A small bird, on average 7.5–13.0 cm long (but at 5 cm long the bee hummingbird (Mellisuga helenae) is the world’s smallest bird), that is able to hover and fly at more than 50 km/h by beating its wings at about 50 Hz, making an audible humming that gives it its common name. They feed on nectar, which they obtain by their long bills, and their metabolic rate is so high that at night they go into torpor to conserve energy. They occur only in the New World, where there are 325–40 species comprising the family Trochilidae.