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.
