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.
Musculoskeletal system

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
Skeletal modifications
Head (Fig. 13.3)
Leg and foot
Wing
Feathers
There are several types of feather (Fig. 13.6):

Flight
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.
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.
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