Musculoskeletal system

The skeleton of the bird (Fig. 13.1) is unlike that of any other group of animals and it has developed from the evolution of powered flight. A combination of a reduction in the total number of bones and the fusion of many joints has resulted in a skeleton that provides a strong base for the attachment of the flight muscles. Although the skeleton must be light enough to enable the downward force of the wing to lift the bird into the air and to keep it airborne, when compared to mammals of a similar size birds are not exceptionally light.

Fig. 13.1 Skeleton of a typical bird (hawk).

(With permission from Colville T, Bassett JM 2001 Clinical anatomy and physiology for veterinary technicians. Mosby, St Louis, MO, p 355.)

Main features

Bone structure

• To lighten the skeleton the cortex of the bone is much thinner than in mammalian bones and many long bones are hollow. Internally, lightweight struts create a honeycombed interior and reinforce the bones (Fig. 13.2).

• Many of the larger bones are pneumatic, i.e. they are filled with air contained in membranous air sacs that connect with the respiratory system. These cut down the weight of the skeleton and so aid flight. The number of pneumatic bones is reduced in diving birds as they restrict the ability to stay under water.

Fig. 13.2 Longitudinal section through an avian long bone. The bony struts serve to strengthen the bone.

Skeletal modifications

• The sternum is extended into a laterally flattened keel (Fig. 13.1), which provides a large surface area for the attachment of the major flight muscles. These are the pectoral muscles responsible for the powerful down stroke and the supracoracoid muscle responsible for the upstroke. Flightless birds, e.g. ostriches and emus, do not have a keel.

• To counteract the action of the flight muscles and to support the wings a large bone, the coracoid, lies between the keel and each shoulder joint (Fig. 13.1).

• The number of joints in the body is reduced. This is particularly seen in the vertebral column and results in a rigid trunk to support the action of the flight muscles.

• The neck is long and mobile and contains more cervical vertebrae than the 7 seen in all mammals – there may be as many as 25 in the swan. A long neck makes up for the lack of flexibility in the body and enables the bird to turn its head right round, creating the wide range of vision necessary for survival, to catch food and to preen all parts of its body.

• The number of caudal vertebrae are reduced and fused forming the tail or pygostyle. Some flight feathers are attached to the tail and play an important part in flight, balance and display. At the base of the tail is the uropygial or preen gland, which is essential for keeping feathers oiled and healthy.

• There is a single body cavity, i.e. birds have no diaphragm dividing the thorax and abdomen.

• There is no floor to the pelvic cavity, which makes the pelvis more distensible to facilitate the passage of eggs to the outside. Egg laying enables birds to reproduce without the need to carry the developing fetus around internally. This would increase the body weight and severely restrict the ability to fly.

Head (Fig. 13.3)

• A lightweight beak covering the mandible replaces the teeth and reduces the weight of the skeleton. Each species has a characteristic beak shape, which is adapted to its eating habits.

• Between the mandible and skull is the quadrate bone, which makes dislocation of the jaw very unlikely.

• Many birds also have a joint known as the craniofacial hinge between the upper beak and the skull. This increases the mobility of the beak during feeding.

• The orbit is large and thin-walled to lighten the skull and to house the large eyes. As much of the bird’s brain is concerned with visual information, the eyes connect with large optic lobes in the brain.

Fig. 13.3 The anatomy of the bird skull (barn owl).

Leg and foot

• Some of the bones are fused, forming a tibiotarsus and a tarsometatarsus, but the stifle joint is similar to that seen in mammals.

• Most of the muscles of the leg lie high up on the leg or on the body itself and control movements by means of long tendons.

• A digital flexor tendon (Fig. 13.4) runs in a tunnel behind the intertarsal joint and supplies each of the digits. It is responsible for bringing about the perching reflex, seen when a bird lands on a branch: the toes automatically flex and tighten on the branch, preventing the bird from falling off.

• Most birds have three toes pointing forwards and one pointing backwards; however, parrots have two toes pointing forwards and two backwards.

• The shape of the feet reflects the lifestyle of the species: e.g. ducks have webbed feet for swimming; raptors have strong talons for catching and killing their prey.

Fig. 13.4 Intertarsal joint in the avian leg. The digital flexor tendon runs in a tunnel within the metatarsal bone. It attaches to the caudal aspect of the digits and is important in the perching reflex.

Wing

• The bones of each forelimb (Fig. 13.5) are reduced to a humerus, a separate radius and ulna, fused carpal and metacarpal bones and two digits:

Digit 3: the main digit is attached to the fused metacarpal bones and carries the primary feathers

Digit 1: forms the alula or bastard wing and carries a few feathers. It is essential for controlling take-off and landing.

• Most of the wing muscles are found on the body or at the proximal end of the wing and long tendons attached lower down the wing control movement. If the flight muscles were located on the wings, the extra weight would impede flapping flight.

• In cross-section the wing is slightly curved from front to back forming an ‘aerofoil’ shape. This creates lift as the bird flaps its wings.

Fig. 13.5 Bird in flight showing the structure of the wing (forelimb).

Feathers

Feathers are the distinctive feature of members of the class Aves. They develop from epidermal cells in a similar way to the hairs of mammals and the scales of reptiles. Feathers are made of keratin and create a strong but lightweight covering over the wing and the body.

All feathers have a similar structure (Fig. 13.6). The central shaft or rachis is filled with blood capillaries during growth but later, as the feather matures, it becomes hollow. On either side of the shaft, the vane consists of barbs and interlocking barbules. These hook together to form a flattened, wind-resistant surface.

Fig. 13.6 A General structure of a feather. B Four types of feather: 1 Primary and contour; 2 Down; 3 Filoplume.

(Adapted with permission from Colville T, Bassett JM 2001 Clinical anatomy and physiology for veterinary technicians. Mosby, St Louis, MO, p 347.)

The feathers must be kept clean in order to function effectively. Birds constantly groom themselves to ‘zip up’ the barbules and to apply the secretions from the preen gland at the base of the tail, which keeps the feathers waterproof.

There are several types of feather (Fig. 13.6):

• Flight feathers – long rigid feathers attached to the wing and the tail:

Primaries: attached to digit 3 and to the fused metacarpal bones (Fig. 13.5). Usually 11 feathers but number varies with species; provide the major thrust during flight

Secondaries: attached to the ulna; shorter than the primaries

• Contour or covert feathers – cover the rest of the wing and the outermost layer of the body to produce a smooth outline; shorter and more flexible; the lower part of the vane (closest to the skin) is fluffy.

• Filoplume and down feathers – lie close to the body underneath the contour feathers, forming an insulating layer; they have no barbs so they are fluffy. Filoplume is designed to break up, creating dust that absorbs sweat and dirt and keeps the bird clean.

Birds always shed dust from their feathers and this may be a source of pathogens. The zoonotic respiratory disease psittacosis is caused by the microorganism Chlamydophila (formerly Chlamydia) psittaci and is spread by feather and faecal dust. Avoid inhaling dust from suspected cases.

Moulting

Adult birds moult once a year, usually after the breeding season. The old feathers are lost over a period and during this time the birds are vulnerable: flight is affected and processes such as egg laying in domestic hens may cease. Production of new feathers requires energy so food intake, particularly the protein requirement, increases. Any nutritional deficiency may result in stunted feather growth or poor colouration. Feathers are used for flight, for insulation and, in some species, for display. Captive birds may be kept for their beauty and for showing.

When handling birds it is essential that the feathers are not damaged. They are only replaced at the next moult so the ability to fly may be seriously impaired, while the owner may be annoyed that their appearance has been affected.

When preparing birds for surgery, avoid plucking the feathers as this will affect the bird’s ability to keep warm until the next moult. Damp the feathers with water or spirit and part them to reveal the skin.

Flight

Most of the surface of the wing is covered with flight feathers, forming a light but strong structure. It is slightly curved, the dorsal surface being longer and convex and the ventral surface shorter and more concave. This aerofoil shape generates maximum lift and minimum drag as the wing moves through the air. Air passing over the dorsal surface has to travel faster than the air passing under the wing, resulting in lower pressure over the dorsal surface, which generates lift. The lift force must equal the weight of the bird if flight is to occur.

Control of flight is brought about by tilting the leading edge of the wing and the formation of slots between the feathers, e.g. separation of the primary feathers and the alula allows some air through, which maintains a smooth air flow on the dorsal surface, thus increasing lift. If the angle of tilt is greater than 15 ° then lift is reduced and the bird stalls or falls out of the sky.

Wing shape affects the type and speed of flight. Soaring birds such as the albatross or buzzard have broad wings that maximise lift while the wings of many garden birds are short and allow rapid bursts of flapping flight. The wings tips of the buzzard are slotted to reduce turbulence while the feathers on the edge of a barn owl’s wing are fringed to reduce the noise of the wingbeat as the bird approaches its prey.

In gliding flight the wing is held still and at a low angle so that the bird gradually loses height. Birds such as the albatross actually lose very little height because of the size and shape of the wing and because they make constant adjustments to the angle of the leading edge. Species such as the vulture and the buzzard make use of thermals or upwards warm air currents which allow them to soar and gain height.

In flapping or powered flight the bird’s body is held almost stationary and the wings are moved rhythmically up and down to generate forward thrust as well as lift. The main power is provided by the large pectoral muscles, which extend from the keel to the humerus and make up to 20% of the body mass. In strong fliers such as the pigeon the muscles are deep red because of their high myoglobin content and their good blood supply. In species such as the domestic fowl and the turkey the muscles are almost white and are capable of producing powerful bursts of flight that only last for a short time.

At take-off maximum lift is helped by running or by launching from a branch. To land the tail is used as a brake and the wings are extended into the stall position. The legs are extended to absorb the force of the impact and in perching birds the flexor tendons contract as the foot grasps the branch.

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