98 



EUPHA FOLEY TUGMAN 



two stimulus areas. The solid line (- 



-) is the curve for 



Male IV and the dashed line ( ) is the curve for Female V. 



Figure 4 gives the results for Male V and Female VI. The 



solid line (- ) represents the results for Male V. The 



abrupt ending is due to the bird dying at this point in the in- 

 vestigation. The dashed line ( ) represents the results 



for Female VI. Both Male V and Female VI, as stated before, 

 were trained to choose the darker of the two illuminated areas. 

 The curves for Male IV and Female V (Figure 3) are not widely 

 different until the threshold was reached for Female V. They 



Figure 3. Number of trials necessary for Male IV ( ) and Female V 



( ) to learn each discrimination. The difference in intensity is shown on 



abscissae. Number of trials is represented on ordinates. Both curves show 

 large number of trials necessary when difference in intensity is small. 



begin practically the same. Both birds had little trouble in 

 discriminating the areas when there was a wide difference in 

 the intensities. When the difference in intensity was .055 c.p. 

 Female V required 135 trials while Male IV made 30 perfect 

 choices without any trouble. Then at the difference in intensity 

 of .036 c.p. Female V made a better record than Male IV. But 

 when the variable light was again shifted and the difference in 

 intensity reduced to .030 c.p. Female V seemed wholly incapable 

 of discriminating even after 540 trials. The difference in in- 

 tensity was increased to .033 c.p. and after 600 trials she dis- 

 criminated. For the descending series Female V had discrimi- 



