140 Adams, Ecological Succession of Birds. [Anril 



this class of birds will be necessary before they can be satisfactorily 

 correlated with their proper avian associations. 



But let us not overlook the fact that even this dominance is only 

 relative, for since the Ice Age even this entire formation has migrated 

 northward, and a true succession has been produced with its attend- 

 ant changes in the conditions and in the composition of the asso- 

 ciations. Just as upon Isle Royale a definite d}'namic trend was 

 given to the complete environment by the falling Lake surface, so 

 in the post-Glacial northward migration there was a northward 

 migrating climate. These conditions determined that on the north 

 side of this immense succession or migration habitats and asso- 

 ciations were developed which are comparable to those attending 

 the downward march of the Isle Royale beach ; and even to-day, by 

 passing from Isle Royale to the tree limit with its zone of aspens 

 and birches, one may find representatives of the various kinds of 

 associations which in all probability moved north, just as to-day in 

 passing from the forest to the rocky beach balsams and spruce are 

 encountered before the aspens and birch. If however, this is only 

 another case of convergence and not at bottom the same or a 

 comparable process, we are then certainly far from an understand- 

 ing of even the general nature of the problem. 



3. Internal Factors. With the idea of succession, as exemplified 

 by Isle Royale, let us turn to other factors which influence the 

 internal relations of the birds within an association or society, 

 because such relations are also necessary to an intelligent under- 

 standing of succession. Some of these general relations have been 

 outlined, but certain others are needed which have been well 

 expressed by Brewster ('06, p. 62-63): "Many if not most birds 

 show a marked preference for breeding in certain regions, through- 

 out which they are more or less evenly and generally distributed, 

 but within which their numbers do not seem to increase beyond 

 fixed maximum limits no matter how carefully the birds may be 

 protected or how successful they may be in rearing their young .... 

 I have observed — as, indeed, who has not! — that few birds — 

 excepting those which, like Swallows, Terns, Herons, and Gulls, 

 are accustomed to nest in colonies — tolerate very near neighbors 

 of their own species during the season of reproduction. Xi its 

 beginning each pair takes pos.session of a definite tract of wood- 



