Why Sex Chromosomes Work Backwards in Birds vs Mammals

What Happened A Reddit user posed a thought-provoking question about sex determination systems across different animal groups. In mammals, females carry two X chromosomes (XX) while males have one X and one Y chromosome (XY). However, in birds, the pattern is flipped: males have two identical chromosomes (ZZ) while females have two different ones (ZW). The user’s question touches on a fundamental puzzle in evolutionary biology: if these groups shared a common ancestor, how did one lineage completely reverse its sex determination system?