ON THF. OLFACTORY SENSE OF BIRDS 



15 



without odour. The NaCl pollution was very effective in contrast to 

 pollution with quinine sulfate as used by Zahn, which in a concentrated 

 solution is not discriminated against by pigeons. The water was offered to 

 the pigeons in flat glass bowls, which were set in somewhat larger bowls, 

 one of which contained the odorous substance. Already in the first tests 

 the pigeons observed even the minutest differences in the bowls such as 



Gate shutters 



box 



Fig. 5. Three-choice apparatus for olfactory experiments on pigeons. 



scratches or small air bubbles in the glass. Thus their score was near 100 

 per cent correct choices necessitating a further series of 1024 trials using as 

 similar bowls as possible, and switching them around irregularly, to avoid 

 the possibility of optical orientation. In this new series the score fell to 

 50 per cent, corresponding to random choices. Though no statistically 

 significant learning scores were obtained, still this series and later ones were 

 important for the evaluation of earlier experiments, because it showed 

 how easily the experimenter can be led astray by the very precise optical 

 alertness of the birds. 



