Mine do the same thing. I have a pie pan under mine to catch most of the spillage. I don't fill the pipe very full, so when they make a mess, I just don't fill the pipe for a day or so and they eat what's on the ground.
So how many birds can this feed? I'm in a jam for space. Build a tractor with detachable run and the coop part is jut a bit too small. I'm looking for a space saving feedfeed (and waterer) for 6 regular size birds.