THEORIES OF MIGRATION 29 



Let us now return to the hypotheses that have 

 been outlined. Objection may be raised to any of 

 them;, but it appears that an explanation of the phe- 

 nomena under discussion may be found in a com- 

 bination of the various forces suggested, working in 

 unison. The entire act of migration is so utterly 

 complex that no single factor may be ascribed as the 

 absolute cause. A logical explanation may, however, 

 be attempted if we hold that the origin of these 

 movements is multiple. It may be advanced as a 

 working hypothesis that migration has arisen from 

 movement induced by seasonal and climatic change 

 developed in certain species until it has become 

 hereditary instinct. 



It will suffice to turn to everyday observations on 

 the habit of brief post-breeding wanderings of some 

 of our common birds to record what may have con- 

 stituted a beginning of migration. A pair of robins, 

 tufted titmice, or white-breasted nuthatches, select 

 a home site, prepare a nest, incubate eggs, and rear 

 a brood of young to maturity. During this period 

 foraging for food is confined to a limited space. As 

 the young grow, the surrounding area must be 

 scanned intensively to procure a sufficient supply of 

 food. When the fledglings leave the nest they may 

 linger for a time in the vicinity; but as the young 

 grow stronger, they begin to wander, at first for a 

 few yards, but gradually extend their area until it 



