[Illustration: _Figure 5. Migration of the Canada goose. The northward movement keeps pace with the progress of spring, because the advance of the isotherm of 35 F agrees with that of the birds._]
[Illustration: _Figure 6. Isochronal migration lines of the gray-cheeked thrush, an example of rapid migration. The distance from Louisiana to Alaska is about 4,000 miles and is covered at an average speed of about 130 miles per day. The last part of the journey is covered at a speed several times what it is in the Mississippi Valley._]
Another example of rapid migration is furnished by the yellow warbler. This species winters in the Tropics and reaches New Orleans about April 5, when the average temperature is 65° F. By traveling north much faster than the spring season progresses, this warbler reaches its breeding grounds in Manitoba the latter part of May, when the average temperature is only 47° F. They encounter progressively colder weather over their entire route and cross a strip of country in the 15 days from May 11 to 25 that spring temperatures normally take 35 days to cross. This "catching up" with spring is habitual in species that winter south of the United States as well as in most northern species that winter in the Gulf States. There appears to be only six exceptions to this rule: the Canada goose, the mallard, the pintail, the common crow, the red-winged blackbird, and the robin.
The snow goose presents a striking example of a late but very rapid spring migration. Most all of these geese winter in the great coastal marshes of Louisiana, where every year over 400,000 spend the winter and congregations of 50,000 or more may be seen grazing in the "pastures" or flying overhead in flocks of various sizes. Their breeding grounds are chiefly on Baffin and Southampton Islands in the northern part of Hudson Bay where conditions of severe cold prevail except for a few weeks each year. The birds are not stimulated to migrate even though the season in their winter quarters is advancing rapidly while their nesting grounds are still covered with a heavy blanket of ice and snow. This suggests the stimulus for spring departure is regulated by an internal mechanism, such as development of the gonads. Accordingly, blue geese remain in the coastal marshes until the last of March or the first of April, when the local birds are already busily engaged in reproduction. The flight northward is rapid, almost nonstop so far as the United States is concerned; although the birds are sometimes recorded in large numbers in the Mississippi Valley, eastern South Dakota, and southeastern Manitoba, there are few records anywhere along the route of the great flocks that winter in Louisiana. When the birds arrive in the James Bay region, they apparently enjoy a prolonged period of rest because they are not seen in the vicinity of their breeding grounds until the first of June. During the first 2 weeks of that month, they pour onto the Arctic tundra by the thousands, and each pair immediately sets about the business of rearing a brood.