (< 1%). Nests known to be parasitized by gadwalls contained on average at least 7 or 8 parasitic eggs while those parasitized by scaup contained an average of only 1.6 ± 0.2 (SE) parasitic eggs. Most parasitized nests were situated in areas of high nest density. Lesser scaup laid parasitically in gadwall nests located in shorter or sparser cover than most other nests, whereas nests parasitized by gadwalls were in vegetation similar to unparasitized nests. Assuming that nests parasitized by gadwalls contained > 14 eggs, intraspecific parasitism significantly reduced nesting success from 76 to 54%, egg success from 74 to 45%, and hatchability of eggs from 97 to 91%. Interspecific parasitism had no significant effect on nesting success or hatchability, but caused a slight decrease (74 to 67%) in egg success. The differential effect of lesser scaup and gadwall parasitism on host reproductive success was likely due to differences in numbers of parasitic eggs laid in the nest of the host by each species. We hypothesize that parasitism occurred because ducks did not continuously occupy the nest during laying, thereby allowing two or more females to lay eggs synchronously at the same site. Parasitic laying by gadwalls was possibly an anomaly, rather than true parasitism.