The holotype, a female, which was captured, examined, and released afterwards on 19 February 2011 has the following measurements: head and bill length 32.9 mm, exposed culmen length 11.5 mm, bill length from gape 12 mm, flattened wing length from chord 134 mm, tarsus length 31 mm, mid-toe length (with claws) 26.5 mm, tail length 57 mm, wing span 330 mm and a mass of 24 g. Two paratypes, a juvenile female and a male which were originally identified as Wilson's storm petrels (Oceanites oceanicus) and collected by Argentinian ornithologist Andor Kovács at El Bolsón, Argentina in 1972 and 1983 have the following measurements: the wing length of the juvenile female is 138 mm, the tail length 61 mm, the tarsus length 31.5 mm, the culmen length is 9.5 mm, the mid-toe length is 26 mm, the wing length of the male is 137 mm, the tail length is 53 mm, the tarsus length 30.5 mm, the exposed culmen length is 10 mm and the mid-toe length is 27 mm.
The head, mantle, scapulars, back and upper rump are blackish-brown. The nape, mantle and scapulars, back and upper rump are washed silvery-gray. Some scapulars and the longest tertials have narrow but distinct white edges.