When presented with resources that differ in quantity, many animals use a numerosity system to discriminate between them. One taxonomically widespread system is the approximate number system. This is a numerosity system that allows the rapid evaluation of the number of objects in a group and which is regulated by Weber’s Law. Here we investigated whether wild, free-living rufous hummingbirds (Selasphorus rufus) possess an approximate number system. The hummingbirds were presented with two experiments. In the first we investigated whether hummingbirds spontaneously chose an array containing more flowers than an alternate array. In the second we asked whether the hummingbirds could learn to use numerosity as a cue to which of two arrays contained the better reward. The birds did not spontaneously prefer an array containing more flowers. After minimal training, however, they learned to choose the more numerous array and could differentiate between arrays of five and seven flowers. These data support the presence of an approximate number system in the rufous hummingbird. It seems plausible that having such a system would enable much more efficient foraging in this species.

We trained marked, wild rufous hummingbirds to feed from artificial flowers. In the first experiment we presented birds with two arrays of artificial flowers, one more numerous than the other (e.g., 7 flowers vs 1 flower). We presented birds with eleven different combination of arrays: 1vs2, 1vs3, 1vs4, 1vs5, 1vs6, 1vs7, 2vs7, 3vs7, 4vs7, 5vs, 6vs7and recorded which array the bird visited first. During the second experiment, the array with a smaller number of flowers had a lower quality reward (5% sucrose solution) than the array with more flowers (25% sucrose solution). After birds were presented with a single numerous array (7 flowers) that was highly reward and then with a single array (1 flower) that was rewarded poorly, the birds were presented with two arrays simultaneously for the eleven different array combinations. We recorded how many times it took birds to visit first the more numerous array three times in a row. The order of presentation of the different tests was alternated and counterbalanced on both experiments.