Figure 6. — Migration of the blackpolled 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 of spring. 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. 



other birds. A study of all species traveling up the Mississippi Valley 

 indicates an average speed of about 23 miles a day. From southern 

 Minnesota to southern Manitoba 16 species maintain an average speed 

 of about 40 miles a day. From that point to Lake Athabaska, 12 species 

 travel at an average speed of 72 miles a day; while 5 others travel to 



25 



