Inbreeding, i.e. the mating of genetically related individuals, can lead to reduced fitness and is considered to be a major selective force of mate choice. Although inbreeding avoidance has been found in numerous taxa, individuals may face constraints when pairing, leading to mating with suboptimal partners. In such circumstances, individuals that are able to avoid factors exacerbating detrimental effects of inbreeding should be favoured. Using the socially and genetically monogamous black-legged kittiwake (Rissa tridactyla), we explored whether the detrimental effects of inbreeding are exacerbated by sperm ageing (i.e. the post-meiotic senescence of sperm cells, mainly occurring within the female tracts after copulation), and whether they can be mitigated by behavioural tactics. First, by experimentally manipulating the age of the fertilizing sperm, we found that hatching failure due to sperm ageing increased with higher genetic similarity between mates. We then investigated whether more genetically similar pairs exhibited mating behaviours that prevent fertilization by old sperm. The more genetically similar mates were, the less likely they were to copulate early in the reproductive season and the more females performed post-copulatory sperm ejections. By flexibly adapting their behaviour in response to within-pair genetic similarity, kittiwakes may avoid exacerbation of inbreeding costs due to sperm ageing.