354 PRACTICAL ZOOLOGY 



ment, lighthouses, and telegraph wires. Over fifteen hundred 

 birds were killed in one night by dashing against the Statue of 

 Liberty 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 mi- 

 gration of birds, such as the temperature and condition of the 



FIG. 234. Eggs of ostrich, hen, and humming bird, showing comparative size. 



(Photo, by E. R. Sanborn.) 



food supply. Other theories attempt to explain how birds find 

 their way during migration. The best of these seems to be the 

 " follow-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. 



Mating. --Mating takes place soon after the birds return in 

 the spring. A few birds, like the birds of prey, remain mated 

 throughout life, but most of them select new mates each spring. 



Nest Building. The nests are built in almost every con- 

 ceivable location, and those of one species resemble one another 

 but differ from those built by other species. Some birds, like 

 the prairie horned lark, build a nest on the ground (Fig. 219); 



