Rapid classification of a study subject's gender is critical for many ecological, behavioral, and conservation projects. We evaluated sexual-size dimorphism of adult Westland Petrels (Procellaria westlandica), a large nocturnal colony-nesting seabird, using linear discriminant function analysis and compared our results to birds classified using standard DNA gender identification techniques. The results revealed a strong pattern of sexual dimorphism (Wilks' Lambda  =  0.43, F_7,29  =  5.6, P < 0.001) in the standard discriminant function analysis despite an unbalanced sex ratio in our sample of adults captured at the breeding colony. Minimum bill depth and head length, of the seven morphometric characters we measured, successfully assigned the correct gender to 95% of all individuals sampled (n  =  37). We provide a canonical classification function of morphological traits that may be used in the field to rapidly differentiate adult females and males of this rare petrel species.