i62 THE STORY OF THE BIRDS 



of this expansion is mostly polewards, but some- 

 times nearly due east or west. I attribute the 

 autumn migration of birds to an overpowering 

 impulse to return to the winter or range base, in 

 which each individual and its descendants has 

 some special spot, to which it is drawn by a 

 variety of local causes. There are thus two 

 points to which a migratory bird is attracted : its 

 non-breeding centre, often an area associated 

 with its gregarious instincts ; and its nesting- 

 place, with the equal attraction of a mate. Most 

 of us know how unerringly birds return season 

 by season to their old breeding-places ; it may 

 not be so widely known, still it is as much a fact, 

 that birds return with as much certainty to cer- 

 tain spots in which they pass the winter. The 

 breeding impulse may be said to be the primal 

 factor in spring; the social instinct, or an equally 

 strong nostalgic impulse, a chief factor in autumn. 

 The habit of returning in autumn, as soon as re- 

 production is accomplished and the moult safely 

 over, to certain areas, appears to be as deeply 

 rooted as the desire to return to other localities 

 to breed in spring. The Impulse to return may 

 be, and possibly is, inherited. But inheritance can 

 go no further than this impulse ; the successful 

 accomplishment of the journey depends upon 



