Why can a gosling imprint on a person or a bright object instead of its mother?

A gosling can imprint on a person or a bright object because imprinting is a form of learning where a young animal fixes its attention on the first conspicuous, moving object it experiences during a specific sensitive period after hatching.12

In natural settings, this object is typically the mother, which helps the precocial young—who are capable of walking independently shortly after birth—stay close to their parent for protection and guidance. However, the mechanism is not innately programmed to recognize only the mother; rather, it is designed to identify a "model of a companion" based on early visual, auditory, or tactile experiences.21

Because the innate constraints on this process are not absolute, a gosling will readily follow a human or an inanimate object, such as a bright ball, if it is the first salient stimulus encountered. Once an object is established as familiar, the young bird develops a preference for it and may show anxiety toward unfamiliar objects, effectively closing the window for imprinting on other subjects.2