A myth I haven't heard since I was small. Adults used to tease children by telling them that if you put salt on the tail of a bird, it could not fly away, and you could catch it.
"True again," said Uncle Pumblechook. "You've hit it, sir! Plenty of subjects going about, for them that know how to salt upon their tails. That's what's wanted. A man needn't go far to find a subject, if he's ready with his salt-box."

The child was supposed to eventually puzzle out the joke, by realizing that if the bird would let you put salt on its tail it would also let you grab it.