314 VERTEBRATE ZOOLOGY 



and released it has great difficulty in orienting itself and only suc- 

 ceeds in getting home if by chance it happens to discover a familiar 

 landmark. Young birds are much less capable of homing than are 

 older birds, and need to follow a leader until they become familiar 

 with the route. Some birds migrate in flocks of great size, others in 

 small numbers or even in pairs. The speed attained by migrating 

 birds may be as high as a hundred miles an hour, but the majority 

 of them scarcely attain half that speed. Even at the rate of fifty 

 miles an hour birds have been known to travel a distance of nearly 

 two thousand miles in two days; for they take little rest while mi- 

 grating, and are often entirely exhausted when they reach their 

 destination. It is during the migrating season that ignorant and 

 lawless people take advantage of the large numbers of fatigued birds 

 and shoot them in vast numbers, displaying in so doing a lack of 

 sportsmanship truly lamentable. 



GEOGRAPHIC DISTRIBUTION OF BIRDS 



Birds of good flying powers are as nearly cosmopolitan as any 

 animal could be; the albatross and the petrels range the oceans from 

 one extreme to the other. Birds of moderate flying powers may be 

 limited to one continent; many birds, for example, are confined to 

 North America, while others breed in North America and winter in 

 Central or South America. Flightless birds are often confined to 

 single islands, such as New Guinea or New Zealand. 



One should carefully distinguish between migration and distribu- 

 tion when dealing with birds; for when we say that a given species 

 breeds in Canada and winters in Central America we do not mean 

 that its area of distribution covers all of the intervening territory, 

 for a large part of this territory is not even passed over by the species 

 in question during the migration flights. 



DEVELOPMENT OF BIRDS 



The classic type for the study of avian embryology is the common 

 fowl. Usually the study of the development of the chick constitutes 

 a major part of college courses in vertebrate embryology; hence only 

 a brief outline of bird development need be given here. There is so 

 little difference between the development of the reptile and the bird 

 that the following sketch will serve to illustrate the salient features 

 of the embryology of the Sauropsida in general. 



