The American Naturalist. 



EXTENSIVE MIGRATION IN BIRDS AS A CHECK 



UPON THE PRODUCTION OF GEOGRAPHICAL 



VARIETIES. 



By Thomas H. Montgomery, Jr. 



Two problems have of late years received much attention 

 from ornithologists, and deservedly, namely, the faunal dis- 

 tribution of species, and their ranges of migration. But to my 

 knowledge, no one has raised the question of the possible ex- 

 istence of a relation between the extent of the periodic migra- 

 tion and the amount of geographical variation evinced by a 

 species. The object of this paper then is to show that such a 

 relation does exist, that extensive migration tends to act as a 

 check upon the production of geographical varieties, or races 

 so-called. 



In the first place, on comparing the amount of faunal vari- 

 ation with the extent of the periodical migrations in a given 

 species, it will be found to be usually, if not always, the case, 

 that those species which undertake migrations of more than 

 the average extent — migrating through 30° or more of lati- 

 tude, have no tendency to give rise to geographical varieties. 

 In order to see how far this law extends, and whether excep- 

 tions to it may be found, I have compared all the species of 

 North American birds with regard to this relation existing be- 

 tween variation and range of migration, using Ridgway's ex- 

 cellent " Manual of N. A. Birds " as my authority for the 

 amount of variation and extent of migration in these species. 

 For our present purposes the North American species may be 

 divided into three groups, based on the extent of their migra- 

 tions : (1) species with exceedingly protracted migrations, but 

 irregular as to the localities traversed ; (2) species with more 

 or less regular migrations, of 30° lat. or more in extent ; and 

 (3) species which undertake migrations less in extent than 30° 

 lat., or species which do not migrate at all. We may now 

 consider each of these groups in turn, with regard to the ques- 



