A BIRDLOVER'S YEAR 



still, millions of them must make their great 

 bi-annual journeys in perfect safety. 



Amongst the migrants are many short- 

 winged and feeble-flying birds who usually 

 do not indulge in long flights ; yet when 

 this instinct possesses them they are capable 

 of performing great journeys. Special at- 

 tention must, therefore, be paid to the fact 

 that migration is performed at such a height 

 as to alter greatly the conditions of air- 

 resistance, buoyancy, &c., owing to the 

 rarefied condition of the atmosphere ; and 

 it is also possible that birds are assisted by 

 selecting favouring wind-strata. Travelling 

 at such a high level would mean an extension 

 of the field of view ; and Mr. Chapman, a 

 well-known ornithologist, worked out a rough 

 calculation which showed that birds flying 

 at an altitude of twenty-five thousand feet 

 would have a tangential extent of vision of 

 two hundred and twenty-five miles, while 

 those flying at thirty thousand feet would 

 have a vision of two hundred and forty-five 

 miles. Thus, " in the first-named altitude," 

 he says, " birds would in daylight see 

 England before leaving Norway, whilst at 

 the higher level those crossing Spain would 

 114 



