APRIL 30. 



Figure 5. — Isochronal migration lines of the gray-cheeked thrush, an 

 example of rapid migration. The distance from Louisiana to Alaska 

 is about 4,000 miles and is covered at an average speed of about 

 130 miles a day. The last part of the journey is covered at a speed 

 that is several times what it is in the Mississippi Valley. 



Their breeding grounds are chiefly on Baffin Island and on Southamp- 

 ton Island in the northern part of Hudson Bay, in a region where con- 

 ditions of severe cold prevail except for a few weeks each year. The 

 birds seem to realize that even though the season in their winter quar- 

 ters is advancing rapidly, their nesting grounds are still covered with 

 a heavy blanket of ice and snow. Accordingly they remain in the 

 coastal marshes until the last of March or the first of April, when the 

 local birds are already busily engaged with the duties of reproduction. 

 The flight northward is rapid, almost nonstop, so far as the United 

 States is concerned, for although the birds are sometimes recorded in 



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