Kin-biased conspecific brood parasitism in a native Mandarin duck population

Conspecific brood parasitism (CBP) is a widespread female alternative reproductive tactic, where clutches tended by a single female contain eggs laid by different females. CBP is disproportionately common in waterfowl, where females, not males, exhibit natal philopatry. This raises the possibility that closely related females cooperate with each other for inclusive fitness benefits and may explain why females expend effort raising offspring that are not their own. Some studies show higher host–parasite relatedness than expected, while a similar number of studies fail to do so. Here we test the hypothesis that host and parasite are closely related in Mandarin ducks ( Aix galericulata ) to further our understanding of the role of host–parasite relatedness in the evolution of CBP. Using microsatellite markers, we found that more than 33.7 % of ducklings were parasites and that more than 70 % of nests contained parasitic offspring. Relatedness between host mother and parasitic ducklings was significantly higher than that of host mothers and ducklings in other nests, showing hosts were, on average, more related to their parasites than to other breeding females. There was a weak negative relationship between pairwise relatedness and distance between nests. However, distance did not contribute to the higher relatedness of host and parasite. Thus, host–parasite relatedness likely involved some form of kin recognition. Based on our results, relatedness plays a role in CBP in Mandarin ducks, though the explicit fitness benefits to both the host and the parasite still need to be determined.