a little bird told sb

/ɐ lˈɪtəl bˈɜːd tˈəʊld ˌɛsbˈiː/

/ɐ lˈɪɾəl bˈɜːd tˈoʊld ˌɛsbˈiː/

used to indicate that one knows about something but will not reveal how

Examples

1. A little bird told me it was your birthday.

2. This expression, a little bird told me, is kind of almost a universal idea I think.

3. You're not saying who told you, but you're just saying a little bird told me that it was your birthday or that your birthday is coming soon.

4. You could just say a little bird told me it's your birthday.