IX] THE ENEMIES OF BUTTERFLIES 119 



would have no opportunity of obtaining. Great cau- 

 tion must, therefore, be exercised in the interpretation 

 of feeding experiments made with birds in captivity. 



It appears to be generally assumed that colour 



perception in birds is similar to what it is among 



human beings, but some experiments made by Hess^ 



render it very doubtful whether this is really the case. 



In one of these experiments a row of cooked white 



grains of rice was illuminated by the whole series of 



spectral colours from violet to deep red. Hens which 



had been previously kept in the dark so that their 



eyes were adapted to light of low intensity were then 



allowed to feed on the spectral rice. The grains 



illuminated by green, yellow, and red were quickly 



taken, but the very dark red, the violet, and the blue 



were left, presumably because the birds were unable to 



perceive them. Again, when the birds were given a 



patch of rice grains of which half was feebly illuminated 



by red light and the other half more strongly by blue 



light, they took the red but left the blue. Previous 



experiment had shewn that with ordinary white light 



the birds always started on the best illuminated grains. 



It seems reasonable to conclude, therefore, that in the 



red-blue experiment the feebly illuminated red grains 



were more visible than the far more strongly lighted 



blue ones. It might be objected that the birds had a 



prejudice against blue, but, as Hess points out, this is 



almost certainly not the case because they took grains 



1 C. Hess, Handbiich der vergleichenden Physiologie (herausgegeben 

 von H. Winterstein), Bd. 4, 1912, p. 563. 



