Black-capped Swallow is a small swallow with a moderately long, deeply forked tail. The plumage pattern is two-toned: dark above and white below. The upperparts generally are dusky brown or blackish, faintly glossed with blue, and with a darker, black crown. The throat and upper breast are white, flecked with brown. The lower breast and belly are pure white. The flanks are brown, and the vent and undertail coverts are blackish. The sexes of Black-capped Swallow are similar, but juveniles are duller; the throat and breast may be washed with pale cinnamon, and the upperparts are scaled with pale cinnamon.
The rapid flight style and deeply forked tail of Black-capped Swallow help to distinguish it from other species. Tree Swallow (Tachycineta bicolor) is larger, is bluer above, does not have a deeply forked tail, and has a white vent. Violet-green Swallow (Tachycineta thalassina) does not have a deeply forked tail, has a white vent, and is greener above with a white face and white patches on the side of the rump. Northern Rough-winged Swallow (Stelgidopteryx serripennis) does not have a forked tail, lacks the black cap, and has a white vent.
The following description is based on Ridgway (1904), Turner and Rose (1989), and Howell and Webb (1995):
Adult: Sexes similar. Crown, nape, and sides of the head and neck glossy black, with a faint bluish luster. Back and scapulars sepia brown, becoming darker on the rump and deepening to black or dark sooty brown on the uppertail coverts. Rectrices dull black or sooty black. Lesser wing coverts glossy black or brownish black; rest of wing brownish black, fading to dark grayish brown or sooty on secondary and innermost greater wing coverts. Malar region, chin, and throat white, spotted or flecked with sooty brown. Center of breast and belly white. Flanks and under surface of wing plain grayish brown. Vent and undertail coverts sooty black or very dark sooty brown.
Juvenile: Browner and less glossy. Head brown; chin and throat buffy, with no dark flecking. Edges to feathers of upperparts and of tertials pale cinnamon.
Apparently undescribed in Black-capped Swallow. Swallows in general follow the Complex Basic molt strategy (Howell 2010).
Tarsi and toes: brown
Bare parts color data from Ridgway (1904) and Turner and Rose (1989).
Total length: 11.7-12.3 cm (Ridgway 1904), 12 cm (Land 1970, Turner and Rose 1989), 12.5-13.5 cm (Howell and Webb 1995)
male (n =2; Ridgway 1904)
wing length (chord): mean 95 mm (range 94-96 mm)
tail length: mean 53.5 mm (range 53-54 mm)
depth of fork (tail): mean 18.5 mm (range 17-20 mm)
bill length (exposed culmen): mean 5 mm (range 5-5 mm)
tarsus length: mean 10 mm (range 10-10 mm)
female (n =1; Ridgway 1904)
wing length (chord): 94 mm
tail length: 53 mm
depth of fork (tail): 20 mm
bill length (exposed culmen): 5 mm
tarsus length: 10 mm
sexes combined? n = ? (Turner and Rose 1989)
wing length (flat): 95.5 mm (range 93-97 mm)
tail length: mean 53.9 mm (range 51-58 mm)
depth of fork (tail): mean 16.8 mm (range 15-20 mm)
bill length (from base): mean 7.9 mm (range 7.4-8.4 mm)
tarsus length: mean 10.8 mm (range 10-11.6 mm)
Mass: mean 12.2 g (n = 5, sexes combined; Dunning 2008)