Saturday, November 24, 2012

Irrational choices in hummingbird foraging behavior

    This week's article focuses on  primary research entitled, "Irrational choices in hummingbird foraging behavior" by Melissa Bateson, Susan Healy & Andrew Hurly.  This research focuses on hummingbird decisions and preferences between competing choices.  This is an important area of research because it gives us insight into how animals think and make decisions, especially in the wild, when foraging for food.  The entire research is based on presenting hummingbirds with two choices -- a target and a competitor -- and seeing their preference.  The target and competitor were two artificial flower types that differed in the volume of nectar and the sucrose concentration.  The two flowers were designed to give an equal rate of energy intake, however.  The material used was a Plexiglass plate with 18 wells drilled into it, arranged with flowers.  Birds visited the flowers every 10 minutes throughout the day and were allowed to feed from as many wells as they wanted to.  The preferences the birds made were calculated based on the final 100 choices they made for each treatment condition.  The first choices were discarded, because it took at least 30 flower visits for birds to begin showing preferences.  This procedure was followed by presenting three choices - a target, competitor and decoy flower - and seeing how the birds' preferences changed.  In the trinary condition (three flowers) the decoy was adjusted to have the lowest rate of energy intake, by a wide margin.  This was done because (in human literature) it is known that the effect of the decoy is strongest when the target and competitor are initially similar, and the decoy is noticeably different from the rest.  
    What I found strange about this study is that in the trinary condition (three flowers) they adjusted the target to have a lower rate of energy intake than the competitor, because they were hoping for the birds to show a preference for the Competitor in the Binary treatment, but switch to preferring the Target in the Trinary treatment.  If the energy intake is lower in the target flower, then why would they prefer that one over the competitor in the trinary treatment?  
     The results showed that in the binary treatment (two flowers), birds chose the Competitor flower significantly more than the Target flower.  In the trinary treatment, the birds chose the competitor more than the target, and the target more than the Decoy.  The real interesting thing about this, however, is that the choice of Target flower dropped off more in the trinary condition than the drop off for the competitor, meaning that the preference for the Competitor over the Target was higher in the Trinary condition than in the Binary condition, which violates the principle of irrelevant alternatives.  This principle states that, by introducing a third choice [X] that is irrelevant to the first choices [A and B], then the preferences for A and B should not be changed.  In other words, even though the preferences for A and B may come down (as was seen with the hummingbirds), the ratios and relationships should stay relatively constant.  So what they saw in these hummingbirds (as is often seen in humans as well) is that an irrational choice was made because the inclusion of a third, irrelevant variable changed the preferences.  The hummingbirds ended up decreasing their preference for the treatment condition more than expected.
     One explanation for these results is that the hummingbirds did not use an "absolute currency" to evaluate the options.  In other words, the weight in preference they had for each option depended on the circumstance, and they didn't assign an absolute value to each option, regardless of other choices available.  What I think was going on here is that since the hummingbirds already weren't that attached to the target flower, they were more likely to make choices of the third option, "just to test it out".  But, they were pretty happy with the competitor flower, and so their preference for it hardly declined... they were loyal supporters of that flower.  With humans, it has been found that a Decoy actually has the opposite effect, where it increases preference for the Target.  The target is defined as the option which dominates the decoy in two dimensions (in this case the target had more volume and concentration than the decoy).  The Competitor, on the other hand, had a lot more volume than the Decoy, but slightly less concentration, therefore it dominated the decoy in one dimension.  The hummingbirds actually continued preferring this competitor, so maybe they were able to distinguish between concentration and volume, and they preferred volume.
     Another explanation is that the Decoy option in this example is most similar to the Target on the volume dimension, and therefore, takes choices disproportionately from the Target option than the Competitor option in the Trinary Treatment.  If two options are very similar, then they are more likely to flip-flop between the two, and only show a slight preference for one of the two.  This does not fall in-line with the rational choice theory, which is interpreted as wanting more, not less of a good thing.  In this case, the hummingbirds are taking less of the good thing (The Target option), in exchange for the Decoy.
     As the researchers also admit, further experiments need to be conducted to see what exactly is causing this irrational choice in hummingbird behavior.  There is one characteristic of the study that sticks out, that is, the variance in concentration between the three options.  In order to adjust for rate of energy intake, the volume of the competitor option is three or four times the target and decoy options, respectively, while having a lower concentration %.  Maybe the birds are detecting upon this variance and making choices based solely on the volume characteristic.  Another field to investigate is the idea that maybe comparative evaluation may be more efficient than absolute evaluation mechanisms, and would, therefore, be favored by natural selection.  Comparative evaluation would be what we saw in this example, birds are making judgments between options and sticking to what they prefer, while decreasing their preference for the other option disproportionately.  To investigate these, further information would be needed about the fitness benefits of these choices.

Bateson, Melissa, Susan D. Healy, and T. Andrew Hurly. "Irrational choices in hummingbird foraging behaviour." Animal Behaviour 63.3 (2002): 587-596.

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