In a word, they fast. 

But
don’t they have to eat continuously to prevent starvation? And if the night is more
than a couple of hours long wouldn’t they starve? 

Yes, that’s true, but they have a
clever trick they can use to make their energy supplies last the entire night.
They allow their metabolism to slow and their body temperature to drop, a
condition called torpor. It’s like hibernation, but it only lasts overnight instead of over
several months. When daylight approaches they spontaneously warm up by
shivering their flight muscles. When they reach their normal operating
temperature they take off in search of food.

Twenty-five years ago I was
fortunate to spend a couple of days banding hummingbirds under the supervision
of the late Bill Calder. Bill had set up mist nets (nets with a mesh so fine
that it is not noticeable to birds) in a flower-filled, montane meadow at ~9000
ft. elevation in the Elk mountains of western Colorado. We arrived to set up
the nets early in morning, just as the sky was lightening but before the sun
was up. As we walked through the meadow we could hear the buzzing flight sounds
of numerous hummingbirds, already actively foraging for nectar in the very chilly early morning mountain air. 

As
soon as we deployed the nets we started to hear the distress calls of
hummingbirds trapped in the mesh. With our head lamps on we rushed to find the
captured hummers and gently extricated the birds from the mesh. We then placed them in bags
made from Bill’s wife’s nylon hosiery, and took them to Bill who weighed
them, checked them for bands and, if they were unbanded, clamped a tiny,
numbered band around one leg. They were then released. From capture to release
took at most five to ten minutes. We worked continuously for several hours
until the rate of capture fell to zero. By an hour or two after sunrise the
ravenous hummers had drunk their fill and could slip into their daily routine
patrolling their trap lines and stocking up on enough nectar to last them
through the next night.

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?

larger. If this is not intuitive, imagine two
animals shaped like cubes. Call them “cubeys.” Look at the small
cubey on the left and compare it to the larger cubey. The larger cubey is twice
as big, if you measure bigness as height or length. Notice how many cubes make
up the larger cubey. There are 8 of them. Also notice that only 3 of the faces
of the little cubes in the larger cubey are exposed to the external surface,
but 6 faces are exposed in the smaller cubey. This means that the smaller cubey
loses heat twice as fast as the larger cubey because it has relatively more
surface area for its size than the larger cubey. The same holds true for real
animals. Their shapes may be different but the relationship between surface
area and volume (weight) holds true: larger animals have relatively less surface
area than smaller one. 

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.

Hummingbirds drink more than
their weight in nectar each day. This requires hundreds of visits to flowers.
If they are deprived of nectar for just a few hours they are in danger of
starving. This high rate of consumption is required because they have the
highest recorded metabolic rate of any vertebrate, both while resting and
during activity. This high rate of metabolism is due to several factors: their
massive flight muscles, which make up 30% of their body weight and their tiny
size, which causes them lose heat more rapidly than a larger bird does.

Hummingbirds stake out a
group of flowers that are producing nectar and then defend them,
trying to prevent other hummingbirds from taking “their” nectar. The
flowers they defend are like a trap line. The “owner” checks each
blossom in a sequence, taking nectar when it is found and then moving on to the
next flower in line. This behavior is called “traplining,” in
reference to how humans hunt beaver and other fur-bearing animals by setting out a
large number of traps and them checking them in sequence at periodic intervals.
Hummingbirds can learn how long they have to wait to get more nectar from the
same kind of flower. So after emptying one blossom they visit others and don’t
return until they first has had enough time to replenish its supply of nectar.

Plants use the energy in
sunlight to make sugar from carbon dioxide and water — the process that’s
called photosynthesis. Most of the sugar is made in the leaves, the plant organ
that is specialized to gather sunlight. From the leaves this sugar travels through
the plant’s conducting tissues to the other parts of the plant — the roots,
stems and flowers. These plant parts then remove the sugar from the conductive
tissues and use it to fuel all their metabolic processes. In the flowers there are specialized cells, usually found at the base of
the flower, around the ovary. These cells secrete the sugar into a sweet droplet of fluid that we call nectar. That’s what you taste when you
pick a honeysuckle blossom and suck on it. And that sugary solutions is what the hummingbird as
well as many insects are after. The flower produces nectar at a rate that
varies with the temperature and time of day, and the nectar will accumulate if no bird or insect visits visits the flower. So the amount of nectar present in the flower depends on how
rapidly it is produced and how often it is removed by hummingbirds or bees. When
most of the nectar is removed it takes a while for the supply to be
replenished.

Some birds, like hawks that are searching for prey, can
hover to a limited extent, but it is very awkward and they are unable to fly forward or backward while hovering. No other kind of bird can hover like a hummingbird, staying motionless (except for their moving wings) in a single spot and then flying forwards and/or backwards. This ability is
due, in part, to the shoulder joint. It is a ball-and-socket joint that only
one other type of bird has: the swifts, that are closely related to the hummingbirds. This shoulder joint
allows the wing to rotate 180 degrees and is the secret to the hummingbird’s
hovering ability. The wing can supply lift not only during the forward stroke,
but also on the back stroke, by flipping over. To see how this works hold your
arms straight out to your sides, palm facing down. Now tilt your hands up a
little (about 10 – 15 degrees above horizontal). Your hands should be tilted so that the thumb side is higher than the little finger side. Now move your arms forward, still keeping them straight. This is the same
motion you use when you are treading water. To continue treading water you
would rotate your hands downward so that the thumb side was lower than the little finger
side and then sweep your arms back. This makes the little finger side of your hand the leading edge on the backstroke. A hovering hummingbird is like a person
treading water, but it is “treading air.” And it doesn’t tilt its
hand like you did on the backstroke. Instead it rotates its entire wing
counter-clockwise as it begins its back stroke. It’s as if when treading water
you, instead of simply tilting your hand in a different direction, you actually
turned your arm over counter-clockwise, so that the thumb side was always the
leading edge. It is the ball-and-socket shoulder joint that permits the wing to
flip like this. Birds without that type of joint can’t flip their wings over so
they can’t generate the lift necessary on the backstroke to maintain their position in the air —
they can’t hover.