The poem's moral is stated in the poem by the villainous frog himself:
Well, poor bird – she should have known
That your song must be your own.
The nightingale, when she first arrived in the bog, had a beautiful, original voice that all the bog loved. She enjoyed her singing; it made her happy. The frog, jealous and greedy, seized upon her innocence and found a way to both make money and to rid himself of his competition.
He took her talent and attempt to perfect it. In a sense, he attempted to fix what wasn't broken. He made singing unpleasant for her, forced her into multiple practice sessions in poor conditions. Finally, these sessions killed her.
There will always be someone willing to take away a person's beauty and joy. Most people do not understand that a coach is pushing for personal fame or a parent is pushing to relive his youth through a child until it is too late. The result could be hatred of a sport one first loved, injury, or even death.
"The Frog And The Nightingale" by Vikram Seth is a strongly ideological and moralistic poem in which Seth, himself a son of wealthy and successful Indian parents, and a graduate of elite public schools, Oxford, and Stanford, uses allegory to portray teachers, literary critics, British colonial powers, and generally the elites of which he is a member and which pay him quite well for his writing, as oppressive and stifling creativity.
The pure inspired nightingale, with its beautiful untutored song, is the heroine and victim of the poem. The frog is the evil and untalented oppressor whose criticisms of the poor nightingale cause her to lose her singing powers, become prey to self-doubt, and eventually die. The point of the poem is that the natural inspired untutored voice is most authentic and that creativity wilts under the influence of discipline and learning.
The poem, as well as having a general allegorical theme and conclusion, is probably intended as a reference to John Keats, a Romantic British poet who died young of tuberculosis and who wrote a poem called "Ode to a Nightingale." Romantic biographical accounts of Keats emphasize that the initial poor reception of his poems contributed to his early demise.
In the conclusion, the frog comes out clearly stating its intentions in a sarcastic and evil manner. By stating that
I tried to teach her,
But she was a stupid creature -
Far too nervous, far too tense.
Far too prone to influence.
The frog is being sarcastic stating that the nightingale had it coming because she came and took over the frog’s territory with regard to singing. The frog used the nightingale’s stupidity against her, which, as stated, came with the bird being too prone to influence. If the nightingale believed that what she delivered was good (as confirmed by the other animals) then she should have rejected the unnecessary influence from the frog. The nightingale had never heard the frog put on a performance or even heard any of the animals praising the frog, so why would the nightingale believe in the frog’s prowess?
The nightingale failed to own its success and agreed to be influenced by a “fake” expert who was ruthless enough to lead it to its demise.
It is important for someone to own their success; not all advice is valid, and sometimes it could be intended for your destruction.