If an egg rolls out of a nest, a mother greylag goose will retrieve it by nudging it with her beak and head. if researchers remove the egg or substitute a ball during this process, the goose continues to bob her beak and head while she moves back to the nest. explain how and why this behavior occurs.

The proximate explanation for this fixed action pattern might be that nudging and rolling released by the sign stimulus of an object outside the nest, and the behavior is carried to completion once initiated. The ultimate explanation might be that ensuring that eggs remain the nest increased the chance of producing health offspring.