12 DISTRIBUTION OF WARBLERS 



America. Icteria also extends southward over the Mexican tableland 

 and, with Vermivora, is more North American than Mexican, 

 though doubtless of Mexican origin. 



This leaves 7 genera whose breeding range still includes an area 

 in the tropics. Of these the following 6 enter North America 

 through Mexico: Compsothlypis, Peucedramus, Geothlypis, Chamce- 

 thlypis, Cardellina, and Setophaga. With the exception of the forms 

 of Geothlypis trichas, which have apparently reached the Bahamas 

 through Florida, none of these genera is known to be represented in 

 the breeding season in the West Indies. 



On the other hand, Dendroica is evidently a West Indian genus. 

 Excepting members of the widely distributed Yellow or Golden 

 Warbler group, Mexico has no species of this genus which are not 

 found in the United States, although 3 of our species extend south- 

 ward into Mexico as geographic forms (i. e. D. auduboni nigrifrons, 

 D. a. goldmani, and D. grades decora, the latter reaching Honduras). 



The West Indies, however, without including the Golden War- 

 blers, have 7 resident species of Dendroica, 5 of which are represented 

 in North America by closely related forms (i. e. D. adelaida and 

 D. delicata, by our D. dominica and D. gracia; D. vigorsii achrustera 

 and abacoensis, by D. v. vigorsi; D. vitellina, by D. discolor). 



In this evident West Indian origin of Dendroica, we have a prob- 

 able explanation of the numerical abundance of the birds of this genus 

 in the Eastern states as compared with the Western states. Of the 23 

 North American species, only one, the phenomenally distributed Yellow 

 Warbler, is found in both the Eastern and Western states, 6 occur in 

 the west but not in the east, one appears to be restricted to east central 

 Texas, and 15 are found in the east but not in the west. 



This restriction of forms of West Indian origin to the Eastern 

 states, in connection with their confinement to these islands in 

 winter, leads us to consider Helinaia and Helmitheros, both confined 

 to the east, as of West Indian rather than of Mexican origin. 



As might be expected, therefore, forms of Mexican origin (e. g. 

 Icteria and Geothlypis), which spread both to the east and the west, 

 are likely to occupy a larger area than those which enter our limits 

 at their extreme southeastern border. In other words, we share with 

 the west many of the Warblers of Mexican origin, but give her in 

 return few or none of those which have been received from the West 

 Indies. 



Continuing the comparison begun under Dendroica, we find, in 

 the first place, that the west has only 2 genera of Warblers not repre- 



