148 Adams, Ecological Succession of Birds. \_\mi\ 



aries generally somewhat shorter. Without further knowledge than 

 that obtained from a comparison of skins, it might be put down as 

 an accidental variety. But the field naturalist soon recognizes it 

 as a most distinct species. It has a different voice, a differently 

 shaped nest; and, while the common species breeds in the plains, 

 this one always resorts to the hills. The Spanish shepherds on 

 the spot recognize their distinctness, and have a name for each 

 species." 



Many examples of similar character might be cited to show the 

 scientific value of a knowledge of the environmental relations of 

 birds, and a moment's reflection w^ill show that the problem of 

 succession is only a small part of the general problem of environ- 

 mental relations of plants and animals. Attention has already 

 been directed to the relation which this general subject bears to 

 evolutionary problems. 



It is not at all unlikely that succession is very closely related to 

 some of the causes of bird migration, and that with advance in this 

 subject much light would be thrown upon migration. Migration is 

 doubtless another illustration of convergent phenomena. In all 

 probability, migration has originated not only independently in very 

 diverse kinds of birds, but perhaps repeatedly, from different 

 causes, even in the same group. The causes of migration must be 

 numerous, varying with different ecological groups, which appear 

 to be the true natural units for study and comparison. Thus the 

 comparative study of migrations of different kinds of associations, 

 as formations and societies, should lead not only to a better under- 

 standing of the various associations, but should also contribute to 

 the general subject of migration which seems to have shown a 

 tendency toward stability in the current methods of study. It 

 scarcely seems probable that with the diverse formations inhabited 

 by birds, and with their ecological diversities there should be only 

 a few causes of the phenomena. 



To keep pace with successions animals must either adjust them- 

 selves, change their habitat, or migrate. From such relations it is 

 evident that various supposed environmental responses must be 

 tested primarih/ within the association and environment to ivhich the 

 animal normally belongs. To this class belongs protective colora- 

 tion and allied phenomena. To be of fundamental value, the 



