86 



would be necessary in order to distinguish a Willow Warbler at 

 500 yards distance. Why Herr Gatke should have placed so 

 high an estimate on the altitude of the migration path in the 

 case of smaller species, without more direct evidence, it is diffi- 

 cult to imagine. All he tells us on this point is " in the case of 

 the vast majority of migrants, both on arrival and departure, the 

 .vertical elevation at which they appear and disappear invariably 

 represents the limit to which human vision is able to penetrate." 

 When we read, however, (p. 76) " that the migrations of most 

 species proceed at a height of at least 20,000 feet " under the 

 so-called normal conditions, we see what a very wide margin 

 there is between the latter estimate and the elevation at which 

 all but the largest species would disappear from our view. 



It may be further noted that this power possessed by birds of 

 floating about or soaring upwards, which is said to be indepen- 

 dent of the wings, in the opinion of Herr Gatke, finds its antithesis 

 in the means by which certain aquatic birds are enabled to swim 

 with the body completely immersed. To the writer it appears, 

 that in comparing the two performances he quite overlooks the 

 fact of the great difference in the density of the surrounding 

 mediums. Whilst the inflation of the air-sacs in flying may 

 make a very trifling difference to the buoyancy of a bird, their 

 deflation in the vastly denser medium of the water, in conjunction 

 with minor actions, may fully explain the means by which the 

 latter act is accomphshed. In needs very little reflection, to at 

 once see that to deflate the lungs and the air-sacs, and at the 

 same time expel the warm air, by compressing the outer coating of 

 feathers, from the downy covering of an aquatic bird, means greatly 

 lessened buoyancy. As an illustration of this, no better example 

 could be found than that of shot birds, whose respiration in the 

 meantime having ceased, and the latent heat of the body rapidly 

 being lost, a few minutes after death lose all their buoyancy and 

 merely float on the surface of the water like any other Ught object. 

 Birds are generally supposed to have descended from some half- 

 reptilian progenitor of aquatic habits, and the opinion may 

 perhaps be hazarded that the air-sacs may, to some extent, be a 

 survival of organs which found their greatest use in remote ages. 



In commenting on the possibility of migration being retarded 



