Which figure of speech is used by the author to describe the humming bird?

Directions: Read the passage and answer the following question:

South America is the home to some of the strangest, some of the loveliest and some of the most horrifying animals in the world. There can be few creatures more improbable than the sloth which spends its life in a permanent state of mute slow motion, hanging upside down in the tall forest trees: few more bizarre than the giant ant eater of the Savannahs with its absurdly disproportionate anatomy, its tail enlarged into a shaggy banner and its jaws elongated into a curved and toothless tube. On the other hand, beautiful birds are so common as to become almost unremarkable. Gaudy macaws flap through the forest, their splendid plumage contrasting incongruously with their harsh cries; and humming-birds, like tiny jewels, flit from flower to flower sipping nectar, their iridescent feathers flashing the colours of the rainbow as they fly.
Many of the South American animals inspire the fascination which comes from revulsion. Shoals of cannibal fish infest the rivers waiting to rip the flesh from any animal which tumbles among them, and vampire bats, a legend in Europe but a grim reality in South America, fly out at night from their roosts in the forests to suck blood from cows and men.