February 



iinches, etc., which reverse this order, and 

 only come at tlie approach of cold weather or 

 in mid-winter, and disappear in the spring. 

 He may be equally unfamiliar with the fact that 

 still other species, like some of the thrushes, 

 finches, warblers, and greenlets, can be seen 

 only for a kw weeks at a time at two different 

 periods in the year, and is perhaps unaware 

 that a very few species are to be found in the 

 woods throughout the whole year. 



These various movements are not due to the 

 special peculiarities of the several classes, but to 

 the law controlling them all equally, and the 

 apparent complexity of the law resolves into 

 the utmost simplicity when it is understood. 



Bird-migrations are all in the direction of 

 north and south, and the underlying cause 

 of this is that they are determined chiefly by 

 the two considerations of temperature and food- 

 supply. With uniform climate and abundant 

 subsistence, birds would, doubtless remain in 

 their several localities, or approximately so, the 

 entire year. In that case, such of the war- 

 blers as find their summer home in northern 

 New England and Canada would also remain 

 there throughout the winter, and we should not 

 be likely to see them except at the personal in- 



49 



