ALTITUDE OF THE MIGRATION FLIGHT 53 



equal volume of water is, however, still further increased by the 

 covering of dovm or feathers, which, permeated by warm air, 

 surrounds the body as already mentioned. 



Hence, after all that has been said above, the question how the 

 body of a bird is enabled to sink and remain for any length of time 

 beneath the surface of a specifically heavier medium, like water, 

 becomes as difficult of explanation as that of the capacity for soaring 

 up into a specificallj' lighter medium, such as the air, unassisted as 

 these faculties are, in both instances, by mechanical aids, or currents 

 of air, or water, respectivel}-. 



The capacity of birds for rising to great heights in the air is 

 undoubtedly turned to account to a certain extent by some or per- 

 haps by many species, even during the ordinary activities of their 

 everyday life. Thus Vultures, and — according to von Middendorff 

 {Isepij>tesen) — Common Ravens, ascend to astonishing heights for 

 the pur^Jose of finding food. In general, however, this peculiar 

 faculty is brought into full and continued requisition only during the 

 migration fiight, which is in fact the only occasion on which it can 

 be fully turned to account. It is consequently beyond denial that 

 this faculty must have been imparted to birds solely for jDurposes 

 of migration. Observations in Nature most convincingly testify to 

 the truth of this conclusion, since birds, without exception, on start- 

 ing for their great migration journeys, immediately rise to heights 

 elevated far above the regions of their daily flights — heights, more- 

 over, which in the case of the vast majority of species, are com- 

 pletely beyond the range of all human perception. 



In the case of our small Warblers— Thrushes, and the like, — this 

 limit of visible elevation may perhaps not amount to much. It is 

 otherwise with the larger-sized birds, as, for instance, the Stork, 

 or 23referably the dark-plumaged Crane, a bird which rises into the 

 clear sky with an expanse of wings of from seven to eight feet, until 

 it becomes almost indiscernible to a keen eye (Naumann), at a 

 height which can hardly be estimated at less than 15,000 to 20,000 

 feet. A dark-coloured flag, from seven to eight feet in length, on a 

 ship, may still bo distinctly recognised at a distance of a mile. At 

 the same time it is well to remember that the conditions for seeing 

 far-off objects are far more favourable in vertical than in horizontal 

 distances. 



The most astonishing results in regard to the heights to which 

 birds will rise spontaneously, and at which they are capable of 

 remaining for any desirable length of time, have been furnished by 

 the observations made on the Condor by Humboldt in the Andes, 

 according to which this bird wheeled about in the air in that district 

 for hours at a height of 22,000 feet (Ansichten der Natur, ii. p. 52). 



