Like all species of Cypseloides, Spot-fronted Swift is blackish overall with a square tail. It is a medium sized Cypseloides, smaller than White-chinned Swift (Cypseloides cryptus) but larger than Chestnut-collared Swift (Streptoprocne rutila). The original description, written by Robert Ridgway in 1893, states that "this apparently new species needs no comparison with any other, the peculiar white markings of the head being sufficient to at once distinguish it". This is certainly true: given adequate views, the white spots in front of and behind the eyes are distinctive. Some birds show white scaling on the belly, but this does not seem to be related to age or sex (Collins 1980). At least one bird seen in eastern Ecuador had significant white scaling on the leading edge of the wing (Seitz 2011).
Spot-fronted Swift is most similar in appearance to White-chinned Swift (Cypseloides cryptus), but given adequate views, the white spots on the face of Spot-fronted Swift are enough to distinguish it. Beware that in strong light the white forehead and chin of White-chinned Swift can appear bolder than normal, so look especially for the white spot behind the eye of Spot-fronted. At a distance, when the details of the facial pattern can not be seen, Spot-fronted is very difficult to distinguish from other species of Cypseloides. If viewing conditions are poor, Spot-fronted also can be confused with Chestnut-collared Swift (Streptoprocne rutila), especially young birds that lack a chestnut collar. Vocalizations might be the most reliable way to distinguish the two.
The following description is based on Chantler (1998); see also Ridgway (1893), Collins (1980), and Marín and Stiles (1992, 1993):
Adult: Sexes similar. The plumage is overall dark. Some individuals appear browner as opposed to black. It is unknown if the browner aspect is caused by retained juvenile plumage, or is a distinct first basic plumage. Lores dark black with bright white spots on either side of base of bill; smaller white spots behind eyes. Chin white. Entire body blackish brown. Remiges paler grayish below, contrasting with darker median and lesser underwing coverts. Some birds show pale frosty scaling on belly (juveniles?)
Juvenile: Similar to the adult, but white spots on head smaller (although never absent).
Little information. Birds captured in Colombia in May were in the early stages of primary molt, and birds in Ecuador in June were slightly further along in primary molt. This timing appears to be 2-3 months earlier than in Costa Rica (Marín and Stiles 1993). Birds examined in Venezuela in November "showed advanced or nearly complete wing and body molt" (Collins 1980). At least in Costa Rica, molting overlaps with breeding, with molt commencing during incubation or while adults are provisioning nestlings (Marín and Stiles 1992).
Bare part color data from Stiles and Skutch (1989), in part.
Total length: 14 cm (Stiles and Skutch 1989, Ridgely and Greenfield 2001b)
Linear measurements (from Marín and Stiles 1992; live birds, sex undetermined):
wing length: mean 128.90 mm ± 2.54 mm (n = 11)
wing area: mean 44.91 cm ± 3.48 cm (n = 10)
tail length: mean 44.22 mm ± 1.80 mm (n = 11)
bill length (exposed culmen): mean 5.97 mm ± 0.38 mm (n = 11)
tarsus length: mean 12.90 mm ± 0.39 mm (n = 8)
Mass: mean 22.90 g ± 1.47 g (n = 11, sex undetermined; Marín and Stiles 1992)