THE DIPPER 303 



of rocks will form quite a different opinion. It both walks and runs 

 with ease. And the strength of its foothold, as Macgillivray did not 

 fail to point out, is remarkable. One may frequently see it walking, 

 or rather wading, up or down the steep face of a mossy rock over 

 which the stream is swiftly flowing, pushing its beak for food into the 

 moss beneath the current, regardless of the little white cascade that 

 tosses and sparkles over its dusky head and back. It is this feeding 

 habit of the dipper that helps to explain one of its structural peculi- 

 arities that is well worth notice. It is its use, not only to protect but 

 to clean the eyeball, of the upper eyelid, instead of the transparent 

 third eyelid, the so-called nictitating membrane. In the case of the 

 dipper this membrane appears to have fallen into disuse, its function 

 having been taken over by the upper eyelid. This, it seems to me, is 

 partly, at least, due to the greater strength of the latter, further 

 increased by a covering of tiny white feathers, which renders it con- 

 spicuous whenever the bird blinks, and it does so frequently. 



The change may have been further assisted by the position of the 

 eyelid, for when the bird happens to be feeding with its head under 

 and opposed to a strong current, it would require not only a stout 

 cover for its eyeball, but would find the upper lid the best adapted 

 for the purpose, owing to the fact of its closing downwards, that is in 

 the direction of the current, which would thus help by its downward 

 pressure to keep the lid in place. The nictitating membrane, on the 

 other hand, which does not move entirely horizontally across the eye, 

 but, as stated by Newton, obliquely from "the outer lower toward 

 the upper inner angle," and consequently in an upward and forward 

 direction, would act in direct opposition to the current, which would, 

 therefore, tend to push it open. There would be the same tendency 

 in the case of the lower lid. 1 



Some of the Continental representatives of the dipper have been 



1 Newton, Dictionary of Birds, p. 234. For the fact that the dipper uses the upper lid 

 when blinking, I am indebted to Mr. Frank Finn's observations on a bird kept by him in 

 captivity (Country Life, August 25, 1906). The feathering of the lid can be seen by examining 

 any skin of the bird. 



