JG RANGE 



FIG. 10-10 Migration 

 longest known, of the ■ 

 (Lincoln 1950). 



gration periods, neither breeding nor wintering in the 

 area ; and accidentals, species that are rare or irregu- 

 lar in occurrence. The bird population in most com- 

 munities reaches peaks during the vernal and autumnal 

 aspects, as transients arrive and temporarily swell 

 fKDpuIations. The autumnal peak is usually the greater 

 because adult populations are incremented by the large 

 number of young birds hatched during the breeding 

 season. Latitudinal bird migrations also occur in the 

 southern hemisphere but are less conspicuous because 



of the small continental land masses there and small 

 populations of birds involved. 



There are many causes of bird migration, varying 

 in relative importance with different species. Aquatic 

 species must leave northern areas before their food 

 supply is cut off by the freezing of the lakes, ponds, 

 and rivers. Insectivorous species unable to change to 

 other types of food must migrate before insects go into 

 hibernation or disappear. The metabolism and food 

 requirements of many song birds are so high that even 



Dispersal, migration, and ecesis 157 



