Sex roles in birds describe sex differences in courtship, mate competition, social pair-bonds, and parental care. Different explanations have been put forward to explain these differences but none are based on a comprehensive study. Therefore, an international team of experts set out to analyze data on 1,800 of the approximately 9,000 different species of birds as their study organisms. The results show that adult sex ratios are the main driving force for differences in sex roles. A paper on this study was published in Ecology Letters on 25 February.