124 
THE CONDOR 
VOL. VI 
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, 113'^ 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 nort Invest to the Missis- 
sipi Valley, thence north to Manitoba, thence northwest to the valley of the Mac- 
kenzie, 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 main- 
tained; southern Indiana and Missouri are reached the first week in May, south- 
ern 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 da3' during the latter part of the journe3^ Thirty days are thus 
occupied in traveling the 1,000 miles from the Gulf of Mexico north to southern 
Minnesota, and scarceh' half that time in traversing the 2,500 miles thence 7 iorth- 
7 vest to x\laska. The direction of migration is emphasized because this change of 
direction is intiniateh’ 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 da3". Sixteen species maintain a daily average of 40 miles from 
southern Minnesota to southern Manitoba, and from this point 12 species travel to 
Lake x\thabasca at an average speed of 72 miles a da3^ 5 others to Great Slave Lake 
at 1 16 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 in- 
creases 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. 
i\t 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 temper- 
atures 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, an3' 
given isotherm, as it moves north during the spring in the Mississippi Valley, con- 
tinually 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. x\long 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 progre.ss is 
