I believe for the majority of the summer the zoo will be open from 9am to 9pm. Access should remain open to most if not all of the zoo, but the aviaries close when the sun goes down and some animals may be put in before closing.
Yes the zoo is open 9am-9pm from late June through August and this includes the entire grounds (except perhaps the walk-in aviaries). The front plaza and gift shops remain open until at least 9:30pm and possibly as late as 10pm.
I can speak from (unfortunate) experience that at least some of the aviaries do close early - last time I was there late on a summer evening, I saved the Hummingbird Aviary (that's barely home to hummingbirds any more) for one of my last exhibits only to find it closed...
