. Bird migration . s on May 30 they find there an averagetemperature of only 45° F. VARIATIONS IN SPEED OF MIGRATION. The immense variation in the speed with which migrants traveldifferent parts of the broad bird highway extending from the Gulf tothe Arctic Ocean by way of the Mississippi and Mackenzie Valleys isa recent determination of special interest. The black-poll warbler 44 BULLETIX 185^ U. S. DEPARTMENT OF AGRIGULTUKE. furnishes one of the best examples (see fig. 5). Wmtering in north-central South America and migratmg in April across the West Indiesto Florida, some individuals pass on
. Bird migration . s on May 30 they find there an averagetemperature of only 45° F. VARIATIONS IN SPEED OF MIGRATION. The immense variation in the speed with which migrants traveldifferent parts of the broad bird highway extending from the Gulf tothe Arctic Ocean by way of the Mississippi and Mackenzie Valleys isa recent determination of special interest. The black-poll warbler 44 BULLETIX 185^ U. S. DEPARTMENT OF AGRIGULTUKE. furnishes one of the best examples (see fig. 5). Wmtering in north-central South America and migratmg in April across the West Indiesto Florida, some individuals pass on northwest to the MississippiValley, north to Manitoba, northwest to the valley of the Mackenzie,and thence almost due west to western Alaska. From the Gulf ofMexico to Minnesota a fairly uniform average speed of 30 to 35 milesa day is maintained; southern Indiana and Missouri are reachedthe first week in May, southern Iowa early in the second week, andsouthern Minnesota is entered by the middle of the month. Then. Fig. 20.—Migration of the Canada goose {Branta canadensis). An example of migration keeping pace withthe advance of spring. The earliest Canada geese arrive in central Illinois when the average temperat: reis about 35° F., and they reach their most northern breeding grounds at about the same temperature,having advanced northward at approximately the same rate as the advance of spring. (See p. 41.) comes a spurt; within another week the black-polls appear in thecentral part of the Mackenzie Valley, and the following week theyarrive in northwestern Alaska, many individuals undoubtedly averag-ing more than 200 miles a day during the latter part of the days are thus occupied m traveling the 1,000 miles from theGulf of Mexico north to southern Minnesota, and scarcely half thattime in traversing the remaining 2,500 miles northv/est to directions of migration are emphasized because the change ofdirection is intimately connected with the great i
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