Sources : Crane
Aristotle [ca. 350 BCE] (De animalibus Book 8, chapter 14.2): [Cranes] travel from Scythia to the marshes in the higher parts of Egypt, from which the Nile originates. This is the place where the Pygmies dwell; and this is no fable, for there is really, as it is said, a race of dwarfs, both men and horses, which lead the life of troglodites. [Book 8, chapter 14.5] The crane migrates from one extremity of the earth to the other, and they fly against the wind. As for the story about the stone, it is a fiction, for they say that they carry a stone as ballast, which is useful as a touchstone for gold, after they have vomited it up. - [Cresswell translation, 1887]
Lucan [1st century CE] (Pharsalia, book 7, verse 975-976): "Cranes who yearly change / The frosts of Thracia for the banks of Nile...".
Pliny the Elder [1st century CE] (Natural History, Book 10, 30): Cranes are continuously at war with the race of Pygmies; hostilities cease only when the cranes leave. Cranes travel large distances from the eastern sea. They start their journey at a time agreed to by all, fly at a great height so they can see their route, choose one of the flock as a leader to follow, and post some of their number at the end of the line to shout orders and keep the others together. When they rest at night they have sentries who hold a stone in their claw; if a sentry begins to fall asleep the stone falls and wakes him. All the other birds sleep with their head under their wing, except for the leader, who keeps watch. Before cranes fly across the Black Sea they ballast themselves with sand; when they reach the middle of the sea they drop the pebbles from their claws, and when they reach shore they remove the sand from their throats. (Book 10, 31): There is a place in Asia called Pythonos Comen with wide plains where cranes meet in assembly; the last one to arrive is attacked with claws. (Book 10, 42): Cranes grow black in old age.
Aelianus [170-230 CE] (On the Characteristics of Animals, Book 1, chapter 44): The screaming of cranes brings on showers, so they say, while their brain possesses some kind of spell that leads women to grant sexual favours - if those who first observed the fact are sufficient guarantee. [Book 2, chapter 1] When cranes are about to leave their Thracian haunts and the frosts of Thrace, they collect on the river Hebrus, and when each one has swallowed a stone by way of food and as ballast against the onslaught of winds, they prepare to emigrate and to set out for the Nile, longing for the warmth and for the food that is to be had there during the winter. And just when they are on the point of rising and moving off, the oldest crane goes round the entire flock thrice and then falls to the ground and breathes his last. So the others bury the dead body on the spot and fly straight to Egypt, traversing the widest seas on outstretched wing, never landing, never pausing to rest. And they fall in with the Egyptians as they are sowing their fields, and in the ploughlands they find, so to speak, a generous table, and though uninvited partake of the Egyptians' hospitality. [Book 3, chapter 13] Resting their bills upon each other's tail-feathers they form in a sense a continuous chain of flight, and sweeten their labour as they repose gently one upon another. And in some distant land when they light upon some water-spring they rest for the night and sleep, while three or four mount guard for all the others; and in order to avoid falling asleep during their watch they stand on one leg, but with the other held up they clutch a stone firmly and securely in their claws. Their object is that, if they should inadvertently drop off to sleep, the stone should fall and wake them with the sound. - [Scholfield translation]
Gaius Julius Solinus [3rd century CE] (De mirabilibus mundi / Polyhistor, Chapter 10.12): It is indeed obvious that in winter cranes congregate in great numbers in this northern region. It will not grieve me to recall how they direct their journeys. They go forward like a military company under a standard. Lest the strength of the breeze blow them off course, they consume sand, and ballast themselves with small stones to a moderate weight.  Then they hasten aloft, so they can mark the lands they are making for, as if from a tall watchtower. A bird confident of the course goes in front of the flock. She castigates idleness of flight with her voice, which gathers the troop together. When she has grown hoarse, another takes her place.  Preparing to cross Pontus, they aim for the narrowest passage. This (it is easy to discern by looking) is between Taurica and Paphlagonia — that is, between Carambis and Criumetopon. When they know themselves to have passed the middle of the channel, their feet drop the bundles of small stones.  Sailors report that from this circumstance they are often rained upon with stony showers. The cranes do not bring up the sand again before they are settled as securely as possible. The birds are of one mind in caring for those who become weary; indeed, if some falter, they all flock together and continuously bear up the exhausted ones until they recover their strength by resting. They are not less zealous on land.  They divide the night watches so that at any one time ten of them are wakeful. The watchers clasp little weights in their claws, which if by chance are let fall, convict them of slackness. When they perceive something to take precautions against, they make a clamour. Their colour reveals their age: they grow dark with old age. - [Arwen Apps translation, 2011]
Saint Ambrose [4th century CE] (Hexameron, Book 5, chapter 15.50-51): How well do the cranes carry out their guard duty at night without orders and without compulsion ! You may note the watchers at their appointed places. Again, while the rest of the flock is at rest, some make the rounds and make certain that no attack is attempted from any quarter. With unabated vigilance they render complete protection. When the watcher has completed his period for guard duty, he prepares for sleep, after arousing with a warning cry the sleeper who is destined to take his place as the next sentry. The latter willingly accepts his lot. He does not act as is the custom with us when, under such circumstances, we are loath to give up our sleep. Rather, he rises eagerly from his resting place, performs his duty, and repays with equal care and courtesy the favors that he has received. Hence there are no deserters, because their loyalty is a natural one. Hence their guard duty furnishes real protection, because their wills are free. They also follow this procedure when in flight. In this way they alleviate fatigue as they perform in turn the function of leadership. At a certain prescribed time one takes, a position ahead of the rest, in advance of the banners, so to speak. Later, he turns back and yields to a successor the task of leading the flock. - [Savage translation, 1961]
Isidore of Seville [7th century CE] (Etymologies, Book 12, 7:14): Cranes (grus) took their name from their particular call, for they whoop with such a sound. When they fly, they follow a leader in formation like a letter [Greek lambda, ^]. They seek the heights, from which they can more easily see which lands to make for. [Book112, 7:15[ The one leading the flock chides with its voice, but, when it begins to grow hoarse, another crane takes its place. At night they divide up the watches, and alternate their vigils; they hold stones in their extended claws to let it be known if they fall asleep – the noise indicates the need to be on the alert. Their color reveals their age, for they turn black as they grow old. - [Barney, Lewis, et. al. translation]Gerald of Wales [c. 1146 – c. 1223] (Topographia Hibernica, chapter 10): Cranes assemble in such numbers, that a hundred, or about that number, are often seen in one flock. By natural instinct they keep watch in turns at night for their common safety, perched on one foot, and holding a stone in the other featherless claw, that if they should fall asleep, the fall of the stone may rouse them to renew their watch. These birds are emblems of the bishops of the church, whose office it is to keep watch over their flock, not knowing at what hour the thief will come. ... This bird also gives notice of danger by its cries. In like manner the pastors of the church drive the wolves from the fold by sounding the alarm from the holy oracles. ... The liver of this bird is also of such a fiery heat, that, when by any chance it swallows iron, its stomach digests it. So bowels inflamed with the fire of charity subdue and soften iron hearts which were before indurate. - [Forester translation, 1863, chapter 10]