I don't recall whether it says usually or absolutely, but I don't think an anthropomorphic murder of crows maintains the Swarm subtype, since anthropomorphic makes it a small creature (which, I believe, is too big to swarm). This was probably updated in rules compendium, though, and given that I don't have that book... meh.
A Weremurder of Crows, however, is perfectly legal, and the visual is pretty cool.