124 Adams, Ecological Succession of Birds. [April 



not closely correspond with current faunal areas, although there is a 

 very close correlation in some cases. An avian formation may, in 

 general terms, be considered the analogue of a vegetational forma- 

 tion, although this does not imply that they necessarily have the 

 same boundaries. 



As the literature treating of the vegetation of these areas is exten- 

 sive and scattered, a few papers will be cited as an index to others : — 



1. Arid Deserts; Bray, '06; Coville and MacDougal, '03. 



2. Grasslands or Plains; Clements, '05; Pound and Clements, '00. 



3. Southeastern Hardwoods; Cowles, '01; Harper. '06; Transeau, '05. 



4. Eastern Canadian Conifers; Whitford, '01; Transeau, '03, '05- "06; 



Ganong, '03, '06; Harvey, '03. 



5. Rocky Mountain and Pacific Conifers; Whitford, '05; Gray and 



Hooker, 'SI; Piper, '06; Young, '07. 



6. Alpine; Merriam, '90, '99; Coville, '93; Femald, '07. 



These environmental unit areas as found to-day, are the result of 

 many successions which, in some cases at least, reach rather far 

 back into the past. This is because some occupy ancient land 

 areas, such as much of the Southeastern Hardwood area* On the 

 other hand, some occupy relatively new regions, that is, at least 

 with regard to the dominant factors now in control, as in the 

 glaciated part of North x\merica and on the Coastal Plain. So far 

 as the present is concerned such relations clearly show that the.se 

 areas are only the end results of extensive past changes or succes- 

 sions which represent the terminal branches and cross sections of 

 development. It is to the study of such regions and associations 

 that we must turn for the fundamental organization or associa- 

 tional relations of the various elements which compose not only 

 the environments but also the associations of animals. 



In order to make as definite as possible the structural and eco- 

 logical characteristics of these formations, certain general relations 

 are here formulated. Throughout this paper it should be remem- 

 bered that the individual birds and associations of given areas form 

 the units of comparison. Such a distinction is necessary because 

 many species show considerable geographic variation in habits 

 and in the habitats frequented. The writer clearly recognizes the 

 risks and difficulties of such an attempt. They are deliberately 

 put in their present form to invite criticism and qualification from 



