The plumage of adult Spix’s macaws is dull blue with a faint greenish tinge on the breast and abdomen. The upper side of the back and tail are a deeper blue, the bare pores and cheeks are dark grey, the ear-coverts and forehead are pale grey-bluish. The underside of the tail and wing-coverts are dark grey. Their bill is blackish, smaller, and less curved than that of close relatives. Their irises are pale yellowish, and the feet are grey. Sexes are alike. They weigh 360 g and are 55 cm long, on average. Their wingspans are 1.2 m and their basal metabolic rates are 1.245 cm^3 oxygen/hour.