Apostatic selection, in which predators overlook rare prey types while consuming an excess of abundant ones, has been assumed to contribute to the maintenance of prey polymorphisms *RF 1-3*. Such an effect requires predators to respond to changes in the relative abundance of prey, switching to alternatives when a focal prey type becomes less common [4,5]. Apostatic selection has often been investigated using fixed relative proportions of prey [1,6], but its effects on predator-prey dynamics have been difficult to demonstrate . Here we report results from a new technique that incorporates computer-generated displays [8,9] into an established experimental system, that of blue jays (Cyanocitta cristata) hunting for cryptic Catocala moths . Digital prey images from a virtual population are presented to predators. The relative numbers that escape detection determine the subsequent abundance of each prey type. If apostatic selection does promote stability, the system should converge on an equilibrium in which each prey type appears at a characteristic abundance. Our results show that the detection of cryptic prey does involve apostatic selection, and that such selection can function to maintain prey polymorphism.