Birds fly in a variety of ways, ranging from
gliding to soaring to flapping flight to
hovering. Of these, the simplest type of flight
A gliding bird uses its weight (mass) to overcome
air resistance to its forward motion. To do this
effectively, of course, requires a certain mass
and, as a result, only large birds, such as
vultures, glide on a regular basis. When gliding,
a bird loses altitude at some 'sinking speed'
(Vs) while traveling forward at some 'flight
speed' (V). A bird's glide ratio equals V/Vs (the
distance traveled forward per unit of altitude
lost). Some of the best 'bird gliders' (such as
Black Vultures) may travel up to 20 meters for
every meter of altitude lost (or, a glide ratio
A soaring bird (e.g., Turkey Vultures) maintains
or increases its altitude without flapping its
wings. One way to do this is to take advantage of
Updrafts are generated when a steady wind strikes
a hill, cliff, or building, & this is
referred to as obstruction lift.
Thermals, or updrafts caused by the uneven
heating of air near the earth's surface. Air over
fields heats faster than air over a forest or
lake. The warmer air over a field is lighter than
the surrounding cooler air and, therefore, rises.
However, at high altitudes the warm air begins to
cool & sink. As a result, birds using
thermals for lift typically fly in circles (to
stay in the area of rising air).
Over the open ocean, large birds like the
Wandering Albatross take advantage of wind
velocity gradients in a type of soaring called
Of course, most birds flap their wings when they
fly. Flapping flight involves up-and-down
movement of the wings and, during such flight,
different parts of a wing have different
* the proximal part of the wing (basically the
half closest to the body) moves less &
provides most of the lift
* the distal part of the wing moves through a
wide arc and generates most of the thrust that
propels a bird forward.
During the downstroke (power stroke), a wing
moves downward & forward. As a result, the
trailing edge of the wing bends upward (due to
the air pressure) and this transforms the wing
into a 'propellor' & moves the bird forward.
During the upstroke (recovery stroke), the tips
of the primaries separate & these 'slots'
allow passage of air through them (which reduces
friction as the wing comes up). Also, the wing is
partially folded at the wrist & elbow and
drawn in toward the body to reduce drag.
Most species of birds do not flap their wings
continuously during flight. Rather, they exhibit
one of two intermittent flight patterns:
flap-gliding and flap-bounding. Mathematical
models predict that flap-bounding is
energetically cheaper than continuous flapping
flight at high speeds, while flap-gliding is more
efficient than continuous flapping at low speeds.
However, few species of bird exhibit both types
of intermittent flight, so flap-bounding may be a
compromise between the need to maintain muscle
contractions at an optimal velocity and the need
to vary power output and flight speed. In
addition, the primary flight muscle, the
pectoralis, of many small birds is composed of a
single muscle fiber type, further limiting the
range of useful strain rates for these species.
Thus, a "fixed-gear hypothesis"
suggests that the only economical method for
small birds to vary power output is to
intermittently bound. However, investigators at
the Flight Lab at the University
of Montana have found that some small birds,
such as Budgerigars and European Starlings, do
exhibit both types of intermittent flight, with
flap-gliding being used at lower speeds, and
flap-bounding at higher speeds. This suggests
that some small birds are capable of optimizing
their flight styles despite the theoretical
constraints of their muscle composition.
As flight speed increased in a wind tunnel,
budgerigars that exhibited intermittent flight at
all speeds tended to flex their wings during
intermittent non-flapping periods, apparently in
response to increased profile drag.
A few birds using hovering flight. Some birds,
like American Kestrels, 'hover' or remain in
place by flying into the wind at a speed equal to
that of the wind, and other birds hover
momentarily while foraging. But hummingbirds are
able to remain in the same place in still air as
long as they wish -- they are true hoverers. A
hovering hummer keeps its body at about a 45
degree angle to the ground and moves its wings in
more or less a figure-eight pattern, with the
"eight" lying on its side.
Hummingbirds, unlike other birds, can also fly
Hovering is hard work for most birds - Ever seen
a songbird hover over a crowded feeding station,
waiting for a perch to open up so it can land and
eat? Looks like hard work, doesn't it? It is,
which is why hovering is something most birds
don't like to do -- or can't do -- for very long.
Kenneth P. Dial of the University of Montana and
colleague surgically implanted strain gauges in
the wings of three Black-billed Magpies. The
devices measured the force exerted by the main
flapping muscle with each wing beat. The birds
then flew in a wind tunnel at a range of speeds.
The strain gauge allowed the scientists to
calculate the power (the amount of work done per
unit time) required to maintain a given speed.
Hovering took nearly twice as much power as
flying at average speed, the researchers found.
Even when the magpies flew at top speed, they
expended far less power than they did when they
hovered. Evidence suggested that when they
hovered, the birds were working at their physical
limits. Their wing muscles appeared to be
employing anaerobic metabolism, a source of
energy that can't be sustained for long. There
are clearly exceptions to this. Hummingbirds, the
authors note, have an unusual shoulder design
that allows them to generate lift on both
down-beat and up-beat. But birds with a body
design similar to magpies are likely to have
strict limits on their abilities to fly standing
Some birds, like geese & cranes, are often
observed flying in V-formation. The reason is
wingtip vortices. The birds take advantage of the
upwind side of the vortex shedding off the bird
in front of them. This updraft actually lifts the
bird up, making the flight a little easier.
Air moves from the area of high pressure (under
the wing) to the area of low pressure (top of the
wing) at the wing tips. Birds flying in
V-formation use these vortices of rising air.
All birds have high metabolic rates, and flying
birds have even higher rates. The metabolic cost
of flight depends on the type of flight (gliding,
soaring, flapping, or hovering), wing shape, and
speed. Of course, flapping flight and hovering
are the most costly types of flight. Laboratory
studies of birds trained to fly in wind tunnels
indicate that the metabolic 'cost' of flapping
flight can be anywhere from about 7 to 15 times a
bird's basal metabolic rate.
Speed influences the cost of flight, with low
speed flight (such as when taking off or landing)
requiring more energy. Some information also
suggests that bird's flying at maximum speeds
also use more energy than at 'medium' speeds. Low
speed flight is more costly because there is more
drag (induced drag). This is true because air
flow past the wings is more turbulent at low
speeds. High speed flapping flight is more costly
because greater speed requires a higher rate of
Birds, of course, get around in ways other than
flying. In fact, some birds are flightless and
depend entirely on walking, running, or swimming
to get from place to place.
Some birds spend most of their time on or in
water. Birds have special adaptations of the
legs, feet, & wings for terrestrial and
aquatic (swimming and diving) locomotion.
Walking, running, hopping, & waddling
- birds that travel along the ground regularly
often have relatively long legs. Among the
ratites, such as Ostriches and Emus, there has
been a reduction in the number of toes (less
weight at end of the limb = more efficient
Climbing - birds that climb,
like woodpeckers and nuthatches, have sharply
recurved claws to help grip the substrate (e.g.,
bark of a tree).
Swimming - aquatic birds
* low specific gravity (lightweight so they are
* feathers with lots of barbules & hooklets
(less permeable to water)
* well-developed uropygial gland (secretions help
keep feathers in good condition)
* webbed feet that act like oars
Diving - birds that frequently
dive under water, such as grebes, cormorants,
& loons, have:
* relatively high specific gravities (heavier and
* feet located well back on the body to permit
better propulsion and maneuvering underwater
and/or smaller wings that permit 'flying'
underwater (e.g., scoters, petrels, and, of
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