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.
You are very keen sending your military out to deal with that! I would only engage from my surface fortifications… if I had them ready.
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.