What metaphors are used in Fear by Gabriela Mistral?

When the speaker of this poem infers that someone who is unidentified, referred to as "they," could turn her little girl into a bird, she is speaking metaphorically, by referring to aspects of the swallow that the girl would have if she went through such a transformation. In this case, the fear expressed is that the girl will, as she grows up, start socializing with others of her kind and fly away, traveling with them rather than staying near her home.