There are a few levels of wildlife photo shooting:
Level 1 - Zoo where everything is there right in front of you and best is during the feeding times.
Level 2 - Jurong Bird Park where the birds are there, but you got to have longer lens and somewhat better techniques to handle longer lens.
Level 3 - Botanic Garden where you can find insects and birds or swans and ducks, etc.
Level 4 - Sungei Buroh, where you need much more effort to get there, and even greater effort to be able to get any decent pictures of birds. Chek Jawa at Pulau Ubin - I haven't been there yet, after they construct the board walk.
And there are others like AH butterfly park, a butterfly park at Sentosa, underwaterworld at Sentosa, MOG, etc.
BTW, anyone been to the Butterfly park at Sentosa? Expensive? Worth it?