Imprinting is the English translation by Konrad Lorenz (1937) of his term Pragung (1935), for the distinctive process by which some newly hatched birds learn the visual characteristics of their species. In the case of nidifugous birds such as fowl and waterfowl, the young hatch with fully developed senses and are freely mobile, so that when the parent leaves the nest the young immediately follow and stay in close proximity. Distinctive calls are usually given by the parent while it is in the nest and as it leaves the nest. Lorenz described a number of instances show ing that such newly hatched birds would ap proach and follow almost any sizable moving object that they saw and, within minutes or hours of such visual experience of one object, would no longer follow any other but instead would show avoidance and flight (i.e., fear be havior). The initial attention, approach, and following are clearly facilitated by vocalizations or sounds coming from the parental object, and Lorenz noted that these sounds may have to be of a specific kind. Furthermore these positive filial responses and the learning of the parental object are confined to a short sensitive period after hatching and are replaced by fear re sponses, so that there is a critical period for imprinting.