Use of number by crows: Investigation by matching and oddity learning

Hooded crows were trained in two-alternative simultaneous matching and oddi ty tasks with stimulus sets of three different categories: color (black and white), shape (Arabic Numerals 1 and 2, which were used as visual shapes o nly), and number of elements (arrays of one and two items). These three set s were used for training successively and repeatedly; the stimulus set was changed to the next one after the criterion (80% correct or better over 30 consecutive trials) was reached with the previous one. Training was continu ed until the criterion could be reached within the first 30 to 50 trials fo r each of the three training sets. During partial transfer tests, familiar stimuli (numerals and arrays in the range from 1 to 2) were paired with nov el ones (numerals and arrays in the range from 3 to 4). At the final stage of testing only novel stimuli were presented (numerals and arrays in the ra nge from 5 to 8). Four of 6 birds were able to transfer in these tests, and their performance was significantly above chance. Moreover, performance of the birds on the array stimuli did not differ from their performance on th e color or shape stimuli. They were capable of recognizing the number of el ements in arrays and comparing the stimuli by this attribute. It was conclu ded that crows were able to apply the matching (or oddity) concept to stimu li of numerical category.