Hummingbirds are New World birds that constitute the family Trochilidae. They are among the smallest of birds, most species measuring in the 7.5 13 cm (3 5 in) range. Indeed, the smallest extant bird species is a hummingbird, the 5-cm Bee Hummingbird.
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, but possibly as high as 200 times per second, allowing them also to fly at speeds exceeding 15 m/s (54 km/h; 34 mph), backwards or upside down.
Hummingbirds have the highest metabolism of any homeothermic animal. To conserve energy when food is scarce, they have the ability to go into a hibernation-like state (torpor) where their metabolic rate is slowed to 1/15th of its normal rate. The smallest species of hummingbird weighs less than a penny.