Swinhoe's Storm Petrels (Oceanodroma monohris) show no apparent sexual dimorphism in size and color

Abstract Sex determination is often difficult in small seabirds in the field. External measurements and feather coloration were compared between 23 male and 24 female Swinhoe'm Petrels Oceanodroma monohris, following molecular sex determination. No sex differences were detected in any of the nine size parameters. Although females exhibited higher ultraviolet and visible color reflectance from the abdomen feathers and males exhibited higher visible reflectance from the crown feathers, these sexual differences in coloration were barely detectable. These results suggest that neither external measurements nor coloration are reliable for sex determination in this species.