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Bird Feathers
The most noticeable feature of a bird is its feathers. Feathers
are perfectly designed or their function. They are light but very
strong, and they are flexible but very tough. Feathers do not
grow all over the bird.
Beaks and eyes have no feathers and most birds have featherless
legs and feet. Scavenger birds that feed on dead animals, such as
vultures, have very few or no feathers on their heads and necks
because they would get dirty when they bird ate. Also, ostriches
and the sacred ibis have featherless heads and necks because the
live in hot climates and need the bare skin to get rid of excess body
heat.
A bird's body appears to be covered with feathers, but not so.
Feathers grow only in certain areas called feather tracks. The
contour feathers overlap so they appear solid, but limiting the
number of feathers helps to keep down the body weight. In between
the feather tracks, the down feathers or the semiplumes grow. It
is interesting to note, that in flightless birds like the ostrich
and it's relatives and also in penguins, feathers grow all over
the body.
Why are feathers light?
There are several reasons. They are made
of a tough, flexible material called keratin. Since it is not
living, it doesn't need blood, nerve, or tissue supply. Feathers
also look solid, but they are not. The spine down the middle,
called the shaft, is hollow. Strengthening struts run across the
hollow core like steps on a ladder. The vanes, which are the two
halves of the feather that spread out from the shaft, are made of
thousands of branches called barbs. These angle toward the tip of
the feather. Each barb has tiny parallel branches called
barbules. Because a feather has all these tiny parts with spaces
in between, a feather has as much air as matter.
How do feathers grow?
Each feather grows from a papilla or bump
in the skin, which is surrounded by a pit of cells consisting of
several layers. Each papilla is supplied with a vein and an
artery. Growth begins with the tip of the feather. As an embryo
the tip grows out toward the skin and eventually breaks through.
When a bird hatches, the tip separates and appears as fuzz on the
baby bird. This soft down is not a real feather, but works as a
temporary covering. After hatching many changes take place in the
papilla which then begin to produce the real feathers. The thin
,outermost layer forms a tough, protective sheath. Inside this
sheath, the contour feather begins to grow from the pulp cells.
First the central quill is formed. As the tip continues to grow,
the downy fuzz is pushed ahead of it. Each feather is tightly
rolled inside the sheath. It is called a pin feather at this
stage because it appears long and pointed. The feather continues
to grow and the pulp, vein, and artery follow along inside the
quill to make the feather longer and broader. When the feather is
full grown, the sheath splits open, falls off, and the feather
uncurls. The vein and artery are sealed off in the papilla. The
feather dies and the pulp shrivels, leaving a hollow shaft. The
quill is stiff but springy and tightly anchored in its socket,
surrounded by skin and muscles. Months later, when the feather is
worn out, the process begins again.
Feathers and Bird Survival
A bird's survival depends upon the condition of its feathers.
Consequently, they spend a lot of time caring for their feathers,
a process called preening. Using their feet and beaks, they
arrange the feathers to keep them smooth. Wind and brushing
against things "unzips" the barbs, so they nibble each feather
from the base to the tip to realign and hook the barbules. Birds
also bathe frequently, because dirt adds weight. Most birds have
an oil gland at the base of their tail and frequent oiling keeps
the feathers water-resistant and shiny.
Eventually feathers wear out and need to be replaced. Molting,
the process of loosing old feathers and growing new ones, occurs
in most birds once or twice a year. Feathers are lost in a
specific order and at varying rates, depending upon the species.
Most take about two months to molt. Predatory birds molt slowly
because they need most of their feathers to fly to hunt. Some of
their flight feathers last two or three years. Other birds, like
penguins, loose all of their feathers in a two week period of
time, after the new ones have begun to grow. Water birds loose
all of their flight feathers at once. They must hide until the
new ones come in. During breeding season, most birds loose the
feathers on their lower breasts and thus take advantage of the
bare skin to warm their eggs.
Types of Bird Feathers
Birds have between 1,000 and 25,000 feathers, depending upon the
species. Obviously, larger birds have more feathers; the swan,
with its long neck, having the most. Feathers can be divided into
six categories. The contour feathers are the most abundant and
cover the outer surface of the bird, giving the smooth, sleek
profile so important to flight. All contour feathers have the
same basic structure, with modifications depending upon placement
and function. There is a central rachis, or shaft, which is
hollow. Inside, dried remains of the pulp form strengthening
struts which run crosswise like ladder rungs. The vanes are the
two halves of the feather that spread out from the shaft. They
are made up of hundreds of branches called barbs, angling toward
the tip of the feather. Each barb has tiny, parallel branches of
its own called barbules.
The vanes are smooth because of the structure of the barbs and
barbules. The barbules on the edges of the barbs facing the tip
of the feather, have microscopic hooks. The barbules on the side
facing the base of the feather have rounded ridges. The hooks on
the barbules of one barb catch on the ridges of the next barbule,
holding the barbs together in a zipper like fashion. It is in
this way that the solid looking, almost airtight surface of each
feather is formed.
As the feather bends and twists during flight, the barbule hooks
slide back and forth retaining the smooth but flexible shape.
However, a reverse twist from the tip to the base will separate
the barbs and make the feather ragged with spaces in between.
Preening "re-zips" the barbs and barbules. The barbs at the base
of a contour feather have no hooks and so they appear fluffy.
The second type of feather is the semiplume. It is shaped like a
contour feather, but its' shaft is not as stiff and its' barbs
have no hooks so are fluffy.
The third type is the down feather. It is fluffy like the
semiplume but has a very short shaft. Both the semiplume and the
down feathers are important for keeping the bird warm. Air
becomes trapped between the fluffy barbs and is held against the
skin by the overlying contour feathers, thereby, preventing loss
of body heat.
Some contour feathers combine the advantages of the down feathers
on a single feather. They have an afterfeather, which is a second
shaft coming from the main quill like a branch. It is fluffy. One
feather performs two functions: smooth covering as well as
insulation while decreasing overall weight.
The fourth feather type is called the filoplume. These are tiny
and delicate with only a few barbs on the tip. They are sparsely
scattered amoung the other feathers. Because there are nerve
endings at their base, scientists believe their purpose is to
sense the positions of the contour feathers, so the muscles in
the skin can keep them in place.
Bristles are stiff, hairlike feathers found only in some birds.
Their function is specific in each species. Bristles are found
around the mouths of birds who scoop insects out of the air.
There are several theories as to their function, but it is now
thought the bristles help keep the insects out of the bird's
eyes. Bristles are found covering the nostrils of woodpeckers,
presumably to help keep the wood dust out of their nose. In
ostriches bristles form eyelashes.
The final feather type is found only in a few birds. It is the
powder-down feather. This feather grows continually. The tip
breaks off forming a water resistant powder. The metallic sheen
of the heron is caused partly by this powder down.
Bird Wings
A bird's wing is the basic structure for flight. It is covered
with contour feathers that are specialized for flight. It is the
shape of the wing that enables a bird to fly, and the shape is
determined by the feathers.
The actual wing is a modified forelimb, with a skeletal structure
like an arm. It is broad and mostly feathers, with very little
skin and bone. The upper bone, closest to the body, is where the
flight muscles attach. The actual wing is V-shaped with two bones
in the next section. The outer portion of the V is made up of
long wrist and fused finger bones. Part of the thumb is present
as a projection called the aulula, which protrudes from the front
of the wing. This is very important during flight and will be
examined in more detail in the mechanics of flight.
Flight Feathers
The feathers on the wing are the flight feathers, specialized
contour feathers. They have an especially rigid shaft. The vanes
provide a light weight, broad surface that pushes on the air to
make flight possible. There are three kinds of flight feathers on
each wing. The primaries are attached to the outer part of the
wing, to the hand and finger bones. There are between nine and
twelve on each wing. The inner part of the wing has the
secondaries, Again the number varies by species: hummingbirds
have 6 of 7 and large birds have as many as 32. The tertiaries
are attached to the upper wing.
It is critical that air flows evenly around and over the wings
during flight so that friction and drag are kept at a minimum.
The surface of the wing is kept smooth by the overlapping
placement of the flight feathers. Each feather is also shaped so
the the side facing the wind is narrower and stiffer than the
trailing edge, which makes it stronger. In addition, along the
leading edge (front) of the wing, are smaller contour feathers,
called coverts, which cover the base the flight feathers. These
feathers give the wing the airfoil shape that make flight
possible.
Although the curved airfoil shape of the wing is necessary to
lift the bird into the air, feathers contribute much more during
flight. On the upstroke of the wing, the feathers tilt so that
air can pass between them, lessening drag. On the downstroke, the
larger vane of the trailing edge bends upward, pushing down the
leading edge. Each feather performs like a propeller with every
wing stroke. In addition, with every stroke, the wing is
positioned either slightly forward or backward, not just up and
down. The hummingbird can move his wings in a figure 8 pattern
which enables him to hover. Large, slow flying birds have
additional features on their flight feathers.
In order to prevent stalling, which occurs when the airspeed over
the wing slows, each flight feather has a notch on the leading
edge, about 2/3's of the way to the tip. The narrowing of the
vane leaves spaces between the feathers when the wing is spread
which helps even out the airflow. Also, these birds can spread
the feathers on the alula to create and additional slot. Fast
flying birds do not have these notches on their feathers.
Feathers give the wing its shape and there is a direct
correlation between form and function. Birds who fly fast in open
air have long, narrow wings. They have difficulty taking off, but
can stay in the air indefinitely, once airborne. Woodland birds
must fly slowly to maneuver between branches and trees, as well
as take off frequently, have short, broad wings and wide
feathers. They cannot fly as fast or as long as birds with
longer, streamlined wings. Finally, birds that soar, have broad
secondary flight feathers which greatly increase the surface area
of the wing so they can ride easily on the warm air currents.
A Bird's Tail Feathers
The last place feathers play a vital role during flight is on the
bird's tail. The tail acts as the rudder, balancing and steering
the bird. The tail feathers have vanes of equal size and can be
tipped in different directions for stability. While soaring, the
tail feathers are spread to increase the surface area and get
more lift. The entire tail can be twisted to change direction.
And last, to aide the bird when stopping. the tail is turned
downward and acts like a brake.
It is truly amazing the critical role feathers play in the flight
of a bird. No other animal or machine has duplicated the
intricacies of a single feather. But feathers serve other
functions too. They protect the bird's skin from cuts, bruises,
sun and rain. Predators often end up with feathers instead of
bird for their meal. Woodpeckers have extra stiff, pointed tail
feathers which they use to stick against the tree for additional
support while they drill for insects. The sand grouse in Africa
soaks up water with its breast feathers to feed to is babies.
Feathers insulate. A penguin's body temperature is 110 degrees,
while the surrounding air can be -75 degrees so their feathers
cover their entire body and are almost hairlike. Feathers can be
fluffed up in the winter or squeezed down in the summer. Some
birds grow more feathers in winter. Ostriches can make their
contour feathers stand up to release excess body heat.
The list of adaptations of feathers to suit a particular bird's
life style is endless. Feathers are used to line nests which
helps to hold in heat. Song birds collect old feathers, sparrows
pull feathers from other birds, and ducks and geese pluck their
own down feathers and then use the bare skin on their breast to
warm the eggs. Hatchlings that are tree nested are born without feathers
and, therefore, must be cared for by the parent. Those who nest
on the ground are born with fluffy down, are not dependent upon
the parent and can feed themselves. Water birds have special oil
glands to make their feathers more water resistant. The exception
are birds that dive; they have no oil glands because they cannot
be buoyant. They must dry their feathers in the sun.
Owls are nocturnal hunters. The undersurface of their wing
feathers are fluffy and velvet-like, the leading edges have
unlinked barbs, and fringed trailing edges, all to muffle the
sound of the air as it passes over the wing. They also have very
densely packed face feathers which help the owl gather and focus
sound so they can hunt in total darkness. The final significance
of feathers is appreciated not only by the birds themselves, but
also by man. The variety of colors found in a bird's plumage is
not only magnificent but functional. Because birds fly, they
don't need as much camouflage and can have much more colorful
skin coverings.
Bird Feather Color
Color is important in mating. Birds can see color, where most
mammals do not. Colored plumage is used to attract a females
attention or is used to be visible to other males as a warning.
The amount of color found in different birds is dependent upon
life styles. If both male and female sit on the nest, both will
have dull brown coloration. Feathers used to camouflage can have
disruptive coloration which are spots or patches of color, they
can totally change color (like the Ptarmigan in winter) or that
can utilize counter shading where the top of the bird has darker
feathers and the underside tends to be a lighter color. In the
tropics where there is lush vegetation and dense foliage, birds
have the most brilliant colors because there are plenty of places
to hide.
How are the colors produced in feathers? Light comes from the sun
in waves. Each color has a specific wave length. When something
appears colored, the object absorbs all the wave lengths of light
except the one it reflects. The reflected light is the color one
sees. Substances that produce colors are called pigments. Black,
brown, some red, and dull yellow in feathers are produced by
pigments called melanin. Melanin are microscopic particles found
in the shaft and barbs of feathers. Black melanin is rod shaped
but the others are oval shaped. The more melanin particles
present, the darker the color. Feathers with lots of melanin are
stronger. Consequently, it is not uncommon for white birds to
have wing tips that are black, because the wingtips are
constantly battered by the wind and need to be stronger.
The brilliant colors- red, gold, yellow, and orange - are formed
by pigments called carotenoids. These are not individual
particles, but, rather, pigments dissolved in fat globules that
are left behind when the feather stops growing and dies. Birds
that are colored by carotenoids use the pigments in their food
and change them into the form used in feather formation.
Flamingos are perhaps the most widely known birds whose feathers
are colored with carotenoids.Their natural diet contains the
pigment that makes their feathers pink, but when in captivity,
zoo food lacks the proper pigment and their feathers are white
after they molt.
The color blue is not produced by pigment. Rather, it is produced
in the same manner that makes the sky appear blue. There are
tiny, air filled pockets in the feathers that scatter the blue
wave lengths. In the barbs of a blue jay's feather, these hollow
cells form a layer over the melanin. The melanin absorbs all the
other colors, making the blue appear more intense. Green is
produced in a similar manner, only a transparent layer of yellow
cells lie over the particles that scatter the blue wave lengths.
White is a mixture of all colors and white feathers have no
pigments at all. Some, however, do have air bubbles that reflect
back the light making the white appear even brighter.
The iridescent colors found in hummingbirds and peafowls, are
produced in yet a different way. In peafowls, the barbules of the
tail feathers have three very thin layers of keratin which
reflect the colors much like a soap bubble does. In hummingbirds,
the barbules are twisted so the flat sides face out. These are
coated with at least three very thin films of oval, plate-shaped
structures, which hold tiny air bubbles. These reflect the light
in different wave lengths producing the iridescent colors found
in the feathers.
Find out: What is a Bird?
