A sequence assembly of the chicken W chromosome enables reconstruction of the gene content of the W chromosome across 14 bird species and shows striking similarities in the maintenance of broadly expressed and dosage-sensitive genes on highly degenerate sex chromosomes in both birds and mammals. However, the chicken W chromosome is not enriched for genes with expression in female-specific tissues, providing an intriguing contrast to the acquisition and amplification of genes with testis-specific expression on mammalian Y chromosomes and suggesting that the inheritance of chromosomes solely through females or males can lead to different evolutionary outcomes.