A HISTORY OF VERTEBRATES: BIRDS AND MAMMALS 483 



perching birds, the tendons of the foot are so arranged that the weight 

 of the body automatically causes the toes to flex and grasp the perch 

 when the bird alights upon a branch. The woodpeckers have sharp 

 claws and the fourth toe is turned backward with the first to form a foot 

 ideally suited for clinging onto the sides of trees. Swimming birds have 

 a web stretching between certain of their toes— the three anterior toes 

 in loons, albatrosses, ducks, gulls and many others; all four toes in 

 pelicans, cormorants and their relatives. The marsh-dwelling jacana of 

 the tropics has a foot with exceedingly long toes and claws that enable 

 it to scamper across lily pads and other floating vegetation. Swifts and 

 hummingbirds have very small feet barely strong enough to grasp a 

 perch. These birds spend most of their time on the wing and almost 

 never alight on the ground. 



Reproduction. Birds have developed elaborate behavioral patterns 

 and structural modifications associated with reproduction, many of 

 which have been carefully studied. For example, in most of the common 

 species each male bird stakes out for himself a well delineated nesting 

 territory which he vigorously defends against all rivals and into which he 

 hopes to attract a female. The distinctive song of the male during the 

 breeding season advertises the territory to females of the appropriate 

 species and warns rival males to stay away. The brilliant plumage of the 

 male birds plays a similar role, serving both to warn rivals and attract 

 and stimulate females. An advantage of this territorial organization of 

 breeding birds is that it ensures a reasonably uniform distribution of 

 mating pairs in the inhabitable area. This facilitates finding food with- 

 out going far afield, and helps to get and to keep the parent birds to- 

 gether. 



Once a female has been attracted to the territory, courtship begins. 

 Sometimes it is accompanied by elaborate display rituals which appar- 

 ently serve as a sexual stimulant leading to nest-building and copulation 

 (Fig. 24.9 B). A brief cloacal apposition is sufficient to transfer sperm to 

 the female reproductive tract; only a few male birds, chiefly primitive 

 species, retain the reptilian copulatory organ. Further courtship and 

 copulation may occur after the eggs have been fertilized and laid. Pre- 

 sumably this aids in keeping the parents together for the tasks of incu- 

 bating the eggs and caring for the young. Voung chickens and some other 

 birds are precocial. They are covered with down, and can run about 

 and feed for themselves when hatched. But most of our song birds are 

 altriclal, and are naked and helpless when they first emerge from the 

 eggs. Such birds need close parental care to supply food and warmth 

 during the critical period of their infancy. Either or both parents care 

 for the young. As Professor Young, of University College, London, has 

 so aptly put it, "In birds, as in man, the 'procreation of children' is not 

 accomplished by a single act of fertilization." 



Migration. The capture of sufficient food and the reproductive 

 process are the motivations responsible for most of birds' activities. Some 

 birds are able to fill all of their needs in the general area in which they 

 were hatched, but others have taken full advantage of their power of 

 flight and go considerable distances in their search for favorable nesting 



