THE MIGRATION OF NORTH AMERICAN BIRDS 19 
busily engaged with the duties of reproduction. The flight north- 
ward is rapid, almost nonstop, so far as the United States is con- 
cerned, for although the birds are sometimes recorded in large num- - 
bers in the Mississippi Valley, including eastern South Dakota, and 
in southeastern Manitoba, there are few records anywhere along the 
route of such great flocks as are known to winter in Louisiana. When 
the birds arrive in the James Bay region of Canada they apparently 
B4505M 
FIGURE 9.—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 per day. The last part of the journey is covered at a speed that is several times what it is in the 
Mississippi Valley. 
enjoy a prolonged period of rest, as they are not noted in the vicinity 
of their breeding grounds until the first of June. During the first 2 
weeks of that month they pour into the tundra country by the 
thousands, and each pair immediately sets about the business of 
rearing a brood. 
The robin has been mentioned as a slow migrant, and as a species 
it takes 78 days to make the 3,000-mile trip from Iowa to Alaska, a 
