How is the Vane of Flight and Contour Feathers Formed Into a More Continuous Surface

Birds of Seabrook Island

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Structure

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Feather Structure Contour Feathers Flight Feathers Color Molt Feather Care	Feather Function Flight Thermoregulation	Reproduction Development Brood Patch	The Skeleton Legs and the Ankle Joint Feet Bills and Nostrils Muscles and Bones	Internal Structures and Systems Control Systems Digestive and Respiratory Systems - Circulation Urogenital System - Osmoregulation

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Feathers - Structure

Feathers define birds. They are part of the integument (skin) and are responsible for the two fundamental avian characteristics - flight and thermoregulation.
Mature feathers (like our hair) are dead. They are, however, retained (also like our hair) in follicles in the skin and can be positioned by muscles that surround and attach to each follicle - "filomotion." When birds are cold, they erect their body (contour) feathers and become little insulated balls with a thick layer of air trapped by the feathers and conserving heat. When they are hot, they depresses their feathers to minimize the thickness of their insulating shell and maximize heat loss. Flight feathers are anchored and constrained but the control of their position can be quite intricate giving the individual feathers optimal positions to provide thrust and lift.
Feathers (and scales) are largely composed of beta-keratin - a strong fibrous protein.
Birds have two types of feather - contour and flight.
 For a light but informative source of information on feathers, read
Hanson, T. 2011. Feathers: The Evolution of a Natural Miracles . Basic Books.

Contour Feathers

  Contour feathers attach to the skin in regular tracts and define the outer surface of the body (contrast the appearance of a live chicken with a plucked chicken ready to cook).
Contour feathers that cover the body have a basal calamus that is encased in a follicle in the skin. This gives rise to a central rachis and two opposite, lateral loose vanes that form a flat feather. The vanes of flight feathers (below) are usually asymmetrical, those of contour feathers are symmetrical and their barbs do not interlock.

J. van Tyne and A. J. Berger. 1976. Fundamentals of Ornithology.
2nd ed. John Wiley & Sons, New York.

© University of Michigan, Museum of Zoology.

  Beneath the contour feathers, some birds (especially larger and aquatic birds) may possess soft and fluffy down feathers which typically lack a rachis and do not form a  vane - they are clusters of barbs originating from the calamus. Down is not necessarily restricted to the tracts (pterylae) that the other contour feathers occupy - they may form in areas that do not bear contour feathers (apteria).
Note that not all birds have down. Woodpeckers, kingfishers, and a few passerines hatch naked and their first feathers are like those of adults. In others (most passerines) young have downy tufts on the back and head but they are not really helpful in thermoregulation - they are quickly replaced by contour feathers. In pelicans, the young hatch naked but develop a downy coat covering their body as they grow. In some groups (penguins, petrels, owls, and a few others), the first coat of nestling down is replaced by a second coat. As juvenal feathers develop, they push the down out and replace it with typical contour feathers. Down is important in providing insulation (and water-proofing) in those birds that have it.
There are also several special contour feathers. Semiplumes have a rachis that is longer than the barbs (which are often reduced) - they fill out the body contours and may be involved in courtship (these form the display plumes in egrets). Filoplumes are small, hair-like feathers with a few barbs and barbules at the tip that monitor the position of the flight feathers - they provide sensory information for aerodynamic control and may monitor air speed. Bristles consist of a rachis with only a few basal barbs and barbules. They are sensory and are usually found on the head - they may form eyelashes, nostril coverings, and, located around the mouth, may extend the gape (as rictal bristles), serving sensory functions and aiding in capturing food by enlarging the functional mouth opening (gape).
Primitively, the follicles of contour feathers have two foci for growth, an outer and an inner focus, and two feathers (vanes) emerge from the same follicle - the shaft (outer) and the after-shaft (inner). The after-shaft is usually smaller and usually provides  insulation while the shaft gives contour to the body. The after-shaft is well developed in gallinaceous birds - pick a body feather from a chicken, turkey, pheasant, or quail to see it... It is also present in diurnal birds of prey, herons and their relatives, and parrots. It is reduced in most other birds and is greatly reduced (to a tuft of barbs) or is absent. In suboscine passerines it is completely lacking. In most oscines, the aftershaft is reduced and has very little rachis.
Birds have thousands of feathers - a typical songbird has 2,000 - 4,000 feathers with 30-40% of them on the head and neck. Numbers range from >1,000 feathers (940) in a Ruby-throated Hummingbird to 25,216 (20,177 on the neck and head) of a Tundra Swan. The feather coat may weigh two to three times as much as the bony skeleton (which has hollow bones with extensions of the air sacs {"pneumatic"} to reduce mass). [Remember that the skin is also our most massive organ.] Interestingly, the skin of some birds (e.g., Brown Pelican) is also "pneumatic" (has open spaces that may communicate with air sacs and the respiratory system).

Flight Feathers

  Flight feathers attach to bone.

   Remiges ("oars") - singular remix - are flight feathers of the wing. Rectrices ("rudders") - singular rectrix- are flight feathers of the tail.
Extending from the basal calamus (located in a follicle in the skin anchored to bone), flight feathers have a stiff central shaft or rachis to which attach barbs that form a flat, aerodynamic (often asymmetrical) vane. Barbules on each barb interlock to form the vane (distal barbules have hamuli - tiny hooklets - that interlock with the filamentous proximal barbules). If you haven't played with a feather recently, pick up a shed feather on the beach and separate the adjacent barbs. Them rub them back together - they reengage to form an intact feather. This is what a bird does when it preens...

© University of Michigan, Museum of Zoology

   In the wing, primaries (9-12 with 10 being the commonest number) attach to bones of the hand (the phalanges of the second and third fingers and all metacarpals fuse to form a flat plate on which they insert). They form the "hand" wing. Primaries provide the power for flying. They are numbered from the wrist out (thus reduction of the 10th primary means that the outermost feather is the one that is reduced). These feathers usually molt from the wrist out (proximal to distal).
There are one or two small flight feathers that attach to the phalanges of the first  finger (not fused with the hand) to form the "alula" or "bastard quills" in many birds. These short feathers move independently of the primaries and provide "slotting" at slow speeds when landing or taking off. (They increase the flow of air over the wings like wing flaps in airplanes.) See the picture of an Osprey hovering to see them in action.
Attaching to the ulna (one of two bones forming the forearm) are secondaries (8 or more depending on the length of the wing). Secondaries are numbered from the wrist in (toward the body - distal to proximal).

© University of Michigan, Museum of Zoology

   Secondaries usually molt from the wrist in. In birds with larger numbers of secondaries, molt may proceed from both ends of the series toward the middle.
Secondaries form the inner or "arm" wing and provide lift during flight.
Each remex is typically covered at its base on the upper surface by a greater covert with successive rows of smaller feathers moving away from the leading edge - the median coverts, several rows of minor coverts and a number of marginal coverts. The coverts of the lower surface are similar but are less fully developed and some rows of lesser coverts are incomplete. Passerines and woodpeckers lack greater secondary coverts under the wing.
Coverts and flight feathers all overlap to give a fared aerodynamic wing used in flight.
As and aside, because of developmental allometry, some groups of long-winged birds lack the 5th secondary (the upper covert is present but the feather is missing) - this condition is known as diastaxic as opposed to the condition when the fifth secondary is present - eutaxic. Most of the waterbirds and waterfowl, herons, raptors, shorebirds, parrots, and owls are diastaxic. Gallinaceous birds, cuckoos, woodpeckers and passerines are mostly eutaxic. The condition is variable in a number of orders.
A few large birds have tertiaries that attach to the humerus. However, this term is often incorrectly used to describe proximal coverts on the wing or the scapulars (a flight feather must attach to bone).
Even though the hand and arm wings form a continuous flight surface, it is easy to "feel" the separation of primaries and secondaries at the wrist when the bird is in your hand.
Note that remiges are asymmetrical - the outer vane is smaller than the inner vane and the entire feather may be typically curved or arched. This trait is useful when looking at feathered fossils and trying to decide if they could fly. In contrast, rectrices are more nearly symmetrical in most birds.

Rectrices, the flight feathers of the tail attach to fused bones (vertebrae) that form the pygostyle ("pope's nose"). Most birds have 12 (6 pairs) of largely symmetrical tail feathers. Some have 10 (hummingbirds, swifts, most cuckoos, etc.). One cuckoo genus, some rails and some grebes have only 8 and some very small birds 6. One adult hummingbird has but 4 tail feathers (young have 10). Many have more than 12 feathers including guineafowl (14), ptarmigans and lyrebirds (16), White Pelicans (24), the White-tailed Wattled Pheasant (32), etc.
Tail feathers are overlain by greater and lesser coverts (there are fewer coverts than rectrices in most passerines and more coverts than rectrices in some swimming birds. When missing, it is usually the central coverts that are lost. The undertail coverts are less well defined but appear to approximate the number of rectrices.
Rectrices assist flight by providing lift and facilitating steering (birds are able to fly without their tail feathers but they don't really look right!). Tail feathers are usually molted from the innermost pair to the outermost (there are some variations in birds that use the tail as a prop on trees - scansorial birds may retain the innermost feathers as a prop while the others are molted).
You need to watch graceful Swallow-tailed Kites soaring to really appreciate the dynamics of the tail in flight. Barn Swallows are similar in their use of the tail but they fly much more quickly so adjustments are harder to appreciate.
The base of all flight feathers and their bony junction is "faired" or streamlined by one or more series of contour feathers known as coverts and identified above (coverts attach to the skin and are contour feathers by definition, not flight feathers)

Color

   The color of feathers is a combination of pigment and structure. The most common pigment is melanin, an insoluble pigment which produces black-brown colors. There are also lipid soluble carotenoid pigments that produce reds, yellow, and orange. With one exception, however, blues and greens are produced by combinations of pigments (melanin and carotenoids) with feather structure. If you look at feathers with true pigment in transmitted light the original color shows (try a red feather from a cardinal). If, however, you pick up a Blue Jay feather (blue) and hold it toward the sun it looks brown - you have to look at it in reflected light to see the blue which is caused by an interaction of the surface microstructure and pigments in the feather.
You might note that many birds with white feathers such as the White Ibis have black primaries. Melanin strengthens the physical structure of feathers and those exposed to accelerated wear are often black.

Molt

Because they wear, all feathers are shed and replaced ("molted") once or twice a year (or breeding cycle). [In many birds, flight feathers are molted once - usually after breeding - and contour feathers, especially those worn in feeding, may be molted twice. Some birds have only one annual molt. [The young of some species may have a post-juvenile molt to replace their first contour feathers before the non-breeding period - others retain their first feathers (especially the flight feathers) to the first annual molt.]
Contour molt may lead to different feather patterns that characterize "breeding" or adult (alternate) plumage patterns vs. "non-breeding" (basic) patterns in juveniles and adults in winter. Color change also occurs with wear (e.g., European Starlings loose their spots and acquire their iridescent breeding plumage as contour feathers abrade - they only have one molt/year). Dark feathers (those impregnated with melanin) wear more slowly than white or colored feathers containing fat-soluble pigments. Many white birds have black primaries - their feathers subjected to the most wear (e.g., Wood Stork).
Because of their importance to flight, feathers of the wing and tail are not molted synchronously but are shed, one at a time, in a definite pattern. Thus, when one feather is shed, the previous feather may be half grown, and the one before that is fully grown. This means that the bird is minus only a small portion of the surface necessary to provide lift. You have probably noted this in various soaring birds in the summer and fall. A complete molt usually begins with the innermost primaries - then the outermost primaries are lost and the process spreads through the remaining feathers. When the primary molt is well underway, the secondaries molt from the outside in. At the same time, contour feathers of the body are replaced. The tail feathers are also replaced two at a time. Molt usually begins at the center and moves outward (centrifugal). In colies, some toucans, woodpeckers, and a few passerines, molt begins with the center remiges and progresses outward (centripetal).
In many species, there is a complete molt after breeding - the post-nuptial molt. In many species, there is also a partial molt (often involving only worn contour feathers) before breeding - called the pre-nuptial molt. The "summer" plumage is then known as the nuptial or breeding plumage and the winter plumage as the winter plumage.
However, there are a variety of molt patterns (loons and ducks, for example, undergo an accelerated post-nuptial molt into an "eclipse" plumage and are often flightless - before fall migration they molt into their nuptial plumage - an adaptation that facilitates pair formation during the winter). Thus it is probably better to call the non-breeding plumage the "basic" plumage and the breeding garb as the "alternate" plumage. Thus, the molts become pre-alternate molt (normally in the spring) and pre-basic molt (normally in the fall) in the majority of cases. In those species that take several years to reach maturity, the basic and alternate plumages may differ and can be identified as "first basic," "second alternate," etc...
Molt consists of two processes - shedding of a feather (ecdysis), and growth of its replacement (endysis). At the appropriate time, the tight connection of the base of the feather to its follicle becomes loose and the feather is shed (hormones?). The follicle engorges with blood and keratinized barbs or a rachis with lateral barbs begins to grow at the base. As it extends above the surface of the skin, the follicle remains sheathed and is highly vascularized ("pin feather"). As the new feather grows, the vascular bed recedes and the feather erupts from the keratinized sheath which becomes abraded away. When the new feather reaches its appropriate length, growth stops, the feather looses its blood supply and "dies" but remains attached in the follicle until the next molt. Complex striated muscles often attach to the follicle and are responsible for feather position (filomotion).
If you have a pet bird and accidentally pull a feather, it will regrow regardless of molt cycle (the loss of the feather stimulates the follicle to become active). If, however, you cut a feather (flight feathers may be cut to keep domestic waterfowl from flying), it will not regrow until the next regular molt.

Feather Care

Because they wear, all feathers are shed and replaced ("molted") once or twice a year (or breeding cycle). [In many birds, flight feathers are molted once - usually after breeding - and contour feathers, especially those worn in feeding, may be molted twice. Some birds have only one annual molt. [The young of some species may have a post-juvenile molt to replace their first contour feathers before the non-breeding period - others retain their first feathers (especially the flight feathers) to the first annual molt.]
Contour molt may lead to different feather patterns that characterize "breeding" or adult (alternate) plumage patterns vs. "non-breeding" (basic) patterns in juveniles and adults in winter. Color change also occurs with wear (e.g., European Starlings loose their spots and acquire their iridescent breeding plumage as contour feathers abrade - they only have one molt/year). Dark feathers (those impregnated with melanin) wear more slowly than white or colored feathers containing fat-soluble pigments. Many white birds have black primaries - their feathers subjected to the most wear (e.g., Wood Stork).
Because of their importance to flight, feathers of the wing and tail are not molted synchronously but are shed, one at a time, in a definite pattern. Thus, when one feather is shed, the previous feather may be half grown, and the one before that is fully grown. This means that the bird is minus only a small portion of the surface necessary to provide lift. You have probably noted this in various soaring birds in the summer and fall.
In many species, there is a complete molt after breeding - the post-nuptial molt. In many species, there is also a partial molt (often involving only worn contour feathers) before breeding - called the pre-nuptial molt. The "summer" plumage is then known as the nuptial or breeding plumage and the winter plumage as the winter plumage.
However, there are a variety of molt patterns (loons and ducks, for example, undergo an accelerated post-nuptial molt into an "eclipse" plumage and are often flightless - before fall migration they molt into their nuptial plumage - an adaptation that facilitates pair formation during the winter). Thus it is probably better to call the non-breeding plumage the "basic" plumage and the breeding garb as the "alternate" plumage. Thus, the molts become pre-alternate molt (normally in the spring) and pre-basic molt (normally in the fall) in the majority of cases. In those species that take several years to reach maturity, the basic and alternate plumages may differ and can be identified as "first basic," "second alternate," etc...
Molt consists of two processes - shedding of a feather (ecdysis), and growth of its replacement (endysis). At the appropriate time, the tight connection of the base of the feather to its follicle becomes loose and the feather is shed (hormones?). The follicle engorges with blood and keratinized barbs or a rachis with lateral barbs begins to grow at the base. As it extends above the surface of the skin, the follicle remains sheathed and is highly vascularized ("pin feather"). As the new feather grows, the vascular bed recedes and the feather erupts from the keratinized sheath which becomes abraded away. When the new feather reaches its appropriate length, growth stops, the feather looses its blood supply and "dies" but remains attached in the follicle until the next molt. Complex striated muscles often attach to the follicle and are responsible for feather position (filomotion).
If you have a pet bird and accidentally pull a feather, it will regrow regardless of molt cycle (the loss of the feather stimulates the follicle to become active). If, however, you cut a feather (flight feathers may be cut to keep domestic waterfowl from flying), it will not regrow until the next regular molt. Banner - Forster's Tern. North Beach.

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Source: https://birdsofseabrook.app.clemson.edu/topics/feather_structure.html

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