They are among the smallest of birds, most species measuring 7.5–13 cm (3–5 in). Indeed, the smallest extant bird species is a hummingbird, the 5-cm bee hummingbird weighing less than a U.S. penny (2.5 g).
They are known as hummingbirds because of the humming sound created by their beating wings which flap at high frequencies audible to humans. They hover in mid-air at rapid wing-flapping rates, typically around 50 times per second, allowing them also to fly at speeds exceeding 15 m/s (54 km/h; 34 mph).
Hummingbirds have the highest metabolism of any homeothermic animal. To conserve energy when food is scarce, and nightly when not foraging, they go into torpor, a state similar to hibernation, slowing metabolic rate to 1/15th of its normal rate.