THEORIES OF MIGRATION 3^ 



For further understanding of the problem of the 

 origin of migration let us turn to Australia, where 

 data are available through records furnished by an 

 increasing number of keenly observant naturalists. 

 This island continent, while it does not have the 

 extremes of climate found in the larger land masses 

 of the northern hemisphere, has sufficient extent 

 for a considerable difference between north and 

 south. The winter in the south, though marked by 

 lowered temperature, is not rigorously severe. Some 

 forms of birds are regularly migratory, but many 

 others may be termed only nomadic, as across broad 

 areas their presence depends upon rains. In some 

 sections, during periods of drought extending some- 

 times over several years, bird life practically dis- 

 appears. Rains come and turn the country once 

 more green, trees flower, and plants mature their 

 seeds. Flocks of parrots return to regions from 

 which they have long been absent; honey-eaters 

 appear in the eucalyptus; coots swarm in marshes 

 and swamps, and with ducks and other waterfowl 

 proceed to breed and rear their young. Proper con- 

 ditions may continue for several years, when these 

 birds will remain common. With the shifting of 

 rains to other sections the birds dependent upon 

 them follow, and desert the area that has given 

 them temporary sustenance. 



Though this seems mere vagrancy, it is migration 



