Very little is known about the breeding biology of Collared Crescentchest, other than observations by Gressler and Marini (2007) and Kanegae et al. (2010a). The nest is globular, with a side entrance, and placed near the ground, such as inside a tussock of grass. The nest is composed of woven dry grass, mixed with a leaves from trees or shrubs. The entrance to one nest was 4.2 cm (width) x 3.7 cm (height); this nest itself had dimensions of 13.5 cm (height) x 10.0 cm (width) x 10.5 cm (length).
The clutch is two; the eggs are ovoid, greenish blue and unspotted. Egg dimensions (mean; n = 2, Kanegae et al. 2010a) are 19.05 x 14.95 mm, with mean mass of 1.55 g. The incubation period is estimated as 15-18 days (Gressler and Marini 2007) or as 12-16 days (Kanegae et al. 2010a), and the nestling period is 12-14 days (Kanegae et al. 2010a). Both sexes contribute to incubation and to care and provisioning of the nestlings. Male Collared Crescentchest have a brood patch (Kanegae et al. 2010b).