How does a hummingbird survive the night?

By switching itself most of the way off. A hummingbird's metabolism is so extreme that it must feed every ten or fifteen minutes through the day and cannot possibly store enough to run through a night - so at dusk it enters torpor, letting its body temperature fall towards the air's, its heart rate drop from over a thousand beats a minute to perhaps fifty, and its breathing become intermittent enough to look dead.

A torpid hummingbird can be picked up without waking. It takes twenty minutes or more to come out of it, shivering violently to rebuild heat, which is a dangerous interval - and the bird has to gamble each evening that nothing will find it. Andean species at altitude go furthest, dropping body temperature to around three degrees Celsius, the lowest recorded for any bird.

It is a nightly version of hibernation rather than a mild doze, and the trade is explicit: torpor saves perhaps two thirds of the energy the night would otherwise cost, at the price of being defenceless and slow to start. Birds that have fed well enough go into it less deeply, which suggests the decision is made each evening on the state of the fuel tank rather than by the clock.