22 THE MIGRATIONS OF BIRDS 



But little different is the theory that migration is 

 due to changes in temperature through which, with 

 the colder air of autumn warning of the approaching 

 extremes of winter, birds pass south, to return when 

 the season has changed again, so that a period of 

 warmth is approaching. In this hypothesis some 

 have held that the migratory birds originated in the 

 north, were driven south by the advance of ice in the 

 Pleistocene, and have returned to the north with the 

 coming of milder conditions. A love of birthplace 

 calls them now each year to return to the natal home. 

 Observation shows, however, that many birds re- 

 main a very brief period on the breeding-ground, — 

 merely long enough to permit the rearing of young, 

 — and then immediately begin the return journey 

 south, — so that the period spent in the northern 

 home is short. Many in fact return south long be- 

 fore there is need for them to do so; so that the argu- 

 ment of love of birthplace hardly seems substan- 

 tiated. 



According to a somewhat different hypothesis, all 

 species have arisen in the south and have spread to 

 the north through the natural impact of individual 

 upon individual in the struggle for existence; that 

 this pressure is at its highest point during the period 

 of reproduction, so that individuals then must fare 

 afield to find space in which their families may be 

 reared, and that with this duty accomplished they 



