ECOLOGY OF ISLE ROYALE. 135 



OF mixed in character, bordering and in the depressions ; but is mineral, 

 stony and residual elsewhere. The combined shore and beaches are 

 extensive, largely stony and gravelly, and contain but little sand; much 

 of the shore line is rocky and precipitous; many outlying islands. 

 Vegetation, herbaceous in shallow inland waters and as a ground cover 

 except where the shade is too dense, and upon rocks; shrubs on pro- 

 tected beaches, in more open places in the forest and in bums; the 

 forest consists of Tamarack, Black Spruce and Arbor Vitse in bogs; and 

 elsewhere in mesophytic conditions of Balsam Fir, Arbor Vitse, White 

 and Yellow Birch, and rarely Sugar Maple. Upon the dry ridges, Jack 

 Pine; and in burned areas, Aspen and Paper Birch. Climate, seasonal 

 changes very pronounced; winters very long and cold, and summers 

 short and cool; a relative humidity of about 80% in December and 

 of about 70% in July [of. Johnson, '07) ; a mean temperature for 

 January 7.97° F.; and for July, 62.24° F. (Port Arthur data). Early, 

 deep snows. Prcdaceom animals, as the Lynx, Marten, weasels. Red 

 Squirrel and bats are directly in competition with the birds for food, 

 or prey upon the birds. 



The above environmental factors are dominant features and give us 

 a general picture of the conditions, largely in terms of common ex- 

 perience. In the life of the birds, however, a complete reassortment 

 and change of intensity in these factors occurs when they are com- 

 bined as habitats. The surrounding lake, the numerous bays, small 

 lakes and ponds compose the aquatic habitat and make it a characteris- 

 tic feature. The very irregular and extensive shore line and limited 

 beach area characterize the coastal border, while inland, excepting the 

 main bodies of the few larger lakes, the encroachment of the bog vegeta- 

 tion upon the shores is such as to prevent an extensive development of 

 sandy open beaches. The above mentioned habitats are open unforested 

 areas; the remainder of the island, with the exceptions of the bare 

 rocky ridges, the clearings and burned over areas, are fostered. Very 

 extensive swamp forests abound in the elongate valleys and the borders 

 of the water bodies, and are composed of Tamarack, Black Spruce and 

 Arbor Vitse. Tlie mesophytic forest occurs on drained areas and is 

 characterized by Balsam Fir, White Spruce and Paper Birch; the 

 burned areas by second growths of aspens and Paper Birch. Then there 

 are also influences Vhich are exerted upon the bird life in general, as 

 for example, migi?ation. In this case, undoubtedly both external condi- 

 tions and the habits and the behavior must be correlated. Another 

 general and dominant influence should be reiterated here, and that is 

 that all open areas tend to become invaded with vegetation and finally 

 forested, whether they are lakes, ponds, bogs, rock openings on the 

 ridges, burns or clearings. The mesophytic Balsam-spruce forest tends 

 to monopolize all habitats, and gives a definiteness to all succession 

 upon the island. 



From a genetic standpoint the past and present dominance of the 

 surrounding Lake must be recognized. This formerly stood at a level 

 much above that of the highest ridges upon the island, as is clearly 

 evidenced by the abandoned beaches on the north shore of Lake Superior. 

 Such relations prove that Isle Royale was once a rocky reef in the 

 lake, which, as the Lake level was lowered (it is quite unlikely that the 



