Hummingbirds' wing bones are hollow and fragile, making fossilization difficult and leaving their evolutionary history poorly documented. In traditional taxonomy, hummingbirds are placed in the order Apodiformes, which also contains the swifts, but some taxonomists have separated them into their own order, the Trochiliformes. Juvenile Anna's hummingbird with tongue extended To conserve energy when food is scarce and at night when not foraging, they can enter torpor, a state similar to hibernation, and slow their metabolic rate to 1/15 of its normal rate. Hummingbirds have the highest mass-specific metabolic rate of any homeothermic animal.

During courtship, some male species dive from 30 metres (100 ft) of height above a female at speeds around 23 m/s (83 km/h 51 mph).

Of those species that have been measured during flying in wind tunnels, their top speeds exceed 15 m/s (54 km/h 34 mph). They hover in mid-air at rapid wing-flapping rates, which vary from around 12 beats per second in the largest species to around 80 per second in small hummingbirds. They are known as hummingbirds because of the humming sound created by their beating wings, which flap at high frequencies audible to humans. The common ancestor of extant hummingbirds is estimated to have lived 22 million years ago in South America. Hummingbirds split from their sister group, the swifts and treeswifts, around 42 million years ago. They are specialized for feeding on flower nectar, but all species also consume flying insects or spiders. The largest hummingbird species is the 23 cm (9.1 in) giant hummingbird, weighing 18–24 grams (0.63–0.85 oz). The smallest extant hummingbird species is the 5 cm (2.0 in) bee hummingbird, which weighs less than 2.0 g (0.07 oz). They are small birds, with most species measuring 7.5–13 cm (3–5 in) in length. With about 361 species and 113 genera, they occur from Alaska to Tierra del Fuego, but the vast majority of the species are found in the tropics around the equator. Hummingbirds are birds native to the Americas and comprise the biological family Trochilidae.

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