The question posed in Robert Frost's "The Oven Bird" is found in the last line of the poem.
What to make of a diminished thing.
The body of the poem speaks to everything a bird has seen and how everyone has heard the song of a bird. The imagery depicted shows a time where the leaves are falling and the flowers, once full of color, are close to disappearing as well.
The importance of the question comes when the bird seems to question why everyone notices the natural elements dying with the onset of fall, but they fail to recognize the lack of the bird's song.
The bird is, essentially, questioning why no one notices the loss of the bird's voice, but only notices the loss of the leaves and flowers, the drying and dusty roads.
In the end, the bird seems to be questioning singing at all. The bird wants to be considered in the same why which other things in nature are: recognized, missed, and reflected upon.