Magpies are two-tone colored birds known for their tendency to take and hoard (mostly shiny) things. They're considered to be pretty impressively intelligent as far as birds, and even animals in general, go. Generally considered a nuisance for their thieving habits, the noise they make and having something of a mean streak toward other birds. Not unlike another certain black-white.
The parallel's been made before in a fanfic at least once. made me chuckle when i first read Marisa being called "the magpie witch".
