200 CHARLES A, COBURN 



The intensities of the stimulus areas and the general illumi- 

 nation of the chambers were varied in these tests by the use 

 of milk glasses as described on page 194. The only visual factor 

 which was constant during the thirty trials, was that of form. 

 It is evident, therefore, that this was the cue which enabled 

 crow no. 1 to make a perfect record in these series. 



Lack of time prevented further work on the perception of 

 form. The last two days of work were devoted to a further 

 study of size discrimination. The purpose was to obtain, if 

 possible, more conclusive evidence of the crows' ability to dis- 

 tinguish sizes, and, incidentally, to learn if the improvement in 

 the method (introduction of gates) would increase the percentage 

 of correct reactions to differences in size. 



Thirty tests were given, the results of which appear in Table 

 5. During the first series, the crows appeared to be confused 

 by the sudden change in the problem presented to- them. They 

 worked rather slowly and quietly, but their choices were not 

 made with the usual defmiteness. It was evident that they 

 (especially crow no. 1), did not clearly appreciate what was 

 required of them. 



TABLE 5 



Discrimination of Size 



Correct choices 

 Date No. of tests Crow No. 1 Crow No. 2 



Conditions of Discrimination 



5 centimeter — 3 centimeter circle 

 Sept. 19 10 4 7 



" 19 5 5 4 



5 centimeter — 4 centimeter circle 

 Sept. 19 5 5 5 



5 centimeter — 4.5 centimeter circle 



Sept. 20 5 4 4 



" 20 5 5 5 



In the second series, the indefiniteness and hesitation in 

 their behavior were lacking. In every case no. 1 went quickly 

 and directly to the correct exit. Crow no. 2 made a mistake 

 in the first test. Its decisions, however, were made clearly and 

 definitely thereafter. This clear-cut, decisive type of reaction 

 continued, with both crows, during the remaining tests, even 

 when the discrimination was between the 5 centimeter and the 



