Bring out the irony in the frog's statement - your song must be your own.
The frog tells the nightingale that her song must be her own while he himself does not let her use her skill and talent in this field. He has restricted her entire freedom and that is when the irony occurs. In the beginning when the nightingale confesses that her song was original he immediately demeans her and tells her that it was nothing to boast about. Then after killing the nightingale, the frog puts the blame on the nightingale saying that she should have been original.