Sex determination of Red Knots Calidris canutus roselaari using morphometrics

Researchers often lack tools to classify sex for monomorphic and weakly dimorphic species in the field, an important component of many avian ecology and demography studies. The Red Knot Calidris canutus roselaari exhibits minor differences in size and plumage patterns between sexes, although overlap is considerable and sex is not readily apparent in the field. We captured, measured, and molecularly sexed 198 individuals (68.7% males, n = 136; 31.3% females, n = 62) at two breeding sites and one migratory stopover site and found significant differences between sexes, with females having a longer total head, culmen, and wing than males. We used a jackknifed cross-validated discriminant function analysis (DFA) to correctly identify sex of 85.9% (95% CI 80.2–90.3%) of all roselaari knots (94.8% of males and 66.1% of females). When restricting the probability of group membership to >0.7, we increased our classification accuracy to 92.3% for females (n = 26) and 90.3% for males (n = 121) while leaving 25.7% unclassified (n = 51). We conclude that DFA provides a means for sexing roselaari Red Knots when molecular sex determination is not feasible, and we provide a formula for researchers to use in the field.