CHAPTER VIII 



MIGRATION 



§ I. Main Facts regarding Migration. § 2. Problems in Process of 

 Solution. § 3. Nature of the Migratory Custom. § 4. Imme- 

 diate Stimuli liberating the Migratory Impulse. § 5. The 

 Problem of Way-Finding. § 6. Possible Origin of the Migratory 

 Custom. 



Migration is a seasonal mass-movement of a large contingent 

 of a species from the breeding place, always in the colder 

 part of the range, to the winter-quarters, and back again. 

 It is not to be confused with the overflow movements of an 

 over-crowded population, as in locusts and lemmings, or 

 with a restless following of a mobile food-supply and the 

 variations of physical conditions, as in the case of herring. 

 From a slightly different point of view we may regard 

 migration as an adaptive reaction to the difficulties presented 

 by the seasons in particular areas. Just as some mammals 

 circumvent the winter by passing into the state of hibernation 

 to which they have become constitutionally adapted and 

 predisposed, so many birds evade the winter by migration, 

 and that not in virtue of individual initiative, but because of 

 constitutional rhythms and impulses. 



§ I. Main Facts regarding Migration 



(A) In the Northern Hemisphere the occurrence of 

 migration is very general. But the extent of the phenomenon 

 varies greatly. A flock of curlews may migrate from the 

 moorland to the shore ; the Virginian Plover may migrate 

 from Labrador to Brazil. In October, 1882, a vast multitude 

 of Gold Crests struck our shores from the Channel Islands 



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