194 



THE BIOLOGY OF BIRDS 



little differences proving that birds and mammals are on 

 divergent evolutionary tacks. 



The song-box or syrinx is a transformation of the lower 

 end of the windpipe or trachea, usually helped by the upper- 

 most rings of the bronchial tubes — an illustration of that 

 familiar method of evolution which consists in making a new 

 thing out of some structure actually very old. The bird's 

 syrinx is a distinct novelty, and yet the materials for making it 

 are present in the reptile. 



The syrinx is anatomically intricate, but the essentials 

 are : (i) the firm, bony framework, sometimes dilated into 

 an accessory resonating sac, as in the diving ducks ; (2) the 



vibrating internal membranes over 

 which the out-breathed air passes 

 rapidly, producing all the many 

 sounds ; (3) a variable number of 

 muscles which alter the tension of 

 the membranes, and may also move 

 the windpipe as a whole ; and (4) 

 the controlling nerves, notably the 

 two hypoglossals from the brain. 



The song-box is usually more 

 complicated in birds with a re- 

 iterated, modulated song with 

 several phrases than in those with 

 From a simple calls and pipings. But it 

 muscle is not possible to establish any 

 exact correlation between the in- 

 tricacy of the song and the com- 

 plexity of the instrument. Thus 

 crows have a very complicated 

 song-box, but are not songsters, 

 though it may be said, of course, that the complexity is here 

 associated with talkativeness and large vocabulary. The 

 fact is that the musician has to be considered as well as the 

 instrument, part of him though it is. 



Fig. 37. — Song-box or syrinx 

 of a diving duck 

 specimen, 

 windpipe; M. 

 moving the trachea ; sy,, 

 the syrinx or song-box con- 

 taining the vocal cords ; 

 R.S., a special resonating 

 sac ; BR., the bronchial tubes 

 leading to the lungs. 



