Fixed action patterns are instinctive programmed responses in different species that are triggered by a specific external sensory stimulus. These stimuli are called sign stimuli. When the sign occurs, the animal starts a sequence of acts in response to the stimulus and continuous until the series of actions is completed. If the animal is interrupted for some reason, it continues its job from where it was interrupted; either the original stimulus is or is not present.

A chick’s gaping mouth induces a feeding response in a parent bird: The stimulus is the gaping mouth, which makes the parents to instinctually respond foraging more and feeding the chick.

A male fruit fly waves his wings at a female fruit fly during courtship: In courtship, the original stimulus is the presence of the female. One of the male´s responses actions is the wave of its wings.

A baby garter snake captures and eats a slug: For the baby snake, the sign stimulus is the slug scent. The snake then responses by capturing and eating the slug.

A European cuckoo hatchling pushes the host bird's egg out of the nest: The stimulus for the cuckoo chick is the presence of the host bird´s egg. When the cuckoo bird emerges from its egg, even before it opens its eyes, it pushes the host out of the nest.

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