For those who are not residing in Singapore, they may think that the birds in the Singapore bird series are common to Singapore in the wild. Not entirely true. The tern depicted on the $1 is not so common as these are migrating birds. Usually found in coastal areas stretching from China to Australia. I have yet to see one in Singapore.
As for the sharma ($50), bulbul ($5), these are song birds kept by Singaporean. They are not in the wild. The $20 sunbird is a common bird in Singapore. The female are greenish yellow while the male ones with purple throated. I once caught a male bird. After studying it for 2 days and fed with honey, I released it.
There are a number of species of kingfisher found in Singapore. The $10 white collar kingfisher is a common bird in Singapore. They have distinctive calls. Even if you don't see them, you can hear them. You can find them easily in places where there are water.
As for the $100 bee eater, I have yet to see one. Maybe I did not search hard enough. Here is a photo of the colourful blue tailed bee eater spotted and photographed in Changi by Charlie.