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Birds are an incredibly diverse group of animals. They are found on every continent and in almost every land habitat on earth. As a result, birds have a wide range of body designs. One feature that has allowed birds to spread across the world is the ability to fly. With the exception of bats (which are mammals), birds are the only vertebrates that can fly.
Flight is such an important aspect of birds’ lives that their entire body is built to allow them to fly. Even flightless birds (ostriches, penguins, etc) retain some features meant to sustain flight. The two main features that birds have for flight are wings and feathers. Let’s look at how these features fit into birds’ ecology.
Basic wing shapes
There are four basic wing shapes that apply to most flying birds.
Feathers appear to have evolved from scales and are composed of B-keratin. Scales and feathers develop in a similar fashion. In actuality, birds have both feathers and scales. You can find scales on the legs and feet of most birds.
Feathers are incredibly strong and yet are incredibly flexible. To allow both lift and forward movement, feathers can bend at almost a right angles.
Feathers are made of a shaft, called the rachis and the vanes on either side. Vanes are made of barbs that are arranged side by side up the shaft of the feather. Barbules grow from the barbs, which have tiny hooks that interlock in a similar way to hook-and-loop fasteners. The short bare part at the base of the shaft is called the calamus. If viewed crossways, the calamus is basically hollow.
There is an opening at the very tip of the base where the blood supply entered the feather when it was growing. Once a feather is fully grown, the supply of blood is sealed off and the feather itself becomes “dead,” similar to the ends of human nails.
On most flight feathers, the vanes are of unequal length. This gives feathers the ability to twist under air pressure. The large flight feathers are attached to bone by connective tissue, and have little or no down at the base. All other feathers are attached to muscles below the skin.
Feathers are adapted for different roles:
• thermoregulation (keeping warm and cool)
• protection from impact
• defense (both physical and visual)
• incubation of eggs
• brooding of young
• display (both visual and aural)
• hunting by touch
• carrying water (in some cases)
Types of feathers
There are seven types of feathers:
1. Contour feathers: small feathers that cover the body, wings, and tail.
2. Flight feathers: feathers used in flight
a. Remiges: large wing feathers
i. primaries are attached to the bones of the “hand”
ii. secondaries are attached to the bones of the “forearm”
iii. tertiaries are attached to the humerus or upper arm bone.
b. Retrices: large tail feathers
(Not all long feathers off the rear of a bird are retrices. The huge, beautiful tail feathers of the peacock or cockerel are not tail feathers. They are contour feathers that have grown out of proportion to the bird and are only for display.)
3. Semi-plumes: feathers used to create shape for the bird and provide some warmth.
4. Down feathers: soft under-feathers that provide insulation for warmth for a bird.
5. Bristles: feathers used as a touch sensor or funnel that make the bird reflexively snap up food.
6. Filoplumes: specialized kind of bristle feathers many nerve endings that act as sensory organs, registering pressure and vibration. They help keep feathers in place and adjusting them for flight, insulation, and bathing.
7. Eyelashes: feathers similar to human eyelashes.
Most birds have an annual moult, which is when the bird drops old feathers and grows in new ones. Moulting can take from four to 16 weeks to complete and normally occurs during or after the breeding season. Large birds take longer to moult, such as eagles and large vultures that take two to three years to complete a moult.
Some birds that partake in long migrations, will delay their moult until after they complete their migration. Eleonora’s falcons moult in their wintering grounds, probably because they breed very late in the summer. Others, like some of ducks, moult their flight feathers all at once and regrow them right away. This is called “going into eclipse,” but these species have the element of water as well as land to aid them in feeding, hiding, and defense.
Birds use their beaks to preen or clean and arrange their feathers. Enormous concentration and time is spent oiling and cleaning feathers, which is a bird’s most vital tool. Birds will often bathe to assist with cleaning. Some vultures will even fly up to 32 kilometers (20 miles) to find water to bathe in after feeding.
Many colors in bird plumage come from a combination of two pigments. Melanins are responsible for blacks and dark colors, while carotenoids produce yellow and red. Melanins are made by the bird’s body, but carotenoids are obtained from the diet. This is obvious when flamingos lose their pink color if they do not eat the correct diet.
Feather colors are also classified as iridescent or non-iridescent. Iridescent colors are seen only when like strikes feathers at certain angles. These colors are actually created by prism-like structures in the feathers, rather than color pigments. Iridescent colors are found on hummingbirds, some kingfishers, and some turkey vulture feathers. Non-iridescent colors, made from chemical pigments, are seen from any angle and are found on most songbirds.
Feathers may have both pigments and structures that create colors. In the green magpie, the green is the result of a yellow pigment overlying a blue structural color. Because yellow fades in the sun, magpies living in open areas are much bluer than those in shaded forests.
Some birds will change color prior to the breeding season. This color change is often the result of molting. Other birds achieve this color change by wearing away parts of the feather leaving a different color for the breeding season. Darker pigments are stronger and more wear resistant.
Some birds use a partial molt, where they drop and re-grow the display feathers, which tend not to be flight feathers. This uses less energy than re-growing all the feathers.
Plumage color is also vital for camouflage. Colors help owls and nightjars blend resemble the bark of a tree and woodcocks fade into the forest undergrowth. Upland birds, such as the ptarmigan, take on speckled plumage as they to imitate the autumn color transition, eventually becoming completely white to hide in the winter snow. Female snowy owls are speckled with black marks (while the male is entirely white) which help camouflage females while they incubate the eggs on the ground, after most of the snow has melted from the tundra. The white feathers on fishing eagles may even break up their silhouette when they hunt for fish low over water.
Many birds have juvenile and adult plumage. Some species take several years to gain their adult plumage and have a sub-adult phase, too. Bald eagles have dark juvenile plumage, become mottled and streaked as sub-adults, and acquire the white head and tail as adults.
Numbers of Feathers
Feathers on a bird are not evenly distributed. They grow in distinct tracts or lines (called pterylae), with large areas of bare skin (apteria). The apteria help cool the bird. Apteria vary in size and are typically covered with down or semi-plumes for warmth. Water birds have small apteria and penguins have almost none.
The number of feathers on a bird varies from species, size, sex, age, health, season, and temperature of habitat. Most songbirds have between 1500 and 3000 feathers. The lowest recorded number is 940 for a hummingbird and the highest is 25,216 for a swan. Most birds have more feathers in winter for additional insulation against the cold.
The number of flight feathers also varies. Most birds have 10 primaries and 10 secondaries, but this can vary from nine to 12 primaries and six to 35 secondaries. Remiges typically number from 10 or 12, but can range from six (in some songbirds) to 32 (Bulwer’s Pheasant).
Feathers and Humans
For thousands of years, humans have admired birds and their feathers, although not always to the birds’ benefits. For hundreds of years, Bald and Golden Eagle feathers have been used by indigenous people of North America for ceremonial headdresses. Birds of Paradise feathers are utilized in the same way by tribes in the southern hemisphere.
In the early 20th century, hunting birds to provide feathers for ladies’ hats lead to legal protection of the birds in the UK and the U.S. Now we are able to admire and be amazed by birds and their feathers through film, photography, and the chance to see birds as vital parts of our environment. Feathers are without doubt one of nature’s wonders.