52 THE BIRDS OF HELIGOLAND 



out of the question that the bird could have held on to it with its 

 feet. In both cases the birds remained perfectly motionless, since 

 the least motion would, at so short a distance — at most some thirty 

 paces — have at once betrayed their hiding-places. Naumann, in 

 vol. ix. of his great work, relates similar experiences in regard to 

 this small Diver. 



Another extremely valuable opportunity of observing this quiet 

 immersion of the body was presented to me many years ago in the 

 case of a Cormorant in a pond in the Zoological Gardens at Ham- 

 burg. For the purpose of catching some of the swallows which 

 ■were roving in fairly large numbers over the surface of the water, 

 the bird had innnersed itself to such a depth that only its head 

 remained visible above the surface ; in this position it remained 

 perfectly motionless, for the least movement of its feet Avould have 

 been at once betrayed on the perfectly smooth surface of the water. 

 The swallows, foreboding no ill, frequently came up very close to it, 

 and when the bird thought it could reach one of them, it would, 

 quick as lightning, protrude its neck and make a snap at it. After 

 four or five unsuccessful attempts, it actually did manage to catch 

 one of them, which it swallowed after givino; it a few shakes in the 

 water. It then quietly re- immersed its body as before, and with 

 neck drawn in continued to lie in ambush for further prey. 



This immersion of the body in and below the water witnessed 

 in such birds must not be confused with their diving for food, which 

 may be seen every day. In the performance of this operation the 

 body is placed in a neai'ly vertical jjosition, and is then driven 

 beneath the water by powerful upwardly-directed strokes of their 

 swimming feet; hence the desired result is obtained simply by the 

 expenditure of mechanical force, just as the ordinary flight of 

 birds through the air is attained by rapid and powerful strokes of 

 the wings. 



To enable the bird, however, to immerse its body under the 

 surface of the water slowly, and to maintain it in that position 

 without movement, it is necessary that its specific weight slioidd be 

 increased considerably above that of the water, and it is quite 

 impossible to see how this can be effected. The total bulk of the 

 solid parts of the body of a Great Northern Diver may be estimated 

 at about one cubic foot : hence, in order to be enabled to sink, the 

 weight of this bulk should exceed that of the same volume of sea- 

 water. It does not, however, actually amount to the fourth part of 

 this weight ; for the heaviest of these Divers I have ever examined 

 weighed fifteen pounds, whereas a cubic foot of water from the 

 North Sea weighs sixty-two pounds. This already considerable 

 difference between the weight of the bird's body and that of an 



