I 1 6 Loomis on Birds of Chester County, South Carolina. I April 



of northward movement. If this were the real cause birds of 

 unstable local distribution, like the Dickcissel, would be without 

 incentive, still they are regular migrants. A cause must cover 

 the whole gi'ound to be the fundamental cause. It is not 

 unlikely that love of home is an important factor governing local 

 distribution in many species, but causes that merely influence the 

 selection of place of local abode are not to be confounded with 

 the causes that occasion migration. As has been stated under 

 'Physical Cause,' northward migration is viewed simply as a 

 return movement that is restrained only by the snow and ice. 

 There are outward conditions that necessitate it. On the part 

 of the birds, as in early southward migration, there is intelligent 

 adaptation to these conditions — how far-reaching it is, and to 

 what extent heredity enters as a factor, the limits of our present 

 knowledge leave undetermined. 



Finally, to sum up in brief the conclusions reached in the 

 preceding remarks : — 



(i) That migration begins with the southward movement, 

 commencing south and north before August and progressing 

 gradually, the two movements each extending over a period of 

 nearly six months. 



(2) That the young do not precede their parents in the south- 

 ward movement. 



(3) That all southward movement of birds is enforced depart- 

 ure from the region of their birth (enforced evacuation of territory 

 capable in winter of supporting but a small portion of its summer 

 life), and that all northward movement is return from exile at 

 the earliest opportunity, necessitated by pressure from the south 

 (by the need of dispersal and occupancy of all available food 

 areas), and perhaps, in some species at least, by requirements as 

 to climate during the breeding season. 



(4) That the earlier southward movements are anticipatory, 

 and necessarily so, and the later directly resultant of the con- 

 ditions of winter. 



(5) That irregular occurrence in winter birds is ascribed, 



(a) To variableness in the location of isolated communities, 

 independent of failure of food or severity or mildness of season. 



(6) To sudden cold contracting the food area and forcing birds 

 southward (cold being the remote cause and failure of food the 



