THE EVOLUTION OF MIGRATIONS 97 



partridges. The starling was released in New York 

 city in 1890 and 1891. It has now reached north 

 to Canada, south to South Carolina and west almost 

 to the Mississippi. It has as a rule appeared spo- 

 radically some years ahead of permanent establish- 

 ment in the various places it has progressively 

 reached. Such spreading is assured if a bird is once 

 successfully established and has an adequate rate 

 of reproduction and survival. The demands of 

 territory, competition for nesting sites and food and 

 other factors inevitably bring it about. 



The Hungarian partridge is non -migratory. As 

 yet it has encountered no obstacles in its rapid 

 northward spread. How far north winter conditions 

 will continue tolerable remains to be seen. In vari- 

 ous parts of the west the mourning dove (Zenaidura 

 macroura) and black-billed cuckoo (Coccyzus eryth- 

 rophthalmus) are at present extending their range 

 northwards. These birds are migratory. New ac- 

 cessions to their breeding range must be vacated for 

 the winter. Their slow rate of progress is in great 

 contrast to that of the partridge. As illustrations 

 to the argument outlined below they are the more 

 fitting. But they are migrants of long standing 

 and their present rate of spread must be infinitely 

 faster than it was in the bygone centuries when 

 migration, as a custom, was first being established. 



