In Shakespeare's time it was commonly believed that pelicans fed their young on their own blood. This superstition is referred to in Act 4, Scene 5 of Hamlet, where Laertes is leading a rebellious mob against King Claudius to revenge himself for the death of his father Polonius (and later the mental breakdown of his sister Ophelia). Laertes mistakenly believes Claudius was responsible for his father's death, but Claudius asks:
If you desire to know the certainty
Of your dear father's death, is't writ in your revenge
That, swoopstake, you will draw both friend and foe,
Winner and loser?
None but his enemies.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
To his good friends thus wide I'll ope my arms
And like the kind life-rend'ring pelican
Repast them with my blood.
So when King Lear refers to Goneril and Regan as "pelican daughters" the Elizabethan audience would understand that he was thinking of them as something like vampires who were devouring his blood and whom he had nourished all his life with his blood. What troubles Lear so much throughout the play is the fact that he could be so mistreated by his own flesh and blood.
this makes no sense because this means that to know why lear refers to his daughters as pelicans in kingLears you must have read Hamlet??