Do magpies maraud? It sounds good, but I've googled the collective noun for a group of magpies. It came up with the following:
magpies a conventicle of magpies
magpies a gulp of magpies
magpies a mischief of magpies
magpies a tidings of magpies
magpies a tittering of magpies
Based on this board, it has to be a mischief or tittering!