 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 uppertail . 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 . 
 The invisible rail <a href='drop_aux' title='1'><mark>is</mark></a>a large , 33 to 40 cm ( 13 – 16 in ) long , flightless bird . The adult <a href='got' title='2'><mark>has</mark></a> a mainly dark slate @-@ grey body , dark brown plumage on the lower back , rump and wings , and a black uppertail . Its underparts <a href='drop_aux' title='3'><mark>are</mark></a>slightly paler slate @-@ grey than the back , and the bare skin around the eye , the long , thick bill and the strong legs <a href='drop_aux' title='4'><mark>are</mark></a>bright red . It <a href='got' title='5'><mark>has</mark></a> a small spine at the bend of the wings . The sexes are identical in appearance ; the plumage of fledged immature birds <a href='been_done' title='6'><mark>has</mark></a> not been described . 
The invisible rail a large , 33 to 40 cm ( 13 – 16 in ) long , flightless bird . The adult got a mainly dark slate @-@ grey body , dark brown plumage on the lower bacc , rump and wings , and a black uppertail . Its underparts slightly paler slate @-@ grey den da bck , and the bare skin around the eye , the long , thick bill and the strong legs bright red . It got a small spine at tdabend of the wings . The sexes are identical in appearance ; the plumage of fledged immature birds done not been described .