Pigeons learn the concept of an "A".

Pigeons learned to discriminate between "A"S and "2"s in eighteen different typefaces. They subsequently showed excellent transfer to twenty-two typefaces that they had not previously seen; one pigeon was tested with handwritten letters and responded correctly to them also. Pigeons' responses to "A's and "2's with parts removed suggested that their performance was controlled by several features, none of which alone could be considered necessary or sufficient. A test in which birds were shown other letters of the alphabet supported this conclusion. It appears that the original discrimination was learned as what Ryle calls a "polymorphous concept".