Criteria for scoring the intensity of sexual selection in birds.

All species were scored from literature by applying this classification system to textual descriptions of mating systems and breeding behaviour. Where available, we used quantitative estimates of polygamy and EPP to refine our scores. The thresholds for scores 0–3 were based on previous studies [39–41]. We treated lekking as an extreme form of polygyny based on genetic evidence of very high variance in mating success in most well-studied cases (see Methods; Fig 2). Where scores differed among criteria, we selected the highest score. Scores for all species are presented in S1 Data along with literature sources and estimates of data certainty.