From my years of experience these owls seems to like using any unused houses that are with openings like windows or doors.
Swiftlet bird house is one of the favourite.
What I think they do every evening before they start their hunting works at night is to find a nice place to prepare their night.
First is to groom and while doing so will shit and urinate.
You will find their white urine and a kind of ball dung which they will release every day when they visit your BH.
They will enter your building even there is not a single bird nest inside.
Their main effect is when your BH is with nests population.
Once they attack the house all your birds will be reduced to 1/3 its original population.
They will continue to harass all your birds and one day nothing is left.
So what should be done?
Right now this special gadget is the most effective.
This month alone the gadget captured three owls at two BH locations.
In fact in one of the BH I installed two units and both captured one.
I call it as the owl trap.
It comes with this special spring action frame which when released will instanly trap any creature that sits of a piece of wooden bar.
A string is attached to the wood bar end and will pull the hook that holds the spring loaded frame.
The moment the hook is released the loaded coil will snap the frame into action.
Have a good look at these pictures.
The first was from a BH in Ajil Trengganu while the second was from Kepala Batas, Penang.
Caught two in one visit.
An amazing gadget which I think every BH should have.