ANATOMY OF FLIGHT

The development of flight has been the single major factor underpinning the evolution of avian anatomy. Freed from the limitations of terrestriality, birds have been able to colonise an enormous range of habitats, from deep in Antarctica to the harshest of deserts.

Flight allowed birds to escape predators (except for other birds, of course) and triggered the development of elaborate colouration and noisy vocalization systems for communication.

Wing structure

Wing structure in relation to flight is best understood in terms of the aspect ratio: length divided by average width.

Typical shapes are as follows:

High aspect ratio – gliding flight (eg gulls, albatrosses). The wing is long, narrow and flat. Some accomplished gliders like albatrosses use surface wind shear effects over waves to glide for miles without flapping and may even be grounded on calm days as they lack the strength to take off.

High aspect ratio wings for high speed flight (eg swallows, raptors). Typically the wings are tapering with little camber and swept-back. Some forms of swift are so well adapted for flight that the legs are vestigial and almost all activities apart from brooding take place aloft.

Low aspect ratio for rapid take-off and manoeuvrability (eg pheasants). The wings are stubby and highly cambered. The outer primary feathers may be slotted for added lift and control in twisting flight. Primary feathers attach on to the digits proper, while secondary feathers attach to the arm.

Low aspect ratio for soaring flight (eg vultures, eagles, storks). The wings are broad with well-developed slots for lift. Each primary feather may be used as an individual aerofoil.

Feathers

Feathers are unique to birds, and probably arose as modified scales for thermoregulation. Use in flight probably came along as a secondary adaptation. Below figure (redrawn from Kardong (1995) shows a hypothetical intermediate stage. Some lizards use similar flattened scales to reflect radiation and to alter body outline (eg Chameleons).

Details of feather structure (redrawn from Freethy, 1983)

Details of feather structure were covered in FMHV in Year 1. Each feather develops from a hollow precursor that unfolds to form a complex interlocking keratinous structure.

The basic structure is a proximal supporting calamus or quill: this extends out to a rachis or shaft bearing barbs with interlocking barbules forming the vane. The calamus is a hollow structure with a superior and an inferior umbilicus embedded in a cutaneous follicle. The inferior umbilicus is continuous with the growing tissue of the dermal papilla. During growth the calamus is filled with mass of spongy tissue that dies away as the feather matures to leave a hollow structure. The dermal papilla remains as a bud of living tissue that will replace the feather after it is shed at moulting.

Four main types of feather are recognised.

Down feathers totally lack barbules and lie close to the skin.

Filoplumes have got long shafts but relatively little vane and are generally for display. A variant, the bristle feather, is used for mechanical sensory purposes — normally around the face.

Flight feathers have a long rachis and prominent vane, and are attached to the major limb bones. The main flight feathers on the forelimb are the remiges, divided into the primaries on the manus and the secondaries on the antebrachium. Those forming the balancing and steering surfaces of the tail are the retrices.

Contour feathers form the aerodynamic shape of the bird and may have extensive fluffy segments lacking interlocking barbules, for insulation. Those found at the bases of the remiges and retrices both dorsally and ventrally are called coverts.

Feather design allows for extremely efficient aerodynamics as well as rapid repair of superficial damage. If the barbules should become separated, the bird can easily preen them back into place with the aid of the secretions of the uropygial gland. Thus feathers are much less vulnerable to injury than, say the membranous wings of bats. Nevertheless, they are regularly replaced in annual moult cycles.

Feathers do not continuously cover the skin surface. They are arranged in characteristic tracts or pterylae, leaving exposed skin areas that may be used for heat exchange. The abdominal brood patches are important specialisations of featherless skin where the dermis thickens and becomes highly vascular.

Pterylae (redrawn from Freeethy, 1983)

The Skeleton

Apart from the lightness of the bones and other adaptations such as air sacs to reduce weight, the ability to fly imposes certain specialisations on the thoracic and shoulder regions.

Thorax

There is extensive fusion of the thoracic vertebrae to provide a rigid upper cage to the thorax. Essentially only one or two vertebrae have free articulation. The cranial fused vertebrae form the notarium, while the caudal ones form the synsacrum. There are several free caudal vertebrae in the tail, giving flexibility to this structure for steering in flight, and the whole terminates in an upturned pygostyle of 5-6 fused vertebrae.

Sternum

The sternum is an essential component in the anchoring of the flight muscles. It is either flat, as in the Ratite birds (ratis: a raft) or with a pronounced keel as in the Carinate birds (carina: a keel). The sternum articulates with the vertebral column by a series of sternal ribs that are involved in respiratory movements (qv)

Pectoral girdle

The diagram below (redrawn from Freethy, 1983) will serve to illustrate the structure of the shoulder joint and the muscles that act on it. The main depressor muscle is the pectoralis, running laterally to the keel of the sternum. Its action is opposed by the elevator muscle: the supracoracoideus, running medially to the joint and passing through the triosseal canal, where the scapula articulates with the clavicle and coracoid bone.

The shoulder (redrawn from Freethy, 1983)

The shoulder joint and thorax are highly specialised as they have to bear the weight of the bird in flight and the articulation needs to be able to tolerate a wide range of movements including rotation. The following diagram (redrawn from Freethy, 1983 ) summarises some of the range of specialisations that can be seen. The upper diagram is the wing skeleton of a pelican, while the lower is that of a hummingbird.

Hummingbirds show extraordinary manoeuvrability: they can hover, move in all three dimensions and even turn upside down. In addition they can accelerate and stop very rapidly.

Wing bone anatomy in a pelican (top) and a hummingbird (redrawn from Freethy, 1983)

This range of movements is reflected in the following anatomical features:

The elevator muscles are about 50% of the mass of the depressors, compared with 5-10% for most birds;
The manus is greatly enlarged relative to the arm area with a corresponding variation in the relative importance of primary and secondary flight feathers

Flight and weight considerations

A number of features of the bird’s skeleton are specialised for flight and for the alternation between flying and walking or perching. These include:

Short body with a centralised centre of gravity;
A rigid but light skeleton with extensive areas of fusion (eg vertebrae) and overlapping and interlocking areas (eg strap-like projections of the ribs);
Bracing adaptations in the pectoral girdle to support the flight muscles;
A pronounced keel to the sternum for anchoring the same muscles;
Light but strong bones with extensive air spaces;
Relatively light heads and flexible necks

Disorders of the skeletal system

A number of disorders are seen and are of economic importance especially in domestic chickens. If you are interested, follow up on the following.

Spondylolisthesis (kinky back) is ventral displacement of the free T4 vertebra; in its most severe form it may lead to paraplegia;
Scoliosis is lateral bending of the vertebral column, usually thoracic;
Kyphosis (humpback) occurs in the lumbosacral region in rickets (vitamin D3 and Calcium deficiency) and in staphylococcal osteomyelitis;
Osteomalacia (cage layer fatigue) may result in fractures of the thoracic vertebrae;
The most commonly affected vertebra is the “weak link” of the single free vertebra between the notarium and the synsacrum.

Questions

What governs moulting?

How do feathers replace themselves?

What happens to feathers if birds don’t preen them?

Penguins have a keeled sternum despite being flightless. Why?

Call (410) 603-1400