Have you ever heard or personally seen that the swiftlet house is full of headless birds, with wings left? This situation is most likely caused by bats.
There are many species of bats. As you can see in the picture below, they hang in groups in swiftlet houses. This kind of species generally does not directly eat swiftlet, but badly interferes the swiftlet.
No matter what kind of bats or other strange birds, as long as they appear around the swiftlet house, they will affect the growth of swiftlet. Don't keep complaining that the population of swiftlet not increasing. After all these details are done, adjust the sound and apply some pheromone, Can the number of swiftlet population not increase?