THE STRUCTURE OF AVIAN SKIN AND FEATHERS
N Harcourt-Brown BVSc Dip ECAMS FRCVS
NH&FM Harcourt-Brown, 30 Crab Lane, Harrogate, North Yorkshire HG1 3BE
INTRODUCTION
The current consensus puts the total number of avian species over 9,000. Fortunately,
relatively few species from only several families are seen by the veterinary surgeon in general
practice. The anatomy, biochemistry and physiology, as well as pathology and diseases differ
from family to family, as they do in mammals.
Birds are covered with feathers. The feathers are responsible for protection, colouration,
locomotion, sensation and sexual selection. The skin covers the bird and contains and gives
rise to the feathers but over the majority of the bird’s body it is too flimsy to be protective. In
some areas it is greatly strengthened for protection or modified to form the claws and beak.
NB In avian anatomy there has been a traditional confusion of terminology. Many authors
liked to coin their own terms for anatomical structures. In 1971 the International Committee
on Avian Anatomical Nomenclature (ICAAN) was founded. This committee was responsible
for the production of Nomina Anatomica Avium (NAA). Unlike Nomina Anatomica
Veterinaria, all the anatomical features that are described and named are illustrated. A second
edition of this manual appeared in 1993. It is of immense help if veterinary surgeons use
NAA II terms, especially in publications.
SKIN
The skin of birds is of two types, one is very much thinner than mammalian skin and this is
overlain by the feathers. This skin offers little physical protection and is usually supple, with
underlying musculature for support and movement of the feathers. In areas associated with
trauma, such as the feet or around the face, feathers are often lacking; the epidermis is much
thicker and is often overlain by thick protective scales.
Epidermis
In the chick embryo the epidermis is derived from a single cell layer of ectoderm that divides,
early on, into a cuboidal germinal layer and an outer periderm. The periderm protects the
embryo during its growth in the egg but its cells die, keratinise and slough a few days before
hatching. The cuboidal layer beneath it divides and gives rise to the epidermis; this is several
cells thick by the time the bird hatches.
The epidermis has a deep layer of dividing cells, stratum germinativum, and a superficial
layer of keratinised cells, stratum corneum. Stratum germinativum is separated from the
dermis by a basal layer whose cells continually divide. These cells enlarge and form an
intermediate layer, homologous to the prickle cell layer in mammals. The cells are held
together by desmosomes but spaces between the cells are permeated by tissue fluid. As these
cells mature and are carried towards the surface of the skin, they flatten to form the
transitional layer that occupies the same position in the epidermis as the stratum granulosum
of mammals. In birds the keratohyalin granules within these cells cannot be seen with light
microscopy. Once flattened the epidermal cells become keratinised and die forming the horny
stratum corneum. In feathered areas of the body this layer is only two or three cells thick. In
other areas, such as the feet, this layer is much thicker and can be greatly modified.
The feet (and tarsometatarsal areas in many birds) are covered in shield like plates of keratin:
the scutes. Scutes vary in size from minute scales (cancella) small scales (reticula),
intermediate scales (scutella) to very large solid scales (scuta). In most birds the dorsal aspect
of the feet is covered with larger scales than the plantar aspect. In areas of greater movement
the scales are usually smaller.
The scales are formed from epidermis that gives rise to layers of keratin. This keratin is very
hard. In between the scales the epidermis forms a sulcus and in this region the keratin is
softer.
The plantar aspect of the foot has a more flexible layer of epidermis and the reticulate scutes
are covered with a thin layer of hard keratin that rests upon a thick layer of more pliable
keratin regarded as intermediate to the hard and soft keratins found dorsally. The germinal
layer is also thicker and capillaries run into this region reaching as far as the transitional
layer. In these more supple areas intercellular fibres can be seen running between the
laminae. The plantar aspect of the foot also has a modification in the dermis and collagen and
elastin fibres are present in abundance. The dermis is closely attached to the subcutaneous
layers allowing the bird to grip firmly. The foot has various digital pads to aid the grip. The
digital pads are situated at each interphalangeal joint and not only aid grip but protect the
digital flexor tendon as it passes over the slight prominence of the joint.
Dermis
The dermis lies between the epidermis and the subcutaneous tissues. This layer of connective
tissue can be divided into superficial and deep layers; the deep layer being further subdivided
into loose and compact layers. The deepest layer (loose) is attached to an elastic lamina that
is the junction with the subcutaneous tissue. This layer contains the apterial muscles (smooth
muscle) and fat bodies. The compact layer is made of a mesh of collagen and elastin fibres,
the collagen is mainly arranged horizontally (in mammals it is random). The superficial layer
of the dermis shows great differences between the species, but in the main there are more
fibroblasts and less dense collagen fibres than in the deep layer of the dermis.
The dermis is very pliable. There are scattered elastin fibres and collagen throughout but in
addition, the dermis contains elastin fibres that are thicker and joined together to form
tendons. Each tendon runs into the end of a smooth muscle belly. These muscles run
throughout the dermis interconnecting between the feather muscles which move the feathers
and the apterial muscles that interconnect the feather tracts.
The subcutaneous tissue is divided into superficial and deep fascia. There are abundant fat
bodies in the superficial fascia. The subcutaneous tissue contains striated subcutaneous
muscles (mm. subcutanei), which run between the skeleton or somatic muscles and the skin.
Sternal bursa and related structures
Poultry have a bursa like structure that forms between the skin and keel. This seems to be
present in heavier birds and is absent from other birds such as quail, pigeons and ducks. The
sternal bursa can become infected and this causes the carcase to lose value as it has to be
trimmed or even discarded.
Other birds often have a sternal cushion that is a fatty layer between the dermis and the keel.
Ratites have similar reactions to pressure: prominent callosities in the ostrich are located over
the most ventral aspect of the sternum and over the bony prominence produced by the
cranioventral projection of the pubic bones. These callosities bear the weight when the bird
rests in sternal recumbency. Rheas and emus have only the sternal callosity.
Keratinisation
This is reviewed by Spearman and Hardy. There are some differences between mammals and
birds.Once the cells have matured and keratinised they are shed from the surface of the skin
in one of two ways. The keratinised cells from the areas of supple skin are shed in the same
manner as mammals: desquamation of single cells or groups of cells. These cells and their
accompanying sebum move through the feathers, distributing their sebum as they go. In areas
of scaly skin, the horny layer is shed in relatively large sheet like fragments. This can be
obvious in some birds like the puffin that shed their ornamental beak shells, or grouse that
shed the frond-like scales on their feet: in both species this is an annual event. Other birds
shed their tarsometatarsal scales periodically. In captive hawks this can be very obvious. This
is a totally different process to sloughing of the skin by reptiles.
Lipogenesis
Keratin-bound phospholipids occur throughout the stratum corneum in birds forming a waterrepellent layer that covers the whole bird. Living avian epidermal cells contain fat globules
and produce far more lipids than mammalian or reptilian cells. These oil droplets are
removed during conventional histological processing.
Lipid droplets form in the basal cells. They increase in number and coalesce into larger
globules in the intermediate layer. The amount of lipid varies, the greatest amount occurring
in the interdigital webs and foot pads; the least is present in the tarosmetatarsal region. As the
stratum corneum forms, and the cells flatten and become keratinised, the fat globules pass
through small pores in the keratin into the spaces between the cells. The whole of the surface
of the bird becomes coated with this layer of oil. The lipids that are present in the stratum
corneum make a much more waterproof barrier than keratin alone. The oil also coats the
feathers. In most species the lipid coating is augmented with secretions by glandular tissue.
Cutaneous glands
There are small wax-secreting glands in the external wall of the auditory meatus, small
mucus-secreting vent glands and a large uropygial gland. The uropygial gland is a bilobed
holocrine gland. It is found dorsally at the base of the tail. The two lobes are separated by a
septum that is continuous with the capsule of the gland. Each gland is lined with numerous
secretory alveoli that open into a central cavity. The shape of the uropygial gland varies
between species and families of birds. It has been used as a taxonomic marker. In a few birds
the gland is absent, notably, the Ostrich (Struthio camelus), also Amazon parrots and their
close relatives, the Pionus parrots. The tip of the uropygial gland (papilla) has two openings,
one for each side of the gland, often surrounded by a tuft of down feathers. The secretory
alveoli are supplied with oil formed by modified epidermal cells that do not become
keratinised. Each epidermal cell has a prominent Golgi apparatus (generally associated with
lipogenesis) and as it matures the cell becomes filled with lipid droplets. Finally the lipids
coalesce to form a globule and cytolysis causes the cell to dissolve completely and release its
products into the duct. When massaged, a small amount of greasy secretion exudes from the
tip of the gland. It is held on the tip of the gland by the tuft of feathers. The bird wipes its
beak on the papilla and spreads the resulting secretion over its body.
There are many species variations in the nature of the sebum and there are also differences
between sebum formed by the skin and sebum from the uropygial gland. One of the main
functions of the sebum is waterproofing the feathers and skin. The uropygial gland is most
developed in waterbirds. A pelican’s uropygial gland can be the size of a hen’s egg. Sebum
from the uropygial gland seems to be more water repellent in aquatic birds. All birds need
some degree of waterproofing but only 7% of sebum on a pigeon is derived from the
uropygial gland. The sebum is also required to keep the plumage smooth and supple, vital for
maintaining full aerobatic performance. It also enhances feather colour, has antibacterial and
antifungal properties, and contains vitamin D precursors.
In some birds the secretion is coloured, the Great Hornbill (Buceros bicornis) has a yellow
secretion that helps to colour its beak. In other birds (Hoopoes and Musk Ducks) the secretion
is odorous. Green Woodhoopoes (Phoeniculus purpureus) use a foul smelling secretion from
their preen gland as a predator deterrent. In this species, the oil gland has no feathers on the
papilla, the birds sleep in a communal nest hole and if threatened they all turn round so that
their preen glands face the entrance and the gland is stimulated to emit secretions.
Sweat glands
Birds do NOT possess sweat glands. Feathers are an extremely efficient insulator. Heat is lost
from the respiratory tract and by radiation from featherless surfaces. When flying in a wind
tunnel and viewed through a thermal imaging camera, the feet and legs of starlings loose a
substantial amount of the body’s heat. Heat loss at night is also a probable reason for sleeping
with their head (really their beak) under their wing, and one leg raised.
Feathers
An obvious characteristic of birds is their covering of feathers. It has been suggested that
birds have descended from theropod dinosaurs and that feathers were derived form scales.
There is evidence in some Chinese fossils that the early feathers appear to have the potential
for insulation only and that they were not connected with flight. Modern bird’s feathers vary
in size from the small feathers that cover penguins so closely, to the huge tail feathers seen on
peacocks or Argus Pheasants. However in spite of their varied morphology, feathers are
constructed in a similar manner. The feather is a very complicated structure; it is a tough,
keratinised, cellular derivative of the epidermis and is formed in a follicle that penetrates
deep into the dermis. The feathers are arranged in distinct areas known as feather tracts
(pterylae). The featherless spaces (apteria) may be bare, or covered with semiplumes and
down feathers, however these featherless spaces are usually covered by overlying contour
feathers. A featherless space on the ventral abdomen, the brood patch, may occur seasonally.
Its occurrence appears to be dictated by incubation habits: in some species it is found in both
sexes, in others it is found in the males or the females. The brood patch allows close contact
between the egg and parent and it forms by feather loss, thickening of the dermis and increase
in the vascularity of the area.
There are several types of feather:
Contour feathers form the majority of the external plumage. The various groups of contour
feathers are of great importance to the ornithologists. As these groups of feathers and their
colours are used to describe differences in species, subspecies, and hybrids, as well as the
bird’s sex and age there are names for all of them. It is also possible to predict avian hunting
strategies (and make taxonomic judgements) from feather shape in the absence of the live
bird. Contour feathers can be divided into flight feathers, primary, secondary, and in some
birds tertiary retrices (wing feathers), remiges (tail feathers), and body feathers. Contour
feathers that cover the base of the remiges and retrices are called coverts (tetrices). Each
feather has a long central stalk (scapus). The initial part of the stalk, buried in a tightly fitting
follicle is the calamus. The tip of the calmus is embedded in the follicle and has an opening,
the proximal umbilicus that is attached to the follicle’s dermal papilla. The rest of the stalk is
called the rachis. On the ventral aspect of the rachis a groove runs the whole length from the
tip to the junction between the calamus and the rachis where there is a small hole, the distal
umbilicus that leads inside the calamus. The area around the distal umbilicus may have a
small tuft of feathers or even a complete hypopenna, or after-feather. The rachis forms the
support for a series of parallel stiff filaments arranged at about 45º to the rachis; these are the
barbs. The barbs are further divided into the proximal and distal barbules, which hold onto
their adjacent barbules with hamuli or hooklets. In a clean, normal healthy feather this system
forms a strong interlocked sheet, the vane. The vanes of the wing feathers are asymmetric, the
leading edge being narrower. This helps to produce an aerodynamic shape. The contour
feather follicle is also richly supplied with sensory nerve endings that appear to be stimulated
by movement of the rachis.
Down feathers have a short rachis and long soft barbs. They are often the first feathers on a
newly hatched chick. In adult birds they are found under the contour feathers where they
form an insulating undercoat. Their distribution varies with families, for instance covering the
whole body in ducks and penguins, uneven distribution in gulls and owls, or complete
absence in hummingbirds and passerines.
Semiplumes are characterised by a wholly fluffy vane and a long rachis, always longer than
the longest barb. They are found along the margins of the contour feather tracts or sometimes
singly in the feathered areas. The are insulative and in some aquatic species aid buoyancy.
Powder down feathers are found in pigeons (Columbiformes) and parrots (Psittaciformes),
and some other families. These white feathers grow continuously and the barbs at the tip
constantly break off. This produces a fine white dust made of keratin particles about 1mm in
diameter. The powder down is a waxy coating that covers the whole bird and helps
waterproof the feathers. In pet parrots powder down will also cover the surroundings; this is
very obvious in white cockatoos
Hypopennae are usually small and not usually associated with remiges and retrices. In Emus
and cassowaries the hypopenna is often as large as the main feather forming a double body
feather
Bristle feathers are the short spiky feathers around the eye in most species and loral region in
some. They are protective and to aid this function have lots of sensory nerve ends attached to
their follicles.
Filoplumes are present in nearly all birds (not ratites), although they are not usually apparent
until the bird is plucked. The filoplume is a long spike with a tassel of barbs at its tip and is
usually associated with a contour feather. Filoplumes are assumed to be sensory detectors as
they are closely associated with sensory nerve ends and also Herbst corpuscles. It is thought
that these feathers probably assess the strains and movement of the contour feathers: vital for
flight. Each flight feather has a number (up to 10) of closely associated filoplumes. A few
birds e.g. bulbuls, have long modified filoplumes in their neck regions that extend beyond the
contour feathers. They are thought to be decorative only.
Feathers are important tactile organs. Not only are the filoplumes sensory, the feather follicle
wall is supplied with numerous sensory nerve endings.
Feather attachment and movement
From the outside the skin shows feather tracts and featherless spaces. The feathers arise from
follicles. When viewed from inside the bird, the feather follicles are moved by a series of
muscles that span the featherless spaces. The feather muscles and the apterial muscles are
smooth muscle and are limited to the dermis.
Feather muscles are attached to the outside of the follicle by elastic tendons. The muscles are
arranged around the follicle and run to adjacent follicles in a manner that produces X shaped
cross-struts. This pattern of muscles allows elevation, depression and rotation of the follicles
or pulls them closer together. The fulcrum is provided by the fat bodies between the feather
follicles. There are two layers held in tension by elastic sheets. The muscles also tense the
feathers against movement, especially on the wing during flight. In spite of the fact that this
muscle system is smooth muscle it is supplied with nerves from the spinal cord and is under
conscious control as well as autonomic control. The apterial muscles are composed of
bundles of smooth muscle that are joined end to end by elastic tendons and arranged as a
sheet across the apteria. This muscular sheet can have the ability to tense the skin. Both sets
of muscles are similar and at the junction of their tracts they tend to merge together. The
muscles and elastic tissues make the skin different to biopsy than that of mammals. When a
biopsy is cut out the pull by the muscles and elastin fibres on the remaining skin makes the
hole much larger whilst the biopsy is contracted.
Retrices can be grouped into primary and secondary feathers. The carpus is the point of
division: carpus to tip of hand is occupied by the primaries, carpus to elbow are secondaries.
Some birds, such as vultures, have tertiary retrices that arise from the humerus. The primary
feathers are closely attached to the bones of the hand. The secondary feathers are more
loosely attached to the ulna but even so they are still firmly attached and in some species
there are bony prominences on the ulna where they attach. The remiges are attached to the
dorsal aspect of the wing bones. It is impossible for the bird to move the flight feathers on the
wing without moving the bones of the wing.
Conversely the retrices are very mobile either by the bird moving its caudal vertebrae or by a
set of intrinsic muscles that allow the tail to be spread as a fan. The retrices steer the bird
during flight or can be used for communication/display. The vertebrae end in a special bone,
the pygostyle that supports the feathers, and there is a set of complicated muscles that are
used to support and move the tail feathers.
The massive and gaudy tail feathers of the Peacock are the tetrices or coverts, the retrices are
brown and relatively short and are used to support the fan.
In some families there is a space (diastema) between the 4th and 6th secondary feathers on the
wing that is meant to correspond to a missing 5th feather.
The integument is modified to produce the beak and claws. There are also modifications of
the integument in other areas to produce structures that are secondary sexual characteristics,
spurs, wattles etc.
Beak
In many birds this is a hard, keratinised epidermal structure, although in waders
(charadriforms) it is leathery and in ducks (anseriforms) it is hard around the margins but soft
over the rest of the surface. In most birds it is a single sheath like structure although some
seabirds, fulmars, gannets, albatrosses (procellariforms) have the beak divided into a number
of distinct sections. The base of the beak in some birds is modified on a permanent basis e.g.
hornbills and turacos, and in others it is temporary, e.g. puffins.
Histologically the beak resembles skin. The outer layer of the beak is formed as the skin is
and can be divided into the same layers. The stratum corneum is very thick and its cells
contain calcium phosphate, hydroxyapatite crystals, and abundant keratin as well as the
‘usual’ keratin-bound phospholipids and calcium that are found in the rest of the epidermal
cells in unmodified skin. The crystalline structures give the beak its durability. The dermis is
closely attached to the periosteum and contains abundant collagen and elastin fibres. The
periosteum is made of a thick layer of fibroblasts and collagenic tissues directed towards the
bone surface. The periosteum blends into the bone.
The integument of the beak grows continually. Beak growth is from two areas. The beak
arises from the base, culmen, and is directed rostrally so that there is continuous passage of
the horny beak from base to tip, as in a horse’s hoof. In a large macaw this it takes under a
year. Also the thickness of the beak increases as it grows rostrally due to keratin being laid
down by the whole of the epidermis. The shape of the beak is dictated by the bony (maxillary
or mandibular) support. The beak is worn away by abrasion from food and other materials but
also by the bird ‘grinding the lower beak against the inner surface of the upper beak.
Deformities of the lower beak cause corresponding deformities in the upper beak. Beak shape
is vital for feeding. The shape and size of the beak dictates what food the beak can cope with.
This has been examined in finches and related to seed hardness and crossbills where it is
related to the ability to extract seed from pine cones. Various sieves exist in flamingoes,
ducks and geese.
The beak is also a sensitive structure. The area of major sensitivity varies between species. In
ducks it is situated at the tip of the upper and lower beaks in an area known as the nail.
Running through the nail are numerous canals that open on the occlusal surface. These canals
contain dermal papillae carrying mechanoreceptor nerve endings: Herbst corpuscles. In the
bone that is overlain by the nail are numerous pits that also contain dermis and sensory nerve
endings. The upper and lower nails and their associated dermal structures are involved in
food recognition and discrimination. The lower beak of the parrot also has a well-developed
sensory area. Waders have lots of Herbst corpuscles in their beaks and have been shown to be
able to sense food items deeper in the substrate than they are able to reach with their probing
beak. There are many species as well as family differences. There are other sensory nerve
endings such as Grandry corpuscles that closely resemble Merkel cells. All these nerve
endings are connected to the trigeminal nerve.
The base of the upper beak in owls, parrots and pigeons is swollen, featherless, soft and
highly sensitive. It is known as the cere. In some species it is a sign of gender, e.g.
Budgerigars and a subspecies of Mountain Parakeet.
In baby birds there is an egg-tooth that is a cone of horny cells that contain hydroxyapatite. It
is shed some time after hatching. Although beaks differ widely, egg-teeth are all the same.
The baby birds use it to help break out of the egg.
Claws
The digits terminate in a claw known as a talon by falconers. The claw fits over the germinal
epithelium like a thimble and the germinal epithelium overlies most of the last phalangeal
bone. The dorsal plate (scutum dorsale) of the claw is keratinised and calcified and is the
hardest part of the claw. It also is the thickest and grows more quickly thereby promoting the
curvature of the talon. The plantar plate (scutum plantare) of the claw is relatively soft.
There are also diminutive claws found at the end of the digits of the hand. In some adult
birds, such as falconiforms, they are insignificant, in ratites they are well developed but
relatively functionless. However in fledgling Hoatzins they are well developed and have
functional muscles; the young birds use them as an aid to their arboreal clamberings before
they can fly. Archaeopteryx had three prominent claws on each wing.
Spurs
These hard, keratinised conical structures occur on the caudal tarsometatarsal area of
Phasianidae where they are well developed in the male and usually insignifcant in the female.
Some birds have spurs on the carpometacarpus, such as the Spur-winged Goose and Spurwinged Plover.
Comb wattles, rictuses, frontal processes (snoods) and ear lobes
These ornamental outgrowths of skin are characterised by a thickened and extremely vascular
dermis. The colour of these structures in poultry is produced by blood.