Today feeding people blood inevitably calls to mind vampires. But when Shakespeare was writing, vampires weren’t really invented yet. They weren’t yet a THING. A bird that feeds with its blood is just a noble bird, not a weird creature sacrificing itself to vampires.
I’m not sure one could talk about blood feeding in this day and age without conjuring a vampire image, so vivid is the vampire in the cultural imagination. Pelicans, not so much.