Ruby-crowned Tanagers build cup nests that are usually less than 2 m off the ground in dense foliage such as thickets, coffee bushes, or new shoots out of a tree stump (Euler 1900, Holt 1928, Isler and Isler 1987). The nests are commonly built next to streams (Isler and Isler 1987). The filaments of a species of fungus in the genus Marasmius is frequently used in the bird’s nest (Aubrecht et al. 2013). A nest found on 21 January consisted of broad grass leaves and plant stems, lined with fine rootlets (Holt 1928). This nest had two eggs with well-developed embryos.
They lay between 2-3 eggs per nest, which are blunt oval and slightly glossy, with a whitish base color tinged salmon-pinkish or yellowish with smudges of reddish-brown to chocolate-brown particularly at the large end, and may have black scribbles (Euler 1900, Ogilvie-Grant 1912, Isler and Isler 1987, Walters 2006). Four eggs measured (mm): 23.6 x 18.0, 22.9 x 17.0, 22.6 x 17.3, and 21.8 x 17.3 (Ogilvie-Grant 1912), and two eggs measured (mm): 24 x 17 and 15 x 17 (Holt 1928). In captive research, the eggs were incubated for 13 days before both parents fed the fledglings for 9 days until they left the nest (Norgaard-Olesen 1974). Breeding usually happens in summer from late September to January (Short 1971, Isler and Isler 1987). A male collected in December had enlarged testes (Belton 1985), and two males collected on 29 September had testes measuring 12 x 7 mm (this bird actively singing) and 8 x 6 mm (Short 1981).