during the nesting season in the northern part of the range, while 

 during their sojourn in the south, dayHght is continuous. During 

 several months of the year they have 24 hours of daylight and during 

 the other months considerably more daylight than darkness. 



Routes of Migration 



While it is beyond question that certain general directions of flight 

 are constantly followed by migratory birds, it is well to remember that 

 the term "migration route" is to some extent a theoretical concept con- 

 cerned entirely with the lines of general advance or retreat of a species, 

 rather than the exact course followed by individual birds. Even the 

 records of banded birds usually show no more than the place of band- 

 ing and recovery, and one must have recourse to intermediate records 

 and to reasoning based on probabilities to fill in details of the route 

 actually traversed between the two points. 



There is also infinite variety in the routes covered during migration 

 by different species. In fact, the choice of migration highways is so 

 wide that it seems as if the routes of no two species coincide. Differ- 

 ences in distance traveled, time of starting, speed of flight, geographical 

 position, latitudes of breeding and of wintering grounds, and in other 

 factors, all contribute to this great variation of migration routes. 

 Nevertheless, there are certain factors that serve to guide the avian 

 travelers along more or less definite lines, and it is possible to define 

 general lines of migration for the majority of species. 



It has been frequently observed that migrating birds have a ten- 

 dency to follow major topographic lines on the earth's surface when 

 their trend is in the general direction of the birds' journey. Bird 

 migration is generally thought of as a north-and-south movement, 

 with the lanes of heavier concentration following the coasts, mountain 

 ranges, and principal river valleys. To a considerable extent this is 

 the case, particularly in North America, where the coast lines, moun- 

 tain chains, and larger rivers in general run north and south. In cases 

 where the migration is a long one, however, the notion must be aban- 

 doned that the birds' flight is always restricted to narrow routes that 

 follow river valleys and the like, as many species seem to disregard 

 utterly such apparently good natural highways. For example, the 

 Arkansas River has a general east-and-west course for a great part of 

 its length, and while it does constitute a route for many perching birds 

 en route from the Mississippi Valley to the Rocky Mountain region, 



41 



