The Tetrapoda: Amphibia, Reptiles, Birds 617 



food has been the development of highly efficient sense organs, particu- 

 larly the eyes. 



While fertilization is internal, the eggs are never retained within 

 the body. They are usually laid in special nests constructed by the 

 parents, and they are incubated at a constant temperature. Essentially 

 the eggs are identical with those of the reptiles, having the typical em- 

 bryonic membranes and the large amount of yolk. 



The birds have developed complex behavior patterns associated 

 with their mating habits, their search for food, and the care of the young. 

 In contrast to their reptilian forebears, the birds are very vocal animals. 

 They have special mating calls and songs. These unusual sounds are 

 made possible by a special syrinx at the base of the trachea. 



The Classification of the Class Aves. — Birds have been divided 

 into two main subclasses : 



Subclass 1. Archaeornithes. The lizardlike birds which possessed an elon- 

 gate tail, and teeth in sockets, and had the metacarpals not fused. Ex- 

 amples : Archaeopteryx, Archaeornis. 



Subclass 2. Neornithes. The caudal vertebrae are reduced in number, the 

 teeth are lacking or in furrows, the metacarpals are fused. Examples : 

 Hesperornis, a fossil bird, and all recent birds. 



The Biology of Birds 



At the present time, birds are probably at the zenith of their de- 

 velopment and have evolved species specialized for life in nearly all pos- 

 sible habitats. Since there are some 30,000 different forms recognized, 

 it is to be expected that there is almost infinite variation of certain 

 structures as well as of their habits. 



Structural Variations. — -As a group, the birds are structurally 

 quite homogeneous ; such variations as occur are adaptive to specific 

 food habits or ways of life. These variations are most evident in the 

 bills, wings, and feet. 



Since the forelimbs of the birds are useless for food getting or 

 other purposes, the bill performs many diverse functions. The bird 

 uses the bill not only for food getting, but for nest building, preening 

 of feathers, and defense. Seed-eating birds such as the sparrows have 

 heavy, conical bills adapted for crushing and hulling their hard food. 

 Insect-eating birds must search for their food in cracks and crevices or 

 in the soil ; associated with this, their bills tend to be longer or wider. 

 The warblers have long slender bills while the whippoorwill has a veri- 



