212 BIRD BEHAVIOUR 



groups, the species which go farthest north to breed 

 often winter farthest south, as exemplified among 

 the Swifts and the Turtle-Doves ; among Ducks, 

 too, the arctic-breeding Pin-tail and Wigeon go 

 farther south than the temperate-zone-haunting 

 Mallard. None of the Geese, however, which 

 mostly breed in the high north, go very far south. 

 Many naturalists seem puzzled as to why birds 

 which have escaped starvation by going towards 

 equatorial lands should retrace their course again 

 to the countries they have left ; but this is because 

 they have generally little practical acquaintance 

 with any but the birds of the Northern Hemisphere, 

 where only, in Europe and the United States, do 

 ornithologists much abound. To any one who 

 knows the tropics, the problem does not seem so 

 wonderful. In the less well-watered parts of these 

 countries, the heat of the hot season produces 

 effects not so very different from the frosts of the 

 temperate winter. Herbage and leaves are parched 

 up, with the result that both insect and vegetable 

 food become scarce, and the water supply is much 

 restricted. Under these conditions reptiles, frogs, 

 and even fish " aestivate," i.e. go in for a summer 

 sleep, in the case of the aquatic creatures burying 

 themselves in the mud, and native birds of aquatic 

 habits have to leave these districts for better- 

 watered ones. 



Obviously, then, the migrants are no better off, 

 and have every reason to get out of a country in 

 which the residents have all they can do to survive. 



