Hummingbirds belong to the Trochilidae family of birds, which are endemic to the Americas. There are around 360 different species. They may be found from Alaska to Tierra del Fuego, although the tropics are home to the great majority of the species. The majority of species are tiny birds, measuring 7.5–13 cm (3–5 in) in length. The 5 cm (2.0 in) bee hummingbird, which weighs less than 2.0 g, is the smallest extant hummingbird species (0.07 oz).

The 23 cm (9.1 in) giant hummingbird, weighing 18–24 g (0.63–0.85 oz), is the biggest hummingbird species. They are adapted for nectar feeding. However, all species eat insects and spiders.

Around 42 million years ago, hummingbirds diverged from their sister species, the swifts, and treeswifts. Extant hummingbirds’ common ancestor is thought to have lived 22 million years ago. Hummingbirds get their name from the humming sound made by their beating wings, which flutter at high frequencies that humans can hear.

They hover in mid-air with fast wing-flapping speeds that range from approximately 12 beats per second in the biggest species to over 80 in the tiniest. Top speeds of those species observed in wind tunnels surpass 15 m/s (54 km/h; 34 mph), with some species diving at speeds above 23 m/s (83 km/h; 51 mph).

The weak song of most male hummingbirds is enough to attract a female. Males in a few species go on elaborate aerial displays. The tiny cup-shaped nest is generally adorned with moss or lichen and fastened to the thin branches of a tree or bush by webbing. However, a few species place their nest beneath a big leaf, such as a palm frond. The female constructs the nest, incubates the clutch of two white eggs and feeds the nestlings in all species.

Any homeothermic animal with the greatest mass-specific metabolic rate is a hummingbird. They may go into torpor, a state akin to hibernation, and decrease their metabolic rate to 1/15 of their usual pace to preserve energy when food is limited, and they are not hunting at night.

A total of 360 hummingbird species have been identified. The hermits (subfamily Phaethornithinae) and the normal hummingbirds have historically been split into two subfamilies (subfamily Trochilinae, all the others). However, molecular phylogenetic investigations have revealed that the hermits are related to the topazes, rendering the Trochilinae non-monophyletic as previously defined.

The topazes and jacobins, the hermits, the mangoes, the coquettes, the brilliants, the gigantic hummingbird (Patagona gigas), the mountaingems, the bees, and the emeralds are the nine main clades of hummingbirds. With the remainder of the hummingbirds, the topazes and jacobins have the oldest separation. Hummingbirds have the third-largest number of species among all bird families (after the tyrant flycatchers and the tanagers).

Hummingbird fossils have been discovered in the Pleistocene of Brazil and the Bahamas; however, neither has been formally documented, and only a few current species have been identified. Older fossils could not be positively identified as hummingbirds until recently.

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