I think that the diary entry from the point of view of the nightingale would express a certain amount of joy at finding a "mentor" in the frog. The nightingale sincerely believes that her new teacher will guide her in being a better singer. Certainly, I think that part of her reaction to this is gratitude and this could come across in the diary entry.
I do think that some level of sadness, melancholy, or even obliviousness could be seen in other diary entries when the nightingale details the gradual erosion of her voice. I am not sure if the entries could move towards realizing what is happening, for the nightingale is shown to be purely naive about these realities. Yet, there seems to be room in a diary entry to authentically talk about how her voice is eroding. It is not as if she woke up one morning with no voice. It took time through the "training" of the frog. The nightingale could provide some unique insight in a diary entry or two about how her "training" is actually causing pain to her voice. There is something tragic about how working towards an end of improvement, appropriating the world in accordance to a pure notion of subjectivity, has the very opposite effect. This would be true in the case of the nightingale.