We describe an experiment designed to distinguish between two models of risk-sensitive feeding behaviour: the variance discounting model and the z-score model. The variance discounting model assumes that mean reward levels do not affect preferences over reward variability, but the z-score model assumes that mean reward levels do affect preferences over variability. We presented two choices to feeding rufous hummingibrds, Selaphoruous rufus. One alternative had a higher mean and a higher variance than the other. After measuring preference, we increased the mean of both alternatives by adding the same amount to all possible outcomes. The variance discounting model predicts that such a general shift should not change preferences, but the z-score model predicts that preferences will change. Our results support the z-score model. The variance discounting model's assumption of constant risk-aversion fails.
Bibliographical noteFunding Information:
We are grateful to C. L. Gass, T. Phillipi, A. E. Sorensen, S. Tamm and Z. Tooze for their help and advice. We thank C. P. L. Barkan, T. Caraco, E. L. Charnov, M. E. Ellis, J. R. Lucas, G. H. Orians, L. A. Real, C. T. Snowdon, R. C. Ydenberg and the reviewers for their comments on the manuscript. This work was partially supported by NSERC grant 674876 to C. L. Gass. DWS was supported by a NATO postdoctoral fellowship, and SRP was supported by a NSERC studentship. We thank NATO and NSERC for their support.