84 THE MIGRATION OF BIRDS. 



previous suggestion. That a captive balloon floated 

 above some spot where migration is notoriously 

 prevalent^ as for instance at Spurn Point on the 

 Yorkshire coast, in the Wash, on the Sussex 

 Downs, or better still, over Heligoland, would 

 result in priceless information concerning the annual 

 movements of birds, is absolutely certain. The 

 more I study Migration, the more I feel convinced 

 that it is a nocturnal drama of the air, and that only 

 a faint conception of its wonders can be formed 

 from terrestrial scrutiny. I shall be happy to 

 assist in any such investigation. Will the British 

 Association for the Advancement of Science, which 

 has already done so much for the elucidation of the 

 Pilgrimage of the Birds, still further lend its all- 

 powerful assistance towards establishing such aerial 

 posts of observation ? 



The various notes uttered by birds on migra- 

 tion must not be forgotten. Some species are 

 much more noisy on passage than others, and many 

 birds keep up quite a chorus of cries as they 

 wing their way along. That these notes serve the 

 important purpose of keeping migrating flocks to- 

 gether seems unquestionable. The cries of birds 

 may be heard repeatedly at night during the two 

 seasons of passage, as flock after flock of migrants 

 crosses over the darkened sky. These notes may 

 also serve as guides to the young and inexperienced^ 

 keeping them to the true course ; whilst the birds 

 that fly in silence may be kept in touch of the 

 route by hearing the cries of more noisy species.: 



