When we had our propane furnace & water heater venting upgraded the installer just left the pipes sticking straight out from the wall < no 45 down elbow > with no grill of any kind. When I questioned him he said "not needed" .. The next spring my neighbour said he saw birds coming & going from one of the pipes - swallows nest about 6 feet in ! < bank swallows I assumed ? >
I let them fledge and cleaned it out & put a plastic grill over. John T.