472 



THE BIRDS 



XVI. 24 



trachea. The latter is often long and coiled, perhaps to warm the air. 

 The tracheal rings are bony and complete. The voice is produced in 

 the syrinx, a slight enlargement at the lozver end of the trachea, con- 

 taining a pair of semilunar membranes with muscles that alter the 

 pitch of the sound. The apparatus is simple in many birds, but the 

 muscles are very complicated in the singing birds and are especially 



s.cerv 



s clav 



s. thor 

 ant 



s Lhorpost 



s.abd. 



Fig. 283. Diagram of lungs and air-sacs of pigeon, seen from ventral side on left, 



dorsal on right. On the left side only the ventral surface of the lungs and the 



expiratory bronchi and air-sacs are shown (dotted). On the right are the inspiratory 



bronchi and air-sacs (in black). 



B. main bronchus; C. cervical ventrobronchus; M. mesobronchus; V. vestibule; s.abd. 



abdominal air-sac; s.cerv. cervical air-sac; s.clav. clavicular air-sac with diverticulum (ax.) 



in axilla; s. thor. ant. and post, thoracic air-sacs. (After Brandes and Ihle.) 



large in the males. Many varieties of sound are produced, from simple 

 cries appropriate to each sex to elaborate songs. In many species the 

 song is given in its full complexity by individuals that have had no 

 opportunity of hearing others sing, but in some the song is largely 

 learnt by the young and may show considerable local variation. The 

 voice is used for communication in various ways, including, in social 

 birds such as rooks, the giving of warning and the frightening away 

 of intruders. The language may include as many as fifteen sounds used 

 under different circumstances (chaffinch). The more elaborate song of 

 male birds is used in courtship both as a sexual stimulant and as a 

 threat to other birds invading the chosen territory (p. 503). 



