A comparison of five methods for assignment of sex in the takahe (Aves : Porphyrio mantelli)

The takahe (Porphyrio mantelli) is an endemic New Zealand avian species whi ch was once thought to be extinct. Its rediscovery in 1948 heralded an inte nse species recovery programme, which is ongoing. Early in this programme, difficulties in the assignment of sex became apparent. Subsequently several approaches have been used in order to develop accurate methods to identify males and females in this species. We discuss the value of these approache s and report the results of our studies into DNA-based, as well as morphome tric and hormonal methods. We demonstrate that sex-linked DNA sequence vari ation within both the chromo-helicase-DNA binding (CHD) and ATP synthase a- subunit, (ATP5A1) genes allows the correct assignment of known-sex adults. We also report the development of a discriminant function using body weight , bill depth, shield width, length from nares to bill-tip, tarsus length, m id-toe length and culmen length. This method had a maximum accuracy of 89% on a sample of 37 known-sex takahe, using culmen and tarsus lengths. In rel ation to hormonal methods of sexing takahe, plasma testosterone and oestrad iol concentrations in adult and juvenile male and female takahe were measur ed in spring. The oestradiol/testosterone ratio discriminated between all 1 5 adults sampled within 38 min of capture. However, males experiencing incr eased capture stress had reduced testosterone levels and hence an uninforma tive oestradiol/testosterone ratio. Testosterone and oestradiol levels were generally undetectable in juvenile takahe less than 10 months old. Consequ ently, plasma sex steroid analyses in the breeding season are useful for th e determination of sex in recently captured adults only.