Sex identification in birds

Abstract

Visually identifying the sex of a bird can be difficult. It cannot be done in half the world's species when they are adults, and virtually none can be sexed as chicks. Despite this, the sex of a bird is vital for aviculture, scientific research, and conservation. Over the years, surgery and chromosome inspection have been used for sex identification. At present, these techniques are being eclipsed by the use of DNA. DNA can be used because a male bird has two Z sex chromosomes, whereas the female has a Z and a W. Consequently, the presence of a W unique DNA sequence will identify a bird as female. However, the isolation of a W-linked marker can be laborious. This is being remedied through the use of two DNA amplification reactions, known as random amplification of polymorphic DNA (RAPD) and amplified fragment length polymorphs (AFLP). Unfortunately, the markers they yield are often nonfunctional DNA. This type of DNA is a mild parasite and makes up about 95% of the genome. It also evolves rapidly so that a test based on such a sequence will only work in a single species or a few close relatives. More recently, a W-linked gene has been found. This is named CHD1-W, and it evolves far more slowly than nonfunctional DNA. It can be used to sex all birds, barring ratites such as the ostrich. The technique is simple, cheap, and effective. To conclude, DNA sex identification is demonstrated in three situations: an experiment that has shown that birds can control the sex of their offspring; a race to identify the sex of the last wild Spix's Macaw from moulted feathers; and as a valuable aid to breeding in zoos.