142 MICHIGAN SURVEY, 1908. 



Goldien-crowned Kinglet, Red-breasted Nuthatch, Canada Jay, Downy, 

 Hairy, Arctic Three-toed and Plicated Woodpeckers, and the White- 

 winged Crossbill. Here again the association becomes small in variety 

 of species and comparable with the small society which must have been 

 associated with the complete dominance of the Lake waters. Thus there 

 has been a development of diversity from simplicity, with later a return 

 to simplicity. To these birds of the forest should also be added those 

 species of general distribution!, as the Eagle, Swift, Swallows, etc., a 

 class of birds whose predaceous, insect-feeding and wide ranging habits 

 make them particularly diiBcult to properly associate. A careful study 

 of 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 migrate"! 

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

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

 tions. Just as upon Isle Royale a definite dynamic trend was given to 

 the complete environment by the falling Lake surfa,ce, so in the post- 

 Olacial northward migration there was a northward migrating climate. 

 Tliese conditions determined that on the north side of this immense suo- 

 oession or migration habitats and associations were developed which 

 are companaMe to those attending the downward marcli of the Isle 

 Royale beach; and even today, 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 today in passing from the forest to the rocky beaich 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 understanding 

 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 understanding of suc- 

 cession. Some of these general relations have been outlined, but certain 

 others are needed which have been well expressed by Brewster ('06, p. 

 G2-63) : "Many if not most birds show a marked preference for breed- 

 ing in, certain regions, throughout 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. At its beginning 

 each pair takes possession of a definite tract of woodland, orchard, 

 swamp or meadow, which the male is ever on the alert to defend against 

 trespassers of his own kind and sex, although he often seems qaite 

 willing to share his domain with birds of other and perhaps closely re- 

 lated species. The extent of the area thus monopolized varies exceed- 

 ingly with birds of different S])ecie8. An apple orchard wliich affords 



