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Bird flight
From Wikipedia, the free encyclopedia
Flight is the main mode of locomotion used
by most of the world's bird species. It is important to birds for
feeding, breeding and avoiding predators.
An African
Hawk Eagle in flight
Evolution
and purpose of bird flight
The origin of bird flight
is still somewhat unclear, even though most paleontologists agree that birds evolved
from small theropod dinosaurs. It seems likely that they evolved from species
that lived on the ground, with flight developing after the evolution of feathers.
It seems likely in this case that flight evolved as a result of benefits in the
pursuit of small airborne prey items (such as insects), possibly subsequently
becoming useful as a predator-avoiding behavior.
Flight is more
energetically expensive in larger birds, and many of the largest species fly by
soaring and gliding (without flapping their wings) most of the time. Many
physiological adaptations have evolved that make flight more efficient.
Today birds use flight for
many purposes. It is still used by some species to obtain prey on the wing in
addition to foraging, to commute to feeding grounds, and to migrate between the
seasons. Flight's importance in avoiding predators (and its extreme demand for
energy) can be shown in the frequency with which it is lost when birds reach
isolated oceanic islands that lack ground-based predators. It is also used by
some species to display during the breeding season and to reach safe isolated
places for nesting.
Basic
mechanics of bird flight
The lift
force has both a forward and a vertical component.
The fundamentals of bird
flight are similar to those of aircraft. Lift force is produced by the action
of air flow on the wing, which is an airfoil/aerofoil. The lift force occurs
because the air has a lower pressure just above the wing and higher pressure
below.
When gliding, both birds
and gliders obtain both a vertical and a forward force from their wings. This
is possible because the lift force is generated at right angles to the air
flow, which in level flight comes from slightly below the wing. The lift force
therefore has a forward component. (Weight always acts vertically downwards and
so cannot provide a forward force. Without a forward component, a gliding bird
would merely descend vertically, exactly as a parachute does).
Forces
acting on a wing
When a bird flaps, as
opposed to gliding, its wings continue to develop lift as before, but they also
create an additional forward and upward force, thrust, to counteract its weight
and drag. Flapping involves two stages: the down-stroke, which provides the
majority of the thrust, and the up-stroke, which can also (depending on the
bird's wings) provide some upward force. At each up-stroke the wing is slightly
folded inwards to reduce upward resistance. Birds change the angle of attack
between the up-strokes and the down-strokes of their wings. During the
down-stroke the angle of attack is increased, and is decreased during the
up-stroke.
There are three major
forces that impede a bird's aerial flight: frictional drag (caused by the
friction of air and body surfaces), form drag (due to frontal area of the bird,
also known as pressure drag), and lift-induced drag (caused by the wingtip
vortices).
The
wing
The bird's forelimbs, the wings,
are the key to bird flight. Each wing has a central vane to hit the wind,
composed of three limb bones, the humerus, ulna and radius. The hand, or manus,
which ancestrally was composed of five digits, is reduced to three digits
(digit II, III and IV), the purpose of which is to serve as an anchor for the
primaries (or metacarpo-digitals), one of two groups of feathers responsible
for the airfoil shape. The other set of flight feathers that are behind the
carpal joint on the ulna, are called the secondaries or cubitals. The remaining
feathers on the wing are known as coverts, of which there are three sets. The
wing sometimes has vestigial claws, in most species these are lost by the time
the bird is adult (such as the highly visible ones used for active climbing by Hoatzin
chicks), but claws are retained into adulthood by the Secretary Bird, the screamers,
finfoot,ostriches, several swifts and numerous others, as a local trait, in a
few specimens. The claws of the Jurassic therapod-like archaeopteryx are quite
similar to those of the hoatzin nestlings.
Wing
shape and flight
A male
mallard mid-flight, demonstrating that wings don't necessarily move in unison
The shape of the wing is an
important factor in determining the types of flight of which the bird is
capable. Different shapes correspond to different trade-offs between beneficial
characteristics, such as speed, low energy use, and maneuverability. The planform
of the wing (the shape of the wing as seen from below) can be described in
terms of two parameters, aspect ratio and wing loading. Aspect ratio is the
ratio of wing breadth to the mean of its chord, or mean wingspan divided by
wing area. Wing loading is the ratio of weight to wing area.
Most kinds of bird wing can
be grouped into four types, with some falling between two of these types. These
types of wings are elliptical wings, high speed wings, high aspect ratio wings
and soaring wings with slots.
Elliptical
wings
Elliptical wings are short
and rounded, having a low aspect ratio, allowing for tight maneuvering in
confined spaces such as might be found in dense vegetation. As such they are
common in forest raptors (such as Accipiter hawks), and many passerines,
particularly non-migratory ones (migratory species have longer wings). They are
also common in species that use a rapid take off to evade predators, such as pheasants
and partridges.
High
speed wings
High speed wings are short,
pointed wings that when combined with a heavy wing loading and rapid wingbeats
provide an energetically expensive high speed. This type of flight is used by the
bird with the fastest wing speed, the peregrine falcon, as well as by most of
the ducks. The same wing shape is used by the auks for a different purpose;
auks use their wings to "fly" underwater.
Soaring
wings with deep slots
These are the wings favored
by the larger species of inland birds, such as eagles, vultures, pelicans, and storks.
The slots at the end of the wings, between the primaries, reduce the turbulence
at the tips, whilst the shorter size of the wings aids in takeoff (high aspect
ratio wings require a long taxi in order to get airborne).
Hovering
Hovering is a demanding but
useful ability used by several species of birds (and specialized in by one
family). Hovering, literally generating lift through flapping alone rather than
as a product of thrust, demands a lot of energy. This means that it is confined
to smaller birds; the largest bird able to truly hover is the pied kingfisher,
although larger birds can hover for small periods of time. Larger birds that
hover do so by flying into a headwind, allowing them to utilize thrust to fly
slowly but remain stationary to the ground (or water). Kestrels, terns and even
hawks use this windhovering.
The ruby-throated
Hummingbird can beat its wings 52 times a second
Most birds that hover have
high aspect ratio wings that are suited to low speed flying. One major
exception to this are the hummingbirds, which are among the most accomplished
hoverers of all the birds. Hummingbird flight is different to other bird flight
in that the wing is extended throughout the whole stroke, the stroke being a
symmetrical figure of eight, with the wing being an airfoil in both the up- and
down-stroke. Some hummingbirds can beat their wings 52 times a second, though
others do so less frequently.
Take-off
and landing
Take-off can be one of the
most energetically demanding aspects of flight, as the bird needs to generate
enough airflow under the wing to create lift. In small birds a jump up will
suffice, while for larger birds this is simply not possible. In this situation,
birds need to take a run up in order to generate the airflow to take off. Large
birds often simplify take off by facing into the wind, and, if they can,
perching on a branch or cliff so that all they need to do is drop off into the
air.
Landing is also a problem
for many large birds with high airspeeds. This problem is dealt with in some
species by aiming for a point below the intended landing area (such as a nest
on a cliff) then pulling up beforehand. If timed correctly, the airspeed once
the target is reached is virtually nil. Landing on water is simpler, and the
larger waterfowl species prefer to do so whenever possible.
Adaptations
for flight
The most obvious adaptation
to flight is the wing, but because flight is so energetically demanding birds
have evolved several other adaptations to improve efficiency when flying. The bird
skeleton is hollow to reduce weight, and many unnecessary bones have been lost
(such as the bony tail of the early bird Archaeopteryx), along with the
toothed jaw of early birds, which has been replaced with a lightweight beak. The
skeleton's breastbone has also adapted into a large keel, suitable for the
attachment of large, powerful flight muscles. The vanes of the feathers have
hooklets called barbules that zip them together, giving the feathers the
strength needed to hold the airfoil (these are often lost in flightless birds).
The large amounts of energy
required for flight have led to the evolution of a unidirectional pulmonary
system to provide the large quantities of oxygen required for their high respiration
rates. This high metabolic rate produces large quantities of radicals in the
cells that can damage DNA and lead to tumours. Birds, however, do not suffer
from an otherwise expected shortened lifespan as their cells have evolved a
more efficient antioxidant system than those found in other animals.
Wikipedia
http://en.wikipedia.org/w/index.php?title=Bird_flight&action=history
Flight is the main mode of locomotion used
by most of the world's bird species. It is important to birds for
feeding, breeding and avoiding predators.
An African
Hawk Eagle in flight
Evolution
and purpose of bird flight
The origin of bird flight
is still somewhat unclear, even though most paleontologists agree that birds evolved
from small theropod dinosaurs. It seems likely that they evolved from species
that lived on the ground, with flight developing after the evolution of feathers.
It seems likely in this case that flight evolved as a result of benefits in the
pursuit of small airborne prey items (such as insects), possibly subsequently
becoming useful as a predator-avoiding behavior.
Flight is more
energetically expensive in larger birds, and many of the largest species fly by
soaring and gliding (without flapping their wings) most of the time. Many
physiological adaptations have evolved that make flight more efficient.
Today birds use flight for
many purposes. It is still used by some species to obtain prey on the wing in
addition to foraging, to commute to feeding grounds, and to migrate between the
seasons. Flight's importance in avoiding predators (and its extreme demand for
energy) can be shown in the frequency with which it is lost when birds reach
isolated oceanic islands that lack ground-based predators. It is also used by
some species to display during the breeding season and to reach safe isolated
places for nesting.
Basic
mechanics of bird flight
The lift
force has both a forward and a vertical component.
The fundamentals of bird
flight are similar to those of aircraft. Lift force is produced by the action
of air flow on the wing, which is an airfoil/aerofoil. The lift force occurs
because the air has a lower pressure just above the wing and higher pressure
below.
When gliding, both birds
and gliders obtain both a vertical and a forward force from their wings. This
is possible because the lift force is generated at right angles to the air
flow, which in level flight comes from slightly below the wing. The lift force
therefore has a forward component. (Weight always acts vertically downwards and
so cannot provide a forward force. Without a forward component, a gliding bird
would merely descend vertically, exactly as a parachute does).
Forces
acting on a wing
When a bird flaps, as
opposed to gliding, its wings continue to develop lift as before, but they also
create an additional forward and upward force, thrust, to counteract its weight
and drag. Flapping involves two stages: the down-stroke, which provides the
majority of the thrust, and the up-stroke, which can also (depending on the
bird's wings) provide some upward force. At each up-stroke the wing is slightly
folded inwards to reduce upward resistance. Birds change the angle of attack
between the up-strokes and the down-strokes of their wings. During the
down-stroke the angle of attack is increased, and is decreased during the
up-stroke.
There are three major
forces that impede a bird's aerial flight: frictional drag (caused by the
friction of air and body surfaces), form drag (due to frontal area of the bird,
also known as pressure drag), and lift-induced drag (caused by the wingtip
vortices).
The
wing
The bird's forelimbs, the wings,
are the key to bird flight. Each wing has a central vane to hit the wind,
composed of three limb bones, the humerus, ulna and radius. The hand, or manus,
which ancestrally was composed of five digits, is reduced to three digits
(digit II, III and IV), the purpose of which is to serve as an anchor for the
primaries (or metacarpo-digitals), one of two groups of feathers responsible
for the airfoil shape. The other set of flight feathers that are behind the
carpal joint on the ulna, are called the secondaries or cubitals. The remaining
feathers on the wing are known as coverts, of which there are three sets. The
wing sometimes has vestigial claws, in most species these are lost by the time
the bird is adult (such as the highly visible ones used for active climbing by Hoatzin
chicks), but claws are retained into adulthood by the Secretary Bird, the screamers,
finfoot,ostriches, several swifts and numerous others, as a local trait, in a
few specimens. The claws of the Jurassic therapod-like archaeopteryx are quite
similar to those of the hoatzin nestlings.
Wing
shape and flight
A male
mallard mid-flight, demonstrating that wings don't necessarily move in unison
The shape of the wing is an
important factor in determining the types of flight of which the bird is
capable. Different shapes correspond to different trade-offs between beneficial
characteristics, such as speed, low energy use, and maneuverability. The planform
of the wing (the shape of the wing as seen from below) can be described in
terms of two parameters, aspect ratio and wing loading. Aspect ratio is the
ratio of wing breadth to the mean of its chord, or mean wingspan divided by
wing area. Wing loading is the ratio of weight to wing area.
Most kinds of bird wing can
be grouped into four types, with some falling between two of these types. These
types of wings are elliptical wings, high speed wings, high aspect ratio wings
and soaring wings with slots.
Elliptical
wings
Elliptical wings are short
and rounded, having a low aspect ratio, allowing for tight maneuvering in
confined spaces such as might be found in dense vegetation. As such they are
common in forest raptors (such as Accipiter hawks), and many passerines,
particularly non-migratory ones (migratory species have longer wings). They are
also common in species that use a rapid take off to evade predators, such as pheasants
and partridges.
High
speed wings
High speed wings are short,
pointed wings that when combined with a heavy wing loading and rapid wingbeats
provide an energetically expensive high speed. This type of flight is used by the
bird with the fastest wing speed, the peregrine falcon, as well as by most of
the ducks. The same wing shape is used by the auks for a different purpose;
auks use their wings to "fly" underwater.
Soaring
wings with deep slots
These are the wings favored
by the larger species of inland birds, such as eagles, vultures, pelicans, and storks.
The slots at the end of the wings, between the primaries, reduce the turbulence
at the tips, whilst the shorter size of the wings aids in takeoff (high aspect
ratio wings require a long taxi in order to get airborne).
Hovering
Hovering is a demanding but
useful ability used by several species of birds (and specialized in by one
family). Hovering, literally generating lift through flapping alone rather than
as a product of thrust, demands a lot of energy. This means that it is confined
to smaller birds; the largest bird able to truly hover is the pied kingfisher,
although larger birds can hover for small periods of time. Larger birds that
hover do so by flying into a headwind, allowing them to utilize thrust to fly
slowly but remain stationary to the ground (or water). Kestrels, terns and even
hawks use this windhovering.
The ruby-throated
Hummingbird can beat its wings 52 times a second
Most birds that hover have
high aspect ratio wings that are suited to low speed flying. One major
exception to this are the hummingbirds, which are among the most accomplished
hoverers of all the birds. Hummingbird flight is different to other bird flight
in that the wing is extended throughout the whole stroke, the stroke being a
symmetrical figure of eight, with the wing being an airfoil in both the up- and
down-stroke. Some hummingbirds can beat their wings 52 times a second, though
others do so less frequently.
Take-off
and landing
Take-off can be one of the
most energetically demanding aspects of flight, as the bird needs to generate
enough airflow under the wing to create lift. In small birds a jump up will
suffice, while for larger birds this is simply not possible. In this situation,
birds need to take a run up in order to generate the airflow to take off. Large
birds often simplify take off by facing into the wind, and, if they can,
perching on a branch or cliff so that all they need to do is drop off into the
air.
Landing is also a problem
for many large birds with high airspeeds. This problem is dealt with in some
species by aiming for a point below the intended landing area (such as a nest
on a cliff) then pulling up beforehand. If timed correctly, the airspeed once
the target is reached is virtually nil. Landing on water is simpler, and the
larger waterfowl species prefer to do so whenever possible.
Adaptations
for flight
The most obvious adaptation
to flight is the wing, but because flight is so energetically demanding birds
have evolved several other adaptations to improve efficiency when flying. The bird
skeleton is hollow to reduce weight, and many unnecessary bones have been lost
(such as the bony tail of the early bird Archaeopteryx), along with the
toothed jaw of early birds, which has been replaced with a lightweight beak. The
skeleton's breastbone has also adapted into a large keel, suitable for the
attachment of large, powerful flight muscles. The vanes of the feathers have
hooklets called barbules that zip them together, giving the feathers the
strength needed to hold the airfoil (these are often lost in flightless birds).
The large amounts of energy
required for flight have led to the evolution of a unidirectional pulmonary
system to provide the large quantities of oxygen required for their high respiration
rates. This high metabolic rate produces large quantities of radicals in the
cells that can damage DNA and lead to tumours. Birds, however, do not suffer
from an otherwise expected shortened lifespan as their cells have evolved a
more efficient antioxidant system than those found in other animals.
Wikipedia
http://en.wikipedia.org/w/index.php?title=Bird_flight&action=history