The two BHs I recently visited have almost the same problem.
The two owners claimed that ever since those owls visited their BH the number of birds staying went down drastically.
The first thing that came to their mind was how to block away the owls from entering.
Some goes to the extend of shutting down the main entrance hole to the minimum while the other got this idea of installing those steel bars about 3 inches apart top to bottom.
Both are good to block those owls but what about your tenants (swiftlet)?
Would these owl protection ideas affect your swiftlet from flying in?
Have you ever consider what it will do to your BH?
Do you have any other alternatives?
To make matter worst both owners did nothing to clean all those markings left on the floor and the window areas. (You need to clean and remove their smell)
This little thing might not looks important but it does change your BH activities.
For the sake of your BH you need to think first before you select your option.
Why don't you sit down and find out what are the various options you have to prevent those owls from entering again.
If you have the list in front of you, try to figure out which one is the best and ask yourself will they affect your tenants,
Can you think of the best way to modify the gadget so that when you install it will only affect those owls from entering and will not prevent your swiftlet from entering and leaving?
Think carefully and don't be so drastic until you shoo away your own tenants.
If you are not very sure call 017 7551318.
Maybe this fellow can give you the proper advise or perhaps he might have some new ideas on how to prevent those owls from screwing up your BH.