This is because ravens bear the character of Saturn, the author of such calamities.--Athenian Oracle (supplement, 476).
Ravens forebode pestilence and death.
Like the sad-presaging raven, that tolls
The sick man’s passport in her hollow beak,
And, in the shadow of the silent night,
Doth shake contagion from her sable wing.
Marlowe, The Jew of Malta (1633).
Ravens foster forsaken children.
Some say that ravens foster forlorn children.
(?) Shakespeare, Titus Andronicus, act ii. sc. 3 (1593).