This is the most austral hummingbird in the world, the Green-backed Firecrown (Sephanoides sephanoides). It inhabits the austral temperate forests from western patagonia, in both sides of the Andes. In Argentina it ranges from Neuquen province south to Tierra del Fuego. It feeds mainly on nectar, but occasionally it consumes small arthropods. In winter, when most plants don't flower, only quintral (tristerix corymbosus) has flowers. This makes of this plant the only food source for hummingbirds during that time of the year.
The photo was taken using five flashes, one of them for the background.