30 



BULLETIN" 185, U. S. DEPARTMENT OF AGRICULTURE. 



cross each other in Georgia at approximately right angles. It is 

 possible to trace the routes of the palm warblers because those nesting 

 to the east of Hudson Bay differ enough in color from those nesting 

 farther west to be readily distinguished even in their winter dress. 



WINTER HOME 



EAST AND WEST LIMITS 



OF MIGRATION ROUTE 



Fig. 13.— Distribution and migration of the rose-breasted grosbeak (Zamelodia ludoviciana). An example 

 of a narrow migration route. The breeding range has an east and west width of 2,500 miles, while in 

 migration the birds converge until they leave the United States along a line of the Gulf coast only 800 

 miles wide. (Seep. 24.) The opposite of this (a wide migration route) is shown by the distribution 

 map of the redstart, fig. 12. 



It must always be remembered, however, that from a common 

 ancestry these two groups of palm warblers came to differ in appear- 

 ance because they gradually evolved differences in breeding grounds 

 and in migration routes and not that they chose different routes 

 because they were subspecifically different. 



