My example differs - my birds routinely go over a 4' gate to get to the other side - or thru the hole at the 3' level, but I don't have old commercial reds or the CX eating machines, and my mutts tend towards lighter/flighty - its the predator awareness I'm preserving in them.
As to septic, if it works properly, assuming you aren't ingesting chemicals that worry you if they should build up in your birds over time (I'm not asking, that's something you can research on your own), you have nothing to worry about. That said, a recent study into red algea/red tide blooms off the FL Coast used tracing of human birth control medications in the water supply to differentiate between nitrogen rich run off from commercial fertilizer use (Everglades sugar procution) and the effects of widespread older (and sometimes failing) septic fields in south west Florida.
Spetic is an excellent, but imperfect, solution to waste treatment. There are no perfect solutions. Leave it to you to determine where your risk tolerance lies.