A DNA test to sex most birds

Birds are difficult to sex. Nestlings rarely show sex‐linked morphology and we estimate that adult females appear identical to males in over 50% of the world's bird species. This problem can hinder both evolutionary studies and human‐assisted breeding of birds. DNA‐based sex identification provides a solution. We describe a test based on two conserved CHD (chromo‐helicase‐DNA‐binding) genes that are located on the avian sex chromosomes of all birds, with the possible exception of the ratites (ostriches, etc.; Struthioniformes). The CHD‐W gene is located on the W chromosome; therefore it is unique to females. The other gene, CHD‐Z, is found on the Z chromosome and therefore occurs in both sexes (female, ZW; male, ZZ). The test employs PCR with a single set of primers. It amplifies homologous sections of both genes and incorporates introns whose lengths usually differ. When examined on a gel there is a single CHD‐Z band in males but females have a second, distinctive CHD‐W band.