Pausanias Description of Greece 5.14.1

[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 ¹ the flies out of Olympia .

[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 Hēraklēs, the son of Alkmene, 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 Alpheios. It is said that in the same way, the Eleians too sacrifice to Zeus, Averter of Flies, to drive the flies out of Olympia.