Through thick and thin: Sexing Bristle-thighed Curlews Numenius tahitiensis using measures of bill depth

Birds often exhibit diagnostic traits that differ among individuals of the same species with regard to factors like sex, age, or breeding status. Shorebirds exhibit a wide diversity of colors, shapes, and sizes of their bills, and these traits are commonly used to determine the sex of individuals. In curlews (genus Numenius), length alone accurately separates the sexes in some species, but the shape of the bill has not typically been assessed for this purpose. We collected a suite of measurements on the bills of known-sex Bristle-thighed Curlews N. tahitiensis and determined that standardized measurements of bill depth separated the sexes with high accuracy. A model incorporating the length of a bird’s diagonal tarsus and multiple measurements of the bill was 93.1% accurate in predicting the sex of individual Bristle-thighed Curlews. Simpler models involving only the values of the bill depth near the tip and the base of the bill, however, produced similarly accurate results and are preferred for their parsimony. We advocate the use of one such model that is 93.4% accurate in determining the sex of Bristle-thighed Curlews. As a simple heuristic, a value for the ratio of the bill depth near the tip to that at the base of >0.5 indicated a female, providing an easy field calculation to help determine the sex of Bristle-thighed Curlews. In general, the bills of female Bristle-thighed Curlews are deeper and taper relatively less than those of males. Other observers have qualitatively noted apparent sex-specific differences in the shape of curlew bills, but the generality of our quantitative findings remains to be examined in other curlew species.