Hummingbirds are birds native to the Americas and constituting the biological family Trochilidae. They are the smallest of birds, most species measuring 7.5–13 cm in length. The smallest extant bird species is the 5 cm bee hummingbird, which weighs less than 2.0 g.
They are known as hummingbirds because of the humming sound created by their beating wings, which flap at high frequencies audible to humans. They hover mid-air at rapid wing-flapping rates, which vary from around 12 beats per second in the largest species, to in excess of 80 in some of the smallest.
Of those species that have been measured in wind tunnels, their top speeds exceed 15 m/s and some species can dive speeds in excess of 22 m/s.
Hummingbirds have the highest
mass-specific metabolic rate any homeothermic animal. To conserve energy when food is scarce, and nightly when not foraging,
they can go torpor, a state similar to hibernation, slowing their metabolic rate to 1/15th of its normal ratHummingbirds are specialized nectarivores and are tied to the ornithophilous flowers upon which they feed. Some species, especially those with unusual bill shapes, such as the sword-billed hummingbird and the sicklebills, are co-evolved with a small number of flower species.
However, even in the most specialized hummingbird-plant mutualisms the number of food plant lineages of the individual hummingbird species increases time.