Whether measured by length of body or by weight, no Costa Rican hummingbird is smaller than the Scintillant Hummingbird (Selasphorus scintilla). This species is endemic to Costa Rica and western Panama and is found at middle elevations of the Central and Talamanca mountain ranges.
Raúl Vega‘s stunning photo below, from the collection of the Asociación Ornitológica de Costa Rica, shows exactly what I saw today on the steep hillside between the villages of El Carmen and El Banco.
This bird is so small that you can easily miss it by supposing it to be an insect of some kind. I found this one at the same rabo de gato hedge that had recently yielded Black-crested Coquette. The shining, bright orange gorget of the male, which is not always illustrated to advantage in the guides, is often not seen, but the size of the bird, the rufous tones and the absence of a white rump band exclude almost all other possibilities.
To close, here’s a very nice pic of a female, courtesy of friend John Beer: