Yellow Collar On Javan Myna

I first noticed Javan myna with yellow patches on the neck last month when I was at Dairy Farm area. Unconvinced of what I saw, I went back last week after a heavy rain and I found more of this myna with a yellow collar!
I took this photo last month and that was my 1st time seeing a myna with yellow collar!

I saw more of mynas with yellow collar! As it was raining earlier, some of their feathers in fact are still damp.

From their appearance, no doubt they are Javan myna (except with the yellow patch on the neck, and white instead of yellow eyes). But, why only the mynas over at this area shown yellow collar?

Could this be the answer to the mystery? The pollen from the African tulip tree?? The African tulip tree is very common everywhere, but why only the myna around this area show this yellow patch?? ...