latitudes of the broad region between the Gulf of Mexico and the 

 Arctic Ocean. The blackpoll warbler again furnishes an excellent 

 example (Fig. 3). This species winters in northwestern South 

 America and starts to migrate north in April. When the birds reach 

 the southern United States, some individuals fly northwest to the 

 Mississippi Valley, north to Manitoba, northwest to the Mackenzie 

 River, and then almost due west to western Alaska. A fairly uniform 

 average distance of 30 to 35 miles per day is maintained from the Gulf 

 to Minnesota, but a week later this species has reached the central 

 part of the Mackenzie Valley, and by the following week it is observed 

 in northwestern Alaska. During the latter part of the journey, 

 therefore, many individuals must average more than 200 miles per 

 day. Thirty days are spent traveling from Florida to southern 

 Minnesota, a distance of about 1,000 miles, but scarcely half that time 

 is used to cover the remaining 2,500 miles to Alaska. Increased speed 

 across western Canada to Alaska is also shown by many other birds 

 (Figs. 2,4,6). A study of all species traveling up the Mississippi Valley 

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

 Minnesota to southern Manitoba 16 species maintain an average 

 speed of about 40 miles per day. From that point to Lake Athabaska, 

 12 species travel at an average speed of 72 miles per day, while 5 

 others travel to Great Slave Lake at 116 miles per day, and another 5 

 species cover 150 miles per day to reach Alaska. This change is in 

 correlation with a corresponding 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 with 

 a rush, and, during the height of migration season in Saskatchewan, 

 the temperature in the southern part of the Mackenzie Valley just 

 about equals that in the Lake Superior area, 700 miles farther south. 

 Such conditions, coupled with the diagonal course of the birds across 

 this region of fast-moving spring, exert a great influence on 

 migration and are probably factors in the acceleration of travel 

 speed. However, it should be remembered that the birds are getting 

 closer to the breeding season and may be stimulated to travel faster 

 for this reason. 



Thus it has been shown that the rate of migration varies greatly 

 under varying circumstances. Radar investigations along the 

 eastern coasts of the United States and England indicate spring 

 migration is several miles per hour faster than in the fall. Also, 

 directions of migrations in spring were much less diverse than in the 

 fall, which suggests less time lost in passage (Tedd and Lack 1958; 

 Nisbet and Drury 1967a). King and Farner (1963) found the same 

 species put on more fat preparatory to migration in the spring. This 

 would give the migrants greater energy reserves for longer flights at 

 that season. 



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