THE MIGRATION OF NORTH AMERICAN BIRDS A 
From that point to Lake Athabaska, 12 species travel at an average 
speed of 72 miles a day; while 5 others travel to Great Slave Lake 
at 116 miles a day; and another 5 species cover 150 miles a day to 
reach Alaska. This change is in correlation with a corresponding 
z=— APRIL 2\0 
» 
B4495M 
FIGURE 11.—Migration of the blackpoll warbler. The solid isochronal lines show the places at which 
these birds arrive at the same time. As the birds move northward these lines become farther apart, 
showing that the warblers move faster with the advance ofspring. From April 30 to May 10 the average 
speed is about 30 miles a day, while from May 25 to May 30 it is increased to more than 200 miles. 
variation in the isothermal lines, which turn northwestward west of 
the Great Lakes. 
As has been previously indicated, the advance of spring in the 
northern interior is much more rapid than in the Mississippi Valley 
and on the Gulf coast. In other words, in the North, spring comes 
