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.