17 cm. 18 g. Small size. A large white and triangular patch on the rump is distinctive. The uppertail appears rather dull white because of dark feather-shafts. The rest of the plumage is blackish brown with a paler diagonal bar across upper surface of the wings. Bill and legs dark. Males and females are similar.
Wilson´s Storm-Petrel (Oceanites oceanicus) is larger, has yellow legs; the U-shaped patch on rump just reaches the center of the black tail. In Band-rumped Storm-Petrel (Oceanodroma castro) the white of rump forms a straight band on black tail and extends to the flanks and underneath the feathers that cover undertail. Leach´s Storm-Petrel (Oceanodroma leucorhoa) rump varies, from almost entirely white (more or less divided with fuscous feathers) to completely dark.