a little bird told (or whispered to) - To get the dirt from some shady, faceless, probably stoned s

To get the dirt from some shady, faceless, probably stoned source.

I know your mom's having an affair because a little bird told me.

A little bird whispered to me that your lunch was stolen by a greasy kid.

A little bird told me you failed math, and it wasn't even a little bird.