228 BIRD BEHAVIOUR 



there seem to be any connection between colour 

 of iris and visual powers, both Owls and Hawks, for 

 instance, being found with either dart 6r yellow 

 eyes ; this is, of course, not surprising, as the iris 

 itself plays no direct part in vision, but some 

 correlation might have conceivably existed. 



That some birds have powers of sight surpassing 

 those of man is known from the use of the Great 

 Grey Shrike by the Hawk-catchers at Valkenswaard 

 in Holland, who are informed by their tethered 

 Shrike of the appearance of a bird of prey before 

 they could see it themselves, and when nearer, 

 of its character, whether Harrier or Buzzard on the 

 one hand, or Falfcon or Goshawk, by the degree 

 of agitation shown by the sentinel. 



The Indian Roller (Coracias indica), too, sitting 

 lazily on a telegraph wire in the glare of the Indian 

 sun, perceives and flies down to an insect or other 

 small prey at a distance at which human eyes 

 would certainly not perceive such an object ; while 

 many must have wondered how a Kingfisher, or 

 even more a Gannet, discovers a fish and is able to 

 judge its distance from the surface sufiiciently well 

 to make a successful Swoop. It must be admitted, 

 however, that the Kingfisher at all events makes 

 at least as many misses as hits, not having so much 

 judgment as some of the birds of prey I have been 

 mentioning j and, speaking of most birds, they do 

 not behave as if their power of sight were any 

 better than our own. 



Diving-birds, however, may enjoy better sight 



