i-,6 THE MIGRATION OF BIRDS. 



kind, that the probability is, very few birds indeed 

 can be regarded as thoroughly sedentary. Even 

 in the lowlands of the equatorial regions, where the 

 usual type of migration is unknown among the 

 resident avifaunas, considerable movements take 

 place according to season. 



We will deal with the phenomenon of Internal 

 Migrations first. These may be divided very 

 naturally into two great groups, viz. Vertical Migra- 

 tion, or the regular passage of birds from the plains 

 to the hills ; and the Northern Flights of various 

 species breeding in the temperate portions of the 

 Southern Hemisphere, and whose order of progres- 

 sion is exactly the reverse of what takes place in 

 the Northern Hemisphere. The amount of vertical 

 migration is enormous, and is mcst prevalent in 

 hot countries, although there is a very perceptible 

 vertical movement even in such temperate districts 

 as the British Islands and Scandinavia. Perhaps in 

 no other country is vertical migration more pro- 

 nounced or more widespread than in India. Here 

 great numbers of species retire from the plains to 

 the slopes of the Himalayas to breed, ascending 

 thousands of feet above sea-level, and returning to 

 the lowlands as the cold season approaches. The 

 Common Woodcock {Scolopax rusticola) goes at 

 least to an elevation of 10,000 feet to breed in these 

 mountains, and winters on the plains. The move- 

 ments of these birds are just as regular as those 

 of species whose fly-lines extend for thousands of 

 miles, although the altitude visited varies a good 



