With propitious gaze the caladrius looks at the sick man,
When Lachesis twists her thread with favouring hand.
With eyes averted it raises a warning cry of sorrow
As often as it perceives the day of death approaching.
Its flesh restores the bright keenness of vision which has grown dim.
The colour of its wings is said to be milk-white.

Alexander Neckam's De laudibus divinae Sapientiae

The Caladrius

(aka Caladre, Caladrio, Calandre, Calandrius, Caradrius, Charadrius)

The caladrius is a mythical bird described as being pure white and gifted with the ability to foretell whether or not the sick would recover. When placed at his side, if a man's illness was fatal, the caladrius would turn its head from him. If the man was destined to recover, the caladrius would look into his face and take the sickness upon itself. It would then fly toward the sun, where the illness would be destroyed and scattered. One seeking a caladrius would be advised to befriend royalty, as the birds were found exclusively in the courts of kings. The excrement of the caladrius was said to have the power to cure cataracts and other ailments of the eyes when applied as an ointment, although in some manuscripts it is the marrow of the bird’s thighbone that possesses this ability.

Versions of the caladrius appear throughout classical literature, including Aristophanes's The Birds and the works of Plato. Perhaps the earliest reference alluded to is a line written by the writer Hipponax (546-520 B.C.), as quoted by Suidas in the twelfth century.

The charadrius is a bird of such nature that if those who are suffering from jaundice look at it, as report goes, they more easily get rid of that disease. For which reason also the sellers (of the bird) hide it, lest those who are suffering from jaundice should be cured for nothing. "Why, he is hiding it: like a man with a charadrius to sell," as Hipponax says; whence has arisen the proverb: "Imitating the charadrius," said of those who hide anything.
Another interesting passage, taken from Book XVII, ch. 13 of Aelian's De natura animalium (third century A.D.), is one of the earliest recorded versions of the bird's method of curing illness:
Now this is the natural power of the charadrius, which by Zeus it is not right to despise. If a man has his body full of jaundice and then looks keenly at the bird, and the bird looks back at him very inflexibly, as though being made angry with him in return, then this mutual gaze cures the man of the aforesaid complaint.
It was during the Middle Ages, however, that the caladrius became most prominant, when Christian moralists founded upon it a religious allegory. It was popularized as a symbol for Jesus Christ, as described in the following extract from a translation of the Aberdeen Bestiary (circa thirteenth century).
The caladrius represents our Saviour. Our Lord is pure white without a trace of black, 'who did no sin, neither was guile found in his mouth' (1 Peter, 2:22). The Lord, moreover, coming from on high, turned his face from the Jews, because they did not believe, and turned to us, Gentiles, taking away our weakness and carrying our sins; raised up on the wood of the cross and ascending on high, 'he led captivity captive and gave gifts unto men, (Ephesians, 4:8).
It is for its religious connotations that the caladrius is most well-known by historians and literary scholars.

The Caladrius and its Legend, Sculptured upon the Twelfth-century Doorway of Alne Church, Yorkshire. by Geoge C. Druce and found at http://bestiary.ca/etexts/druce1912/Druce%20-%20Caladrius%20and%20its%20Legend.pdf (read this, it's excellent)