[Numerousness judgement in hooded crows: generalization by the "more than" concept].
Four carrion crows were trained to choose the greater array from pairs of numerousness discriminanda in the range of 1-12. In the process of training all irrelevant attributes of the arrays (geometric form, size and color of elements, patterns of distribution) were varied. Several control procedures were employed to make extraneous cues unlikely. In particular, in order to preclude the use of cumulative area or other magnitude cues the ratio of cumulative area of elements to their number was varied (in a half of presentations the greater array consisted of bigger elements while the lesser array consisted of smaller elements and in the other half the greater array consisted of smaller elements while the lesser array consisted of bigger elements). All the crows demonstrated high accuracy of comparisons (75.3 +/- 2.4%). Correct performance dominated also under conditions with minimal difference between the compared arrays. It was concluded that these crows were able to compare the arrays in the range of 1-12 by numerosity itself. When arrays in the new range of 10-20 were presented all the crows demonstrated successful transfer of acquired reaction without any additional training (71.5 +/- 2.3%). However, when the ratio of cumulative area of elements to their number varied only two of the four crows performed successfully (71.9 +/- 3.6). Taken together these data suggest that crows are capable of forming the concept "more than" based on numerical discrimination up to 20.