Krabbe and Schulenberg (2003) reported spontaneous singing from October to January in northwestern Ecuador, and in October and March in eastern Ecuador. Until recently, the only direct observations of breeding, apart from one "immature" and one "juvenile" in November in Pichincha (hylodroma, Fjeldså and Krabbe 1990, Knox and Walters 1994) were those of Greeney and Nunnery (2006), who witnessed an adult (hylodroma) feeding a fledgling pieces of an earthworm on 19 April 2001 in northwestern Ecuador (near Tandayapa, Pichincha).
The first description of the nest and breeding biology of Giant Antpitta comes from a nest studied in northwestern Ecuador in November 2007 (Solano-Ugalde et al. 2009). Over the course of 13 days, a pair of Giant Antpittas were observed carrying nesting material (mainly small dead branches, mosses, lianas, ferns, Sellaginella sp., dark rootlets, fibers from petioles and leaflets of Ceroxylon palms) to a nest built about 7 m above the ground on top of a fairly dense tangle of thin vines and partially supported and attached to a small (18 cm DBH) tree. Bulkier matter was placed first (dead branches and larger fibers), with medium sized materials (ferns, Sellaginella sp. and rootlets) added subsequently. These finer materials began to shape the cup which was firmed up with the final addition of moss and thin vines. Casual observations suggested that one individual gathered most of the material, while the other spent more time arranging material in the nest. Adults apparently used their feet and bills to arrange materials and shaped the cup by pressing their breast into the inner depression, with wings pushed rearward in a manner described for other passerines during nest-cup shaping (Greeney et al. 2008b). At this nest the process of construction lasted at least 13-14 days, possibly not much longer than this and potentially shorter if the eggs were laid earlier than was confirmed (see below).
The nest was partially obscured by overhanging epiphytes and difficult to detect from most angles. The deep, bulky cup had the following dimensions: outer diameter 27.5 cm; outer height 24 cm, with an additional 3 cm of fibers and moss draping below in a loose tail; inner diameter 13 cm; inner depth 8.2 cm. The habitat surrounding the nest was seasonally humid, montane cloud forest, with a fairly dense understory and subcanopy and a canopy height of around 25 m. Dominant trees in the area of the nest included Cecropiaceae, Chloranthaceae, Lauraceae, Melastomataceae, and Rubiaceae. The nest tree was on fairly steep terrain and located around 15 m from a 2-3 m-wide stream. At this nest, by 20 December, adult behaviors suggested that incubation was underway, an assumption that was confirmed only two days later using video monitoring of the nest. At this nest, clutch size was two. The eggs were subelliptical in shape and uniform turquoise in color, though they were not inspected closely. Because they were not able to approach the nest frequently, only a rough estimation of incubation period (11-18 days) was possible. Solano-Ugalde et al. (2009) reported adults vocalizing at the nest, documenting both the full song and shorter modifications of the typical song, but did not confirm if both sexes vocalized. Both sexes did participate in incubation, however, as confirmed by observed switching of adults at the nest. They reported that, while incubating, adults occasionally napped, arranged material in the nest, and engaged in rapid probing (Greeney 2004). Though details were not given, they also suggested that adults occasionally fed one another.
Apparently, only one of the two eggs hatched, the other disappearing from the nest several days after hatching of the first. At hatching the nestling's eyes were closed, the skin was dark gray, and the nestling was covered in "fine blackish gray down". Roughly two weeks after hatching the nestling retained some tufts of natal down on the head, but most contour feathers were fully expanded. The contour feathers, however, differed from the adult plumage in having buffy edges and, particularly on the head, were overall browner than those of adults. The mandible was mostly dusky with a paler base and tomia, while the maxilla was dusky with a paler tip. The rictal flanges were enlarged and pinkish orange, while the mouth lining was bright orange. The back feathers had broad dark edges while the feathers of the underparts were slightly more rufescent, with thinner black borders, already showing some scaling. The chin and throat feathers were slightly paler. Flight feathers were almost fully emerged from their sheaths, by around 18 days after hatching differing little from those of the adults, and the wings bore only a few wisps of natal down at this point. Several days after it was last seen in the nest, the chick was found in the dense understory near the nest, where both adults were bringing it large pieces of earthworms. These observations suggested a nestling period of around 22 days.