Book/Source: Silmarillion, LotR (Appendices)
Disclaimer: I do not claim rights to the Tolkien estate.
(Note: No, the books don't actually say anything about birdsong in connection with the choices. I made that up.)
His daughter’s announcement came in the same way as his brother’s, long ago: in the wake of a nightingale’s song.
The cursed bird sang every day without fail at the hour of nightfall. But when a life-changing decision was to be made, it was construed as a sign! It seemed an abjectly Elvish behaviour, this 6000-year-long fixation on a romantic idea. Yet like the greatest Tinúviel, both had chosen mortality.
Because a bird had tweeted.
Well, no more! Elrond signalled to his son, who went in search for arrows.
It was, perhaps, a mightily human response.