Sources : Caladrius
Aberdeen Bestiary [circa 1200 CE] (folio 56v-57r): The bird called caladrius, as Physiologus tells us, is white all over; it has no black parts. Its excrement cures cataract in the eyes. It is to be found in royal residences. If anyone is sick, he will learn from the caladrius if he is to live or die. If, therefore, a man's illness is fatal, the caladrius will turn its head away from the sick man as soon as it sees him, and everyone knows that the man is going to die. But if the man's sickness is one from which he will recover, the bird looks him in the face and takes the entire illness upon itself; it flies up into the air, towards the sun, burns off the sickness and scatters it, and the sick man is cured.
Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Birds 5.24): Caladrius, as Jacobus and Isidore say, is a completely white bird. The inner part of its thigh removes the darkness from the eyes. But the nature of the caladrius is such that, if it is brought several times to a sick person, it makes bystanders certain whether the sick person should die or live. For if it looks down upon the face of a sick person and turns away its eyes, it is a sign of death. But if it does not avert its eyes, it is a sign of life. For it looks to his face and collects all the infirmities of that sick man within itself and flies in the air and there it burns the infirmities and scatters them, and the sick man is immediately healed. Alexander is said to have found these birds in Persia. Indeed, the kings of ancient times kept these birds in their royal courts. Concerning these birds, then, and likewise those which the blessed Brendan found in a certain high and beautiful tree, one of which answered him that they were spirits doing their penance there in the form of birds, but whether it is true or impossible, we leave to the present reader to judge. - [Badke translation/paraphrase]
Bartholomaeus Anglicus [13th century CE] (Liber de proprietatibus rerum, Book12.22): the Philosopher saith, the Birde that is called Kaladrius, is white of coulour, and hath no parte of blacknesse. And the neather part of his leg cleanseth and purgeth dimnesse of the eyen. His kinde is such, when a man is helde in great sicknesse, this birde Kaladrius turneth away his face from him that is sick, & then without doubt the man shall die. And if the sicke man shal escape, the bird Kaladrius setteth his sight on him, & beholdeth him, as it were fauning and pleasing: and this bird is other then the bird that is called Calandra, that singeth as a Thrustle, as the Glose saith, Deut. 14. There it is said, that Calaudreon [lark?] is another then Calundre, &c. - [Batman]