44 



BULLETIN 185, U. S, DEPARTMENT OF AGRICULTUEE. 



furnishes one of the best examples (see fig. 5). Wintering in north- 

 central South America and migrating in April across the West Indies 

 to Florida, some individuals pass on northwest to the Mississippi 

 Valley, north to Manitoba, northwest to the valley of the Mackenzie, 

 and thence almost due west to western Alaska. From the Gulf of 

 Mexico to Minnesota a fairly uniform average speed of 30 to 35 miles 

 a day is maintained; southern Indiana and Missouri are reached 

 the first week in May, southern Iowa early in the second week, and 

 southern Minnesota is entered by the middle of the month. Then 



ISOTHERM or ZS" F. 

 ISOCHRONAL MIGRATION LINES 



Fig. 20. — Migration of the Canada goose (Branfa canadensis). An example of migration keeping pace with 

 . the advance of spring. The earliest Canada geese arrive in central Illinois when the a\'erage temperat; re 

 is about 35° F., and they reach their most northern breeding grounds at about the same temperature, 

 having advanced northward at approximately the same rate as the advance of spring. (See p. 41.) 



comes a "spurt"; within another week the black-polls appear in the 

 central part of the Mackenzie Valley, and the following week they 

 arrive in northwestern Alaska, many individuals undoubtedly averag- 

 ing more than 200 miles a day during the latter part of the journey. 

 Thirty days are thus occupied in traveling the 1,000 miles from the 

 Gulf of Mexico north to southern ]\Imnesota, and scarcely half that 

 time in traversing the remaining 2,500 miles northwest to Alaska. 

 The directions of migration are emphasized because the change of 

 direction is intimately comiected with the great increase of speed, as 

 will be explained. 



