Energy storage keeps a hummingbird from starving, but not for long. The energy stored by the end of a day usually is just sufficient to survive overnight. Fortunately, hummingbirds, like hibernating mammals, can lower their body temperature overnight to conserve energy. However, we found that hummingbirds do not lower their body temperature unless there is a danger they actually may starve. Even with their abilities to save some energy and to conserve energy in an extreme crisis, the impression is that small hummingbirds face big problems because they must eat often.
Hummingbirds spend most of their feeding time visiting flowers to eat nectar.
To the ancient Greeks, nectar was the drink of the gods, thus you might think nectar is pretty special. Actually flower nectar is a simple fluid composed mainly of water and sugar. Anyone who has sucked a honeysuckle or petunia blossom can testify to nectar's sweetness. None of the nectars contained only glucose or only fructose. When we gave hummingbirds a choice between feeders containing sucrose and those with only glucose or only fructose, they preferred the sucrose.