Breeding of Black-capped Swalllow is reported from February-April in Mexico (Wagner 1951), April-July in Guatemala (Dearborn 1907, Griscom 1932, Skutch 1960, Baepler 1962) and in May and June in El Salvador (Thurber et al. 1987).
The nest of Black-capped Swallow is in a cavity or crevice, such as in vacant nest burrows of Blue-throated Motmot (Aspatha gularis) (Skutch 1960), in crevices in a rock sink (Baepler 1962), in burrows in the bank of a road cut (Thurber et al. 1987), and possibly on cavities in a building (Dearborn 1907). Griscom (1932) also mentioned these swallows nesting in a cave, but the nest sites themselves were not observed. Thurber et al. (1987) imply that these swallows can excavate their own burrow, and so are not dependent upon occupying burrows excavated by other species. The nest itself, within the burrow, is a shallow cup of pine needles and leaves (Skutch 1960, Baep) or of twigs and mud, sometimes with feathers in the lining (Baepler 1962); nest construction is by both sexes, and takes five days (Skutch 1960). Nests may be solitary (Skutch 1960) or in small colonies (Baepler 1962). The eggs are white and unmarked; the clutch is four (Skutch 1960). Mean egg dimensions are 16.6 x 12.8 mm (range 15.7-17.3 x 12.7-13.1 mm, n = ?; Turner and Rose 1989). Both sexes provision the nestlings, and both sexes carry away fecal sacs (Baepler 1962). One pair made 36 visits to the nest during a period of one hour (Baepler 1962). The lengths of the incubation and nestling periods are not known.