Unique among avians, hummingbirds can be thought of as nature’s little hoverboards—except, of course, they don't explode into flames. Their fighter-jet acrobatics are as dazzling as their plumage, and they’re the only birds that can fly sideways and backwards. But the same physics that sets them apart also exacts a hefty price. As the world's smallest birds, hummingbirds have a high surface-area-to-volume ratio. That means they lose a lot of heat through their skin (the same way, for example, small ice cubes melt faster than big ones). The problem is compounded by a lack of downy feathers, the fluffy insulation that keeps most birds warm. Skipping the down shaves weight, but the birds must compensate by refueling constantly, consuming two or three times their body weight every day. That's why hummingbirds love nectar—energetically speaking, it's like rocket fuel. They will visit multiple flowers in a minute, lapping up three to seven calories daily. That...