The extent to which two parents are related to each other can strongly influence the fitness of the offspring. Although females should choose mates based on the level of genetic similarity, current evidence suggests that behavioural kin discrimination in birds is limited and may preclude this possibility. We investigated the potential for cryptic sperm selection in the polyandrous, lek-breeding ruff, Philomachus pugnax. In direct sperm competition between two males, the one that fertilized the majority of offspring was most often least genetically similar to the female. This was not due to intrinsic male qualities such as sperm mobility or ejaculate size, because the predictive variable is the genotype of one male relative to that of the female and of the other competing male. This is an extrinsic quality and an individual male may be favoured in one competitive triad (a female and two competing males) but not in another. This finding has implications for the extent to which birds can adaptively optimize outbreeding, even in the absence of the behavioural ability to discriminate genetic similarity and has broad implications for sexual selection in birds, a taxon in which high levels of genetic polyandry are the norm.