622 The Animal Kingdom 



there are four definite migratory routes : one extending along the east 

 coast, a second through the Mississippi Basin, a third through the Great 

 Plains, and the fourth along the west coast. A great many of the 

 smaller birds do most of their flying at night and rest and feed during 

 the day ; many of the waterfowl fly in great flocks. Most of the smaller 

 birds have definite times for their movements, while the waterfowl tend 

 to depend more upon the vagaries of the weather. Usually the forward 

 movement is about 25 miles a day ; however, those birds which cross 

 vast water areas such as the Gulf of Mexico may do the 500 miles in 

 a single night. Even the tiny ruby-throated hummingbird performs 

 this feat ! During their migratory movements, the birds average about 

 30 miles per hour, but there is great variation in this rate of speed. 



The greatest migrator of all as judged from the distance traveled 

 is the arctic tern. It breeds in the far north with the breeding area 

 extending from 7^ degrees south of the pole to Massachusetts. After 

 the young hatch, the birds undertake the long hazardous journey to the 

 Antarctic continent some 11,000 miles away. In this journey, they cross 

 the Atlantic to Europe and head southward along the African continent, 

 finally terminating the trip in the islands of the Antarctic. The golden 

 plover has a migration path nearly as remarkable as the arctic tern. 

 Its nesting ground likewise is in the far north. In late August it starts 

 the long trek toward the pampas of Argentina. At times it covers as 

 much as 2,400 miles without stopping. This route towards Argentina 

 goes along through the lesser Antilles, the Guianas, and Central Brazil. 

 The birds remain in Argentina until the following March. When re- 

 turning to their breeding grounds, the birds fly northward through 

 Central America and up through the Mississippi fly way to northern 

 Canada. 



In contrast to these birds are those that scarcely move from the vi- 

 cinity of the nest in which they were hatched. The bobwhite, the cardi- 

 nal, the Carolina wren, and some woodpeckers are examples of these. 



Many theories have been formulated to account for these amazing 

 mass movements. None of them, however, seems entirely satisfactory. 

 Clearly the fact that a bird makes its first trip often without the adult 

 indicates that migration is an inherited behavior pattern of great com- 

 plexity. Factors such as length of day have been shown definitely to af- 

 fect the gonads, and secondarily to "trigger" the start of the migration. 



The birds of the South American continent migrate northward dur- 

 ing the winter season, but these movements are not so extensive as those 



