One reason hummingbirds have such high metabolic rates is because they are so small. It hinges on the relationship between body size and surface area. As the size of an animal increases its relative surface area becomes smaller. And as animals become smaller their relative surface area gets
|Two equally warm cubeys. Which needs to eat relatively more?|
This means that a small, warm-blooded animal will have to generate more heat per unit of time than a larger, warm-blooded animal. To do this it will have to eat relatively more for its size than the larger animal. Thus the hummingbird consumes more than its weight of nectar in a day while we get by with just a small fraction of our body weight. Notice that it is relative amounts that are involved. The hummingbird only weighs 2.5 grams, but consumes 3 or more grams of nectar per day. We obviously eat more in absolute weight per day, but far, far less than a hummingbird relative to our body weight.
This surface area/body size relationship also explains why you need to bundle up your babies and small children even when the weather is mild. A tiny infant or child will chill much faster than an adult because it loses heat more rapidly in relation to its body size than an adult would.