SUMMARY 127 



environment and changing conditions. As Seebohm 

 puts it, ; The desire to migrate is a hereditary 

 impulse, to which the descendants of migratory 

 birds are subject a force almost, if not quite, as 

 irresistible as the hereditary impulse to breed in 

 the spring" (44). 



The route is simply the course followed between 

 the breeding area and winter quarters ; it is more 

 or less restricted by the size of the area in which 

 food is to be found ; it is usually the most direct 

 way from one food-base to the next, in a general 

 direction from the seasonal bases. Most birds 

 move between north and south, but migrations are 

 regularly followed in other directions by some 

 species. 



Routes may follow coastlines, these providing 

 visible landmarks, and also, for many species, 

 plentiful food ; islands, capes, estuaries and inlets 

 are landmarks, asylums, food-bases, and sites for 

 congregation and departure for cross-sea passages ; 

 at these places migration is often specially notice- 

 able. Overland routes may suggest ' ; broad front ' 

 migration, when there are no particular restricting 

 influences and the species have no special need for 

 hurry. Migration at great elevations and at high 

 rates of speed is proved, but the highest and quickest 

 possible is as yet unascertained. It may also, under 

 other conditions, be performed at low elevations 



