Sooty Swift is a medium-sized swift. Typical of Cypseloides swifts, it has blackish brown plumage and overall is a featureless bird. This species also has a square tail, similar to most other members of the genus. There is no discernable sexual dimorphism.
Sooty Swift is similar to most other medium to large Cypseloides swifts, such as Great Dusky Swift (Cypseloides senex) and Rothschild's Swift (Cypseloides rothschildi). The distribution Sooty Swift broadly overlaps that of Great Dusky Swift, but Great Dusky Swift is larger, is paler brown, and the feathers of the head have pale tips, creating a "frosted" appearance. There is little or no geographic overlap with Rothschild's Swift, which is very similar to Sooty Swift but is paler brown. In the hand, the tips of the rectrices of Sooty Swift are bare and protruding; these bare protruding rectrix shafts are lacking in Rothschild's Swift.
The following description is based on Chantler (2000):
Adult: Sexes similar. The plumage generally is blackish brown. The head is brown, with a black patch surrounding the eye, and a slightly paler forecrown. The auriculars also are slight paler. The back is dark brown, darker than the head and nape, and slightly darker than the rump. The upper surface of the rectrices is black brown; the under surface is grayer. The lesser and median wing coverts and the outer primaries are blackish brown (fairly similar in color to the back); the inner primaries, the secondaries, and the greater coverts are paler brown. The inner webs of the remiges are paler than the outer webs. There is very indistinct pale fringing on the coverts and remiges. The undersurface of the remiges is paler than the uppersurface.
Juvenile: Similar to the adult, but with indistinct gray fringing on the body and with more distinct grayish white fringing on the remiges, particularly on the secondaries.
Tarsi and toes: black
Bare part color data from Belton (1984).
Total length: 15 cm (Chantler 2000)
wing length: male, 153 mm (n = 1, Brazil; Eisenmann and Lehmann 1962); mean 146.8 mm (range 142.5-146.5 mm, n = 4, Brazil, sex not specified; Chantler 2000)
tail length: male, 49 mm (n = 1, Brazil; Eisenmann and Lehmann 1962); 46-47 mm (n = 2, sex not specified; Roesler et al. 2009)
tarsus length: male, 12 mm (n = 1, Brazil; Eisenmann and Lehmann 1962); 13.2-14.6 mm (n = 4, sex not specified; Roesler et al. 2009)
Mass: male, range 40-44 g (n = 3, Brazil; Belton 1984); female, 41 g (n = 1, Brazil; Belton 1984)