The breeding biology of Scarlet-thighed Dacnis is not well known, especially in the southern part of its range. A well-hidden nest with hatchlings was discovered about 15 m high in a 18 m tall muñeco tree within the dense foliage of the tree and of a parasitic Struthanthus vine in Rivas, Costa Rica on 9 May 1936 (Skutch 1962). One of the nestlings fledged on 13 May, flying as far away as about 60 m from the nest; the next morning this fledgling (likely) was perched atop the nest tree with its sibling still in the nest (Skutch 1962). The nest was collected from the tangle of parasitic vines after the family group had dispersed. The nest is a shallow, frail open cup of tendrils, rootlets, and bits of fern and grass that was fortified and fastened with the support of spider silk. It was primarily supported by two parallel slender branches. The internal diameter of the nest was about 6 cm with a depth of about 2 cm. It was comprised of a thin matrix of coarse wiry materials such as rootlets, tendrils, and fern rachises. The nest was completely covered below by about 10 cm long green pieces of Nephrolepis pendula, which were held in place by the outer strands of the nest and provided excellent camouflage against the parasitic tangle (Skutch 1946). Another active nest was recorded from southern Costa Rica sometime between April and July in 2000 (Lindell and Smith 2003). Individuals in breeding condition have been collected from Panama from March through July, including males in subadult plumage with enlarged testes (Strauch 1977, Wetmore et al. 1984). A young male was reported from Boruca, Costa Rica in May or June (Bangs 1907). Both adults feed young, but possible females feed young more often (Skutch 1962, Wetmore et al. 1984).