"A little bird told me..." is defined by Phrase Finder as:
I was told by a private or secret source.
The phrase in this exact form can be found pretty frequently by, for example, perusing online news sources. Here is one recent example from July 2017:
Maybe I’ll miss the camaraderie of the bond desk, but somehow I doubt it. I glance at my boss’s corner office, which looks like a Siberian ice cap on a winter’s evening - darkened, empty and unloved. A little bird told me he’s on his way out too.
Phrase Finder asserts that the origin goes all the way back to the Bible, but the phrasing is vastly different in the example provided:
Curse not the king, no not in thy thought; and curse not the rich in thy bedchamber: for a bird of the air shall carry the voice, and that which hath wings shall tell the matter.
- Ecclesiastes 10-20 (King James Version)
OED also provides figurative uses of "bird" in a similar phrasing with a similar meaning from very early writing:
I dyd lately here..By one byrd, that in myne eare was late chauntyng.
- John Heywood · A dialogue conteinyng the nomber in effect of all the prouerbes in the englishe tongue · 1st edition, 1546.
Accepting that the general figurative idea of a bird acting as a clandestine messenger is very old, how did the phrase arrive in its current form, a little bird told me? Why exactly did the bird become little?