Hummingbirds are birds that comprise the family Trochilidae. They are among the smallest of birds in the world. Most species are measured in 7.5–13 cm (3–5 in) range. However, the smallest extant bird species is a hummingbird, the 5-cm Bee Hummingbird. They can rapidly flapping their wings 12–90 times per second in the air (depending on the species). These are only birds which can also fly backwards. Their English name derives from the characteristic hum made by their rapid wing beats. They can fly at speeds exceeding 15 m/s (54 km/h, 34 mi/h).