'The cranes of Ibycus' is a proverbial expression suggesting the uncovering of a dark deed through divine intervention; the 'Ibycus' of the story is a famous Greek lyric poet. Schiller has a poem that lays the tale out; you can find it here in an anonymous 1902 translation. I make no claim that the translation captures Schiller's original poetry in any significant way; only that it does get the story about right. You can also read the story in Bulfinch's prose version.
The Cranes of Ibycus
Can blood-guilt scream to heaven, its cry unsated?
And can the gods be blind to living law?
Can murderers find solace by forgetting?
Has memory no more its tooth and claw?
Say no! The gods are watchful and most wary,
but step by step the deserving march to doom,
and in the sky, be it sun-bright or starry,
Nemesis will soar and Sekhmet's shadow loom.
And the cranes that fly so far in gentle peace
will bring to mind the murder that is done;
whatever form they take, what shape they wist,
they recall the darkness to the darkened one.
When deed is done, the sinner shuns the sky
for there the ruthless cranes on wings of judgment fly;
but though he cry for the mountains to hide his head,
the cranes still bear sure vengeance for the dead.