I parked my car and walked to those trees for some investigation.
"There must be something that were produced by these trees to lure those swiftlet to be circling them."
That was what that came to my mind.
The moment I arrived at the spot I notice this sizable tree with lots of fruit on the branches.
A closer look confirmed that both trees were a specie of Fig.
The larger tree have a tiny sized fruit while the second much larger (10 times larger than the 1st).
I remembered writing about Fig tree wasps that help to pollinate the flowers by borrowing the skin of the fruit and squeezed themselves into the fruit.
Once inside the fruit they will start to lay their eggs in some kind of a flower pots.
The eggs will turned into maggots and will start to eat on those sweep saps inside and while crawling from on pot to the other will carry those pollen and pollinate the flowers.
The fig trees and these wasps helps each other their survival. This natural process is called symbiosis.
While at the two trees I managed to capture some pictures to share with my blog readers.
Please help yourself with them:
They will act as your insect barn in a natural way.
Every fruiting season those wasps will breed vigorously and they will be eaten by your birds.
In this way you will attract more birds to your BH.