CONDITIONS OF FLIGHT 241 



The varying amount of sociability in birds whilst on 

 migration, their gregarious or solitary tendencies, have 

 all been described at length in the previous volume, and 

 as the facts apply not only to British birds, but to all 

 species, we need not repeat them. We have also dwelt 

 at some length on the duration of the passage and its 

 varying phases of intensity. The migration is usually 

 characterized by the advent or departure of a few indi- 

 viduals ; then the flight becomes more strongly marked 

 up to its greatest phase of intensity, which may or may 

 not be marked by one or two exceptionally strong 

 movements, after which the passage of the species for 

 the season as gradually dies away as it commenced. 

 The various stages of the journey, and the normal 

 velocity of flight during passage, have also been de- 

 scribed. I have nothing further to add at present to 

 what I have already said in the above-mentioned work 

 concerning the effects of Wind and Temperature on the 

 migration of birds. The remarks just as aptly apply to 

 British birds as to other species. The moulting of birds 

 before they migrate, and the structure of the wings of 

 migratory birds, have also been dwelt upon. 



Of the perils of migration but little more may be said. 

 I have already devoted a chapter of my previous work 

 to the subject. Many more instances of fatalities to 

 migrants might be given, but they would all partake of 

 the same general character : sufficient has been furnished 

 to illustrate very vividly the perils that beset birds 

 whilst on passage. The mortality connected with 

 migration can scarcely be realized. The vast numbers 

 of birds that perish during passage is past belief. And 

 this not only applies to the British Area but to every 



