Adult was uniform bluish slate gray with white undertail coverts, vent, and central lower abdomen, and with large white corners on the long, graduated tail. The bill and tarsi were reddish, and the iris brown or reddish. Males and females were similar in plumage, although males on average were larger. Juveniles were overall buffy on the body, without spotting, and with bare parts paler than adults (Clement 2000, Cory 1886a, Johnston 1969).
Red-legged Thrush (Turdus plumbeus) differs in having the throat of different color than the breast, and striped in most subspecies. The underparts of Red-legged Thrush also are paler than those of Grand Cayman Thrush. Several subspecies of Red-legged Thrush shows a more contrasting pattern on wings than what is seen on extant specimens of Grand Cayman Thrush.
This section is based on Ridgway (1907:87) and Johnston (1969); see also Summary:
Adult: The tail pattern is shown in a drawing in Johnston (1969), where it can be seen that the central two pairs of rectrices have no white (but text states that three (of 13) specimen with unworn tails had traces of white on second rectrix). White is found on the inner vane of the remaining rectrices in increasing amount from rectrices 3 to 6. A small amount of white is found on outer vane of rectrices 4 and 5.
The lores are very slightly darker than the surrounding area of the head. The throat is concolored with the breast and not striped or at least inconspicuously so: under microscope, throat feathers were seen to have white edges on some of the individuals giving a weak gray-and-white striped effect.
Effect of feather wear was that the worn edges became brown even though the feathers originally were slate gray.
Juvenile: Uniform buffy on both upper side and underside except for white on abdomen to undertail coverts. Note that the underside was unspotted. Tail and wing patterns similar to adults.
Molt of tail feathers was noticed on two adult birds taken in early April while worn plumage without molt was noted in specimen from June and August. Fresh plumage was noted for specimen from May and early June. Tail feather molt seemed to have started from the central feathers. All the juvenile specimen taken during summer of 1886 showed signs of postjuvenile molt (Johnston 1969).
Notes by different collectors tell us that the bill, tarsus and orbital skin was coral red and iris brown (Brown) or bill and feet orange, eye reddish (Richardson) in adult, iris reddish or yellow brown, bill orange and legs yellow in juvenile (Johnston 1969). Tibiotarsus slaty gray in adults, buff brown in juveniles (Johnston 1969).
Below are given the five measurements performed on the specimen but excluding for example individuals with strongly worn tails from that measurement (Johnston 1969) . All measurements are in mm. Notice that males are larger than females (wing chord, tail, and tarsus) in agreement with what has been found for Red-legged Thrush (Turdus plumbeus).
Wing chord: adult males (n = 8) 133.2 (1.2), adult females (n = 5) 126.3 (1.3), juvenile males (n = 2) 127.3
Tail length: adult males (n = 10) 125.2 (0.9), adult females (n = 6) 118.9 (1.3), juvenile males (n = 2) 114.8
Tarsus: adult males (n = 11) 41.6 (0.4), adult females (n = 7) 40.6 (0.6), juvenile males (n = 2) 43.1
Bill (culmen): adult males (n = 10) 25.3 (0.4), adult females (n = 6) 25.9 (0.2), juvenile males (n = 1) 24.8
Bill (nostril): adult males (n = 10) 18.2 (0.2), adult females (n = 5) 18.5 (0.3), juvenile males (n = 1) 15.7