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change of air in the air-sacs of birds is unable to facilitate their 

 flight to any considerable extent. Nevertheless, long continued 

 observations in Nature have convinced me that birds must be 

 •endowed with a certain capacity for soaring or floating in the air 

 which is independent of the use of their external instruments of 

 flight." Unfortunately he is unable to throw any light beyond 

 the above suggestions, as to the nature of these aids to flight. 

 That he, however, in the main attributes this capacity to the 

 possession of the air-sacs will be gathered from his further re- 

 marks on p. 72, In the interesting account of his observations 

 on the soaring upwards of a Buzzard and the appearance of 

 Sparrow Hawks as they descended to the island, some light is 

 thrown on another point, viz., the limit at which certain species 

 of birds might be migrating without our being able to see them. 

 Thus we find the before-mentioned Buzzard disappeared from 

 view at a height of at least 12,000 feet, and a Sparrow Hawk 

 appeared as a speck at 10,000 feet. Now bearing in mind that 

 these elevations amount to about half the estimated height at 

 which so-called normal migration is performed by the vast 

 majority of species, it will not be unprofitable to calculate at 

 what heights other kinds of birds would become lost to our view. 

 We may fairly reckon the larger of the European Turdinae at less 

 than half the size of a Buzzard. Thus they should disappear at 

 a height of 6,000 feet or less. The smaller species would equal 

 half the size of a Sparrow Hawk. We should, therefore, lose 

 them at 5,000 feet or so. Chaffinches, Pipits, Wagtails, Fly- 

 catchers, and the larger Warblers are about half the size of a 

 Song Thrush. They would become invisible at 3,000 feet. 

 Whilst the delicate Phylloscopi and Gold-crests would be dis- 

 appearing from view at a height of only 1,500 feet or less. It 

 will be gathered, however, from Herr Gatke's remarks on p. 56 

 that it is not the largest species which migrate at the greatest 

 -elevations. For he includes " species allied to the Snipes, such 

 as Curlews, Godwits, Plovers and their relations, as coming 

 next in respect to the height of their migration flight in order 

 to those already enumerated," i.e., Song Thrushes, Ked-breasts, 

 Hedge Sparrows, and Golden-crested Wrens. 



It will be readily admitted that an exceptionally keen sight 



