the way, and the birds reach the nesting site full of energy, bubbling over with song, and in good condition to assume the cares and labors of house building and brood raising.
VARIATIONS IN THE SPEED OF MIGRATION.
The immense variation in the speed with which migrants travel different parts of the broad bird highway that extends from Gulf to Arctic Ocean, by way of the Mississippi and Mackenzie valleys, is a recently ascertained fact of special interest. The black-poll warbler furnishes one of the best examples of this. It winters in north central South America and migrates in April across the West Indies to Florida. From here some individuals pass on northwest to the Mississippi Valley, thence north to Manitoba, thence northwest to the valley of the Mackenzie, and thence almost due west to western Alaska. From the Gulf of Mexico to Minnesota a fairly eniform average speed of 30 to 35 miles per 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 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 averaging more than 200 miles per 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 Minnesota, and scarcely half that time in traversing the 2,500 miles thence northwest to Alaska. The direction of migration is emphasized because this change of direction is intimately connected with the great increase of speed, as will be shortly explained.
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 per day. Sixteen species maintain a daily average of 40 miles from southern Minnesota to southern Manitoba, and from this point 12 species travel to Lake Athabasca at 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 not far to seek. The speed increases as the birds move northward because the advance of the seasons 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, La., the average daily temperature of January is 54° F., and that of July is 82° F., while at Winnipeg, Manitoba, the corresponding average temperatures are: January, -7° F., July, 66° F. Hence, while the temperature at New Orleans is rising 28 degrees, that at Winnipeg rises 73 degrees. Consequently, any given isotherm, as it moves north during the spring in the Mississippi Valley, continually increases the speed of its advance. The isotherm of 35° F., corresponding to the commencement of spring migration, advances north at a rate of 3 miles per 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 the birds pass northwestward from Minnesota 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 per day. During the next month, under the influence of the chinook winds, its rate of northward progress is
