354 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



Winter renders return migration as imperative as exodus 

 migration.^ Habit in the old birds loses none of its force 

 through erotic promptings to return to the nesting abode. 

 Young birds who have successfully met the difficulties of 

 one journey are not less fitted to follow the old in the return 

 movement. Hence, it is maintained that heredity (implying 

 at most an innate desire for travel and a talent for geogra- 

 phy), education of the young into a knowledge of the way, 

 and habit in the old birds (possibly also foresight), holding 

 them true to time and place, are the paramount inward 

 causes ^ of the return as well as of the exodus — the two 

 movements constituting the adaptation of the bird popula- 

 tion to winter, northern and southern, with its failure of 

 food.^ 



1 For example, the return movements in the Northern Hemisphere are necessitated, 

 it is held, because they are integral parts of the vast movement which sways the bird 

 population northward, relieving the pressure arising from winter in the Southern Hemi- 

 sphere. The complicated system of movements composing this grand movement is 

 believed to be the outgrowth of time. So close is the adjustment of population to the 

 existing food-supply that stability in the established order is essential to the success of 

 migration. (Cf. Calif. W. B. No. IV, p. 314.) 



Some writers, dwelling upon the perils of the journey, envelop migration in a cloud 

 of mystery, losing sight of the fact that it is safer for the birds to go than to stay. There 

 are numerous ordinary undertakings in human life that are not less free from danger 

 than avian migration. Some one has aptly remarked that it is dangerous to live. 



2 As stated in ' No. IV,' p. 315, desire for procreation may be a prompting influence in 

 the return migration. That it is not a paramount cause is further indicated by the 

 existence of sedentary species. 



It was also affirmed in the same paper that there may be, in some species, special 

 physiological demands as to temperature during reproduction. If it requires such nice 

 climatic adjustment to bring the young into the world, it is remarkable that in the space 

 of a few weeks they should be fitted for the changes of climate incident to migration. 

 It may be, however, that a species repairs to a higher life zone not because of the pecul- 

 iar food or temperature of the region, but because in the struggle room has been found 

 nowhere else. Perhaps the solution of this whole question is to be obtained from mi- 

 gration to oceanic islands, such as the Long-tailed and Shining Cuckoos to New Zealand 

 and the Turnstone and Smaller Golden Plover to Hawaii. Applying the interpretation 

 set forth in the preceding pages, it is held that the Cuckoos in the unfolding of migra- 

 tion gained New Zealand as a breeding place, and the two Shore Birds gained Hawaii as 

 a winter resort; that the young, inheriting probably a desire for travel and a talent for 

 geography, learn the way to these habitats from the old; that habit (possibly also fore- 

 sight) holds the old ones true to the way that has been learned, thus perpetuating the 

 intricate movements forming the adjustment to winter, northern and southern, with its 

 failure of food. 



3 While it is contended that the present conditions enforce present migration, 

 it is not denied that the evolution of the seasons was the cause of the evolution of 

 migration. 



