Quantity discrimination in jungle crows, Corvus macrorhynchos

We investigated whether jungle crows can select a given quantity of items (printouts of shapes and symbols) when simultaneously presented with smaller or larger quantities. We presented jungle crows with the choice between two sets of discrete homogeneous quantities (two versus five) in a fixed configural pattern during training. When crows could discriminate five items (15 out of 20 correct choices in two blocks of 10 trials each), they received control tests for non-numerical cues such as element configuration, shape, and total filled area and novel quantity (3 versus 5, 4 versus 5, 5 versus 6, 5 versus 7 and 5 versus 8) tests. During training, jungle crows learned the discrimination task relatively quickly. Additionally, the selections were generally not controlled by the non-numerical cues, indicating that the crows were responding to quantity. Jungle crows selected the familiar larger quantity in smaller sets (3 versus 5 and 4 versus 5) and the novel larger in larger comparisons (5 versus 7 and 5 versus 8) except that of 5 versus 6 quantities. These results suggest that jungle crows have a natural tendency to select the larger quantities and that decisions were affected by the numerical ratio and stimuli magnitude, indicating the use of analogue magnitude mechanism for numerical judgement, as is observed in other animals.