624 COLLEGE ZOOLOGY 



migration, and one can predict with some degree of accuracy the 

 date when it will arrive in a given locality. The speed of migra- 

 tion is, as a rule, rather slow, and a daily rate of twenty-five 

 miles is about the average. 



During their migrations, birds are often killed in great num- 

 bers by striking against objects, such as the Washington Monu- 

 ment, lighthouses, and telegraph wires. Over fifteen hundred 

 birds were killed in one night by dashing against the Bartholdi 

 Statue in New York Harbor. Birds may also be driven out to 

 sea or be killed by severe storms. 



Many theories have been advanced to account for the migra- 

 tion of birds, such as the temperature and condition of the food 

 supply. Other theories attempt to explain how birds find their 

 way during migration. The best of these seems to be the " fol- 

 low-the-leader " theory. According to this, birds that have 

 once been over the course find their way by means of landmarks 

 and the inexperienced birds follow these leaders. 



/. The Nests, Eggs, and Young of Birds 



Some birds, like the hawks and owls, mate for life, but the ma- 

 jority of them live together for a single season only. The nesting 

 period varies according to the species. The eggs of the great 

 horned owl are often deposited before the snow has left the 

 ground, but most birds are forced to wait until April or later, 

 when the supply of insects is sufficient to feed their young. 



The nest site is chosen with considerable care, and is deter- 

 mined upon from the standpoint of protection. As a rule, birds 

 conceal their nests, or else build them in places that are prac- 

 tically inaccessible; for example, the nest of the song sparrow 

 is hidden beneath a tuft of grass, whereas that of the great blue- 

 heron is placed in the top of the tallest tree. 



Many species, like the auk and certain other sea-birds, and the 

 night-hawk and whippoorwill, make no pretence to build a nest, 

 but lay their one or more eggs directly upon the ground. The 

 killdeer and other plovers deposit their eggs in a small, crudely 



