Risk sensitivity in Archilochus colubris: the effects of differences in mean reward and variance on food choice.

Abstract

Risk sensitivity was tested in the Ruby-throated Hummingbird, ARchilochus colubris, to determine if both changes in mean reward and changes in reward variance affect food choice. The experiment was conducted at the University of Michigan Biological Station on the days of August 10-13, 1991. Three different feeders were set up, one containing a low mean, low variance sucrose solution, one containing a low mean, high variance solution, and a third containing a high mean, high variance solution. The foraging behavior of the subjects was observed and the number of visits as well as the number of seconds per visit were recorded for each feeder. The results showed that the hummingbirds spent significantly more time at the low mean, low variance feeder (p=.003) than at the low mean, high variance feeder. A comparison between the low mean, high variance feeder and the high mean, high variance feeder also indicated that a significantly greater amount of time was spent at the latter feeder (p=.008). Another interesting result was that as the stability of the food source decreased, the number of visits to the source increased, indicating that hummingbirds sample a less reliable food source more often. A possible explanation for this is that the birds are trying to determine a pattern of variance. These results offer strong evidence that both differences in mean and variance are determining factors in food choice among hummingbirds.