The American robin has been mentioned as a slow migrant, and, as a species, it takes 78 days to make the 3,000-mile trip from Iowa to Alaska, a stretch of country that is crossed by advancing spring in 68 days. In this case, however, it does not necessarily mean that individual robins are slow. The northward movement of the species probably depends upon the continual advance of birds from the rear, so that the first individuals arriving in a suitable locality are the ones that nest in that area, while the northward movement of the species is continued by those still to come. There is great variation in the speed of migration at different 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.