Differential responses to related hosts by nesting and non-nesting parasites in a brood-parasitic duck

Host–parasite relatedness may facilitate the evolution of conspecific brood parasitism, but empirical support for this contention remains inconclusive. One reason for this disparity may relate to the diversity of parasitic tactics, a key distinguishing feature being whether the parasite has a nest of her own. Previous work suggests that parasites without nests of their own may be of inferior phenotypic quality, but because of difficulties in identifying these parasitic individuals, little is known about their host selection criteria. We used high‐resolution molecular maternity tests to assign parasitic offspring to known parasites with and without their own nests in a population of Barrow’s goldeneyes (Bucephala islandica). We determined whether parasite nesting status, host–parasite relatedness and distance between host and parasite nests affected the probability of parasitizing a host and the number of eggs laid per host. We also investigated whether nesting parasites, conventionally nesting females and non‐nesting parasites differed regarding their age, structural size, body condition, nesting phenology or total brood size. The probability of engaging in parasitism increased with host–parasite relatedness and spatial proximity to host nests for nesting and non‐nesting females alike. However, nesting parasites increased the number of eggs donated with relatedness to the host, while non‐nesting parasites did not do so. Non‐nesting parasites laid fewer eggs in total, but did not differ by any of the other quality measures from conventional nesters or nesting parasites. Our study provides the first demonstration that nesting and non‐nesting parasites from the same population may use different host selection criteria. See also the Perspective by Eadie & Lyon