Chap. 36 BIRDS CONQUEST OF THE AIR 749 



The female organs usually develop to maturity only on the left side, but 

 hawks and some others are exceptions (Fig. 36.17). During the laying season 

 the ovary of an ostrich may weigh three pounds or more and the egg is equal in 

 volume to about a dozen and a half chicken eggs. When an egg reaches full size 

 in the ovary it breaks out of its enclosing sac, is grasped in the soft funnel of 

 the oviduct and begins its travel through the tube. FertiUzation occurs in the 

 upper part of the tube. The albumen or white is laid over the yolk by glands in 

 the middle region of the tube and the shell membranes, the so-called skin, 

 and finally the shell are added in the latter part. Eggs are usually deposited 

 soon after the shell is completed. For comparison of the reproductive processes 

 of other animals see Chapter 18. 



Distribution 



Birds live on all continents, on most islands, and in all seas. They live in all 

 climates, and are abundant in the tropics and through the temperate zone. They 

 penetrate well into the Arctic and penguins thrive in the antarctic cold that 

 mammals cannot endure. One or another species is at home from sea level to 

 heights of 20,000 feet on the slopes of the Himalaya Mountains. Although 

 flight has given birds the vast space of the air, they still conform to the laws 

 of animal distribution, and each species has its own geographic range and 

 particular habitat. Woodpeckers range all over North America below the 

 Arctic, but they hunt insects on tree trunks wherever they are. Emperor 

 penguins endure the storms, cold, and darkness of antarctic winters because 

 they can secure food. Owls and woodpeckers nest in holes in trees and bank 

 swallows and others in the ground, but no birds are subterranean like the 

 woodchucks and ground squirrels. In polar regions, there are few species and 

 many individuals; in temperate regions, many species are resident and many 

 more come and go in different seasons. There are also many species in the 

 tropics, among them various and resplendent parrots and birds of paradise. 



Migration 



Birds outdo all land animals in the distance and regularity of their migra- 

 tions. Not all species are far travelers; chickadees, downy woodpeckers, and 

 blue jays are semipermanent residents in many localities. Yet, individual birds 

 move from one place to another, and bird banding has shown many migrants 

 even among so-called winter residents. Except poor-wills and certain swallows 

 no birds hibernate. They remain in their own locality in full activity, or they 

 leave it and return in a later season. The general trend of migrations is north 

 and south. In the Northern Hemisphere, birds move toward the north where 

 food and nesting places are available during the warmer months, and toward 

 the south to warmth and food in winter. In the Southern Hemisphere where 



