CLASS AVES 



417 



433. Songs. — At the lower end of the trachea, or windpipe, where 

 it branches into the two bronchi leading to the lungs, birds possess 

 an organ known as a syrinx, which is the organ of voice. Both the 

 trachea and the bronchi are held open by cartilaginous rings. In the 

 syrinx these rings are variously modified and give attachment to stretched 

 membranes the vibration of which produces tones. 



434. Migration of Birds. — Among the most remarkable of the 

 phenomena connected with bird life is migration. Many animals 

 migrate but none to such distances and with such regularity as the birds. 

 Their power of flight makes it possible for them to cover great distances 

 in relatively short periods of time and their highly developed cerebellum. 



Fig. 306. — A precocial young, less than two days old, of a killdeer, Oxyechus v. vociferus 

 Linnaeus, "freezing" at the approach of an enemy and thus escaping detection by remain- 

 ing motionless and blending with its background. {Photographed, copyrighted, and contri- 

 buted by Gayle Pickwell.) 



combined with their dependence upon flight, has endowed them with 

 an ability to find their way which exceeds that possessed by any other 

 animal and is difficult for man to comprehend. Not all birds migrate, 

 and every gradation may be found between those which do not and those 

 which cover thousands of miles in their migrations. 



The greatest migration recorded for birds is that of the Arctic tern. 

 Since the breeding range of the Arctic tern extends south to Labrador not 

 all of the individuals of this species make a journey of the maximum 

 length, but those do which nest far north on the shores of the Arctic 

 Ocean, in a region of permanent ice and snow. As soon as the young of 

 these birds are able to fly they start upon their southward journey, and 

 moving at the rate of about 150 miles a day they cover in ten weeks a 

 distance of over 10,000 miles. After spending the southern summer in 



