Analysis of published data on karyotypes of almost 800 avian species, molecular taxonomy, and paleontology showed that the avian ancestral karyotype was formed at least 150 Myr ago, not later than the early Cretaceous. The ancestral karyotype involves three pairs of group A conservative macrochromosomes (Ma), two submetacentrics and one acrocentric; three to four pairs of group B variable Ma; and 34-35 pairs of microchromosomes (Mi). G/R-banding patterns demonstrate that the ancestral karyotype remained almost the same in Palaeognathae (ostriches, rheas, etc.), archaic Neognathae (pheasants, grouses, ducks, and geese), and several families of higher Neognathae classified into a group of higher water birds. Ma linkage groups of these birds were shown to be homologous to conservative linkage groups of medium-size chromosomes of several mammalian species. "Atypical" karyotypes with altered Ma morphology and, sometimes, significantly variable Mi number are characteristic of Passeriformes and all tested representatives of modern higher land birds. The Ma homology between these birds and archaic Neognathae requires further investigation. Karyotypic data confirm the hypothesis that rates of karyotypic alteration differ in different phylogenetic branches of birds.