And it just gets worse. I tried to wait them out, but even when they left a few stayed behind, lurking in the treetops:
I knocked over one tree with a ballista, and those creatures left voluntarily. I took about thirty casualties clearing out the second tree.
A year later, the horde returns, and right now I'm ignoring them because there's lots of work inside the fortress that needs done, and I've got a decent tree farm up and going. I'm thinking it's time for Operation FTW, but I'll have to wait until they leave to set up it for the inevitable third return.
I waited for most of them to leave first. I do have a good crossbow tower setup, but the problem is that crossbows aren't effective against undead.