Aision

Ὁ δὲ ἐν Ὀλυμπίᾳ βωμὸς παρέχεται καὶ ἄλλο τοιόνδε ἐς θαῦμα· οἱ γὰρ ἰκτῖνες πεφυκότες ἁρπάζειν μάλιστα ὀρνίθων ἀδικοῦσιν οὐδὲν ἐν Ὀλυμπίᾳ τοὺς θύοντας· ἢν δὲ ἁρπάσῃ ποτὲ ἰκτῖνος ἤτοι σπλάγχνα ἢ τῶν κρεῶν, νενόμισται τῷ θύοντι οὐκ αἴσιον εἶναι τὸ σημεῖον. φασὶ δὲ Ἡρακλεῖ τῷ Ἀλκμήνης θύοντι ἐν Ὀλυμπίᾳ δι’ ὄχλου μάλιστα γενέσθαι τὰς μυίας· ἐξευρόντα οὖν αὐτὸν ἢ καὶ ὑπ’ ἄλλου διδαχθέντα Ἀπομυίῳ θῦσαι Διί, καὶ οὕτως ἀποτραπῆναι τὰς μυίας πέραν τοῦ Ἀλφειοῦ. λέγονται δὲ κατὰ ταὐτὰ καὶ Ἠλεῖοι θύειν τῷ Ἀπομυίῳ Διί, ἐξελαύνοντες τῆς Ἠλείας Ὀλυμπίας τὰς μυίας.
(Pausanias 5.14.1)

The altar at Olympia shows another strange peculiarity, which is this. The kite, the bird of prey with the most rapacious nature, never harms those who are sacrificing at Olympia. Should ever a kite seize the entrails or some of the flesh, it is regarded as an unfavorable sign for the sacrificer. There is a story that when Heracles the son of Alcmena was sacrificing at Olympia he was much worried by the flies. So either on his own initiative or at somebody’s suggestion he sacrificed to Zeus Averter of Flies, and thus the flies were diverted to the other side of the Alpheius. It is said that in the same way the Eleans too sacrifice to Zeus Averter of Flies, to drive* the flies out of Olympia.

* I take ἐξελαύνοντεςto be a conative present participle; Frazer takes it as an ordinary temporal participle; “when they drive out.”

(tr. William Henry Samuel Jones, with his note)