300 BIOLOGY FOR BEGIXXERS 



development. In all cases, however, they are large in comparison 

 with eggs of other animals. 



The color varies greatly and is probably protective in some cases 

 where nests are open and exposed. On the other hand, eggs laid 

 in burrows and deep dark nests are usually very white, possibly 

 to make them more visible. 



Use. Since the egg is practically a store of food for a young 

 animal, it provides an especially nourishing and concentrated 

 form of human food which has been used by man for ages. Eggs 

 require no cooking, are rich in proteid and fat and are practically 

 all digestible. The egg crop of the United States is worth over 

 $300,000,000 per year. 



Incubation. The time of " sitting " or incubation is in propor- 

 tion to the size of the egg and varies from thirteen to fifteen days 

 for small eggs, to forty or forty-five days in the case of the swan. 

 The female usually sits, but the ostrich is an exception. Some 

 other male birds help in the incubation. The temperature required 

 is 105 degrees and must be kept almost constant. In birds which 

 are helpless and have parental care, the incubation begins as soon 

 as the first egg is laid, and the chicks hatch one after the other, 

 but in those birds like our hens, where the chicks hatch fully 

 feathered and able to feed themselves, all the eggs are laid before 

 sitting begins, so that they may all hatch at once. 



Bird Migration. One of the most mysterious and wonderful 

 instincts hi the world is that which controls the migration of birds. 

 The causes, methods, and means are little understood. Many 

 birds never migrate, such as the ostrich, fish-eaters, and parrots. 

 Crows, owls, jays, woodpeckers, and many others are practically 

 permanent residents. 



Migration may be caused by food supply, climatic changes, or 

 may be made for breeding purposes. It is not easily understood 

 why some species leave abundant food and warmth in the tropics 

 to breed in the cold and barren North. Insect eaters have to migrate 

 as whiter kills their prey; water birds must leave their ponds 

 before they freeze over; fruit eaters follow the season of their diet 

 to some extent, but after all, this does not account for the majority 

 of cases. 



