Ever since their discovery, sex chromosomes have captivated researchers because of their involvement in sex determination, sexual form and function, and sexual conflicts. However, despite a long-lasting fascination, central aspects of their evolution remain poorly understood. This is true even for the relatively well-studied mammalian XY and avian ZW sex chromosome systems, because their ancient formation and pronounced Y and W degeneration prevent detailed analyses. Sex chromosomes experience unique evolutionary conditions, and are affected by drift more strongly than autosomes (due to lower population size) and by both natural and sexual selection. Due to this, sex-linked genes are expected to show different functional variation compared to autosomal genes. We focus on a unique sex chromosome system in birds, the complex neo-sex chromosomes in Sylvioidea passerines, that we have detected and are characterising in my lab. Sylvioidea is a superfamily consisting of ≈1200 species in 22 families that separated from other passerines approx. 24 Myrs ago. Sylvioidea are interesting in the context of sex chromosome evolution because they have enlarged neo-sex chromosomes formed by translocations of parts of several other chromosomes to the ancestral sex chromosomes. There is variation in the number of translocations, with all species sharing one translocation and a few species having additionally one or two translocations. The translocation that is common to all Sylvioidea, a part of chromosome 4A, is particularly interesting because it is homolog to the mammalian sex chromosomes, which means that ‘mammalian-XY genes’ can now be studied as ZW-linked in birds! To date, we have sequenced the genomes of approx. 50 Sylvioidea species, approx. 120 individual great reed warblers (our main study species), and the transcriptome of two species. We have assembled a reference genome of the great reed warbler using PacBio, Chromium and BioNano data (Sigeman et al. 2021 Mol. Biol. Evol.). Our multi-species whole-genome data now enable us to compare sex chromosome evolution and its driving mechanism in this promising species group. Our aims for the present project are: (i) To evaluate the possible occurrence of additional autosomal translocations to the neo-sex chromosome in each of the 22 Sylvioidea families (13 families analysed so far). (ii) To analyse the molecular evolution of sex chromosomes (ZW gametologs) of more than one hundred Sylvioidea species, by using phylogenetic approaches. (iii) To examine whether Sylvioidea species with high degree of sexual dimorphism (indicating strong sexual selection) display enhanced sex chromosome evolution. (iv) To make use of the fact that the a part of the neo-sex chromosome in Sylvioidea is homologous to the sex chromosome in mammals, and study to which degree sex chromosome evolution is repeatable in mammals and birds.