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 



^ C. Hess, Handhuch der vergleichenden Phyaiologie (herausgegeben 

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



