Wired-In Behavior
A Fixed Action Pattern (FAP) is an innate, stereotyped sequence of behavior that, once triggered, runs to completion regardless of further changes in the environment — it is often described as "ballistic," like a fired projectile. FAPs are triggered by a sign stimulus (or releaser): a specific feature of the environment that the animal is innately tuned to detect. Konrad Lorenz and Niko Tinbergen proposed the Innate Releasing Mechanism (IRM) as the hypothetical neural filter that recognizes sign stimuli and unleashes the matching FAP.
Tinbergen's classic demonstration used the male three-spined stickleback fish, which performs an aggressive display toward any object bearing a red underside — even a crude, unrealistic model — because red is the sign stimulus for "rival male," regardless of how little else about the model resembles a real fish.
A supernormal stimulus is an exaggerated version of a sign stimulus that triggers an even stronger response than the natural cue ever does. Herring gull chicks peck at a red spot on their parent's bill to beg for food; Tinbergen found that chicks pecked more at an artificial stick bearing several red stripes than at a realistic model of a real gull's head. Birds will similarly favor incubating an oversized fake egg over their own — the exaggerated cue simply out-competes reality for the releasing mechanism's attention.
Common pitfall: assuming a Fixed Action Pattern can be stopped partway once the animal "changes its mind." By definition it cannot — once the sign stimulus triggers it, the sequence runs to completion, which is exactly what makes supernormal stimuli able to hijack it so completely.
A stickleback rival-response: a neutral grey model with no red mark evokes no lunge; the same model gains a red-belly patch and the focal fish lunges to attack; an exaggerated model with an extra-bright.