I was a bit surprised to encounter a hummingbird around the 11.5k level in Dusy Basin this past week. A quick websearch revealed that some of these little guys are perfectly happy at elevations >16.5k:
http://www.pnas.org/content/95/2/612.full
http://www.earthwatch.org/newsandevents/pressreleases/2002_press_releases/01_23_2002_hummingbirds.html
Also of note is that their vision extends into the UV portion of the spectrum (useful in feeding upon nectar sources not frequented by insects) and they are represented in one of the famous geoglyph constructions in the Peruvian Nazca Desert:
http://en.wikipedia.org/wiki/Hummingbird