BIED MIGRATION, 41 



migration'' in fall. With many birds, probably the majority of land 

 birds, the first individuals of a species to appear at a given locality 

 are old birds that nested there the previous year; these are followed 

 by others that nested in the region just to the north; and the last to 

 appear are those whose homes are in the most northern part of the 

 breeding range. The above statement applies only. to old birds; in 

 what order or at what time young of the previous year migTate has 

 not yet been discovered. If, then, for any species, the southern- 

 nesting birds lead the van in both fall and spring migrations and 

 the rear g"uard in each case is composed of northern-breeding bhds, 

 it follows that some time between October and April a transposal 

 of their relative positions occurs, and that the more southern birds 

 pass over those whose migi'ation 'farther north is delayed by winter 

 still holding sway in then' summer dominions. Just when and where 

 this transposition occiu's is a problem of migration reserved for future 

 solution. Nor is it yet settled whether northem-bred birds remain 

 strictly ^vithin their mnter range until after their more southern 

 congenei's have passed by, or whether they begin an early migration 

 so slowly as soon to be overtaken and passed by their more impetuous 

 cousins. 



Still later m spring another transposition occurs. The northern 

 bhds pass across the southern part of the breedmg range, where the 

 southernmost bhds are already busy with their domestic duties. 

 Spring migration seems, therefore, to be for most species a game of 

 leapfrog, the southern bhds fu'st passing the northern and the northern 

 passing them in turn. 



RELATION BETWEEN MIGRATION AND TEMPERATURE. 



The Canada goose is typical of what may be called regular migration. 

 This bird fulfdls the popular notion of bird migration, i. e., it moves 

 northward m spring as soon as the loosenmg of winter's fetters offers 

 open water and a possibility of food. It continues its progress at 

 the same rate as spi-ing, appearing at its most northern breeding 

 grounds at the earliest possible moment. The isotherm of 35° F. 

 (see fig. 20) seems to be the governing factor in the rate of spring 

 migration of the Canada goose and, as shown on tlic map, the isotherm 

 and tlie vanguard of tlie geese are close traveling companions through- 

 out the entire route. Moreover, the isochronal lines representing the 

 positif>ri of tlie van at Various tiiiu's are approxiiuutoly ('iist-und-W(*st 

 lines during the whole migration pc/iod. 



But this 8C)-call(!d regular migi-a1 ion is iK'ifoiiiKMl hy a very small 

 percentsige of species, the great majority choosing (exactly th(^ ()])i)o- 

 site coui'sc -to reinain in their winter Jionies until s]>ring is far ad- 

 vanced and then reach theii- hiccding grounds by a migi-atioji. nmch 

 nion; rapid l li.m I In- noil h\\ urd ndvaiicc of I he s(>ason. '\^\\(\ sunuuer 



