Vo L" XI l Chapman on the Origin of Bird Migration. ICj 



extension of range. Certainly in the case of the first two there 

 can be no doubt that in this way we can account for their occur- 

 rence in the United States. In them we have two abundant 

 West Indian species, which, with a number of others, have 

 become established on our southern boundaries. Whether they 

 will gradually increase their northern range, as others have done 

 before them, will of course depend upon the conditions they 

 encounter. The second two present a similar case carried to 

 greater extreme. The Pine-creeping Warbler nests from Cuba 

 to New Brunswick, the Parula Warbler nests from Florida and 

 Texas northward to Canada, and in its various closely related 

 forms is found as far south as Brazil. These, like the two pre- 

 ceding, we may consider normal instances of extension of range. 



In our own experience we have seen how readily a species 

 responds to favorable conditions and how quickly it takes pos- 

 session of territory adjoining its habitat when the conditions are 

 favorable. These conditions I think are, first, absence of com- 

 petition with species of similar habits ; second, an abundance of 

 food. Temperature I consider of importance only as it affects 

 the food-supply. 



The Pine Warbler (Dendroica vigorsii) illustrates this. Its 

 habits demand pine forests and it is equally at home during the 

 summer from the pines of western Cuba to those of New Bruns- 

 wick and Manitoba. Its breeding range, therefore, lies between 

 the summer isotherms of 8o° and 64 , — excellent evidence that 

 temperature alone is not the factor which determines its distribu- 

 tion, but temperature as it governs environment. 



The Warblers which nest in the Canadian Fauna are good 

 examples of our second class of migrants, or those whose breed- 

 ing range is entirely north of our southern limits. Here we have 

 species many of which winter in Central or South America and, 

 returning in the spring, pass over thousands of miles to reach 

 the region of their birth. I take it for granted that the members 

 of this second class of more northerly migrants became North 

 American at an earlier period than the members of the first 

 class. This I think is proven by a study of the first class, in 

 which we find species even now entering our limits, and also 

 because it would be unheard of for a species to move its entire 

 habitat thousands of miles, as these Canadian birds would have 



