^°1908^^] Adams, Ecological Succession of Birds. 121 



The following discussion and suggestions on the larger environ- 

 mental units attem])t only an outline of certain phases of the prob- 

 lem, in order to call attention to certain principles which seem 

 useful as a background for the intelligent study of bird habitats and 

 succession. From such a standpoint as this, the dominant influences 

 of given areas and environments are of particular interest and of 

 fundamental value. By focussing attention upon the importance 

 of recognizing these dominant environmental influences, we may 

 hope to escape some of the confusion which appals those who are 

 keenly impressed with the chaos and complexity of the problem. 

 These dominant factors are usually not single isolated forces, but 

 resultants of several or many influences. Thus, as in the case of 

 the vegetation, it is not one factor, but a complex, which influences 

 different birds in different ways. Nevertheless there is what may 

 be called a mass or dominant effect. 



A major habitat unit may be considered as a combination of 

 conditions which are dominant in a certain area. The very domi- 

 nance means that a relativeJij limited number of forces or complexes 

 are operative. With departure from such a center of influence the 

 dominance changes, as other influences are encountered and other 

 dominants are established. 



When we consider that certain ecological groups of birds are 

 world-wide in their environmental relations, it becomes evident that 

 such characters are of fundamental importance. Thus water birds 

 may occur in any part of the world where water is a dominant 

 environmental factor. This is not a simple ecological group of birds, 

 but one of the greater units of association which may be subdivided 

 into many minor classes; as those which frequent the sea, and others 

 the inland bodies of water. The shore birds form another natural 

 ecological group, and also the inland birds a third. There may 

 thus be considered to be three primary ecological groups of birds 

 which are closely correlated with definite and dominant environ- 

 mental influences; thus: — 



1. Water birds. 



Those frequenting the sea and the adjacent rocks on which they 

 nest, and inland waters. 



2. Shore and Marsh birds. 



Those frequenting shores of all kinds, seas, lakes, swamps and 

 rivers. 



