 The invisible rail is a large , 33 to 40 cm ( 13 – 16 in ) long , flightless bird . The adult has a mainly dark slate @-@ grey body , dark brown plumage on the lower back , rump and wings , and a black <unk> . Its underparts are slightly paler slate @-@ grey than the back , and the bare skin around the eye , the long , thick bill and the strong legs are bright red . It has a small spine at the bend of the wings . The sexes are identical in appearance ; the plumage of fledged immature birds has not been described . 
