Green’s Dictionary of Slang

(Aus.) dawn, very early in the morning.

2003		Ridley & Turner That’s Unusual 218: We could be in here till the crack of sparrows.
2010		G. Maxwell Falling & Laughing 202: OK, we’d be back at the crack of sparrows on Monday for a 9 am appointment, but he’d spent his last night in hospital.
2018		P. Rowe Third Language Dict. [ebook] ‘Can you pick me up tomorrow morning?’‘No, sorry; I’ll be out and gorn (gone) by the crack of sparrows’ (it is alleged that sparrows and most beings, fart at dawn).