THE STRUCTURE AND ACTIVITIES OF BIRDS 349 



rate ventricles. Birds are more active than amphibians or rep- 

 tiles, besides being warm-blooded, and they must therefore have 

 a highly developed heart for producing rapid circulation and an 

 especially favorable means of oxygenating the blood. Accord- 

 ingly we find the respiratory system highly organized. In addi- 

 tion to the lungs there are nine large air sacs within the body- 

 cavity which are connected with the lungs and which, besides 

 increasing the amount of air in the body, also decrease the spe- 

 cific gravity of the bird 

 and make flying easier. 

 Furthermore, many of the 

 air spaces in the hollow 

 bones communicate with 

 the air sacs. 



Bird Songs and Call 

 Notes. Connected with 

 the respiratory system is 

 the vocal organ or syrinx 

 with which birds produce 

 their call notes and songs. 

 This organ lies just where 

 the windpipe divides, send- 

 ing a branch to each lung. 

 It is an enlargement of the 

 windpipe containing a valve 

 which vibrates when air 

 is forced out of the lungs and which can be tightened by muscles, 

 thus regulating the number of vibrations and consequently the 

 pitch of the sound produced. 



These sounds may be divided into two kinds, call notes and 

 songs. Call notes form the principal language of the birds, 

 since anxiety, fear, and other emotions can be expressed by 

 them. Songs, on the other hand, are heard most frequently dur- 

 ing the nesting season. Usually only the males are able to sing. 

 The importance of learning the call notes and songs of birds 



FIG. 228. One young cowbird in a 

 vireo's nest. The three young vireos 

 were crowded out by the young cowbird. 

 (Photo, by Hegner.) 



