Ordinality is a numerical property that nectarivores may use to remember the specific order in which to visit a sequence of flowers, a foraging strategy also known as traplining. In this experiment, we tested whether wild, free-living rufous hummingbirds (Selasphorus rufus) could use ordinality to visit a rewarded flower. Birds were presented with a series of linear arrays of 10 artificial flowers; only one flower in each array was rewarded with sucrose solution. During training, birds learned to locate the correct flower independent of absolute spatial location. The birds showed no significant decrease in accuracy depending on which ordinal position (1st, 2nd, 3rd or 4th) was rewarded, which suggests that they used an object-indexing mechanism of numerical processing, rather than a magnitude-based system. When distance cues between flowers were made irrelevant during test trials, birds were still able to locate the correct flower. The distribution of errors during both training and testing indicates that the birds may have used a so-called ‘working up’ strategy to locate the correct ordinal position. These results provide the first demonstration of numerical ordinal abilities in a wild vertebrate and suggest that such abilities could be used during foraging in the wild.

We recorded the visits to all flowers during training and during tests. We recorded the distance between flowers during tests. Both training and test data from the combined visits of all birds to a particular location were analysed using one-sample and paired t-tests to determine the significance of percentage probes to each flower in all arrays for a given position. The effect that position of the flower had on the number of mistakes was analysed with a factorial repeated measures ANOVA. A Repeatability test was used to assess if the number of visits during each training phase was repeatable within a bird. A Friedman’s ANOVA was used to compare the number of trials the birds took to learn the four different ordinal positions. A chi-square test was used to determine whether during the four training trials birds were equally likely to visit the correct flower as to visit the adjacent flowers. We used Spearman’s correlations to test whether the total number of trials or the number of trials when F4 was rewarded were correlated with the test performance of the hummingbirds. We used a binomial test to determine if the birds visited F4 more often than expected by chance at 0.10. All analyses were conducted using R version 3.3.2.