BIRD MIGRATIOK. 43 



necessity compels. They linger in the Tropics so late that when they 

 reach New Orleans, April 5, an average temperature of 65° F. awaits 

 them. They now hasten. Traveling north much faster than the 

 spring does, they cover 1,000 miles in a month and find in southern 

 ^linnesota a temperature of 55° F. In central Manitoba the average 

 temperatm'e they meet is 52° F., and when they arrive late in May 

 at Great Slave Lake they have gained 5 degrees more on the season. 

 Thus durmg the whole trip of 2,500 miles from New Orleans to Great 

 Slave Lake these birds are continually meetmg colder weather. So 

 fast do they migTate that m the 15 days from May 11 to 25 they trav- 

 erse a district that spring requires 35 days to cross. This outstrip- 

 ping of spiing is habitual with all species that leave the United States 

 for the winter and also with most northern birds that winter in the 

 Gulf States. Careful examination of migration records of each species 

 of the Mississippi Valley shows only six exceptions — Canada goose, 

 mallard, pintail, common crow, red-winged blackbird, and robin. 



The robin as a species migrates north more slowly than the openmg 

 of the season; it occupies 78 days for its trip of 3,000 miles from Iowa 

 fo Alaska, while sprmg covers the distance m 68 days. But it does 

 not f oUow that any individual bird moves northward at this leisurely 

 pace. The first robins that reach a given locality in spring are likely 

 to remain there to nest, and the advance of the migration lines must 

 await the arrival of other birds from farther south. Therefore each 

 robin undoubtedly migrates at a faster rate than the apparent move- 

 ment of the species as a whole and does not fall behind the advancing 

 season. This is true of most if not aU of the other seemingly slow 

 migrants. Late and rapid journeys of this kind offer certain advan- 

 tages; fewer storms are encountered, the mortality rate is lowered, 

 food Ls more plentiful along the way, and the birds reach the nesting 

 site full of energy and in good condition to assume the cares and labors 

 of house building and brood raising. 



An extreme example of a late and rapid migration is that of the 

 black-poll warbler (see fig. 5). The birds enter the United States in 

 southern Plorida April 20, when the average temperature there is 

 72° F. Ten days later tlie van lias rcacJiod the central Mississippi 

 Valley, where the temperature is 60° F., and tlio birds continue to 

 advance faster than the progi'css of sj)iijig luitil at tlu^ time they roach 

 their >\laska breeding gi-ounds oji May 'M) they liiul there; an average 

 temperature of only 45° F. 



VARIATIONS IN SPEED OF MIGRATION. 



The immense vaiiatioii in tlic speed willi whieli migrants travel 

 (lifTjTeiit ])arts of tlic hroud biid highway cxteiuruig IVoiii the Gulf to 

 the Arcti<^ Occau by way of the Mississi]»])i and Mjickr-ii/ie Valleys is 

 a recent determination of Hpec-iiil interesl. The black-poll warbler 



