We have always had a good crop of barn swallows in one of our barns. They nest on the floor joists of the loft. We also have 20-30 barn cats, and it frustrates the cats to no end because they get in the loft, right above the nests, and try to reach the birds through the cracks in the floor boards, but can't do it.
Since we have been renovating the barn, I was going to install doors (it hasn't had functional doors in 40 or 50 years). I ended up making dutch doors so that we could let the swallows come and go. I agree, they make a mess, but so do the cats, goats and cows. I do make it a point not to park the tractor under them!