Arist., Hist. An. vi. 6 and ix. 15. So Pliny x. vii. “Nidos nemo attigit: ideo etiam fuere qui putarent illos ex adverso orbe advolare, nidificant enim in excelsissimis rupibus.” cf. also Ælian, ii. 46: γῦπα δὲ ἄρρενα οὔ φασι γίγνεσθαί ποτε ἀλλὰ θηλείας ἁπάσας.