The finer the scale at which a forager can detect patchiness, the greater is its opportunity to allocate time efficiently in response to spatial heterogeneity in resource abundance. There are three advantages of assessing and responding to patchiness at smaller scales. First, an environment may be rendered profitable only when the forager can separate areas of low and high resource abundance. Second, by placing accurate boundaries on profitable food patches, foragers can eliminate the "cost" of over- and underestimating patch size. Third, by biasing search efforts toward rich food patches, the forager can harvest more resources in the same amount of time. We investigate the ability of fox squirrels, Sciurus niger, to detect patchiness at small spatial scales, using experimental food patches subdivided into micropatches. We manipulated micropatch size, food density, and the occurrence of distinct versus vague boundaries. In accord with the theory, increasing the proportion of vague boundaries increased assessment error, while increasing resource density decreased assessment error. Halving patch size combined with doubling resource density had no effect on assessment error. These results indicate that assessing patch size/boundaries is as important as assessing resource density at some scales of patchiness. Other foraging costs, such as predation risk and metabolic costs, may contribute to assessment errors.