This article explores the evolution and adaptation of avian genomes by analyzing the genomes of 48 avian species representing all major extant clades. The avian genome is characterized by its constrained size, which is primarily due to lineage-specific erosion of repetitive elements, large segmental deletions, and gene loss. Despite this conservation, many non-neutral evolutionary changes are detected in protein-coding genes and noncoding regions, reflecting adaptations to different lifestyles and convergent evolution of traits. The study reveals that avian genomes exhibit a high degree of evolutionary stasis at the levels of nucleotide sequence, gene synteny, and chromosomal structure. However, the avian genome has a smaller size compared to other vertebrates, with a reduced fraction of repeat elements and fewer genes. Large segmental deletions and the fission of macrochromosomes into microchromosomes are key features of avian genome evolution. The analysis also identifies selective constraints on certain categories of genes in different avian lineages, with convergent evolution shaping the evolution of protein-coding genes and their regulatory elements. The findings provide insights into the evolution, diversification, and ecological adaptation of tetrapod vertebrates.This article explores the evolution and adaptation of avian genomes by analyzing the genomes of 48 avian species representing all major extant clades. The avian genome is characterized by its constrained size, which is primarily due to lineage-specific erosion of repetitive elements, large segmental deletions, and gene loss. Despite this conservation, many non-neutral evolutionary changes are detected in protein-coding genes and noncoding regions, reflecting adaptations to different lifestyles and convergent evolution of traits. The study reveals that avian genomes exhibit a high degree of evolutionary stasis at the levels of nucleotide sequence, gene synteny, and chromosomal structure. However, the avian genome has a smaller size compared to other vertebrates, with a reduced fraction of repeat elements and fewer genes. Large segmental deletions and the fission of macrochromosomes into microchromosomes are key features of avian genome evolution. The analysis also identifies selective constraints on certain categories of genes in different avian lineages, with convergent evolution shaping the evolution of protein-coding genes and their regulatory elements. The findings provide insights into the evolution, diversification, and ecological adaptation of tetrapod vertebrates.