ioo ANIMAL LIFE UNDER WATER 



that the surface-swimming fish does not detect 

 the approach of its enemy until too late. (This 

 description, of course, merely applies to the 

 appearance of the bird in the area of total 

 reflection.) 



When the fish looks upward he sees the head 

 of the gull above the arc of its " window," * 

 when the head is white it is not easily detected 

 against the sky. The best colour scheme, how- 

 ever, for a bird in this position is a white head 

 and dark shoulders or a dark head and white 

 shoulders. With the plumage marked in this 

 manner, the compressed image of the head and 

 shoulders of the bird appears as an interrupted 

 broad line, and is lost in the " ripple pattern" 

 round the edge of the " window." An illustra- 

 tion of this '* ripple pattern " is shown on the 

 plate opposite page 142. 



How a black and white image is masked 

 when seen above the arc of the- " window " has 

 been explained and illustrated in the chapter on 

 wading birds. 



In the case of the skua there is no necessity 

 for a special arrangement of plumage to conceal 

 it from the fish, as this gull lives by robbing 

 others of the fish they have captured. The bird 



* See lower illustration on folding plate facing p. 3. 



