xxii INTRODUCTION. 



Duck. In such cases the young generally follow the 

 plumage of the adult female. Often, however, whether 

 the sexes are alike or different, the young have a separate 

 first plumage of their own, as in the case of the young 

 Robin, which is spotted. In a few cases the young, 

 as in the Upland Goose of South America (Chloephaga 

 magellanica), show the sex-difference in their first 

 feathering. 



A seasonal change of colour to a greater or a less 

 extent is not uncommon in birds. It may affect only 

 the special plumage of the male, as in the case of the 

 Common Drake, which loses its distinctive hues in 

 summer. Or it may affect both sexes, as in the well- 

 known change of the Ptarmigan to white in winter. 

 Such changes occur amongst tropical birds as well as 

 among those of temperate regions, so that it must not 

 be put down as necessarily an adaptation to a severe 

 winter. 



Considering their powers of flight, it is not surprising 

 that birds are to be found all over the world, even on 

 the remotest islands ; the Emperor Penguin breeding 

 even on the Antarctic ice. In the highest latitudes 

 the species, however, mostly draw their sustenance from 

 the sea, either directly or indirectly, and very few remain 

 during the winter. 



In temperate regions also there is a great migratory 

 movement towards the warmer climates in autumn, 

 and back again to the breeding places in spring. The 

 motive for migration from a temperate climate is 

 obvious enough, food in such zones being scarce during 

 the winter season. The return journey in the spring is 

 less easily explained, but is probably partly due to the 

 strong attachment of birds to their home or nesting 

 site, and partly also in all probability to persecution by 

 the permanent residents in their winter quarters. In 

 the tropics there is a great deal of local migration in 

 relation to the rains ; aquatic and fish-eating species, 



