King Lear calls Goneril and Regan "pelican daughters" because he feels that they are feeding on his heart's blood. In legend, pelican young were depicted as feeding on the blood of their parent. In one version of the legend, by Guillaume le Clerc [13th century CE], it is written that "When the young are born, the parent bird devotes all his care and thought to nourishing them. But the young birds are ungrateful, and when they have grown strong and self-reliant they peck at their father's face, and he, enraged at their wickedness, kills them all." (See The Medieval Bestiary: Pelican.)
Pelican Feeding Her Chicks Her Own Blood Source:
THE FRIENDSHIP OF THE SAINTS
Perhaps such fables arose from observations of how actual pelicans feed their young. See below.
Chick Feeding from Crop Source: