134 Adams, Ecological Succef^sion of Birds. {.'aj^H 



this zone. ^^ itli its onward march there are simultaneous changes 

 in several associations which, while they will vary in their changes, 

 yet all tend to converge in harmony with the dominant factors. 

 These conditions migrate or radiate from the highest land. On the 

 other hand, the -inland marginal zones, which border the smaller 

 water bodies, migrate inwardly; and being closed areas, tend to 

 become extinct. This marginal zone, particularly beyond the 

 upper beach, forms one of the most interesting and complex con- 

 ditions found upon the island. It is not an ecological unit, but is 

 composed of several of them. This is where most of the confusion 

 arises in actual field work of habitat studies. 



b. The Shore and Marsh Association and Habitat. 



As the area of the islands expanded and the shore line was 

 lengthened, the habitat for shore birds increased; but the steep 

 and rocky shores were unfavorable for the development of beaches 

 because loose rock, as tools for the waves, was limited in amount. 

 The local character of the shingle and gravel to-day found in the 

 various coves clearly indicates their local origin; and much the 

 same conditions have obtained in the past. On account of these 

 conditions, the sandy beaches are very conspicuously absent. The 

 dynamic tendencies of the beach are those which cause the extension 

 or restriction of the aquatic and beach habitats, supplemented by 

 the drift which is tossed upon the shore. Where there is shallow 

 water, and mud accumulates, favorable conditions are furnished 

 for invertebrate food for birds. Inland, the numerous lakes, ponds 

 and marshes furnish shore conditions which tend to become extinct 

 through drainage or overgrowth of the vegetation, except in those 

 parts of the larger lakes where wave action tends to scatter such 

 accumulations as rapidly as formed, or to prevent its formation 

 altogether. 



Although observations on this subject arc quite limited, yet it 

 seems fairly safe to consider the Spotted and Solitary Sandpipers 

 as characteristic birds of this association. Upon such a rocky coast, 

 sandy and gravelly beaches are quite exceptional and are confined 

 to protected coves. Additional diversity is produced where small 

 streams enter these coves and produce deltas. 



