The Egg and the Volleyball

Niko Tinbergen placed artificial eggs near oystercatcher nests, varying their size and coloration. The birds consistently preferred artificial eggs over their own viable ones, and when presented with a plaster egg the size of a volleyball — far too large to incubate effectively — the oystercatcher climbed on top of it, spread its wings around as much surface as its body could cover, and sat. It abandoned its own eggs to incubate an object that could never hatch. The maternal response system was functioning correctly; the stimulus had simply been inflated beyond the range the response was designed to evaluate. This experiment became the canonical illustration of supernormal-stimulus exploitation and the central diagnostic image in Barrett's subsequent work on human behavior in engineered environments.