352 



PRACTICAL ZOOLOGY 



breeding grounds in the Arctic Circle the first week in June. 

 The elliptical course they follow is approximately twenty thou- 

 sand miles in length, and this remarkable journey is undertaken 

 every year for the sake of spending ten weeks in the bleak, tree- 

 less, frozen wastes of the Arctic Region. 



Most birds migrate on clear nights at an altitude sometimes 

 of a mile or more. Each species has a more or less definite 



FIG. 231. Nest and eggs of the black tern. The eggs were laid in a 

 slight cavity in the muck of a marsh lined with a few dry stems. (Photo, by 

 Hegner.) 



time of migration, and one can predict with some degree of ac- 

 curacy the date when it will arrive in a given locality. The 

 speed of migration 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- 



