Cooperative breeding has been reported in various bird species, yet its ecological drivers and fitness benefits remain debatable. We found a high rate (32%) of cooperative breeding in the Ryukyu subspecies of the Collared scops owl, Otus lempiji pryeri, where 2 males and 1 female contribute to the rearing of a single nest. We captured and ringed breeding birds for individual identification and monitored the breeding behaviours in detail using nest video cameras and GPS dataloggers, which revealed a parental role division between the sexes. Furthermore, to understand the fitness benefits gained by cooperative breeding, parentage of the chicks was investigated using SNP data obtained from Multiplexed ISSR Genotyping by sequencing (MIG-seq) method. This revealed a presence of extra-pair fertilisation in several broods, which is an evidence that this population can be both socially and genetically polygamous. Kinship between the breeding parents were also investigated, and sibship relationship between the parents was also found. Taken together these results, we discuss the fitness benefits gained by cooperative breeding in this population and the possible ecological factors which may have led to the adoption of this breeding system.