748 EVOLUTION OF ANIMALS Part V 



birds are relatively large, often enormous, and set in exposed positions. The 

 eyeballs are protected by bony plates embedded in the outermost coat. The 

 pecten, a peculiar structure shaped like a half-folded fan, is suspended in the 

 vitreous humor. It is crowded with blood capillaries and nerves. Although its 

 function is not proven it may be connected with nutrition. 



Reproduction. Courtship and mating reflect the bird's generally rapid ac- 

 tivity. Courtships may include brief darting flights, social gatherings and cere- 

 monies such as those of prairie fowls, dances dignified or tempestuous (Fig. 

 36.19). Reproduction in birds is similar to that in reptiles. All young birds 

 hatch from hard-shelled eggs. Paralleling the essentially complete land life of 

 birds fertilization is always internal. Sperm cells developed in the testes pass 

 through coiled sperm ducts that open into the cloaca, and are ejected into the 

 cloaca of the female in the extremely brief mating contact. In the cloaca of 

 very young male chicks there is a small process, the rudiment of a copulatory 

 organ similar to one that is well developed in some reptiles. This structure is 

 the means by which the sex of downy chicks is determined in hatcheries. 



Fig. 36.19. Incidents in the mutual courtship of the great crested grebes, marine 

 diving birds in which the two sexes are strikingly similar in color and form. 1, 

 mutual head shaking; 2, the female is displaying her plumage before the male; 

 3 and 4, further views of the male rising from the water after various dives; 5, 

 both birds have dived and brought up weeds. Then, they meet together and go 

 through a period of head swaying. (Courtesy, Young: The Life of the Verte- 

 brates. Oxford, England, The Clarendon Press, 1950.) 



