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 containing food items that varied in number, mass, or both. In Experiment 1, the symbols 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, and on summation tests, they chose the pair of symbols that summed to the larger total reward. When number of food pellets was varied but mass of reward was held constant in Experiment 2, preference for the larger number symbols failed to appear. When number was held constant and mass was varied in Experiment 3, the pigeons showed a clear preference for the larger mass symbols on single-symbol and summation tests. These findings show that pigeons summate the value of symbols and are more likely to represent symbols by mass of food reward than by number of food items.