Why do we say "A little bird told me"?

Meaning:

I won't name the source of my information

Background:

The earliest recording of this saying is in John Heywood's collection of proverbs (1546) but it is believed that, like so many proverbs, it traces its written origins to the Old Testament:

Do not revile the king even in your thoughts,
or curse the rich in your bedroom,
because a bird of the air may carry your words,
and a bird on the wing may report what you say.
- Ecclesiastes, 10:20, New International Version

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