CLASS AVES 621 



b. The Colors of Birds 



Birds are among the most beautifully colored of all animals. 

 This color is due to pigments within the feathers (chemical 

 colors) or to structural peculiarities, such as prismatic shapes 

 which break up the rays of light into their component colors 

 (physical colors), or to both causes. Nestling birds possess dis- 

 tinctively colored feathers which later give way to the " imma- 

 ture plumage "; this is worn usually throughout the first winter, 

 and is generally dull in color, often resembling the plumage of 

 the adult female. Males and females frequently differ in color 

 (sexual dimorphism) , especially during the breeding season, when 

 the male acquires a brightly colored coat. The attempt to ex- 

 plain this difference has led to the theory of sexual selection. 1 



One important use of color is its protective value to the bird. 

 The colors and color patterns of birds, as well as other animals, 

 are such as to conceal these animals amid their surroundings. 2 



, c. Bird Songs 



The songs of birds, as explained on page 585, are produced 

 by the air passing through the syrinx. For one who wishes to 

 study birds, a knowledge of bird songs is indispensable, since one 

 hears a great many more birds than he is able to see. Songs 

 should be distinguished from call-notes. The former are usually 

 heard during the breeding season, and are generally limited to 

 the males. Call-notes, on the other hand, are uttered throughout 

 the year, and correspond in their meaning and effect to our con- 

 versation. By means of call-notes a bird is able to express anxi- 

 ety or fear, and to communicate to a limited extent with other 



birds. 



d. Bird Flight 



One of the most important functions of birds is that of flight. 

 The bodies of flying birds are structurally adapted so as to offei 



1 Darwin, The Descent of Man and Selection in Relation to Sex. 



2 Thayer, Concealing Coloration in the Animal Kingdom. 



