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29Songbirds have a species number almost equivalent to that of mammals, and are classic 30 models for studying mechanisms of speciation and sexual selection. Sex chromosomes are 31 hotspots of both processes, yet their evolutionary history in songbirds remains unclear. To 32 elucidate that, we characterize female genomes of 11 songbird species having ZW sex 33 chromosomes, with 5 genomes of bird-of-paradise species newly produced in this work. We 34 conclude that songbird sex chromosomes have undergone at least four steps of recombination 35 suppression before their species radiation, producing a gradient pattern of pairwise sequence 36 divergence termed 'evolutionary strata'. Interestingly, the latest stratum probably emerged due 37 to a songbird-specific burst of retrotransposon CR1-E1 elements at its boundary, or 38 chromosome inversion on the W chromosome. The formation of evolutionary strata has 39 reshaped the genomic architecture of both sex chromosomes. We find stepwise variations of Z-40 linked inversions, repeat and GC contents, as well as W-linked gene loss rate that are 41 associated with the age of strata. Over 30 W-linked genes have been preserved for their 42 essential functions, indicated by their higher and broader expression of orthologs in lizard than 43 those of other sex-linked genes. We also find a different degree of accelerated evolution of Z-44 linked genes vs. autosomal genes among different species, potentially reflecting their diversified 45 intensity of sexual selection. Our results uncover the dynamic evolutionary history of songbird 46 sex chromosomes, and provide novel insights into the mechanisms of recombination 47

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