Estimating on the fly: the approximate number system in rufous hummingbirds (Selasphorus rufus)

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.