http://www.scienceperspectives.com/h...a-nutshell.htm
Hummingbirds are among the smallest of vertebrates and have the highest metabolism of all animals (about 30 times the human metabolic rate). This high metabolism (converting food energy to energy the body uses) is due mostly to their small size. The cells of warm-blooded animals produce heat during metabolism. A large animal, such as an elephant, has so many cells inside its body producing heat that the problem becomes getting rid of excess heat. With a small animal, however, heat is lost much more quickly to the environment and the problem becomes getting enough energy to stay warm. In fact, a warm-blooded animal smaller than a hummingbird (or shrew--they are about the same size) could not exist because it could not take in food fast enough to keep itself warm. A hummingbird deprived of food would die within a few hours!
Hummingbirds have evolved a unique method of foraging--hovering. This ability allows hummingbirds to find a food source not available to other birds. Wildflowers typically grow too high off the ground for a standing bird to reach and are too flimsy to support a perching bird.
Hummingbirds get energy by feeding on the sugar-rich nectar of wildflowers, consuming three times their own weight in nectar daily. They also eat a few insects during the day to provide protein and other nutrients.
Conserving Energy.
Because they lose heat so rapidly, hummingbirds are often not able to maintain normal body temperatures during the night. To conserve energy they shut down and become "torpid." Torpidity is a sort of temporary hibernation in which the hummingbird's temperature can drop from the normal 104 F to a mere 54 F and the metabolic rate decreases significantly. Also, studies have shown that the patterns hummingbirds follow when feeding are very efficient. In fact, most hummingbird behavior can be explained in terms of increasing energy efficiency. Although I like to think that hummingbirds will sit on my daughters' fingers while drinking at the feeder because the birds are able to recognize my daughters' kind intentions, I suspect they recognize the opportunity to rest while feeding and know they could escape instantly if they felt threatened in any way.
A Hummingbird Paradox?
If hummingbirds need so much energy, why do they spend so much time perched doing nothing? This question plagued scientists for some time. If you spent a day following a hummingbird (good luck keeping up!) you would find it spent about 75% of its day perching, and only 20% of its day feeding. A group a four scientists (science is seldom an individual effort) led by Jared Diamond discovered the answer. During the perching time, hummingbirds are digesting nectar as rapidly as possible and have nothing to gain by feeding until their storage crops are at least half-empty (Do most people fill up their cars with gas when the gauge reads three-fourths full or wait until it's less than half?). It turns out that with the typical pattern of feed for one minute, perch for three minutes, hummingbirds are taking in energy as fast as their digestive system allows.