MIGRATION AND ORIGINS 283 



grounds shows that there is some association of individuals of the 

 breeding populations apart from the time of nesting. Common ex- 

 perience and some association of members within each arctic popula- 

 tion while away from nesting grounds is also indicated by their 

 synchronous return in spring with all individuals in the same phase 

 of the brief arctic nesting cycle. Since the schedule of a bird's physio- 

 logical preparation for breeding and migration can be experimentally 

 modified by changing the duration of daylight, the physiological 

 synchronization is not a genetically fixable character and it can arise 

 only from similarity in the physical and social environment en- 

 countered while away from the nesting grounds. 



Birds migrating to the Arctic are similarly oriented in time and 

 space. It would be difficult to imagine the whole mechanism of in- 

 dividual behavior, physiology, and social activity which operates to 

 keep these migratory populations on the same coordinates of time and 

 space, but an acquired recollection of locality seems to be the essential 

 instrument of their isolation. Memory is the guide which associates 

 individual birds in populations along their long migratory paths 

 throughout the year. The strength of its influence for return to the 

 nesting grounds at the right time is especially impressive in relation 

 to the small size of some species. Kennicott's willow warblers, for 

 example, weighing only 10 grams, remain a distinct population within 

 their wide ranging species while annually moving swiftly over a large 

 and geographically complex part of America and Asia. 



The results of this behavior permit the migratory races to exploit 

 an arctic area which is only suitable for a brief season. The establish- 

 ment of such migratory races shows that their migratory behavior has 

 met the test of natural selection for value. The essential instrument 

 for their segregation is the memory pattern which acts as if to direct 

 energy of avian activity toward isolating racial characters from the 

 random mixture of the whole species. 



Many maritime species (table 10) nest in arctic Alaska and Yukon 

 without having formed recognizable races. Although the area has 

 long been occupied by these species, they have not become differ- 

 entiated into taxonomically discernible entities. Occupation of the 

 arctic nesting grounds must be useful to the species, or birds would 

 not have become established as migrants to that area. However, 

 either the area is not an isolated one as far as they are concerned, or 

 each individual bird's association with one locality is not maintained 

 consistently enough to segregate the requisite proportion of breeders 

 to allow for their eventual differentiation. It cannot be assumed that 

 those birds lack sufficient homing ability to maintain group solidarity, 

 as this ability is known from experiments in which several species 



