Only few studies have quantified the effects of inbreeding on gamete traits in vertebrates using controlled breeding conditions and a full-sib mating approach. Here, we provide comprehensive evidence for the negative effect of inbreeding on sperm traits in a bird, the zebra finch Taeniopygia guttata. Inbreeding affected sperm motility and percentage of abnormal spermatozoa in ejaculates and results were highly consistent across both study populations, confirming the generality and reliability of our findings.