I think that one of the key traits of the nightingale is loyalty. She demonstrates herself to be loyal to the audience, for when they appreciate their singing, she continues on for their benefit. She really demonstrates herself to be loyal to the frog. The frog is able to manipulate this for his own benefit. He recognizes that the nightingale sees the frog as a "mentor" or "coach." It is because of her loyalty that he is able to manipulate her. She is so loyal to him that she never questions the regimen that he puts her through. She never asks why or to what end it is to accomplish. She never objects to the strenuous abuse of her voice to the point where she ends up dying. It is her loyalty and her misguided sense of trust that defines the nightingale in Seth's poem. It is also this end that allows the reader to understand that while trust and loyalty are admirable traits in characters, they can be easily abused and so individuals must exercise these traits with caution and with a sense of vigilance to not end up like the nightingale.