Generalist herbivores often grow better on mixed diets suggesting that resource types are complementary. We use a consumer-resource framework to develop criteria by which nutritional relationships among foods can be determined from a forager's patch use behavior. Two resources are complementary when at least one yields diminishing returns or the resources interact beneficially with each other. These sufficient conditions may arise when food types contain different plant secondary compounds. We tested for complementarity using fox squirrels (Sciurus niger) and gray squirrels (S. carolinensis) feeding on sunflower seeds treated with tannic or oxalic acid. We used giving-up densities on these two foods in experimental food patches to assay the deterrent effect of each toxin, and to determine whether squirrels treat the toxins as complementary. Squirrels had their highest giving-up densities on oxalate-treated seeds and their lowest on water-treated (control) seeds. Augmenting with oxalate-treated seeds resulted in an increased giving-up density on oxalate-treated seeds relative to tannin-treated seeds. The data suggest that 1) oxalates strongly deter and tannins mildly deter feeding by squirrels, 2) foods containing oxalates or tannins are complementary, and 3) the complementarity of foods containing the two toxins is due primarily to the effect of oxalates. The marginal toxicity of oxalates increases with dose, while that of tannin does not seem to increase with dose.