Summation of symbols by pigeons (Columba livia): The importance of number and mass of reward items

In each of 3 experiments, different sets of 4 pigeons (Columba livia) were trained to discriminate between 2 visual symbols that covered wells contain ing food items that varied in number, mass, or both. In Experiment 1, the s ymbols were associated with 0, 1, 3, 5, 7, or 9 pieces of grain reward. The pigeons learned to choose the symbol corresponding to the larger reward, a nd on summation tests. they chose the pair of symbols that summed to the la rger total reward. When number of food pellets was varied but moss of rewar d was held constant in Experiment 2, preference for the larger number symbo ls failed to appear. When number was held constant and mass was varied in E xperiment 3, the pigeons showed a clear preference for the larger mass symb ols on single-symbol and summation tests. These findings show that pigeons summate the value of symbols and are more likely to represent symbols by ma ss of food reward than by number of food hems.