130 THE STRUCTURE AND LIFE OF BIRDS cuaP. 
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 100 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 wzce versd. 
. Plumage. Iris. 
Angolan Vulture ...... Wing coverts white ... 0.0.0.0... Pale almost to 
whiteness. 
A Cockatoo .........6. Dark Blwe: asi cg: cis aicesneawsany vee Dark brown. 
Diffoi —savcieigsiries Light blue 5 025 s0+c6s ges series avcnea Nearly white. 
