southern California. By May 10 the easternmost birds have advanced 

 only to southern Colorado, while those in the far west have reached 

 northern Washington. Ten days later the northward advance of the 

 species is shown as a great curve, extending northeastward from 

 Vancouver Island to central Alberta and thence southeastward to 



r--r -MM3 



Isochronal Migration Lines 

 Migratbn Route 



Figure 23. Migration of the western tanager. The birds that arrive in eastern A Iberta by 

 May 20 do not travel northward along the eastern base of the Rocky Mountains, 

 because the vanguard has then only reached northern Colorado. Instead the isochronal 

 lines indicate that they migrate north through California, Oregon, and Washington 

 and then cross the Rockies in British Columbia. 



79 



