Hummingbirds are birds native to the Americas and constitute the biological family 'Trochilidae'. They are the smallest of birds, most species measuring 7.5–13 cm (3–5 in) in length. The smallest extant bird species is the 5 cm (2.0 in) bee hummingbird, which weighs less than 2.0 g (0.07 oz).

They are known as hummingbirds because of the humming sound created by their beating wings which flap at high frequencies audible to humans.

Hummingbirds have the greatest mass-specific metabolic rate 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.

Female hummingbirds tend to be larger, requiring more energy, with longer beaks that allow for more effective reach into crevices of tall flowers for nectar. Thus females are better at foraging, acquiring flower nectar, and supporting the energy demands of their larger body size. Directional selection will thus favor the larger hummingbirds in terms of acquiring food.

For nutrition, hummingbirds eat a variety of insects, including mosquitoes, fruit flies, and gnats in flight or aphids on leaves and spiders in their webs. The lower beak of hummingbirds is flexible and can bend as much as 25 degrees when it widens at the base, making a larger surface for catching insects.

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