They are among the smallest of birds: most species measure 7.5–13 cm (3–5 in). The smallest living bird species is the 2–5 cm Bee Hummingbird. They can hover in mid-air by rapidly flapping their wings 12–80 times per second (depending on the species). They are also the only group of birds able to fly backwards. Their rapid wing beats do actually hum. They can fly at speeds over 15 m/s (54 km/h, 34 mi/h).