Breeding of Chestnut-collared Swift is reported from May to August in Mexico (nests with eggs, May to July; nestlings, August) (Rowley 1966, 1984). The egg-laying period in Costa Rica extends from May to July, which corresponds to the early part of the local rainy season; this seasonality is suspected to correspond to the seasonal peak of flying insects (Marín and Stiles 1992). Snow (1962) reported that egg-laying in Trinidad was initiated in April and continued until August, although Collins (1968) observed nest refurbishment by April but did not report eggs until June. As in Costa Rica, breeding on Trinidad corresponds with the local rainy season and an associated abundance of aerial insects (Collins 1968). Belcher and Smooker (1936), however, report nests with eggs on Trinidad until as late as November, although Snow (1962) suggested that the November nest with eggs was an abandoned clutch. A nest with a chick was reported from Ecuador in July (Kiff et al. 1989).
Chestnut-collared Swifts nests are placed on rocky cliffs or similar sites (such as vertical culverts), almost always near water, such as in a gorge above a stream, behind a waterfall, or in a sea cave (Orton 1871, Belcher and Smooker 1936, Snow 1962, Rowley 1966, Nicéforo and Olivares 1967, Collins 1968, Rowley 1984, Marín and Stiles 1992). In Costa Rica, the relative humidty at Chestnut-collared Swift nest sites never dropped below ca 95% (Marín and Stiles 1992). Nests typically are placed low, 0.5-7.6 m above the ground (Snow 1962, Rowley 1966, Collins 1968, Marín and Stiles 1992). The nest is a cup or half cup, placed on a small ledge or protusion, or in a small niche, on a rock wall or under an overhang (Snow 1962, Marín and Stiles 1992). The nest is composed of soft plant material, sometimes with a mud base (Snow 1962, Collins 1968, Marín and Stiles 1992); no saliva is used (Collis 1968, Marín and Stiles 1992). In Trinidad the common plant material used include the liverwort Plagiochilax, the lycopsids Selaginella chladorrhizans and S. cf. arthritica, and the filmy fern Trichomanes membranaceum, all of which grow in the shady, wet areas near streams where Chestnut-collared Swifts prefer to nest (Collins 1968). Nests examined in Costa Rica contained primarily mosses (Marín and Stiles 1992).
Chestnut-collared Swift exhibits strong nest fidelity across years. Old nests are refurbished with a new layer of fresh plant matter at the beginning of the breeding season (Collins 1968).
The clutch size is 2 eggs, and the second is usually laid 2 days after the first (Collins 1968, Marín and Stiles 1992). Eggs are white and unmarked, do not taper, and weigh around 2.5 g (Collins 1968, Marín and Stiles 1992). They are incubated by both parents approximately equally (Marín 2008) for 22-23 days (Snow 1962, Collins 1968), and the two eggs generally hatch within 24 hours of each other (Collins 1968). Chicks are brooded continuously during the day for the first 10-11 days until they are able to maintain their own body temperatures at sufficient levels. The young remain in the nest until they fledge at 5-6 weeks of age (Snow 1962, Collins 1968).