The answer is that barn owls are highly capable of finding potential nesting sites—they are on the lookout much of the year. The dark entrance hole is enough to grab their interest and so all you need to do is make sure the entrance hole is visible from nearby open areas where barn owls will pass through. Placing mulch in the box can be beneficial too—barn owls like to have a softer substrate than the hard floor of a nest box. Other than that, essentially it is “build it and they will come.”