Who said "Sing on, sing on, you gray-brown bird."?

This quote is attributed to Walt Whitman.

Sing on, sing on, you gray-brown bird. - Walt Whitman

"Sing on, sing on, you gray-brown bird."

Walt Whitman Perseverance

Poet American 1819–1892

"Do I contradict myself? Very well then I contradict myself, (I am large, I contain multitudes.)"

"I celebrate myself, and sing myself."

"Keep your face always toward the sunshine, and shadows will fall behind you."

"The future is no more uncertain than the present."

"Perseverance is the key to success."

"Don't give up on what you believe in."

"I've been through worse."

"You learn to live with the pain."