Sources : Ibis
Herodotus [c. 484 – c. 425 BCE] (Histories, Book 2.75) Winged serpents are said to fly at the beginning of spring, from Arabia, making for Egypt; but the ibis birds encounter the invaders in this pass and kill them. The Arabians say that the ibis is greatly honoured by the Egyptians for this service, and the Egyptians give the same reason for honouring these birds.  Now this is the appearance of the ibis. It is all deep black, with legs like a crane's, and a beak strongly hooked; this size is that of a landrail. Such is the outward form of the ibis which fights with the serpents. Those that most consort with men (for the ibis is of two kinds) all the head and neck bare of feathers; their plumage is white, save the head and neck and the tips of wings and tail (these being deep black); the legs and beak of the bird are like those of the other ibis. - [Godley translation]
Pliny the Elder [1st century CE] (Natural History, Book 8, 41): The ibis is a bird from Egypt. It uses its curved beak to purge itself "through the part by which it is most conducive to health for the heavy residue of foodstuffs to be excreted." (Book 10, 40): The people of Egypt invoke the ibis to guard against the arrival of snakes. (Book 10, 45): The ibis is born black at Pelesium, but is white everywhere else.
Aelianus [170-230 CE] (On the Characteristics of Animals, Book 1, chapter 38): ... the Egyptians maintain that all snakes dread the feathers of the ibis. [Book 2, chapter 38] Here is another story relating to the Egyptian Ibis which I have heard. The bird is sacred to the moon. At any rate it hatches its eggs in the same number of days that the goddess takes to wax and to wane, and never leaves Egypt. The reason for this is that Egypt is the moistest of all countries and the moon is believed to be the moistest of all planets. Of its own free will the Ibis would never quit Egypt, and should some man lay hands upon it and forcibly export it, it will defend itself against its assailant and bring all his labour to nothing, for it will starve itself to death and render its captor's exertions vain. It walks quietly like a maiden, and one would never see it moving at anything faster than a foot's pace. The Black Ibis does not permit the winged serpents from Arabia to cross into Egypt, but fights to protect the land it loves, while the other kind encounters the serpents that come down the Nile when in flood and destroys them. Otherwise there would have been nothing to prevent the Egyptians from being killed by their coming. - [Scholfield translation]
Gaius Julius Solinus [3rd century CE] (De mirabilibus mundi / Polyhistor, Chapter 32.22): The ibis bird frequents the same banks. It plunders serpents’ eggs, and carries off the choicest of them as food to their own nests. Thus the success of noxious broods is prevented.  These birds are not useful only within Aegyptian borders. The swamps of Arabia send forth swarms of winged serpents, whose venom is so quick-acting, that, after a bite, death follows more quickly than pain. The ibis, from an innate wisdom, go out aroused and in readiness for battle and devastate this foreign evil before it reaches the borders of their land. Mobbing the pestilential horde in the air, they devour all of it. For which reason, ibis are deservedly held to be holy, and no-one may injure them. They lay eggs with their mouths. Only Pelusium produces black ibis; all other places breed white ones. - [Arwen Apps translation, 2011]
Isidore of Seville [7th century CE] (Etymologies, Book 12, 7:33): The ibis (ibis) is a bird of the Nile river that purges itself by spurting water into its anus with its beak. This bird feeds on snake eggs, carrying the most pleasing food that it takes from them back to its nestlings. - [Barney, Lewis, et. al. translation]