I bought a ultrasonic Humidifier from Carrefour Singapore for about S$50. It is a "Home" brand from Carrefour itself. I guess should be more reliable. The humidifier is 4.5 litter with max 300ml/hour flow rate.
I add in a little bit of aroma together with water in the humidifier. I then used timer to ON the humidifier from 5pm to 7pm. Hopefully the smell of aroma can extract more birds come in.
The next thing I will do is try to modify the humidifier such that the water can be auto refilled into the humidifier.