318 PRINCIPLES OF ANIMAL BIOLOGY 



lians, crossing Bering Strait, might at this rate have reached the Straits 

 of Magellan in 50 years. 



Periodic Migration. — Not all the movements of animals are of the 

 slow, steady, progressive type just described. Many species move in 

 large numbers from one place to another at different times of the year 

 or at different times in their life history. The southward migration of 

 many birds in the fall and their return in the spring is an example of 

 seasonal migration. The great majority of bird species which may be 

 found in the course of a year at a given place in the middle of the north 

 temperate zone, for example, are seen there only at certain times. A 

 small number of species spend the summer there, building their nests 

 and rearing their young but disappearing southward in the fall. A still 

 smaller number are winter residents, some of which have come south 

 from a more northerly summer range. A much greater number are 

 migrants, going north in spring to their breeding range and returning 

 southward as cold weather approaches in the fall. What causes birds 

 to migrate is one of the great biological enigmas. ]\Iigration starts 

 before the situation where they spend the winter or summer becomes 

 unfavorable. In some species the migrating is done correctly by young 

 birds without previous experience and without guidance. Individual 

 birds have often been found to return to the same nesting place in suc- 

 cessive summers, but the way in which they are guided to the spot can 

 only be guessed. It has been suggested that endocrine secretions (page 

 154), particularly those of the pituitary and of the gonads, and the 

 duration of daylight may initiate migration, but how they could guide 

 it is not clear. 



Some other animals migrate seasonally in search of food. When 

 the bison was abundant in the Avestern plains, it wandered in droves 

 north and south as grazing lands developed. The mule deer moves up 

 and down the mountains likewise in search of vegetation. In these 

 instances, however, there is no puzzle, for the animals move slowly, and 

 they wait until the new feeding grounds are needed and are available. 

 They do not anticipate events but direct their movements in relation to 

 what can be actually seen. 



In a few animals the migration is not seasonal but occurs once each 

 direction in a lifetime. The fresh-water eel migrates at times .separated 

 by an interval of years. In its youth this animal ascends the rivei's 

 from the sea and lives there for years but does not breed ; upon reaching 

 maturity it returns to. the sea to breed. The Alaska salmon shows a 

 similar migratory habit. 



Though periodic migration is important in the physiological cyQle 

 of individuals and in the economy of species, i( is not know u to have any 

 influence on species ranges. There is no known peculiarity of distribu- 



