402 METAZOAN PHYLA 



whose wing feathers are highly developed are the birds of paradise; 

 among those which have a highly modified tail are the peafowls, lyre 

 birds, pheasants, and turkeys. 



The colors of birds are in part due to pigment; such colors appear 

 the same when seen under any condition. Other colors are produced by 

 a combination of pigment colors with interference colors resulting from 

 reflection and refraction of light, caused in part by ridges and furrows 

 on the surface of the feather and in part by the internal feather structure. 

 These colors are metallic and changeable. 



Birds' feathers are shed and replaced at intervals, the process being 

 known as molting. Some birds molt but once each year — early in the 

 fall — while others also undergo a partial or complete molt in the spring. 

 The spring molt is usually accompanied by the development of a highly 

 colored breeding plumage. Some changes in the color of birds are due 

 not to molting but to the wearing off of the feather tips, which are of a 

 different color from the rest of the feathers. 



431. 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. 



432. 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, 

 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 the Antarctic, far removed from the northern winter, they return 

 again through the same distance to spend the northern summer at their 

 breeding grounds in the Arctic. Like all birds they nest in only one 

 region and have only one breeding season. As a result of their migra- 



