Simple techniques for sexing nestling hihi (Notiomystis cincta) in the field

Whilst the sexes of adult hihi (Notiomystis cincta) are easily distinguished by their dimorphic plumage, male and female nestling and juvenile hihi appear very similar. Hihi is a threatened bird species, and knowledge of the sex of young is important for conservation management and research. Although molecular sexing techniques exist, an immediate and cheap method for identifying sexes of young birds in the field would be advantageous. In this study, we tested the reliability of morphometric measurements and emerging plumage colour to sex nestlings when they are close to fledging. We conducted a discriminant function analysis on tarsus length, body mass, and head-bill length measurements of 313 individual nestlings of known sex produced on Tiritiri Matangi Island over the six breeding seasons 1998/99 to 2003/04. Typically, the accuracy of discriminant functions for classification is assessed by reapplication to the original sample. Here we first tested the accuracy of the function (included tarsus length and body mass) using a jack-knife reclassification procedure. Second, we applied the function to 85 individuals of the 2004/05 cohort, with similar success (jack-knife reclassification of the six cohorts of 1998/99 to 2003/04,76.4%; 2004/05 cohort, 69.4%). Use of emerging plumage colour on the 2004/05 cohort was similarly effective (72.9% accuracy), but more so when limited to well-feathered nestlings. Depending upon the probability of error acceptable to a researcher, these methods may prove useful, but do not replace the more accurate methods for sexing nestling hihi by molecular sexing or observations of surviving recruits.