114 WALTER NEUHAUS 



the diencephalon is the only well-developed one which is a circumstance 

 of great importance for the interpretation of our experiments (Fig. 4). 

 Judging from morphology and histology only, birds must have the sense of 

 smell. As has already been stated, the experimental evidence on this topic 

 is contradictory. Without attempting any comprehensive review, the 

 following will show that even in recent years conflicting evidence has been 

 published. Positive response to olfactory conditioning in robin redbreast 

 {Erithacus nibecula L.), garden warbler {Sylvia spec), thrush {Turdiis 

 merula L.), greenfinch {Chloris ch/oris L), blue titmouse (Parus caeruleus 

 L.) and Mallard {Anas platyrryncha L.) was obtained by Zahn (1935) and 

 in robin redbreast by Wagner (1939). Contrarily, using the method of 

 Bandurow and Laurin (1935), Walter (1943) could not evoke any olfactory 

 conditioned reflexes in pigeon {Columba livia L.), while the Russian authors 

 obtained positive results. Also Calvin (1957) using a similar method could 

 not obtain positive results in pigeons. Michelsen (1959), however, has 

 reported that two pigeons guided by their sense of smell only, were able to 

 learn how to find food concealed in a complicated choice situation. 



OLFACTORY LEARNING EXPERIMENTS WITH BIRDS 



Our first experiments were also performed using pigeons in a three-choice 

 apparatus. The animals could fly freely from the aviary to a box, from 

 which there was one gate allowing them to get into a compartment contain- 

 ing food, while the other two gates were locked. This, however, the 

 animals could not detect until they tried to pass through the gate (Fig. 5). 

 The gates were closed with grids of a type used by carrier pigeon breeders. 

 A pigeon having made the right choice was rewarded with food and then 

 brought back to the aviary. The gate leading to the reward was marked 

 by the smell of butyric acid on a filter paper in a test tube. As the three 

 gates were enclosed and separated by Plexiglass plates, and a weak, con- 

 tinuous stream of air was caused to flow longitudinally through the appa- 

 ratus, no smell could be detected, at least by a human nose, in the two 

 locked gates, but only in the open one. 



These experiments (Table 1), which include in all 1242 trials with seven 

 male pigeons, do not prove the presence of an olfactory sense in pigeons, 

 however. Even if in five pigeons the number of correct first choices is 33 

 per cent larger than for a random choice, this is not statistically significant. 

 In three animals, however, there seems to be a tendency towards an in- 

 creasing learning, which is shown by the higher percentage of correct 

 choices in the last 30 trials. The experiment was discontinued, because no 

 sure progress seemed to be possible. 



The next experiment with three pigeons combined punishment in the 

 form of NaCl polluted water with the odour of eugenol, against pure water 



