Cypseloides are medium to large sized swifts with uniform dusky or blackish plumage. The tails of Cypseloides are square tipped to slightly forked, and lack protruding "spines" at the tips of the rectrices, and wings are relatively broad. Great Dusky Swift is one of the largest species in Cypseloides, with a square tipped or slightly rounded tail. The plumage of Great Dusky Swift is dusky brown, paler, sometimes almost whitish, on the forecrown, lores, and chin.
Sooty Swift (Cypseloides fumigatus) is smaller than Great Dusky Swift, and has a uniformly dark appearance, lacking the white "frosting" on the head of Great Dusky. White-collared (Streptoprocne zonaris) and Biscutate (Streptoprocne biscutata) swifts are larger and blacker than Great Dusky, and have prominent white collars; the tail of White-collared Swift also is slightly, but noticeably, forked.
The following description is based on Chantler (2000):
Adult: Sexes similar. Feathers of the top of the head chocolate brown, with blackish shaft streaks and distinct pale gray fringes. The pattern on the throat is similar. Consequently the head has an overall pale or "frosted" appearance, especially for fore parts (forecrown, lores, and chin). The back is blackish brown, clearly darker than the head and nape; the rump and uppertail coverts are slightly paler than the back (but darker than the head), and very lightly pale fringed. The rectrices are blackish brown. The greater wing coverts are blackish brown, paler tipped; the other coverts are uniformly blackish brown. The remiges are blackish brown; the inner primaries and the secondaries are paler tipped, especially on the inner webs. The underwing is only slightly paler than the upper surface, and are only slightly paler than the greater coverts. The median and lesser coverts are darker and browner.
Juvenile: Similar to adult, but with very narrow pale fringes to the remiges. On the underwing, median coverts are paler than the greater coverts; the outerwebs of the underwing primary coverts are basally white.
Bill and toes black.
Total length: 18 cm (Sick 1993, Chantler 2000).
Lineear measurements (from Navarro et al. 1992; n = 2, sex?)
wing length (chord): mean 160.5 ± 2.5 mm
tail length: mean 46.0 ± 1.5 mm
tarsus length: mean 20.47 ± 0.05 mm
mean 99.8 g (range 86.1-110 g, n = 33, sexes combined, Argentina; Dunning 2007)
mean 67.8 g (range 56-80 g, n = 8, sexes combined, Brazil; Dunning 2007)