ALTITUDE OF THE MIGRATION FLIGHT 51 



The habit of immersing a part or the whole of their bodies under 

 water, which is peculiar to many, if not all, Divers, is related to that 

 of aerial soaring, although this action is executed in an exactly 

 opposite manner. Great Northern Divers, Grebes, Cormorants, 

 Diving-Ducks, and other related species, if pursued while swim- 

 ming on the sea for any considerable time by shooters in boats, 

 gradually immerse themselves to such a depth that only their 

 heads and the upper parts of the neck project above the water. 

 If, however, they are very hard pressed, they sink completely below 

 the surface, and swim for a distance of from a hundred to a hundred 

 and fifty paces horizontally beneath it, only momentarily exposing 

 their heads and necks for the purpose of breathing. Grebes, indeed, 

 especially if they have been already shot at, will, under such condi- 

 tions, expose nothing but their beaks up to the eyes. 



All these birds when alive and undisturbed (as also do their car- 

 cases) float so lightly on the water that they scarcely make any 

 noticeable depression in it. Nor is this surprising, for all of them 

 have their undersides clothed with a covering of down and feathers ; 

 in the case of a somewhat shrunken specimen of a medium-sized 

 Grebe in my cabinet this covering has nevertheless a thickness of 

 15 mm. (0'59 in.), while in a Great Northern Diver of the same size 

 it reaches a thickness of from 20 to 25 mm. (079 in. to 0'98 in.). It 

 is perfectly easy to understand how birds can, without difficulty, 

 float on the water on an almost weightless support of this nature, 

 which is, moreover, filled with warm air ; but it is difficult to explain 

 how, in spite of such a float, they are able to immerse themselves 

 under the water, and to remain for any desirable length of time 

 beneath the surface. Thus a Little Grebe managed to escape obser- 

 vation on a piece of water about sixty paces in diameter and from 

 two to three feet in depth, by immersing itself up to its beak and eyes 

 in the middle of the pond, and remaining quietly beneath the surface. 

 What is still more surprising, the bird selected for its hiding-place a 

 part of the pond where some dried grass-blades, and wood-shavings 

 about an inch long, were floating about, which entirely diverted 

 one's attention from the insignificant portion of its head and beak 

 which were still visible above the water. On another occasion, a 

 bird of the same species remained quietly immersed at the margin 

 of the same pool, where the water was only about six inches deep, 

 so that only its beak and eyes remained above the surface. It 

 should moreover be noted that, hi the first instance, the depth of 

 the water, as well as the absence of any kind of vegetable growth, 

 completely precludes the supposition that the bird might have 

 obtained some kind of hold or attachment under the water, while 

 in the last case the bottom was so level and hard that it is quite 



