ECOLOGY OF ISLE ROYALE. 133 



lY. Minor Avain Environments and their Associations. 



We have seen that the larger geographic environments or formations 

 are characterized by definite conditions and associations, and at the 

 same time that even throughout these favorable regions the climax as- 

 sociation is not distributed with absolute uniformity because of local 

 variations in the physical features, such as vegetation, water basins, 

 streams, mountains, etc. For the student of local bird life the real work 

 begins when one attempts to examine into the causes and influences ex- 

 erted by these conditions which break the monotony of the formation 

 and make possible a diversified avifauna. But birds do not always re- 

 spond as closely to slight local influences as does the vegetation, and for 

 this reason one must learn by experience just what size of units must 

 be used. Thus in the forest a few wind-falls will attract but little 

 Eittention, but a burn of a few acres will have a noticeable influence in 

 harboring those species of birds which frequent openings; while swifts 

 and swallows ignore many local influences which dominate other species. 



It should also be noted that whenever possible it is of distinct 

 advantage to examine all habitats in their original state, uninfluenced 

 by man. 



Instead of discussing the leading features of local conditions and 

 their societies or associations in detail, only an outline of them will 

 be given, and that in a form to facilitate use and revision. 



1. Minor environments are primarily dependent upon local conditions, 

 and are thus in a sense correspondingly independent of the dominant 

 forces of the region. This is, of course, a relative condition. 



2. Minor environments are, as a rule, relatively limited in area. 

 In general their limited ai'ea favors their short duration, but age is 

 primarily a result of the rate of change. 



3. Marked isolation, even when of extensive linear extent, — as a 

 shore line, along a stream, or an elongate rocky ridge, — is also character- 

 istic of minor environments. 



4. Minor environments tend to become encroached upon by the 

 dominant regional influences and ultimately to become extinct. The 

 succession of societies in local habitats is a declining one, while that 

 of the geographic or climax habitat is an increasing and ascending one. 



5. Local habitats produce most of the variety within the dominant 

 area, and make possible a diversified avifauna. The structural dif- 

 ferentiation within a formation (zones, etc.) is thus largely, in addition 

 to variations in the fprmation itself, of local origin. 



6. Local associations or societies, in general, furnish the essential 

 clues as to their earlier successions which have attended the evolution 

 or development of regional dominance. The variations in these are due 

 both to the kind of life and to the influence of adjacent associations and 

 centers of dominance. 



7. Marginal societies are particularly liable to variation in com- 

 position, due to the combined influences of adjacent formations or centers 

 of dominance as well as to local conditions. 



8. Comparative studies of local habitats will form the most general 

 and practical guide in the determination of the successions in the forma- 

 tion. 



9. Local habitats and societies, in common with the larger environ- 



