128 Adams, Ecological Snccessio?i oj Bij-ds. [\]lrii 



tial dues as to the earlier successions Avhich 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 adja- 

 cent associations and centers of dominance. 



7. Marginal societies are particularly liable to variation in 

 composition, 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 formation. 



9. Local habitats and societies, in conmion with the larger 

 environmental complexes, are characterized by the dominance of 

 few physical and biotic factors, and by a limited number of species. 



V. Avian Succession. 



1. General Remarks. Since the breeding grounds are of funda- 

 mental importance in the ecology of birds, the study of them in 

 such situations furnishes the greatest source of insight into their 

 life relations. By an avian association, formation or society is 

 meant different combinations of species which regularly occur 

 together in the same breeding habitat or area. These breeding 

 grounds must be considered broadly, and include not only the nest- 

 ing site but also the feeding grounds, even when they are physically 

 very different, because ecologically these conditions form a unit 

 during the breeding season. 



It is well known that when a given set of physical conditions are 

 dominant, as in a dense conifer forest, a swamp or an extensive 

 orchard, relatively few individuals and kinds of breeding birds are 

 characteristic of such conditions, except in the case of those nesting 

 in colonies. The field relations of these colonial and isolated 

 breeders are quite different. It is also of importance to recall that 

 abundance is a relative term, with a very different meaning in the 

 case of seed-eating and predaceous species. 



Bearing in mind these conditions, bird succession means a change 

 from the dominance of certain species or associations to that of 

 others. Thus in the beginning a slight change in abundance of a 



