The human X chromosome exhibits four "evolutionary strata" interpreted to represent distinct steps in the process whereby recombination became arrested between the proto X and proto Y. To test if this is a general feature of sex chromosome evolution we studied the Z-W sex chromosomes of birds which have female rather than male heterogamety and evolved from a different autosome pair than the mammalian X and Y. Here we analyze all five known gametologous Z-W gene pairs to investigate the "strata" hypothesis in birds. Comparisons of the rates of synonymous substitution and intronic divergence between Z and W gametologs reveal the presence of at least two evolutionary strata spread over the p and q arm of the chicken Z chromosome. A phylogenetic analysis of intronic sequence data from different avian lineages indicates that Z-W recombination ceased in the oldest stratum (on Zq; CHD1Z PKCIZ and SPINZ) 102-170 million years ago (Mya) before the split of the Neoaves and Eoaves. However recombination continued in the second stratum (on Zp; AD012Z and ATP5A1Z) until after the divergence of extant avian orders with Z and W diverging 58-85 Mya. Our data suggest that progressive and stepwise cessation of recombination is a general feature behind sex chromosome evolution