<p>The adult sex ratio plays a crucial role in shaping breeding systems and traits linked to sexual selection. Recent studies associate adult sex ratio with mate choice, pair-bonding, and parenting, as the rarer sex gains advantages in mate selection and parental investment. However, the causal relationships between the demographic factors that generate adult sex ratio bias and its broader implications remain debated. Skewed adult sex ratios can result from sex-biased mortality and maturation, which influence mating and parental behaviours. Conversely, the costs of these behaviours may further drive sex differences in mortality and maturation, reinforcing adult sex ratio biases. Here, we compile demographic and behavioural data from 261 bird species across 69 families to examine these associations within a phylogenetic framework. Our analyses confirm that demographic traits are associated with adult sex ratio and reveal links between adult sex ratio, pre-copulatory sexual selection, and parenting. Phylogenetic path analyses further demonstrate that sex differences in mortality and maturation drive adult sex ratio biases, which subsequently influence mating and parenting rather than the reverse. This study provides a comprehensive analysis of the interplay between demography, social environment, and breeding systems, highlighting adult sex ratio as a crucial link. Our findings underscore the need for further research on the demographic underpinnings of social behaviour and reproductive strategies.</p>