Emigration 357 



origin with the retention of no specific homing reaction or of one that 

 is operative only after the animal has arrived within the general area 

 of its original home territory. 



Emigration 



When animals leave their home range never to return, the move- 

 ment is spoken of as emigration. This type of migration may take 

 place either as a drift of individuals or of small groups, or as a mass 

 movement. The young of many species of birds— and also to some 

 extent the adults— wander in all directions after the breeding season 

 and before the autumnal migration to the winter range begins. An 

 example of this centrifugal movement is obtained from the returns of 

 banded ducks (Fig. 9.19). Drift emigration of this sort is very wide- 

 spread in its occurrence, but it often escapes notice because the 

 animals or birds move inconspicuously in small numbers. Some kinds 

 of animals wander about for long periods of their lives, seeking pri- 

 marily perhaps food or water, without definite return to any given 

 place; such irregular roaming is spoken of as nomadism. This and 

 other types of animal travels are discussed fully by Heape (1932). 



In contrast to the foregoing, mass emigration is often spectacular 

 although it occurs in relatively few species. At more or less regu- 

 larly recurring intervals such animals as the mouselike lemmings of 

 Scandinavia and Canada, the springbucks of Africa, and the locusts 

 of Egypt and India undertake mass movements involving millions 

 or billions of individuals. The causes of these mass emigrations are 

 not clearly understood: in some instances they may be due to climatic 

 changes; in others they may be due to lack of food which may or may 

 not be the result of overproduction of the species itself. The onset of 

 acute hunger, nervous disorder, or other conditions occurring at the 

 time of mass emigration often profoundly alter the behavior of the 

 animals. For example, according to Heape (1932), some such in- 

 ternal change "transforms the shy and timid springbuck into a fearless 

 creature with no regard for danger of any kind, and converts the no 

 less shy and timid lemming into a truculent swashbuckler." 



An instance of a mass emigration of lemmings was witnessed in 

 1937 by the manager at the Perry River post of the Hudson's Bay 

 Company, Northwest Territory, Canada, and an excerpt from his ac- 

 count follows ( Gavin, 1945 ) : 



• • • While camped at the old post site on April 27 and for about four 

 days subsequently, the onset of the migration was noticed. The whole 

 tundra was a mass of moving lemmings and each time we went into the tent 

 there would be a dozen or more inside. The migration went on night and 



