Laudator Temporis Acti

Plutarch, Life of Marius 17.3 (tr. Bernadotte Perrin):

The affair of the vultures, however, which Alexander of Myndus relates [fragment 26 Wellmann], is certainly wonderful. Two vultures were always seen hovering about the armies of Marius before their victories, and accompanied them on their journeys, being recognized by bronze rings on their necks; for the soldiers had caught them, put these rings on, and let them go again; and after this, on recognizing the birds, the soldiers greeted them, and they were glad to see them when they set out upon a march, feeling sure in such cases that they would be successful.

τὸ δὲ περὶ τοὺς γῦπας θαύματος ἄξιον Ἀλέξανδρος ὁ Μύνδιος ἱστόρηκε. δύο γὰρ ἐφαίνοντο πρὸ τῶν κατορθωμάτων ἀεὶ περὶ τὰς στρατιὰς καὶ παρηκολούθουν γνωριζόμενοι χαλκοῖς περιδεραίοις· ταὐτὰ δὲ οἱ στρατιῶται συλλαβόντες αὐτούς περιῆψαν, εἶτα ἀφῆκαν· ἐκ δὲ τούτου γνωρίζοντες ἠσπάζοντο αὐτούς οἱ στρατιῶται καὶ φανέντων ἐπὶ ταῖς ἐξόδοις ἔχαιρον ὡς ἀγαθὸν τι πράξοντες.

W. Geoffrey Arnott, Birds in the Ancient World from A to Z (London: Routledge, 2007), p. 92:

Their habit of flocking near armies on the march or in battle (Aristotle HA 563a5–12, Alexander of Myndos fr. 26 Wellmann, Aelian NA 2.46) was interpreted as implying that the birds realised battles produced corpses to eat, but is better explained by the fact that pack animals with provisions of meat always accompanied armies.

See M. Wellmann, "Alexander von Myndos," Hermes 26.4 (1891) 481-566 (fragment 26 on p. 553).