BIED MIGRATION. 45 



A similar increase of speed is shown by many other species. The 

 average speed of migration from New Orleans to southern Minnesota 

 for all species is close to 23 miles a day. Sixteen species maintain a 

 daily average of 40 miles from southern Minnesota to southern Mani- 

 toba, and from this point 12 species travel to Lake Athabaska al an 

 average speed of 72 miles a day, 5 others to Great Slave Lake at 116 

 miles a day, and 5 more to Alaska at 150 miles a day. 



The reason for these remarkable differences is very simple: The 

 speed increases as the birds move northward because the advance 

 of the season is more rapid in the northern interior than on and near 

 the southern coast. The farther removed a district is from the ocean 

 the greater the extremes of its temperature. At New Orleans the 

 average daily temperature of January is 54° F., and that of July is 

 82° F., while at Winnipeg, Manitoba, the corresponding average tem- 

 peratures are : January, - 7° F. ; July, 66° F. Hence during the period 

 the temperature at New Orleans is rising 28 degrees, that at Winnipeg 

 rises 73 degrees. Consequently as a given isotherm moves north during 

 spring in the Mississippi Valley it continually increases its rate of 

 advance. The isotherm of 35° F., corresponding to the commence- 

 ment of spring migration, advances north at the rate of 3 miles a day 

 from January 15 to February 15, 10 miles daily during the next 

 month, and 20 miles daily during the following month. 



But an additional explanation must be sought for the wonderfully 

 quickened speed with which birds pass northwestward from Minne- 

 sota to the Mackenzie Valley. Along the eastern foothills of the 

 Rocky Mountains isotherms travel north faster than at corresponding 

 latitudes farther east. From February 15 to March 15 the isotherm 

 of 35° F. (the line of spring) passes along the foothills from New 

 Mexico to northern Colorado at the rate of 12 miles a day. During 

 the next month, under the influence of the Chinook winds, its rate of 

 northward progress is increased to 40 miles a day, so that by April 15 

 it has reached Lake Athabaska. Spring has come with a rush in this 

 western interior country. The result is that during the height of the 

 migration season, from the middle of April to the middle of June, the 

 southern end of the Mackenzie in Saskatchewan has just about the 

 same temperature as the Lake Superior region 700 miles farther 

 south. 



These conditions, coupled with the diagonal course of birds across 

 this region of fast-moving spring, necessarily exert a powerful influence 

 on bird migration. The robin's average temperature of migration is 

 35° F.; that is, the bird puts in an appearance soon after snow begins 

 to melt and streams to open, but before vegetation has made any 

 marked advance. These conditions occur in the central Mississippi 

 Valley about the middle of February, and it is the first of March 

 before spring and the robins cross northern Missouri and arrive 



