The karyotypes of thirty-one birds

The karyotypes of thirty-one bird species are presented and some general aspects of bird chromosomes and the use of the karyotypes in the systematic classification of birds are discussed. It is important to take into consideration all the chromosomes and not just a few of the largest pairs. There is nothing that indicates that the small chromosomes should have a different function than the large ones. It is also important to define the sex chromosomes, since closely related species often have a Z chromosome that is similar in size and arm ratio. Differences between closely related species in some cases are due to a centric-fusion mechanism. In these cases it is more useful to compare the autosomes with the Z chromosome than to compare the other chromosomes with chromosome no. 1. The chromosome number is at least in some cases a little lower in species with putative centric fusion than in related species without it, as in the family Strigidae. The relative length of the chromosome may be more important when identifying corresponding chromosomes in different species, as in the family Anatidae. Two different arm ratios have been observed for a special chromosome within a species. In Vanellus vanellus both chromosomes no. 1 and no. 2 and in Chloris chloris chromosome no. 1 may have either of two arm ratios.