B1HD MIGRATION. 4 1 



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

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

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

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

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

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

 what order or at what time young of the previous year migrate lias 

 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 guard in each case is composed of northern-breeding birds, 

 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 migration farther north is delayed by winter 

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

 this transposition occurs is a problem of migration reserved for future 

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

 strictly within then winter range until after their more southern 

 congeners 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 in spring another transposition occurs. The northern 

 birds pass across the southern part of the breeding range, where the 

 southernmost birds are already busy with their domestic duties. 

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

 leapfrog, the southern birds first 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 fulfills the popular notion of bird migration, i. e., it moves 

 northward hi spring as soon as the loosening of winter's fetters offers 

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

 the same rate as spring, 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 the map, the isotherm 

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

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

 position of the van at various times are approximately east-and-west 

 lines during the whole migration period. 



But this so-called regular migration is performed by a very small 

 percentage of species, the great majority choosing exactly the oppo- 

 site course — to remain in their winter homes until spring is far ad- 

 vanced and then reach their breeding grounds by a migration much 

 more rapid than the northward advance of the season. The summer 



