Feathers

A bird's plumage is all of its feathers, taken together as a whole.  Feathers serve many purposes, including...

  • Providing insulation to control body temperature.
  • Providing aerodynamic power for flight.
  • Using colors for communication and/or camouflage.

Some feathers can also be modified to perform the following functions:

  • Aid in swimming.
  • Producing sound.
  • Aid hearing.
  • Protection.
  • Cleanliness.
  • Repel water.
  • Transport of water.
  • Extend the sense of touch.
  • Support.

A nice video describing the structure, functions, and types of feathers can be found here.

 

Feather Structure

Feathers are made of beta-keratin, a protein similar to that in the skin of all vertebrates (alpha-keratin).  A body feather (aka contour feather or pennaceous feather) is made up of the following parts:


  1. Central Shaft
    •    Calamus (aka Quill): the hollow part at the base, without barbs, that anchors the feather to the skin.
    •    Rachis: the part that supports the vanes.
  2. Vanes
    •    Barb: projections that branch off of the rachis.
      • Ramus: the central shaft of each barb.
      • Barbule: projections that branch off of the ramus.  Single cells, linked end-to-end.
        • Barbicels: small, hooklike projections on the end of some barbules.

Most body feathers are fluffy/downy towards the quill end (for insulation), and transition to a smoother, flatter surface.  The barbules of the downy portion do not have barbicels, which helps to create the downy texture.

An afterfeather is a secondary shaft and vane, attached to the same quill, that is usually all downy.  Its function is to enhance insulation.  A good example of how this works is the ptarmigan (cold-climate grouse) - their winter afterfeathers are almost 3/4 the length of the main feathers, but the summer afterfeathers are much shorter.

Basic barb modifications include...

  • Fused barbs that create a plastic-like appearance, such as on the wing-tips of Cedar Waxwings.
  • Large, closely spaced barbs with extra-long, curved barbicels to repel water, such as in ducks, rails, and petrels.
  • Lack/lessening of barbicels to aid in diving, such as in cormorants and anhingas.  (This requires feathers to be air dried after diving.)
  • Coiled barbicels to absorb and hold water, such as in the desert sandgrouse - which carries water in its feathers to young in the nest.


Vaned Feathers

These include contour (body) feathers and flight (wing and tail) feathers.  The wing flight feathers are called remiges (singular = remex), and the tail flight feathers are called rectricies (singular = rectrix).

Wing feathers, or remiges, are divided into three main groups:

  1. Primaries - outer remiges that attach to the hand and 2nd digit bones.  These provide forward thrust on the downstroke.  They are strongly asymmetrical, with a very narrow leading edge to cut through the air.  Most birds have 10.
  2. Secondaries - inner remiges that attach to the ulna bone.  These form much of the inner wing surface.  Some species show modifications for courtship displays or making sound.  Numbers of secondaries range from 6 (hummingbirds) to 40 (albatrosses).
  3. Coverts - smaller feathers of the wing that form in rows and cover the bases of the remiges.  These fill in any gaps among the base of the flight feathers.

Tail feathers, or rectrices, attach to the pygostyle.  There are usually 12 of these.  These function mainly for steering and braking, but may be modified for display (several tropical birds), sound production (snipes) and support to brace the body (woodpeckers).

 

Downs, Semiplumes, and Bristles - Oh My!

  • Down is a type of soft, fluffy feather that provides insulation.  It usually lacks a rachis (central shaft), except in ducks and other waterfowl.  The barbules trap a layer of air next to the skin, thus providing the insulation.
  • Semiplumes are feathers that have a structure in between down and contour feathers.  They can aid insulation, aerodynamics, and courtship displays.  They have a large rachis with loose down-like vanes.
  • Filoplumes consist of a thin shaft with a few barbs (1 to 6) at the end.  These are used to sense the movement and position of the vaned feathers next to them.  Each flight feather may have 8 to 12 filoplumes around it to monitor its position.  When the tip of the filoplume is disturbed, the movement is transmitted to a nerve bundle at its base.  Filoplumes around flight feathers can aid in flight control; those around contour feathers help monitor airspeed.  Penguins and flightless birds (ratites) do not have filoplumes.
  • Bristles consist of a stiff rachis (shaft) and a few barbs near the base.  They serve both sensory and protective functions.  Protective bristles include the eyelashes of ostriches, hornbills, and cuckoos and the nostril coverings of woodpeckers, jays and crows.  Birds that feed on insects while flying often have bristles around the mouth that function both to sense insects and help catch them.
  • Powderdowns are special feathers that give off a dust made of keratin particles.  There is some debate about the purpose of powderdowns, although some think that they may help to waterproof feathers.

Feather Growth - skipped this section

Evolution of Feathers - skipped this section

 

Feather Colors

Feather colors are a result of two factors:

  1. Biochrome Pigments -  These are natural chemicals that absorb certain wavelengths of light and reflect others.  There are three main types:
    • Melanins - produce grays, blacks, browns, and buff colors.
      • Melanins are present in almost all birds, and are made from an amino acid by pigment cells.
      • The distribution of melanin particles in barbs and barbules create bar or speckled pattersn in the feather.
      • Melanin can make a feather more resistant to wear or fraying (hence the dark wing tips of gulls), and can protect against damage from sand abrasion (in desert species).
      • Melanins can help resist the damage to feathers by bacteria - which is why birds living in wet climates tend to be darker than those in dry climates.
      • Melanin also absorbs radiant energy (sunlight), which aids temperature regulation and drying of wet feathers.
    • Carotenoids - produce bright yellows, oranges, reds, and certain blues and greens (except in parrots, which produce psittacofulvins).
      • Carotenoids come from a bird's diet and are then modified by cells in the feather follicle, as the feather is forming, and depoisited in barbs and barbules.
    • Porphyrins - produce bright browns and greens and a unique magenta color.
      • Porphyrins tend to contain iron (brown) or copper (green).
      • They are easily destroyed by sunlight, so are mostly found in new feathers.
  2. Structural Colors - These result when the structure of the feather alters the components of the light that hits them.
    • Incoherent reflectance - all light waves are scattered to produce white feathers.  Light bounces off of air spaces in the cells of the feathers.  (Rock Ptarmigan)
    • Coherent light scatteringby melanin granules in the barbules.  Hollow melanin granules, arranged in stacks or crystal structures, create iridescent colors that change when looked at from different angles.  (Hummingbirds)
    • Coherent light scatteringby air bubbles in the keratin of the rami (the shafts of the individual barbs).  This is similar to the previous type, but the bubbles are not arranged in any structure, so they look the same form all angles.  (Blue Jays and Bluebirds)

Pigment colors and structural colors interact to create other colors, as well.

Most birds also have feathers that reflect UV light - which birds can see because they have an extra set of cones in their eyes to detect UV.  UV feather colors are usually structural, but can also be caused by some carotenoid pigments.  (This is a relatively new area of study, but birds without UV colors seem to be limited to all-white birds and Rheas.)

UV color seems to be a major factor in selecting mates.  For example, male bluebirds with brighter UV colors tend to produce more young and provide for their young better than males with weaker UV colors.

 

Feather Tracts

The weight of a bird's entire feather coat is typically twice (or more) the weight of its dry skeleton and between 17% and 20% of their total adult body mass.

Feathers are attached to the skin along the feather tracts (areas where feathers are attached in dense concentrations) or pterylae.  There are nine major feather tracts, which are separated by areas with few or no feathers, called apteria.  The feather tracts are not obvious because the feathers spread out to cover the whole body.

The apteria probably allow a greater range of movement of a bird's feathers - which can be for temperature regulation, display, or to adjust buoyancy, wing surface, or tactile sensation (sense of touch).

 

Feather Care

Because feathers are dead structures that do not renew themselves, birds must care for them daily.  This is done by preening, which involves using the beak to rearrange and reposition feathers, straighten out the vanes of feathers, remove parasites, and spread the waxy secretions of the preen gland.  Preen gland secretions...

  • cleans feathers
  • preserves feather moistness and flexibility
  • maintains the insulating and waterproofing qualities of the plumage
  • protect feathers against bacteria and fungi that digest keratin
  • discourage feather lice

Bird parasites include...

  • Chewing Lice which feed on feathers, blood, or tissue fluids.
  • Louse Flies: bloodsuckers that can also spread lice and mites.
  • Feather Mites which live on or inside of feathers.

Feather parasites may reduce a bird's chance of attracting a mate, and may also reduce the feather's ability to insulate the body - causing heat loss and decreased chance of survival in the winter.  Birds preen and scratch their heads as often as once per hour to remove such parasites.

 

Plumage Color Patterns

Bird color patterns have evolved to fulfill one of two major purposes:

  1. To conceal the bird from predators.
  2. To display the bird to potential mates.

Both cryptic and bold color patterns can work to conceal a bird.  Ptarmigan are white in the winter to blend in with snow, black and brown in the summer to blend in with herbs and lichen on rocks.  Bold patterns on wood warblers blend in with small leaves, branches, and lighting on trees.  The greens and reds of parrots blend with tropical leaves and flowers.

Disruptive patterns can break up a bird's outline and reduce its contrast with the environment.  The black and white "collar" on a Killdeer are a good example of this.

Countershading is a type of camouflage in which an animals' dorsal side is a significantly darker color than the underside.  Since the sun's rays strike from above, and are reflected more from the underside than the back, countershading helps to disguise a bird's outline and reduce the effects of shadows that might make the bird easier to spot.

Reverse Countershading is, as the name implies, the opposite of countershading.  Here, the upper parts are lighter and the underparts are darker.  This makes the male of certain species (like Bobolinks) more conspicuous in the breeding season.

In addition to reverse countershading, birds can use the following plumage patterns for signalling and display...

  • Uniform color (Northern Cardinal) enhances the outline.
  • Contrasting edges enhance signal patches.
  • Unusual shapes that do not match the bird's surroundings, especially if they are geometric - like triangular or rectangular wing patches.
  • Regular repetition of spots or stripes. 


Molting and Plumages

Molting refers to replacing the entire plumage.  Most birds begin life with a coat of downy feathers, which are replaced within the first few weeks.  (Although wisps of down may cling to the new feathers for a time.)

A bird is considered a juvenile when its wing and tail feathers grow in.  This is an early step towards independance.

After the juveline stage, immature or adult plumage replaces the juvenile plumage - although many birds do not replace the flight (wing and tail) feathers at this time.

Adult birds usually molt after breeding.  Some birds will keep their new feathers for a full year, others will molt at different times during the year.  Those that only molt annually (once per year) may change in appearance as the feathers get worn down.

Many birds go through two plumages over the course of a year - the Basic plumage (nonbreeding colors) and the Alternate plumage (breeding colors).  Molts are named for the plumages that follow them.  Thus...

   - the Prebasic molt (after breeding) generates the Basic plumage.

   - the Prealternate molt (before breeding) gives the Alternate plumage.

In some birds, it takes a couple of cycles to get to the full adult colors, and the bird may go through an "immature" basic or alternate plumage in their first year.

Most birds replace flight feathers gradually.  Primary flight feathers are typically replaced one or two at a time, beginning with the innermost and working out.  Tail feathers are also replaced in a pattern - usually innermost to outermost, with a few exceptions.  These patterns minimizes the impact of molting on flight ability.  The exceptions to this are those species of ducks in which the male undergoes a rapid molt, leaving them flightless for a few weeks until the plumage grows back in.

 

A five-part video on the evolution of feathers...

   Part 1 - Part 2Part 3Part 4Part 5

Subpages (1): Files for Feathers