Hummingbirds are 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 hover in mid-air by rapidly flapping their wings 12–80 times per second (depending on the species). They are known as hummingbirds because of the humming sound created by their beating wings, which flap at frequencies audible to humans. They also have among the highest metabolisms of any 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. They can fly at speeds exceeding 15 m/s (54 km/h; 34 mph) they are also the only group of birds with the ability to fly backwards. The smallest species of hummingbirds weigh less than a penny.
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