Hummingbirds are birds that comprise 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 can hover in mid-air by rapidly flapping their wings 12–80 times per second (depending on the species). As well to conserve energy while they sleep or 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 are also the only group of birds able to fly backwards. Their English name derives from the characteristic rapid wing beats. They can fly at speeds exceeding 15 m/s (54 km/h; 34 mph).