This bird is a caladrius. It might be based on real bird, based on classical descriptions, but by the Middle Ages, the caladrius had become a mythical bird associated with sickness. It was said that by a look, the caladrius could tell if a sick person would live or die. From the Aberdeen Bestiary, which dates to the twelfth century:
“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. 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). Each day Christ, like the caladrius, attends us in our sickness, examines our mind when we confess, and heals those to whom he shows the grace of repentance. But he turns his face away from those whose heart he knows to be unrepentant. These he casts off; but those to whom he turns his face, he makes whole again.” – Aberdeen University Library, MS 24, f. 57r, https://www.abdn.ac.uk/bestiary/ms24/f57r
Plague Lit 