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In-Depth Information
which is all well and good if the air moves across the wing at a sufficient
speed to create higher pressure on its underside and lower pressure on the
upper surface, thereby creating lift. When the air speed is too low, powered
flight (using wing beats) is the solution. Gliding is undertaken by all birds to
a lesser or greater extent and is particularly useful during the approach to
landing as the bird adjusts its weight and assesses its distance from its final
landing site. Gliding is often interspersed between periods of powered flight
and used in order to save energy. This is most evident when birds fly in flocks.
Soaring
Soaring is perhaps the most efficient way of flying. Depending on the
thermals from the earth requires little more than slight shifts in the
wing to catch the updraft. This requires large surface areas on the wing.
The length of wing on a gliding bird gives some indication of the ratio
of surface area of body to wing. Using the lift from thermals, birds are
capable of achieving altitudes of 18,000 feet and more. The greatest
demand on birds using soaring as a mode of flight is during takeoff, but
once the bird is up, if the lift from thermals can be maintained, very little
energy is required.
Over land, thermals are generated by the nature of the geography. Rocky
outcrops warmed by the sun's rays are often a strong source of lift. At sea the
thermals are far less strong and are created by the waves deflecting wind
upward. Although minimal, this deflected air movement enables the albatross
to remain airborne with hardly a wing beat. It is often necessary for it to fly
very close to the waves to catch these very slight updrafts.
Hovering
Hovering is maintained by striking a fine balance between lift and
gravitational forces. Birds need to create just enough air movement across the
wings and the position in the air will be maintained, though how this is done
may vary. Hummingbirds maintain a hover through very high-speed powered
flight. An animal's energy output per kilogram of body mass decreases as
the body size goes up, and the energy demand is greater for hovering than
for forward motion. This puts size limits on animals that hover. Small animals
such as insects and the smaller of the hummingbirds can hover for extended
periods of time, small birds can hover for shorter periods, medium-sized
birds may hover very briefly, and large birds are totally incapable of hovering.
Smaller wings are easier to flap, requiring less energy, though the smaller the
wing becomes, the faster it must move to create the necessary downward
thrust. The fastest wing beat of any bird is achieved by the purple-collared
woodstar with a wingspan of around 2 inches at 75 beats a second.
The hummingbird's very particular way of flying by moving its wings at very
high speed also places limitations on the size that such birds can achieve; the
largest is around the size of a European blackbird. Birds larger than this are
incapable of moving their wings at the speeds required for this type of hovering.
