^°19^8^^] Adams, Ecological Succession of Birds. 127 



because of local variations in the physical features, such as vegeta- 

 tion, 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 exerted by these conditions which 

 break the monotony of the formation and make possible a diversi- 

 fied avifauna. But birds do not always respond 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 attention, 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, unin- 

 fluenced 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 area 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 characteristic 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 dom- 

 inant area, and make possible a diversified avifauna. The struc- 

 tural differentiation within a formation (zones, etc.) is thus largely, 

 in addition to variations in the formation itself, of local origin. 



6. Local associations or societies, in general, furnish the essen- 



