This is a tricky one! The passage in question (supposedly describing India), for anyone unfamiliar:
Beyond these people, and at the very extremity of the mountains, the Trispithami and the Pygmies are said to exist; two races which are but three spans in height, that is to say, twenty-seven inches only. They enjoy a salubrious atmosphere, and a perpetual spring, being sheltered by the mountains from the northern blasts; it is these people that Homer has mentioned as being waged war upon by cranes. It is said, that they are in the habit of going down every spring to the sea-shore, in a large body, seated on the backs of rams and goats, and armed with arrows, and there destroy the eggs and the young of those birds; that this expedition occupies them for the space of three months, and that otherwise it would be impossible for them to withstand the increasing multitudes of the cranes. Their cabins, it is said, are built of mud, mixed with feathers and egg-shells. Aristotle, indeed, says, that they dwell in caves; but, in all other respects, he gives the same details as other writers (Natural History, 7.2)
Like it says, the earliest mention of Crane Warfare comes from the Iliad:
Now when they were marshalled, the several companies with their captains, the Trojans came on with clamour and with a cry like birds, even as the clamour of cranes ariseth before the face of heaven, when they flee from wintry storms and measureless rain, and with clamour fly towards the streams of Ocean, bearing slaughter and death to Pygmy men (Loeb edition, III.1-6)
but that’s pretty clearly a reference the audience is expected to recognize, rather than the origin of the belief. It does seem to be a reasonably common cultural touchstone, since crane fights show up everywhere from Aesop’s fables to the famous François Vase:
[Image via Wikimedia Commons]
as well as other less famous comic pottery, like this wine jug:
[© Marie-Lan Nguyen / Wikimedia Commons]
My best guess would be that it arose out of a combination of real crane migrations and confused early anthropology, but I 100% have animal-focused tunnel vision and this is brushing up against the edges of my knowledge, so if any classicists have more/better info I’d love to hear it!
[Literary Digression: around the 2nd c. CE, there’s an interesting shift towards mythological explanations for the war that start with Hera/Juno transforming a woman into a crane as punishment for her hubris.
It shows up in Ovid as a passive aggressive warning from Minerva to Arachne during their weaving contest:
And then, to give Arachne an idea / of the reward this upstart can expect / for her audacious bid for praise and glory, / the goddess then expertly represents / in each of the four corners of her work, / a different contest […] in the second corner she depicted / the terrifiying fate of the pygmy queen: / when Juno had defeated her, she ordered / her to transform herself into a crane / and then make war upon her former subjects (Metamorphoses, Charles Martin translation, 6.116-131)
The grammarian Antoninus Liberalis gives the story (probably unintentional) tragicomic notes by having the newly crane-ified woman follow her son around until her tribe gets so sick of the constant moping that they chase her off & declare war on all cranes:
But Hera found fault with Oinoe for not honouring her and turned her into a crane, elongated her neck, ordained that she should be a bird that flew high. She also caused war to arise between her and the Pygmaioi. Yearning for her child Mopsos, Oinoe flew over houses and would not go away. But all the Pygmaioi armed themselves and chased her away. Because of this there arose a state of war then as well as now between the Pygmaioi and the cranes (Metamorphoses, 16)
& my boy Aelian is as extra as always:
a certain woman became queen and ruled over the Pygmies; her name was Gerana, and the Pygmies worshipped her as a god, paying her honours too august for a human being. The result was, they say, that she became so puffed up in her mind that she held the goddesses of no account. It was especially Hera, Athena, Artemis, and Aphrodite that, she said, came nowhere near her in beauty. But she was not destined to escape the evil consequences of her diseased imagination. For in consequence of the anger of Hera she changed her original form into that of a most hideous bird and became the crane of today and wages war on the Pygmies because with their excessive honours they drove her to madness and to her destruction (On Animals, 15.29)]
