The Calliope Hummingbird breeds uncommonly to fairly commonly in the Sierra Nevada and other high mountain ranges in California. In the Sierra, it typically nests above 4,000 feet elevation (Grinnell and Miller 1944; Gaines 1992; pers. obs.), usually near moist meadows or other relatively level and wet sites with a mixture of deciduous and coniferous trees, shrubs, and flowering plants (Grinnell and Miller 1944; Verner et al. 1980; Gaines 1992; pers. obs.).
Although it can be locally fairly common as a migrant in the more arid lowlands during spring migration (pers. obs.), it generally avoids dense forests, dry ridges, or other relatively exposed and hot sites for breeding. Consequently, I was quite surprised to find a Calliope Hummingbird raising young in relatively arid Blue Oak (Quercus douglasii) woodland well below 1,000 feet elevation.