<Li> List of hummingbirds </Li> <P> Hummingbirds are birds from the Americas that constitute the family Trochilidae . They are among the smallest of birds, most species measuring 7.5--13 cm (3--5 in) in length . Indeed, the smallest extant bird species is a hummingbird, the 5 cm (2.0 in) bee hummingbird weighing less than 2.0 g (0.07 oz). </P> <P> 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 in 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 speed exceeds 15 m / s (54 km / h; 34 mph) and some species can dive at speeds in excess of 22 m / s (79 km / h; 49 mph). </P> <P> Hummingbirds have the highest metabolism of any homeothermic animal . To conserve energy when food is scarce, and nightly when not foraging, they can go into torpor, a state similar to hibernation, slowing metabolic rate to 1 / 15th of its normal rate . </P>

How many times do a hummingbird's wings beat per minute
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