132 Adams, Ecological Succession of Birds. [\"'^i 



There are at least five important factors which enter into the 

 composition of the past and present conditions which have moulded 

 and are even now moulding the formation of the habitats upon 

 Isle Royale. These five are: — first, past climatic changes; second, 

 the local topography; third, the falling lake surface; fourth, dyna- 

 mic tendency of the vegetation; and fifth,, Xhe habits and structure 

 of the birds. With these guiding principles, let us now turn to 

 certain details of the resultant succession. 



a. The Aquatic Association and Habitat. 



The expanse of Lake Superior, the irregular shore line producing 

 coves, the inland water bodies and streams, together furnish an 

 extensive and expansive area of habitat. The cutting of the Lake 

 waves encroaches upon the land habitat, and the deposition by them 

 elsewhere causes minor extensions of the land habitat (as at Rock 

 Harbor where a sand spit furnishes a nesting site for a Kingfisher). 

 Inland the encroachment of the vegetation tends to restrict the 

 water areas, as the falling Lake level has, in the past, tended to 

 increase the land habitat. These processes must be recognized in 

 order to grasp the d}Tiamic tendencies of the habitat. 



The characteristic aquatic society is composed of the Herring 

 Gull, Loon, American and Hooded Mergansers, and the Pied- 

 billed Grebe; mainly fish eaters and scavengers. Other species, 

 of greater inland tendencies, are attracted by the fish fond, as the 

 Eagle, Osj^rey and the Kingfisher. The Gulls show a decided 

 preference for the great Lake, and the Loon for the inland waters. 

 The presence of the Kingfisher was influenced by the harbor with 

 its attendant sand banks and bars. As all these water bodies near 

 Isle Royale freeze over in winter, the strictly aquatic birds must 

 normally migrate to secure food. Of course none of these birds 

 nest in the open waters, but on the island beaches (Gulls), near the 

 mouths of streams, and inland in marshy places; but all, as a rule,, 

 nest near the water. The very young soon attend their parents, 

 and are thus in the water at an age when many land birds are vet 

 helpless in the nest, thus confirming their aquatic habits and habi- 

 tat. During migrations many other species frequent this habitat. 



A\here Isle Royale now is, once rolled the open Lake; and it is 



