The genus Nonnula (Galbuliformes: Bucconidae) includes 6 species of nunlets, understory insectivores of humid Neotropical forest. Like the rest of their lineage (Galbuliformes, Piciformes, Coraciiformes, Bucerotiformes, Trogoniformes, and Leptosomiformes), Nonnula were speculated to nest in cavities or burrows, either underground, in mounds of leaf litter, in trees, or in arboreal termitaria. From 2014 to 2020 we studied 17 nests of Rusty-breasted Nunlet (Nonnula rubecula) at Cruce Caballero Provincial Park in the Atlantic Forest of Misiones, Argentina. From the outside, nests looked like the burrows excavated by relatives in Malacoptila, but nunlets excavated only the floor, carpeted it with leaves, built a roof frame of twigs and rachises, and finally covered the whole structure with leaf litter from the surrounding area. A clutch of 4 (occasionally 3) white eggs with mean dimensions 20.1 (SD 0.8) × 16.8 (SD 0.3) mm (2.8 [SD 0.3] g; n = 5) was laid on alternate days, and incubated 17–18 d. The pink, naked hatchlings, with eyes sealed shut, scrambled around the nest chamber and came to the tunnel entrance by day 3, similar to other Bucconidae. Two secretive adults incubated, brooded, and fed nestlings. Fledglings hopped out of the nest at 17–19 d, sometimes returned before leaving definitively, and apparently could not fly. The probability of a nest surviving the 42 d from laying to fledging was 0.40, comparable to some tree-cavity-nesting birds in our study area (but 95% confidence intervals were large: 0.11–0.69). There is no evidence that any Nonnula use tree cavities or excavate burrows. We propose that Nonnula are unique in their lineage in building a dome-shaped nest at ground level. This habit may have evolved from relatives that excavate underground burrows, surround the entrance with a collar of twigs, and cover the excavation site with leaf litter.
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