I looked for that song for a long time, too. Here it is as I remember it. The Robin as sung by Burl Ives https://m.youtube.com/watch?v=6DjeoyAhXWwSung by Burl Ives, from the album "Walt Disney Presents Burl Ives' Animal Folk" from 1963.
THE ROBIN
She came to my window one morning in spring
A sweet little robin, she came there to sing
And the song that she sang was sweeter by far
Than ever was heard on a flute or guitar.
Oh robin, oh robin, oh robin redbreast,
A sweet little robin, she came there to sing
And the song that she sang was sweeter by far
Than ever was heard on a flute or guitar.
Her wing, she was spreading to soar far away
Then resting a moment seemed sweetly to say,
"How happy, how happy the world seems to be!
Awake little man, and be happy with me."
But just as she finished her beautiful song,
A thoughtless young man with his gun came along
He killed and he carried my robin away
She'll sing never more at the break of the day.
Oh robin, oh robin, oh robin redbreast,
A sweet little robin, she came there to sing
And the song that she sang was sweeter by far
Than ever was heard on a flute or guitar.