392 GENERAL ZOOLOGY 



The causes which underlie these great movements cannot 

 yet be stated with absolute certainty. The southerly migra- 

 tion is probably to be associated in general with the failure 

 of the food-supply and the decrease in temperature. It has 

 been stated that the return to the north is due to the desire 

 of the birds to regain their old home, or to their desire for 

 seclusion during the breeding-season. The origin of the 

 habit has undoubtedly to be looked for in the geological 

 history of the world. During the Glacial Epoch (see p. 431) 

 a great part of the northern hemisphere was shrouded in a 

 mass of ice, which came down from the north upon a region 

 which was then almost tropical *ink its climate. With the 

 onward advance of the ice, the birds, like all other forms of 

 life, were driven south, returning whenever the melting of 

 the ice permitted. Geology tells of many periods of alter- 

 nate progression and regression of the ice-sheet, with accom- 

 panying changes of climate. We have already spoken of the 

 discontinuous distribution of the White Mountain butterfly 

 (see p. 48) as dating from this period. It may well be that 

 the northerly and southerly movements then begun among 

 the birds have been continued till to-day. 



Economic Importance of Birds. Leaving out of considera- 

 tion their value to man as a source of food, birds are chiefly 

 important economically in connection with their destruction 

 of insects injurious to vegetation. Of course not all birds 

 are beneficial in this respect ; whether they are beneficial or 

 injurious depends largely on the character of their food. 

 What we know of the food of birds has come not only from 

 the observation of the birds in the field but also from the 

 examination of the contents of their stomachs. The Division 

 of Biological Survey of the Department of Agriculture has 

 performed a most useful task in collecting, tabulating, and 

 publishing observations from all parts of the country, and 

 has recently supplemented this general work by a study of 



