Females of many species mate with several males. According to a recent hypothesis, female promiscuity serves to avoid inbreeding. We tested this hypothesis in a polygynous bird, the great reed warbler (Acrocephalus arundinaceus), in which extra-pair fertilization, inbreeding and inbreeding depression occur. However, the extra-pair males and social males did not differ in relatedness to the promiscuous females, nor did the least related males sire most of the females' chicks. Thus, contrary to recent findings in some insects, birds and reptiles, we found no evidence for inbreeding avoidance among the promiscuous females. Instead, female great reed warblers may seek other potential benefits when cuckolding, such as good gene effects at particular functional genes.