Internal Migration. 81 



chiefly cause birds to move from one place to 

 another (so far as we know), are food-supply and 

 temperature. Now we have only to look at a 

 raised map of Switzerland to see at once how 

 subject the birds must be to such incitements 

 towards change of place. Any one who has been 

 to Switzerland will have noticed that the scenery 

 falls into three great divisions that of the lakes 

 and valleys, that of the Alpine pastures and 

 forests, and lastly, that of the regions on the 

 border-line of perpetual snow, running upwards 

 to the higher snow-fields. The professional 

 mountaineer pays little attention to any but the 

 last of these ; the botanist and ornithologist have, 

 fortunately, much reason to pause and reap a 

 harvest in the lower levels, which are incom- 

 parably more beautiful. For convenience' sake 

 I will call the lowest, No. i ; the second that of 

 the Alpine pastures, No. 2 ; and the highest, 

 No. 3. The distribution of birds in these three 

 regions is continually changing. No. 3, in the 

 winter, is entirely devoid of life and food. The 

 Eagles and the great bearded Vultures, now very 

 rare, can find not even a marmot to prey upon, 



