One very common predator which can harm your BH are those cockroaches.
During daytime they remain hidden behind your nesting planks.
When night falls they will start to hunt for food.
Usually they will be eating your bird's nest.
They will chew until your nests will be with holes or half eaten away.
Belara BH was also facing the same problem.
I managed to kill almost 200-300 of them.
The best method is to get a canister of Ridsect with long straw nozzle.
Hunt for those cockroaches hiding points and spray onto those spots.
You will find many running out of their hiding place and jump all over the place.
I am not surprise that they will start running onto your leg and enter your pant.
One of the most serious consequences of having them is when they shit on your precious nesting planks.
Those planks will not be easily populated by your swiftlet.
I found a good method how to hide them,
I used the bamboo mat that was purchased from a shop in Java.
The 6 inch wide mat can easily be stapled onto the planks.
Have a good look at some photos taken inside the Belara BH.