Many avian species are considered sexually monomorphic, which means it's difficult to tell their sex by looking at them. While most of these species probably have subtle physical characteristics that could be used to distinguish between the sexes, these differences are not readily visible.
Types of Sex Determination Tests
DNA probe-based tests are subject to errors caused by contamination that is inherent with any DNA amplification and detection assay. The greatest potential for contamination occurs when blood samples are collected from a toenail or when feathers are used in lieu of blood. Washing the nail prior to sample collection, as is suggested, does not reduce the potential for contamination. For example, if the target segment of nucleic acid from the W chromosome, which is present in every cell in the heterozygous female bird, were to contaminate the feather of a male and the male's feather was used for sex identification, the male bird could be incorrectly identified as a female. Contaminating a male's sample with nucleic acid from another male would not effect the results.