The western counterpart of the ruby-throated, the black-chinned regularly pumps its tail. Monotypic. Length 3.3–3.8"; bill 16–22 mm.

Best marks for all ages are narrow inner primaries and blunt primaries. Double-rounded tail. Age/sex differences as ruby-throated except as noted. Adult male: black throat with violet-blue lower band. Adult female: black-violet spots rare on throat. Immature male: Throat usually has black-violet spots.

The black-chinned is often confused with Anna’s and Costa’s, which are chunkier and proportionately bigger headed, shorter billed, and shorter tailed; lack the narrow inner primaries of Archilochus; and molt wings in summer. Female/immature Anna’s slightly larger; underparts more mottled, including undertail coverts (mostly whitish on black-chinned); throat often with rose-red spots; wags tail infrequently. Female/immature Costa’s slightly smaller;