A new original method to investigate some cognitive abilities of wild passerine birds was developed. In the course of experiments with five crossbills (Loxia curvirostra), it was shown that the method can be used to evaluate the ability of birds to form concepts. Crossbills were presented with pairs of stimuli each consisting of 1 to 10 equal elements. All the five crossbills learned to choose, within a pair, a stimulus with a larger array of elements. The birds successfully transferred this performance to novel stimuli considerably different from the training pairs comparable with the latter only in the number or area. One of the birds showed a complete transfer of the acquired performance of choice to stimuli consisting of 10 to 20 equal elements. Thus, the ability of forest passerine birds to form concept "larger than" was shown.