BIRD MIGRATION". 43 



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

 reach Now Orleans, April 5, an average temperature of (15° P. awaits 

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



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

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

 temperature they meet is 52° F., and when they arrive late in. May 

 at Great Slave Lake they have gained 5 degrees more on 1 he season. 

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

 Slave Lake these birds are continually meeting colder weather. So 

 fast do they migrate that hi the 15 days from May 11 to 25 they trav- 

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

 ping of spring 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 opening 

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

 to Alaska, while spring covers the distance in 68 days. But it does 

 not follow 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 all 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 is 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 Florida April 20, when the average temperature there is 

 72° F. Ten days later the van has reached the central Mississippi 

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

 advance faster than the progress of spring until at the time they reach 

 then Alaska breeding grounds on May 30 they find there an average 

 temperature of only 45° F. 



VARIATIONS IN SPEED OF MIGRATION. 



The immense variation in the speed with which migrants travel 

 different parts of the broad bird highway extending from the Gulf to 

 the Arctic Ocean by way of the Mississippi and Mackenzie Valleys is 

 a recent determination of special interest. The black-poll warbler 



