Found in residential areas, riparian, open oak woodlands, or chaparral.
Male is told by combination of rufous sides and tail, and green back and crown; also note white breast. Compare with Rufous Hummingbird, as some males of that species might resemble Allen's. Females are generally not separable in the field from female Rufous Hummingbird.
Nests in trees, shrubs or in vines. Nest is a cup-shaped structure built from lichens, leaves, plant down, seeds, and spider's silk. Feeds on nectar from flowers, and small insects.