The breeding season in Ecuador includes March, April, and June (Schulenberg and Gill 1987, Greeney and Gelis 2006). Nests with eggs in southern Peru have been reported from October (Flórez and Londoño 2012).
Unusually for subfamily Emberizinae, the nest of the Olive Finch is large and domed with a side entrance (Schulenberg and Gill 1987). The outer wall is often covered with moss (Greeney and Gelis 2006), ferns, liverworts, Lycopodiaceae, Selaginella, roots, or vines (Flórez and Londoño 2012) for concealment. The inside of the nest is lined with 1-2 cm of woven root fragments, Chusquea bamboo leaves, Asplundia sp. (Cyclanthaceae) leaves, and fresh fern leaves (Greeney and Gelis 2006; Flórez and Londoño 2012). Nests are often placed on stream banks or in vegetation near moving water. On two occasions, nests of Spotted Barbtail (Premnoplex brunnescens) have been found underneath Olive Finch nests (Flórez and Londoño 2012). Of 12 nests in Ecuador, three were built against rock faces and supported by a clump of plants at the base, six were in vine tangles, and three were supported by thin branches (Greeney and Gelis 2006). Five of these 12 nests were supported by multiple support branches (mean 4.8, range 3-8, typical diameter 1 cm), with most passing through the outer mossy walls of the nest (Greeney and Gelis 2006). The outside of the nest may be quite moist because of proximity to water, but the inside remains dry (Flórez and Londoño 2012).
Five nests in Peru were built 1.82 ± 0.49 m above water (mean ± SD) (Flórez and Londoño 2012), and 10 nests in Ecuador were 2.1 ± 0.6 m above a stream (mean ± SD). However, only 3 of the Ecuador nests were directly above moving water (Greeney and Gelis 2006). External measurements of these nests were 148.5 ± 41.8 mm long, 156.58 ± 40.7 mm wide, and 179.54 ± 61.1 mm high (Flórez and Londoño 2012). The entrance holes measured 54.7 ± 11.9 mm by 73.2 ± 10.5 mm. The length from the entrance of the nest to the opposite wall was 106.8 ± 15.0 mm and the height from the entrance to the base of the cup was 47.4 ± 18.3 mm (Flórez and Londoño 2012).
The clutch contains two eggs, which range in color from white with small red-brown speckles that may be concentrated near the larger end (Greeney and Gelis 2006, Flórez and Londoño 2012) to clean white (Schulenberg and Gill 1987). Eggs weigh 4.26 ± 0.42 g (mean ± SD; n = 9) and measure 25.6 ± 1.1 mm by 18.7 ± 0.8 mm (Flórez and Londoño 2012). Additional egg measurements in Ecuador were 24.4 ± 1.1 mm by 17.7 ± 0.1 mm (n = 4; Greeney and Gelis 2006).
Due to lack of sexual dichromatism, it is unclear how the two sexes partition the incubation. During active incubation, the nest temperature is 26.0 ± 1.9°C), whereas when the nest is empty the temperature is on average 24.3 ± 3.3°C (Flórez and Londoño 2012). Although incubation times may fluctuate throughout the day, the eggs are always incubated throughout the night (Flórez and Londoño 2012). The incubation period is at least 14 days (Flórez and Londoño 2012).
Both parents feed nestlings (Flórez and Londoño 2012). Adults average 9.6 ± 2.9 feeding trips per day (n = 19; Flórez and Londoño 2012). The exact diet fed to the chicks is unknown, although the parents were seen provisioning what appeared to be a moth (order Lepidoptera) and another insect (Flórez and Londoño 2012).
The following description of the nestling is based on careful observations of Flórez and Londoño (2012) in Peru unless specified otherwise. On the second day after hatching, the eyes were closed and dark gray down partly covered the brownish pink skin; the chick weighed 12.0 g, tarsus length was 15.7 mm, and wing length was 11.5 mm (Flórez and Londoño 2012). On day five, the eyes were half-open and pin feathers were beginning to emerge on the wings, back and flanks (Flórez and Londoño 2012). By the eighth day, the pin feathers on the tail were nearly gone and 70% of the pins on the back and flanks had developed into dark green juvenile feathers (Flórez and Londoño 2012). By the ninth day, all of the pin feathers on the wings had broken as well as some on the back of the head, and the chick weighed 28.6 g, had a tarsus length of 25.4 mm, a tail length of 3 mm, and a wing length of 29.6 mm (Flórez and Londoño 2012). By the twelfth day, the eyes were open, and its wing length was 42.1 mm (Flórez and Londoño 2012). On the fifteenth day after hatching, the chick’s belly was almost completely covered in olive-green feathers (Flórez and Londoño 2012). By day eighteen, the Olive Finch chick was very active and almost completely covered in feathers. It now weighed 33.0 g and had a wing length of 59.2 mm, and tail length of 21.0 mm. The average growth rate of the chick through day 12 was 2.83 g/day and 1.44 mm tarsus/day, and the tarsus stopped growing after day 12. The wings grew at an average rate of 3.45 mm/day through day 18. The total time from hatching to fledging was 18-20 days (Flórez and Londoño 2012).