Co-parasites preferentially lay with kin and in safe neighbourhoods: experimental evidence from goldeneye ducks

Conspecific brood parasitism (CBP) is an alternative reproductive tactic in several animal taxa. Recently the role of kinship in CBP has been in focus, and some studies have demonstrated high host-parasite relatedness in avian CBP systems. However, high natal and breeding site fidelity of hosts and parasites complicates the interpretation of previous observational findings, and the mechanisms leading to high host-parasite relatedness remain unknown. Some recent findings suggest that broadening the scope of the study of interactions may cast light on these mechanisms. We studied relatedness between parasites laying in the same nest (co-parasites) and factors possibly driving relatedness patterns between co-parasites in the common goldeneye, Bucephala clangula. Based on a field experiment, controlling for site fidelity and host role, we report here that both relatedness and safety of the nest site, and their interaction, affected the likelihood of two females engaging in co-parasitism. At the population level, parasites indeed seemed to lay eggs preferentially with kin. Analyses at a finer spatial scale revealed that high relatedness between co-parasites was not due to the philopatry effect only. Parasites engaging in co-parasitism also laid in safer neighbourhoods than parasites that did not engage in co-parasitism; the number of nondepredated nesting attempts the previous year was higher for the parasites engaging in co-parasitism. However, the interaction between relatedness and safety of the nest site suggests that co-parasitism at dangerous lakes was more likely to involve relatives. Our results provide the first experimental evidence that nest predation risk and interaction between related parasites are associated with kin-biased co-parasitism in a CBP system.