According to optimal diet theory, a predator should eat prey2 only if:
Optimal diet theory ranks prey by profitability, commonly energy obtained per unit handling time, E/h. A lower-ranked prey type should be accepted only when its profitability exceeds the expected rate of gain from rejecting it and searching for the preferred prey. The expression E2/h2 > E1/(h1 + S1) states precisely that condition: prey 2 yields energy fast enough to outperform the combined search and handling return associated with prey 1. As preferred prey becomes scarce, S1 rises and the right-hand rate falls, making prey 2 more acceptable. Energy alone or handling time alone cannot determi
Ref: Ecology: From Individuals to Ecosystems, Begon et al., 5th Ed., Ch. 10