The adult bird reaches only 3 to 3.5 inches (76 to 89 mm) in length. The male has a green back and forehead, with rust-colored (rufous) flanks, rump, and tail. The male's throat is an iridescent orange-red. The female and immature Allen's hummingbirds are similarly colored, but lack the iridescent throat patch, instead having a series of speckles
The female lays one or two white eggs, which she incubates for 15 to 17 days. The young leave the nest about three weeks after hatching. The mother continues to feed the fledglings for several more weeks, then the young are left to fend for themselves.