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Two pigeons learned to respond to four objects while withholding response to two objects. The birds maintained their numerical discrimination when new objects were introduced. They also maintained the discrimination when large and small objects were used to construct "2" and "4," and when different kinds of objects were mixed to make "2" and "4." The subjects responded most often to four objects when tested with one, two, three, four, and five objects in the generalization test after fourth discrimination task. These results suggest the ability of numerical discrimination in pigeons. Two other birds that were trained to peck two objects but not to peck four objects showed, however, severe difficulty in learning this task. Thus, pigeons ' ability for numerical discrimination is affected by training procedure.