Trochilidae are known as hummingbirds because of the humming sound created by their beating wings which flap at high frequencies. They are among the smallest of birds, most species measuring 3–5 in range and are one of the smallest type of animals with a backbone. Hummingbirds have the highest metabolism of any homeothermic animal. To conserve energy when food is scarce, and nightly when not foraging, they go into torpor, a state similar to hibernation, slowing metabolic rate.