102 THE MIGRATION OF BIRDS 



Thus they continually reach colder weather as they 

 travel north. 



The American robin, Turdus migratorius, moves 

 more sedately ; it takes seventy-eight days for its 

 3000 mile trip, whilst spring takes some ten days 

 less to cover the distance. But the individual robins 

 may advance more quickly ; it is the robin as a 

 species which takes this time to cover the area of 

 distribution. The isotherm of 35 F., corresponding 

 to the beginning of spring migration, advances north 

 at the rate of 3 miles per day from January 15th to 

 February 15th ; 10 miles a day is the average for the 

 next month, and 20 for the following month. But 

 along the eastern foothills of the Rockies, isotherms 

 travel faster than in corresponding latitudes farther 

 east ; spring rushes to this western land. In mid- 

 April to mid-June the height of migration the 

 southern portion of the Mackenzie Valley has about 

 the same temperature as the region of Lake Superior 

 700 miles farther south. This, coupled with the 

 diagonal course of the birds across the fast-moving 

 region of spring, exerts a powerful influence upon 

 migration ; the earliest robins reach southern Iowa 

 on March 1st, and travelling northward at about 

 13 miles per day, find in central Minnesota a tempera- 

 ture similar to the one they left. Those which breed 

 near Lake Superior increase their speed to a daily 

 average of 25 miles, and arrive at latitude 52, 



