Why did Keats choose nightingale bird??

A visiting friend named Charles Brown, wrote about the moment Keats wrote his famous poem Ode to a Nightingale.

In the spring of 1819 a nightingale had built her nest near my house. Keats felt a tranquil and continual joy in her song; and one morning he took his chair from the breakfast-table to the grass-plot under a plum-tree, where he sat for two or three hours. When he came into the house, I perceived he had some scraps of paper in his hand, and these he was quietly thrusting behind the books. On inquiry, I found those scraps, four or five in number, contained his poetic feeling on the song of the nightingale.