This is a male. Females are drab. Negritos are distinctive terrestrial short-tailed tyrant-flycatchers (Tyrannidae). They are among the least vocal of any "songbird" in that they have no song at all and only short alarm and contact notes. This is the Austral Negrito which is common in open country in southern South America. It is similar to the larger, paler winged Andean Negrito (Lessonia oreas) which replaces it further north. The two were formerly considered to be the same species under the name Rufous-backed Negrito.
Digiscoped with Panasonic DMC-LX5 | Nikon FieldScope III | 30XWA | hand-held (no adapter).
References:
Austral Negrito (Lessonia rufa), In Neotropical Birds Online (, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. retrieved from Neotropical Birds Online: https://neotropical.birds.cornell.edu/Species-Account/nb/species/ausneg1
Farnsworth, A., Lebbin, D. & Kirwan, G.M. (2018). Austral Negrito (Lessonia rufa). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from https://www.hbw.com/node/57384 on 22 April 2018).