W. S. Gilbert Quote

On a tree by a river a little tomtit
Sang "Willow, titwillow, titwillow"
And I said to him, "Dicky-bird, why do you sit
Singing 'Willow, titwillow, titwillow?'.
"Is it weakness of intellect, birdie?" I cried,
"Or a rather tough worm in your little inside?"
With a shake of his poor little head he replied,
"Oh, Willow, titwillow, titwillow!"

The Suicide's Grave. - Bartlett's Familiar Quotations, 10th ed. (1919)