This chapter compares amodal completion and the perception of visual illusions in birds and primates. Amodal completion concerns the perception of partly occluded figures. Visual illusions, in theory, represent cases in which perceptual systems that are adaptively suited to some situations may suffer in other situations. The examples discussed in this chapter suggest that the visual perceptual system of a particular species may be tuned to its ecological niche: where its members live, how they move, what they eat, etc.