756 PHYLUM CHORDATA : CLASS AVES — BIRDS 



A section through the oviduct shows — a peritoneal invest- 

 ment, longitudinal muscles, connective tissue with blood 

 vessels, circular muscles, connective tissue, a thick layer 

 of convoluted branched tubular glands except in the funnel 

 and the vagina, and most internally ciliated epithelium, 

 except in the anterior part of the funnel. 



In sexual union the cloaca of the male is closely apposed 

 to that of the female ; only in a few^ cases (in ducks and 

 geese, Crax^ Tinamiis, and in the Ratitae) is there a copu- 

 latory organ. The eggs are incubated by the parents 

 for a fortnight, a high tem.perature of about 40° C. being 

 sustained throughout. 



Habits and Functions of Birds 



Flight. — As birds are characteristically flying animals, many of 

 their peculiarities may be interpreted in adaptation to this mode of 

 motion. 



(a) Shape and general structure of the body. — The resistance offered 

 by the air to the passage of a body through it depends in part on the 

 shape of the body, and the boat -like shape of the bird is such that it 

 offers relatively little resistance. The attachment of the wings high 

 up on the thorax, the high position of such light organs as lungs and 

 air-sacs, the low position of the heavy muscles, the sternum, and the 

 digestive organs, the consequently low centre of gravity, are also 

 structural facts of importance. But it must be remembered that the 

 frictional resistance of the air is slight. 



(6) The muscles of flight. — The pectoralis major brings the wing 

 downward, forward, and backward, keeping the bird up and carrying 

 it onward. As it has most work to do, it is by far the largest. The 

 pectoraUs minor raises the wing for the next stroke. There are others 

 of minor importance. On an average these muscles weigh about one- 

 sixth of the whole bird, nearly one-half in some pigeons. Buff on noted 

 that eagles disappeared from sight in about three miniites, and a common 

 rate of flight is about fifty feet per second. In migration many birds 

 fly at a rate of 30-50 miles an hour. 



(c) The skeleton. — The rigidity of the dorsal part of the backbone, 

 due to fusion of vertebrae, is of advantage in affording a firm fulcrum 

 for the wing-strokes, while the arched clavicles (meeting in an inter- 

 clavicle and often fused in front to the sternum) and the strong cora- 

 coids (which articulate with the sternum) are adapted to resist the 

 inward pressure of the down-stroke. As the keel of the breast -bone 

 serves in part for the insertion of the two chief muscles, its size bears 

 some proportion to the strength of flight. It is absent in the running 

 birds, such as the ostriches, and has degenerated in the New Zealand 

 parrot (Stringops), which has ceased to fly and taken to burrowing. 



(d) Air-sacs and air-spaces. — The lungs of birds open into a number 



