Why does Keats call the Nightingale "the light winged dryad of the trees"?

John Keats in his famous Ode, "Ode to a Nightingale", addresses the nightingale as a ''light winged dryad".

           Dryad is a wood-nymph in the Greek mythology which flies from one tree to another very swiftly. Here, the bird, Nightingale is also flying from one beech tree to another with a melodious song in its mouth. So, this bird is termed as 'dryad'.

          "Keats' Hellenic imagination naturally thinks of the bird as a Dryad with its wings not weighed down by the burden of life". This reference gives the bird a Godly status. The poet in such an allusion shows his mythopoeic vision.