Chapter 13 Birds
The birds are a unique group of vertebrates. There are about 8500 known species in the world, all belonging to the class Aves. They are considered to have evolved about 150 million years ago from the thickly feathered Archaeopteryx, which links reptiles to birds. Birds and reptiles have much in common, such as the ability to lay eggs, but no other class of animal has feathers. The possession of feathers, and in the majority of species the ability to fly, is the reason for their success. Flight allows them to colonise new habitats, find new sources of food and escape predators. Much of their anatomy and physiology has evolved to facilitate life in the air.
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.)
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.
(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.
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.
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.