Which is the smallest bird in the world ?

Hummingbirds are New World birds that constitute the family Trochilidae. They are among the smallest of birds, most species measuring in the 7.5–13 cm (3–5 in) range. Indeed, the smallest extant bird species is a hummingbird, the 5-cm bee hummingbird, weighing less than a U.S. penny (2.5 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 in mid-air at rapid wing-flapping rates, typically around 50 times per second, allowing them also to fly at speeds exceeding 15 m/s (54 km/h; 34 mph),backwards.

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 to 1/15th of its normal rate.

Evolution
A map of the hummingbird family tree—reconstructed from analysis of 284 of the world's 338 known species—shows rapid diversification from 22 million years ago. Hummingbirds fall into nine main clades, the Topazes, Hermits, Mangoes, Brilliants, Coquettes, Patagona, Mountain Gems, Bees, and Emeralds, defining their relationship to nectar-bearing flowering plants and the birds' continued spread into new geographic areas. While all hummingbirds depend on flower nectar to fuel their high metabolisms and hovering flight, coordinated changes in flower and bill shape stimulated the formation of new species of hummingbirds and plants. Due to this exceptional evolutionary pattern, as many as 140 hummingbird species are able to coexist in a specific region, such as the Andes Mountains.The hummingbird evolutionary tree shows ancestral hummingbirds splitting from insectivorous swifts (family Apodidae) and treeswifts (family Hemiprocnidae) about 42 million years ago, probably in Eurasia.One key evolutionary factor appears to be an altered taste receptor that enabled hummingbirds to seek nectar. By 22 million years ago, the ancestral species of current hummingbirds became established in South America, where environmental conditions stimulated further diversification.The Andes Mountains appear to be a particularly rich environment for hummingbird evolution because diversification occurred simultaneously with mountain uplift over the past 10 million years.[5] Hummingbirds remain in dynamic diversification inhabiting ecological regions across South America, North America, and the Caribbean, indicating an enlarging evolutionary radiation. Within the same geographic region, hummingbird clades co-evolved with nectar-bearing plant clades, affecting mechanisms of pollination. The same is true for the sword-billed hummingbird (Ensifera ensifera), one of the morphologically most extreme species, and one of its main food plant clades (Passiflora section Tacsonia).
Sexual dimorphisms
Hummingbirds exhibit sexual size dimorphism, with males being smaller than females.The extent of this sexual size difference varies among clades of hummingbirds.Sexual dimorphisms in beak size and shape are also present between male and female hummingbirds,[14] where in many clades, females have longer, more curved beaks favored for accessing nectar from tall flowers.