Hummingbirds are birds that comprise the family Trochilidae. They are among the smallest of birds, most species measuring in the 7.513 cm (35 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 1290 times per second (depending on the species). They can also fly backwards, and are the only group of birds able to do so. 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).