242 THE MIGRATION OF BRITISH BIRDS 



other part of the world. There can be little doubt that 

 the passage across seas is attended by the greatest 

 amount of mortality, and even under the most favourable 

 circumstances the death-rate must be a high one. Of 

 the hordes of young birds that are seen on migration in 

 autumn but a small percentage survive the perils of 

 the journey, and a still smaller percentage return in the 

 following spring. If the season of passage chances to 

 be exceptionally unfavourable — full of storms and fogs 

 — the death-rate will go higher than its normal average, 

 and perceptibly reduce the numbers of a species perhaps 

 for years. An anonymous reviewer of the Migration of 

 Birds says that I greatly over-estimate the perils of 

 migration. Well, after several more years' close attention 

 to the subject I will state that in my previous work I 

 under-estimated the mortality, and that the more I 

 study migration the more I am convinced of its fatal 

 consequences. 



