818 FIELD, FOREST AND FARM 



north. They take possession once more of their 

 groves and forests, their rocks and prairies, which 

 they know how to find with an inconceivable ac- 

 curacy. There they build their nests, rear their 

 young, and gain strength for the coming journey; 

 and upon the return of cold weather they go back 

 again to the lands of sunshine. 



' ' These periodical journeys are called migrations, 

 of which there are two each year, — that of autumn, 

 when the birds leave us and go southward, and that 

 of spring, when they fly northward and come back 

 to us. These semi-annual Sittings take place all over 

 the earth. 



"The various species do not all fix upon the same 

 time for their migration, but each has its own cal- 

 endar, from which it departs only very slightly. 

 Some start well in advance of the increasing chill and 

 the lessening abundance of food, while others do not 

 leave their native land until driven by actual neces- 

 sity, when the cold has become severe. Thus our 

 martin flies away for Africa as early as the month of 

 August, whereas the chimney-swallow lingers until 

 October or even November. 



"The martins forsake our turrets and old walls, 

 our steeples and belfries, while the summer heat is 

 still intense and the small flies on which they feed 

 are still abundant. It is not, then, any lowering of 

 temperature that drives them away, nor is it any 

 lack of food that hastens their departure; but they 

 have a secret presentiment of the change of season 

 that is coming in a few weeks; a deep-seated unrest, 



