The skin covers the body and produces feathers and a variety of other external structures: beak covering, wattles, comb, scales of feet and legs. The skin is thinner than mammals’ and attaches to the skeleton in a number of places: skull, beak, wrist, wing tips, dorsal side of the pelvis, tarsometatarsus, and toes. The general function of the skin is protective, the base of external features, and serves as a large sensory organ with receptors for temperature, pressure, and vibration. One or two glands at the base of the tail (uropygial glands) produce fatty secretions.
Feathers are the one most distinguishing characteristic of birds.
There are five main structural categories of feathers, although there are intermediate forms
1. Contour Feathers
a. Body feathers-typical structure. The central shaft of a feather has two sections. The calamus, the naked portion of the shaft, is implanted in a socket in the skin. Beyond the calamus is the rachis, that bears branches called barbs, on its side. The barbs on either side of the rachis constitute the vane. The vanes are symmetrical on most body feathers but asymmetrical on flight feathers of the wing and tail. The portion of the vanes at the end of the feather nearest the body are downy – soft, loose, and often fluffy and used in products such as down comforters. The vanes at the farther end are firm, compact, and closely knit. The downy parts of feathers as well as feathers that are totally downy provide insulation and filling out body contours. The firmer portion provides an airfoil, protects the body from moisture, the skin from injury, and colors and shapes for displays. Bird flight feathers.
A barb consists of an axis and many closely spaced branches, the barbules which interlock to provide strength.The water repellency of contour feathers is determined mainly by the interlocking structure of the barbules, but a film of uropygial oil gland secretion is also necessary.
Feathers often bear downy outgrowths on the underside, the aftershaft or afterfeather.
b. Remiges and retrices.
The flight feathers of the wings and tail are a type of contour feather. They are large and stiff and may be modified for their roles in flight by being asymmetrical or notched. In some birds the tail feathers are modified for display rather than flight (turkey, peacock, grouse).
c. Ear coverts.
These are small contour feathers that cover the external ear opening. They function to screen the ear opening without blocking sound.
The second type of feather combines a large rachis with mostly downy vanes. Semiplumes are intermediates between contour and down feathers. They function to fill in between contour and downy feathers.
3. Down Feathers.
Down feathers are wholly fluffy feathers that lack interlocking barbicels.
a. Natal Down
Also called nestling down, is found on many birds at hatching or within a few days after hatching. Natal down is a distinct feather generation and tends to precede contour feathers, semi-plumes or adult down feathers. They are generally alike on all parts of the bird, but they differ widely among bird groups.
Birds with adult down feathers among the contour feathers, such as ducks and owls, have a second type of natal down which precedes the adult down.
b. Definitive Down – down feathers on the body after the natal down.
c. Uropygial Gland Down – most birds have an oil gland, the uropygial gland, at the base of the tail. The gland may have a tuft of down feathers that aids in transferring the oil secretion from the gland to the bill.
d. Powder Down – modified feathers of many birds shed an extremely fine white powder formed by the breaking of the barbule tips. These feathers may be found among the ordinary down and contour feathers, in distinct patches by themselves or both in patches and mixed with other feathers. The powder is composed of very small granules and as such is unwettable and is thus a type of waterproofing. It also dims the sheen of a feather.
4. Bristles- bristles in birds are generally feathers with a stiff shaft and barbs only on the proximal portion, if at all. Bristles occur most commonly around the base of the bill, around the eyes, and as eyelashes. They may also cover the nostrils of some birds such as crows and jays.
Filoplumes are hairlike feathers that consist of a very fine shaft with a few short barbs at the end. Typically they are covered by other feathers. Filoplumes appear to function as pressure and vibration receptors – they sense the location of other feathers so they can be adjusted properly. The “hairs” on a plucked chicken are filoplumes.
Also see External Anatomy of Birds for topography of feathers.
Development of Feathers and Follicles
Most adult birds are covered with feathers except on the beak, eyes, and feet. The contour feathers are arranged in rows and groups of follicles.
A follicle ordinarily produces a series of feathers during a bird’s life. If the bird molts, the new feather pushes out the old one. If a feather is lost some other way, the follicle replaces it immediately or at the next molt, depending on the time of the next molt, the health of the bird, its reproductive state, etc. A new feather can be grown in two weeks but it depends on the species of bird, time of year, and type of feather. The number of feathers is relatively constant within a species although they tend to have more feathers in the winter than in the summer. Smaller birds tend to have more feathers per area than larger birds although fewer feathers total. E.g. Ruby-throated Hummingbird- 940 feathers; Canada Goose 33,000.
Feathers are 91% protein, 1.3% fat and 7.9% water.
Plumage and Molt
Birds periodically shed and renew their plumage by molting, usually 2-3 times a year the first year and then 1-2 times annually after that.
Typical Molt Pattern
Natal Down – down feathers on newly hatched birds; may be heavy or sparse depending on the species. Some birds hatch naked (e.g. cormorants) and develop down later; some are hatched naked and grow down feathers simultaneously with contour feather (e.g. woodpeckers) and some have a heavy coat of down at hatching (e.g. waterfowl).
Postnatal Molt – the natal down is pushed out by the tips of the new feathers and the down drops off.
Juvenal Plumage – this plumage is composed of the first true contour feathers. Tends to be loosely textured, giving the bird a fuzzy appearance. Passerine birds develop this plumage before they leave the nest. It’s generally kept only a short time.
Postjuvenal Molt- occurs in the late summer or fall; may be complete or partial.
First Winter Plumage- this plumage has the smooth adult texture as the adult plumage but may have a different pattern than the adult birds; retained over winter.
First Prenuptial Molt -may or may not follow the first winter plumage and may or may not be complete if it occurs.
First Nuptial Plumage- this may be the same, partly different, or wholly different than the first winter plumage. Retained during the breeding season. May be acquired by wearing down of feather tips to expose other patterns/colors (e.g. starlings).
First Postnuptial Molt -occurs right after the breeding season.
Second Winter Plumage – in most species of birds this plumage cannot be distinguished from that of adults of any age. Retained throughout the second winter.
Second Prenuptial Molt, Second Nuptial Plumage, Second Postnuptial Molt, etc.
Some birds may not attain their adult plumage for 3 years (gulls, some hawks) or more (albatross 6-8 years). There are a number of variations on the molting pattern because some birds breed twice a year and others only every other year. And “winter plumage” is meaningless in the tropics. Robins, e.g. molt only once a year.
Molting is usually gradual and feathers are lost and replaced in an orderly fashion.
In some birds such as loons, gulls, and waterfowl, all flight feathers are lost simultaneously.
Why molt? Change in colors for different seasons is related to reproduction, but birds molt primarily because their feathers wear out and need replacement.
Colors are caused by pigments, structural conditions, and combinations of these. Pigments are chemical compounds that absorb light of different wavelengths, whereas structure refers to physical conditions that modify or separate wavelengths.
Functions of Coloration:
1. Insulation from or absorption of heat/light (e.g. light colored desert birds and dark colored cold climate, e.g. juncos)
2. Cryptic coloration
a. mimicking (whip-poor-will, bittern)
b. disruptive (killdeer, burrowing owl)
a. attraction (for groups)
b. epigamic (mates, territories)
In general, males are more brightly colored than females.
Evolution of Feathers
There is a long-held assumption that feathers evolved from scales, but the nature and function of intermediate structures are unknown. There are no genes for feathers; their formation is due to the production of a set of unique protein molecules.
Both the scales and reptiles of birds are composed of keratin,but the keratin of the beak, claws, foot scales, the different types of feathers, and reptilian scales all differ somewhat. So to say reptilian scales gave rise to feathers is an oversimplification, but they are closely related. More information can be found at Feather Identification; Wing Anatomy.