CLASS AVES. BIRDS 



Air space 



443 



Blastodisk 



—Albumen 

 -White yolk 

 Yellow yolk 

 Cholaza 



Blastodisk 



f— Chalaza 



Egg of bird in 

 longitudinal section 



Blastodisk on yolk of 

 freshly broken egg 



Figure 312. Diagram of the structure of a bird's egg. 



milk," a secretion which results from a de- 

 generation of the cells which line the crop 

 of the adult. The pigeon's milk is ejected 

 from the crop of the parent into the mouth 

 of the young nestling. 



It has been demonstrated experimentally 

 that injecting a pigeon with a lactogenic 

 hormone (prolactin) will cause the secretion 

 of pigeon's milk at any time of the year. 



CHARACTERISTICS OF 

 BIRDS IN GENERAL 



Form and function 



The bodies of birds have become adapted 

 to various environments. This adaptation is 

 best shown by the wings, tails, feet, and bills. 



Fusiform shape 



There are a few flightless birds, but the 

 great majority fly. The adaptations that 



make flight possible are significant. One is 

 the matter of body form. So much has been 

 said about streamlining in recent years that 

 nearly everyone knows about air resistance. 

 It is relatively difficult to move a bulky and 

 irregularly shaped body through the air; on 

 the other hand, something that is fusiform 

 in shape or tapering toward the ends will 

 meet with less resistance in both air and 

 water. The body of the average bird ap- 

 proaches this shape as do the bodies of most 

 things that fly or swim (Fig. 313). 



Wings 



The wings of most birds are used as or- 

 gans of flight, and several different types of 

 flight are recognized. Some aerial birds, like 

 the swallows, gulls, and albatrosses, have 

 long pointed wings that enable them to re- 

 main in the air for many hours at a time; 

 whereas terrestrial birds, such as the bob- 



