MIGRATION OF BIRDS. 131 



both of their migratory instincts the finding of food, 

 and of fit places for the rearing of their young. The 

 general motion for these two purposes is in opposite 

 directions they move toward warmer regions in search 

 of food, and toward colder ones in order to build their 

 nests. The winter birds come to us for food, and the 

 summer ones for nidification. The winter ones never 

 are those that feed upon land insects, and but seldom 

 those that feed upon seeds ; because when they come, 

 there are few of these. They are chiefly water-birds, 

 in some sense or other. They frequent the shores of 

 the seas, the inland lakes, or the margins of springs, 

 rivulets, and rivers, and they swim or wade, or merely 

 run along the bank, according to their nature ; and 

 resort to those haunts where their food is to be found 

 with the most unerring certainty. They are all com- 

 mon inhabitants of regions farther to the north, 

 have reared their broods there, and remained till the 

 supply of food began to fail. The extent of their 

 flight southward depends upon the severity of the 

 winter; they come earlier, and extend farther, when 

 that is severe ; and their departure is accelerated by a 

 warm spring, and retarded by a cold one. Though the 

 diffusion of the same species of birds be much more 

 extended than that of the same species of quadrupeds, 

 there is still a variation according to the longitude. 

 The birds of passage which appear in Britain are not 

 exactly the same as those either of continental Europe 

 or of America ; and that accounts for the appearance 

 of the occasional visiters. A strong wind from the 

 east during the time of their flight often wafts a conti- 

 nental bjrd to our shores ; and a strong wind from the 



