There is a hypothesis that this was because young night herons are active at night and because the appearance of its flying in the night sky and singing aloud may have given a weird impression to people.
これはゴイサギが夜行性であり、大声で鳴き散らしながら夜空を飛ぶ様子が、人に不気味な印象をもたらしたためという説がある。