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North Island kokako (Callaeas cinerea wilsoni) appear sexually monomorphic. Females are, on average, slightly smaller than males in most body measurements. Mean tarsus length was significantly smaller among females at all sites and can be used to predict sex of unknown birds with up to 86% accuracy. A simple discriminant function, using tarsus and wing chord measurements, was derived which increased sex resolution to over 90% at some sites. This is sufficient accuracy to provide a useful field technique for kokako research and conservation management. Best discri- minant functions for different sites are presented and their geographical limitations are discussed. Flux, I.; Innes, J. 2001. A field technique for determining the sex of North Island kokako (Callaeas cinerea wilsoni).