When we had a woodpecker on our house years ago, we bought holographic tape from Lee Valley and fastened about two or three feet of it on a strong stick and mounted it with tape near the chimney. I am not sure if that would work for you or not. Taping it so it was about two feet above roof level could work. You might also try raising the vent (although this means getting out of bed a couple of mornings really early) and lowering it, a couple of times to get rid of them. After a couple of days they may be gone.
Just as an afterthought, birds may be a protected species in that area, so be careful if you should choose the pellet gun.