This is a wonderful poem that issues an important lesson for readers of all ages - stay true to yourself. Seth creates this moral through the voices of a frog and a nightingale. As with many fables, these characters present both virtues and vices for the reader.
The frog is the obvious villain in this poem with a villain's classic vices. He is unconcerned that others do not like his singing and seems to continue to spite them. When the nightingale arrives, he is stricken by jealousy and concots a plan to benefit from her. He manipulates her, and he profits from her singing. He forces his style upon her until finally, she snaps. He then resumes his own singing with no thought to her death.
The nightingale is the virtuous example of innocence. She is extremely talented. She is unconcerned with personal gain, singing only for her own joy. She strives to please others, but her own naivete allows her to fall victim to the frog's jealousy and greed.
Sadly the nightingale's desire to please is her downfall. She dies, denying all the rapture of her voice.