Among the most familiar examples of cryptic coloration are underwing moths, which rest motionless on tree trunks during the daytime with their forewings closed over their abdomens. The color patterns of the forewings resemble the surrounding bark, making it very difficult for predators to detect them. An unusual feature of cryptic insects in general and of these forest moths in particular, is that a very large number of them are polymorphic, with one species occurring in a variety of distinctive forms. In these moths, the polymorphism is restricted to the cryptic forewings; the hindwings, which are concealed from predators, are generally uniform within species.

Five forms (or "morphs") of the North American underwing moth, Catocala relicta. (Revised from Barnes & McDunnough 1918). Note the variable fore-wings and the relatively uniform hind wings.

Edward Poulton (1890) first remarked on this phenomenon of cryptic pattern polymorphism, and he suggested that it was an evolutionary response to the foraging behavior of predatory birds, in that it was harder and more time-consuming for a bird to search for several different targets simultaneously than to search for only one. Luuk Tinbergen (1960) directly observed patterns of predation by insectivorous birds and inferred that the birds appeared to be confining their search to only one or a few prey types at any one time. To maximize their rate of detection, they focused on the most common prey available (or the one that they had encountered most recently) and effectively overlooked the others, a process Tinbergen called "hunting by searching image." The hypothesis we have been testing in our laboratory is that hunting by searching image will tend to promote polymorphism in previously monomorphic populations, because it selects against individuals that bear a close resemblance to one another.

Our methods are derived from an established experimental system. In North America, woodland moths are commonly preyed on during the daytime by Blue Jays (Cyanocitta cristata), which are the only avian predators that seem able to break the crypsis and find these insects while they are at rest on tree trunks. In the 70s and 80s, Kamil and his students showed that jays in the laboratory exhibited the same impressive detection abilities when they were required to locate cryptic moths in slide images, and that jays showed clear evidence of hunting by searching image (Pietrewicz & Kamil 1977, 1979).We converted this natural predator/prey system into one that was more amenable to digital manipulation, reducing the resolution of gray-scale images of cryptic moths to 16x16 pixel icons that could be overlaid on textured, fractal backgrounds. The backgrounds were reverse-engineered from the moth images, allowing us to produce a broad range of difficulty levels.

Five digital moths displayed on a uniform gray background (top) and on three different levels of cryptic background. With practice, most blue jays have little difficulty detecting moths even at Level 6, though their performance generally declines at higher crypticities.

We present the moths, one at a time, to blue jays in an operant chamber. In each trial there either is or is not one moth image imbedded in one of the fields of cryptic background on a computer monitor (see below). If the bird finds a moth, it pecks it, the peck is detected by an infra-red touch screen, and the bird is rewarded with a food pellet in the central well below the perch. If the bird does not find a moth, it pecks the green circle, in which case the next trial begins immediately. The bird is never informed if it overlooked a moth, and if it pecks an area of background with no moth present, the time to the next trial is substantially delayed.

Our initial experiment (Bond & Kamil 1999) validated the technique, showing that it produced effects similar to earlier studies using photographic images. In particular, we found that the birds were more accurate at finding moths after a run of the same type of prey than during random presentations of differing prey types. We also found interference effects, in which inducing a jay to search for one type of moth actually reduced the likelihood of its finding an alternative type. This was the first clear demonstration of attentional interference in visual search in animals and unequivocal evidence that the birds were using searching images to find the moths.