130 THE STRUCTURE AND LIFE OF BIRDS chap. 



the eyes are very small, but the Apteryx is the one 

 bird in which the nostrils open at the end of the beak : 

 it trusts more to scent and touch than to sight. 



Birds' keenness of sight is most remarkable. Vul- 

 tures, as I have already mentioned, descry their prey 

 from enormous distances. A Gannet, flying ioo feet or 

 more above the sea, will distinguish a fish near the 

 surface from the surrounding water which it so nearly 

 resembles, and pounce upon it. It is a common 

 amusement on a steamer to feed the gulls that follow 

 the boat with small pieces of biscuit, which, when 

 thrown, float, often invisible to the human eye, in the 

 wilderness of foam which covers all the wake of 

 the ship. The gulls, flying some thirty or forty feet 

 above the water, will swoop down upon them with un- 

 erring aim. Often, when you think they have missed 

 a small fragment, they will at last find it far in the rear 

 of the vessel. 



The colours of birds' eyes are very various. In 

 the Shag the Iris is emerald green ; in the green-billed 

 Toucan, light green of the same shade as the beak ; 

 in the Ariel Toucan, like the tip of the beak, pale blue ; 

 in the Black Stork deep red ; in the Eagle Owl red- 

 orange ; in the Javan Fish Owl light yellow ; in the 

 Indian Kite nearly white. 



The following examples would seem to show that 

 dark plumage implies a dark shade of colour in the 

 Iris, and vice versa. 



Plumage. Iris. 



Angolan Vulture Wing coverts white Pale almost to 



whiteness. 



A Cockatoo Dark blue Dark brown. 



Ditto Light blue Nearly white. 



