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BULLETIN 185, U. S. DEPAETMEISTT OF AGEICULTUEE. 



into Mexico. Thus the migrating hosts present a broad front with 

 an east and west extension of 2,500 miles from Mexico to the Lesser 

 Antilles. 



Fig. 10.— Migration of the western tanager (Piranga ludovkiana). The birds that arrive in eastern Alberta 

 May 20 can not have come by way of Colorado and Wyoming, as would be expected (see fig. 9), for on 

 this date the van of migration along the eastern foothOls of the Rocky Mountains has only just reached 

 northern Colorado. The isochronal lines of migration point to the conclusion that these birds migrate 

 north through California and then cross the Rocky Mountains of British Columbia to Alberta, though 

 at this season (May 20) these mountains are largely covered with snow. (See p. 23.) 



The opposite extreme, a narrow migration route, appears in the 

 case of the rose-breasted grosbeak (see fig. 13). The breeding 

 range extends from Nova Scotia to central Alberta, 2,500 miles, and 

 the migration Unes converge until the grosbeaks leave the United 



