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Lorenz demonstrated imprinting in certain birds was a form of early learning that led to species-typical behavior. This behavior was shown to serve, among other things, species recognition, and the learning was also shown to be very resistant to extinction. Later experimental research showed that the immature birds were reinforced by the growth of the visual image as they moved toward it. Flocking seems to involve the maintenance of a relatively constant space among the animals. The animations below demonstrate that flock members are "picked up" when they find themselves in a certain proximity to moving conspecifics. Imprinting must be present in flocking species, although the mechanism may not have to involve a visual image size, and the reinforcer must relate to perceived characteristics of flock members. Thus, the movement synchrony requires the early experience establishing the common reinforcer.