Please study this article:
Last week decided to test the phenomena in Kepala Batas BH.
The entrance hole was reduced to only one and at the same time the main entrance hole was reduce to only about 1.5' by 1.5'.
While the LMB was reduced in size the floors were modified by additional walls, tweeters and fake nests.
This is more like doing an upgrade to welcome the new year for those swiftlet.
Will report of its progress after a few months into operation.
This was the initial flying in which takes about 4-5 minutes of heavy activities.
This was what happen when the main entrance hole was reduced.
The smaller LMB hole will force those swiftlet to get into a kind of que.
This longer then usual que will generate a kind of sound which can attract other birds that are passing by or from your neighbors.
The que will get longer and longer once the number of birds inside your BH increased.
The longer the que the more will it attract those wild birds.
Need to see its progress after a few months.
Used to be this large size: