134 MICHIGAN SURVEY, 1908. 



mental complexes, are characterized by the dominance of few physical 

 and blotic factors, and by a limited number of species. 



V. AvAiN Succession. 



1. General Remarks. Since the breeding grounds are fundamental 

 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 avain association, formation or society is meant different combina- 

 tions of species which regularly occur together in the same ireeddng 

 haiitat or area. These breeding grounds must be considered broadly, 

 and include not only the nesting 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 species may be 

 noted, with a corresponding decrease in another; and this proportion 

 may continue to change until the intruder becomes dominant and the 

 riVal form may disappear entirely. This process of change, as a rule, 

 is not limited to a single species, but usually involves several or all 

 of the membera of the association, as when a dune invades a swamp 

 and the swamp birds are completely replaced by those frequenting the 

 sand dunes. 



2. Succession on Isle Eoyale. With these preliminary considera- 

 tions in mind, we will turn to the ecological succession of bird life 

 upon Isle Royale, Lake Superior. The field work upon the island was 

 carried on by a party from the University Museum of the University 

 of Michigan, under the direction of the writer. Aside from succession, 

 the general ecological relations of the birds were studied by Otto Mc- 

 Creary and Max M. Peet, and elsewhere detailed descriptions of the 

 region and detailed notes will be published. The writer has based his 

 main records of habitat preference upon their work. For this outline 

 of succession only the primai-y features of the location need be given. 



In the present treatment an attempt will be made to follow the 

 genetic succession, at least in its broader outlines. Various qualifica- 

 tions and reservations have been made, and others will follow, so it 

 is hoped that no confusion will be produced by this method of treat- 

 ment. 



Oeographkalhj, Isle Royale, Michigan, is an island in Lake Superior, 

 near the North Shore, not far from Port Arthur, Ontario. The topogra- 

 phy forms a part of an ancient peneplain of moderate relief, glaciated 

 and with an abundance of elongated low ridges and valleys with numer- 

 ous water basins. The soil, which is locally absent, is generally huniic 



