What is the theme in Wild Swans by Edna St. Vincent Millay?

Uncertainty is a theme. The speaker responds to the swans largely because they seem to fly with such direction and purpose. This direction contrasts with the speaker's uncertainty and confusion, as evident when she looks into her heart and finds "only a question less or a question more." Her heart has apparently been filled with questions for some time ("And what did I see I had not seen before?"), because she is nonchalant about finding nothing but questions there. She is emotionally uncertain, and she feels lost. In this state, she seeks guidance or reassurance, which is why she reacts to the swans so intensely. She sees in them the confidence and purpose she desires for herself.