Foraging theory depicts dietary choice as a function of prey quality and absolute abundance. Ecological processes, however, can depend on the way foragers respond to the relative abundances of available prey types; several models for frequency-dependent foraging adequately describe these responses. Our laboratory experiments with white-throated sparrows investigated preferential choice of two food rewards as we manipulated both reward quality and relative abundance. In any single experiment the two rewards provided the same mean food quantity, but the variances differed. Average energy budgets predicted risk-aversion, so that foraging preference should decrease as reward variance increases. We presented each two-reward pairing at availability ratios of 1:2, 1:1, and 2:1 for three consecutive days. By the third day risk-aversion exceeded preference for reward variance significantly. When relative abundances of the low and high variance rewards were not equal, the birds tended to prefer the rare over the common reward. This response began before the birds had thoroughly sampled the reward distributions. Preference for rarity apparently constrained the birds' economic response to reward variance levels.

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