MIGRATION OF BIRDS 3 



and in some cases in individuals of the same or 

 closely allied species, from the merest change of 

 elevation to a voyage almost as wide as the world 

 itself. The sedentary red grouse nests on the 

 moors, often less than 1000 feet above the sea, 



7 * 



but " when snow-bright the moor expands ' it 

 feeds and resides in the cultivated valley, and as 

 shown by the committee appointed to study grouse 

 disease, not infrequently migrates from range to- 

 range across wide valleys. Many tropical birds, 

 usually considered non-migratory, are subject to 

 short movements, the origin and purpose of 

 which is search for food and safe nesting 

 places. 



The knot breeds in countless numbers in Arctic 

 Greenland and America, so far north that only a 

 handful of ornithologists have traced its home ; 

 it travels south in summer so far as Damara Land. 

 The Arctic tern has a northern breeding range 

 extending perhaps as far north as that of any bird, 

 and it has been taken far to the south of South 

 America in the Antarctic regions ; if the thesis 

 that the further north the bird goes in summer the 

 further south it travels in winter is correct, as it 

 can be proved to be with some species, some of these 

 terns must annually travel about 22,000 miles (21). 

 Between these extremes are an endless variety of 

 distances travelled and methods of migration, 



