432 ZOOLOGY 



Sometimes both take part. The cock may supply the hen with 

 food during incubation or pay no attention to the process. The 

 young may be ready to leave the nest and run about as soon as 

 they are hatched and dry (precocious) or they may be naked 

 and helpless for days (altricial). 



The coloration of birds is most varied and interesting. They 

 are to the vertebrates what the insects are to the invertebrates. 

 As in insects, the color is due to pigment, to physical surface 

 markings which produce luster, or to both. Males and females 

 are often differently colored ; if so the males are the more striking. 

 Young birds, before the sex qualities appear, are more like the 

 females. 



Some birds are differently colored at different seasons. Where 

 there is a difference the more highly colored feathers are pro- 

 duced in the winter and spring as the mating period approaches. 

 Color in birds is believed by some to be due to the high met- 

 abolic activity they show. This activity is believed to pro- 

 duce pigments as waste products. These come to be represented 

 in the coloration of the feathers. The metabolism is more active 

 at the mating season. Color may be useful to birds in two ways. 

 It may serve as recognition marks by which members of the 

 species may quickly recognize each other, and possibly prove 

 attractive to mates; and in some instances at least it serves for 

 concealment because of likeness to the surroundings. 



The eggs of birds are frequently remarkably colored. This 

 color is in the calcareous secreted shell. It is due to* color se- 

 creted by certain cells in the oviduct. The color of eggs of a 

 species is often as distinctive as the color of the bird. It may 

 be uniform, or in definite spots, or in irregular splotches. It is 

 probable that the coloration of eggs is also frequently protective. 



None of the bird's responses to its conditions is more striking 

 than the act of migrating. Some do not migrate, but most do. 

 The value of the instinct is clear enough. Each species has its 

 own customs. The time of starting, the route, the rate of migra- 

 tion, and the termini vary for different species. Usually the 

 rate is not rapid. Some fly pretty steadily for considerable 

 distance. The ducks work their way along the streams and 

 lakes, or from one to another. The warblers flit from forest to 



