I wondered, "Where do these things usually winter?" 
and found this paragraph on a Cornell University website:  http://www.birds.cornell.edu/BOW/towwar/
concerning the existence of two breeding populations with different wintering grounds.  
 
It read:  
"Townsend's Warblers have two separate winter ranges. 
Those that breed on the Queen Charlotte Islands winter on the Pacific coast 
from British Columbia to southern California. Other Townsend's Warblers 
migrate to southeast Arizona and southwest Texas, extensive stretches 
of montane habitat in Mexico, and parts of Guatemala, Nicaragua, Honduras, 
and Costa Rica. Those individuals that winter on the Pacific coast have shorter
wings than those wintering in Arizona, Texas, Mexico and Central America. 
This difference raises the possibility that the two groups should be considered distinct subspecies." 
 
Elise Kreiss
Baltimore City