Irrational choices in hummingbird foraging behaviour

It is conventionally assumed that, when animals evaluate alternative options, the value assigned to an option is absolute and independent of the other options available. It follows that animal choices should exhibit the rational property of regularity whereby the proportion of choices for an option cannot be increased by the addition of further options to the choice set. However, violations of regularity occur in human decision making, suggesting that humans may use comparative evaluation mechanisms whereby the value of an option is computed relative to the other options available. For example, in the asymmetrically dominated decoy effect the preference for a target option over a competitor is altered by the addition of a decoy option that is inferior to the target and competitor on one attribute, but lies between them on a second. We tested whether foraging wild rufous hummingbirds, Selasphorus rufus, would demonstrate violations of regularity in response to an asymmetrically dominated decoy. Sixteen birds chose between three artificial flower types (Target: 15 l, 40% sucrose; Competitor: 45 l, 30%; Decoy: 10 l, 35%) in Binary (Target versus Competitor) and Trinary (Target versus Competitor versus Decoy) treatments. We predicted higher preference for the Target in the Trinary treatment. The birds ranked the three options in the same order in the Binary and Trinary treatments (Competitor>Target>Decoy). Seven birds showed violations of regularity, six increasing their absolute preference for the Competitor in the Trinary treatment. Overall, relative preference for the Competitor over the Target was higher in the Trinary than in the Binary treatment. These changes in preference are incompatible with an absolute evaluation mechanism.