MAY. 83 



wind from continent to continent, for that is com- 

 paratively easy, but also how in bad weather it main- 

 tains that almost ceaseless flight in search of food 

 from dawn to dusk. That in some seasons even 

 these powers fail and thousands die, seems one of the 

 saddest facts of British bird-life ; but it is one of 

 the ordinary risks of the swallow's existence one 

 of the factors which keep the species within its 

 proper bounds. For it is safe to say that nature 

 has now of every species about as many as can be 

 accommodated. Raising two broods in summer, a 

 pair of swallows may increase by autumn into a 

 family of twelve ; and since swallows cannot increase 

 in numbers from year to year, it follows that ten out 

 of every twelve die before the next breeding season. 

 Of every six swallows that fly about in autumn five 

 must die by the spring ! To us, who measure happi- 

 ness by long life, such mortality among our bird 

 friends seems awful. 



LARGER FAMILIES, SHORTER LIVES. 



Swallows, of course, are no worse off than other 

 birds which rear as many young. Indeed, the spar- 

 row, although he steadily increases in numbers and 

 always seems to thrive everywhere, must die even 

 more quickly, for he produces more young. Yet it 

 is not easy to guess by what means at least five- 

 sixths of the sparrows of London are killed off an- 

 nually between the summer and the spring. It is 

 from no defect in vitality that they die, for all kinds 

 of small birds which are kept in cages and carefully 

 protected will live for many years. They will do 



