Little is known about the seasonality of breeding in Cinereous Conebill, which presumably is geographically variable across the wide latitudinal range of this species. Its broad elevational range also no doubt affects its reproductive phenology, as variation in temperature and precipitation may lead to differences between high- and low-altitude populations. Nests with eggs are reported from Bolivia in April (Fjeldså and Krabbe 1990) and from Chile in November (Johnson 1967). Note that the related Tamarugo conebill (C. tamarugense) also breeds in northern Chile, at mid-elevations, from September to December (Estades and Lopez-Calleja 1995). Fledglings are reported from Huánuco, central Peru, in June (Fjeldså and Krabbe 1990); from western Ecuador in June (Fjeldså and Krabbe 1990); from Bolivia in August (Fjeldså and Krabbe 1990); and food carrying is reported from southwestern Peru (Moquegua) in May (Fjeldså and Krabbe 1990). Otherwise, specimens of Cinereous Conebill in breeding condition were collected in February in Colombia (Hilty and Brown 1986), and a territorial dispute and display was observed in May in Ecuador (Moynihan 1963). Based on an assessment of museum specimen data, molt can occur throughout the year in Peruvian populations of C. cinereum that occur between 5 and 12º S; but the majority of molt (including both body and flight feathers) occurs in July and August, which likely marks the end of the annual breeding cycle. Museum of Southwestern Biology specimen data show enlarged gonads in littorale from May thru October, with a pulse in January. However the majority of enlarged gonads were observed in May. Museum samples of cinereum were more limited, but showed the majority of enlarged gonads in August through December with a pulse in March.
The nest of Cinereous Conebill is cup shaped and placed in a tree; it is constructed of root fibers, interwoven with cotton or plant wool, and lined with black horsehair (Johnson 1967). Mean external nest dimensions are 9 cm (diameter) x 6 cm (height), with an internal cup diameter of 5 cm and depth of 3.5 cm (Johnson 1967). The clutch is three, and the eggs are "roundish in shape", pale grayish blue with numerous small, pale purplish gray spots (Johnson 1967). Mean egg dimensions are 16.6 ± 0.34 mm x 13.7 ± 0.04 mm (Johnson 1967). There is no information on the length of incubation or of the nestling period, nor on parental care of the young.