Cypseloides are medium sized swifts with blackish plumage, concolor upperparts, relatively broad wings, and a square or only slightly forked tail. White-chinned Swift is a medium sized to large Cypseloides with a square tipped tail. The plumage of the adult is dark, with white scaling on the chin and forecrown. Juveniles have more extensive white scaling on the lower belly.
Field identification of swifts often is challenging, as the plumage of most species is relatively unpatterned, and swifts usually are observed only a distance, in constant flight. White-chinned Swift is similar to Chestnut-collared Swift (Streptoprocne rutila), especially to female and juvenile Chestnut-collareds. White-chinned Swift is larger than Chestnut-collared, with a larger, more bulbous head extending farther ahead of wings, and no fork or notch in the tail; although usually difficult to see in the field, the white tips of feathers of the chin and forecrown of White-chinned also distinguish this species from Chestnut-collared. Juvenile Chestnut-collared Swift lacks the white belly markings of juvenile White-chinned Swift. White-chinned Swift also is very similar to Spot-fronted Swift (Cypseloides cherriei), but Spot-fronted Swift has small but distinct white spots on the chin, and before and behind the eye.
The following description is based on Chantler (2000; see also Zimmer 1945 and Marín and Stiles 1992):
Adult: Sexes similar. The plumage of adults may be primarily brown or black (Marín and Stiles 1992). It is not known whether the brown plumage, which is similar to that of the juvenile but is darker and more uniform, represents a retained juvenile plumage, or is a distinct (first basic?) adult plumage; regardless, the black plumage is assumed to be the definitive plumage (Marín and Stiles 1992). Crown and auriculars dark brown, with very narrow white tips to feathers of forecrown, lores and short white streak above eye; the pale areas of the head (including the chin) are smaller and less contrasting than in the black plumage (Marín and Stiles 1992). Back and rump blackish brown. Wings and tail slightly blacker than the mantle; the inner webs of the remiges, especially the tertials, are paler than the outer webs. The under surface of the remiges is paler than the dorsal surface, and contrasts with darker median and lesser underwing coverts. The shaft of the rectrices projects slightly past the rest of the feather, forming well developed tail spines. The underparts are slightly paler than the upperparts; the throat is dark brown, but is paler, sometimes whitish, on the chin.
The plumage of adults may be primarily brown or black (Marín and Stiles 1992)
Juvenile: Upperparts blackish brown; underparts dark brown. Feathers of the belly tipped with white, these white tips broader on the lower belly and vent.
Little information. All individuals examined in the hand in Costa Rica in August were in molt (n = 7); molt was initiated in June, during the period when these swifts were incubating (Marín and Stiles 1992).
Bare parts color data from specimens in The Field Museum.
Total length: 15 cm (Stiles and Skutch 1989, Ridgely and Greenfield 2001b)
Linear measurements (from Marín and Stiles 1992; see also Eisenmann and Lehman 1962, Wetmore 1968, and Roesler et al. 2009):
wing length, mean 136.64 ± 1.71 mm (n = 14)
tail length, mean 44.02 ±1.40 mm (n = 10)
tarsus length, mean 15.69 ±0.32 mm (n = 13)
bill length (exposed culmen), mean 5.55 ± 0.43 mm (n = 12)
wing area, mean 48.39 ± 3.22 cm2 (n = 10)
Mass: mean 35.27 ± 1.92 g (n = 13; Marín and Stiles 1992)