Yearlong, diurnal activity. Migrants arrive in late January and early February and mostly are gone by late July, but stragglers occur to late August and September. Rare in winter except in the range of the resident subspecies S. s. sedentarius, and on the coast just east of that range. Nests often clustered. Hummingbirds occasionally are taken by hawks, small mammals, and snakes, but sudden cold periods, which kill food sources, pose the largest threat.
Hummingbirds are important pollinators of many specially adapted flowers.
The status of this species in migration and in winter is uncertain in many areas, because of its similarity in all plumages to the rufous hummingbird. Females and immatures of the 2 species are nearly impossible to distinguish in the field. The avifauna of the South Farallon Islands, California. Studies in Avian Biol. Ehrlich, P. R., D. S. Dobkin, and D. Wheye.