A little bird told me
Definition 1 of 1
Pro Tip 1/3Informal Use
This phrase is used in casual conversations, not in formal writing or speech. 
I won't tell who, but a little bird told me it's your birthday tomorrow!
Pro Tip 2/3Indirect Source
The 'little bird' is not a real bird, but a metaphor for an unnamed or secretive information source. 
A little bird told me you've been promoted. Congratulations!
Pro Tip 3/3Amused Tone
The expression often implies a playful or teasing tone when sharing information. 
So, a little bird told me you've found a new hobby, is that right?