OUR GREATEST TRAVELERS 



301 



way below the horizon and broad day- 

 light continues all night. The birds 

 therefore have 24 hours of daylight for 

 at least eight months in the year, and 

 during the other four months have con- 

 siderably more daylight than darkness. 



the: movements of the robin 



The number of miles traveled per day 

 by a migrating bird varies greatly in dif- 

 ferent parts of the migration journey. 

 These variations are intimately con- 

 nected with corresponding variations in 

 the speed of the northward march of 

 spring, and are based primarily on two 

 facts : First, that the interior of a conti- 

 nent warms up faster than the coasts ; 

 second, that spring is hastened in western 

 North America by the Japan current, 

 while it is as decidedly retarded in the 

 east by the polar current. 



The results of these two causes are 

 strikingly shown in the migration of the 

 robin ( page 362). This bird differs from 

 most others in that throughout its entire 

 course northward it adopts spring's time- 

 table for its own. 



The robin's average temperature of 

 migration is 35 0 F. ; that is, it puts in 

 an appearance soon after the snow be- 

 gins to melt and streams to open, but 

 before vegetation has made any start. 

 These conditions occur in the central 

 Mississippi Valley about the middle of 

 February, and it is the first of March 

 before spring and the robins cross north- 

 ern Missouri and arrive together in 

 southern Iowa. Thence a whole month 

 is consumed by the birds in their slow 

 progress — 13 miles a day — to central 

 Minnesota. There their pace quickens, 

 to keep up with the northward rush of 

 spring, and another 10 days at doubled 

 speed brings them to southern Canada. 



Here they must make an important 

 choice. To the north and northeast lies 

 a land that awakens slowly from its win- 

 ter's sleep, and where the sun must wage 

 a protracted contest against the cold of 

 the ice - masses in Lake Superior and 

 Hudson Bay. To the northwest stretches 

 a less forbidding region, already quick- 

 ening under the influence of the Chinook 

 winds. 



THE EASTERN ROBINS MOVE SLOWLY, THE 

 PACIEIC MUCH EASTER 



Most of the robins from Missouri that 

 pass through western Minnesota elect to 

 turn to the northwest, and now they 

 must not only keep pace with the rapidly 

 advancing season, but must do so while 

 traveling on a long-drawn-out diagonal. 

 Their daily average rises to 50 miles — 

 four times that in southern Iowa — and 

 later, when for the birds bound for west- 

 ern Alaska the course becomes nearly 

 due west, the rate increases to 70 miles 

 a day — more than six times the speed 

 with which the journey began. 



The migration map of the robin shows 

 that these Alaska-breeding birds are the 

 only ones that develop high speed. The 

 robins bound for Newfoundland move 

 leisurely along the Atlantic coast at the 

 proverbially slow rate of the oncoming 

 of spring in New England, and, scarcely 

 exceeding 17 miles a day, they finally 

 arrive at their destination May 6, when 

 their Alaska-bound relatives are already 

 1,200 miles farther north. 



One of the most interesting things in- 

 dicated on the map is the migration route 

 of the robins who nest in southern Al- 

 berta. They arrive too early to have 

 come from the south or the southeast; 

 hence they must have come from the 

 southwest, though this has necessitated 

 their crossing the main range of the 

 Rockies while the mountains were still 

 in the grasp of winter. Robins remain 

 all winter on the Pacific coast, north to 

 southwestern British Columbia, which 

 has about the same winter temperature 

 as St. Louis, 700 miles southward. Hence 

 the wintering robins of British Columbia 

 are already far north at the advent of. 

 spring and do not need any hurried mi- 

 gration to reach Alberta on time. As a 

 fact, they average only 8 miles a day, the 

 slowest rate for the species. 



It may be fairly asked, How do we 

 know that the Alaska robins have come 

 all this long distance from the central 

 Mississippi Valley, instead of the far 

 shorter distance from British Columbia? 

 It happens that the robins of the two 

 sides of the continent are slightly dirTer- 



