408 



THE BIRDS 



ing membranes located on the sides of the syrinx. Air sacs from the 

 lungs press against these membranes from the outside. As the current 

 of air rushes through the syrinx, it pushes these membranes outward, 

 but they are pushed back by the pressure generated in the air sacs and 

 this sets the membranes into vibration. By changing the shape and ten- 

 sion of the membranes, the pitch can be changed. 



Birds have a perching adaptation in the tendons of the legs. If you 

 were to try to sleep perched on the limb of a tree waving in the breeze 

 you would not be likely to stay on the limb for long. The birds are able 

 to do this because the tendons of the leg which are attached to the claws 



Photo by Chas. W. Schwartz 



Fig. 27.2. Male prairie chicken booming before female. These birds have a rather 

 elaborate courtship that may continue for as long as six weeks before mating. 



are pulled when the legs are bent and the claws automatically close. 

 Thus, the weight of the bird's body on the bent legs keeps the claws 

 tightly gripped on the limb. 



Bird Reproduction 



There seem to be two primary objectives in the life of the majority 

 of animals ; first, the getting of food, and second, reproduction. When 

 you have learned how animals accomplish these two vital functions, in 

 all their ramifications, you have an explanation for most of their habits. 

 Food getting in birds is so varied that it cannot be discussed for the 

 group as a whole, although some habits of feeding are discussed later in 

 the chapter. However, reproduction follows a similar pattern in all 

 birds and will be studied for the group. 



