We recently undertook two behind the scenes tours both were very informative and engaging. We have taken the bird and giraffe tours which we have fed giraffes and seen how they train the birds for their free flight shows.
If you can get tickets to these events, do it.
While you're at it, become a zoo friend as well - annual membership gives you free entry :) That way you don't need to take in all of the zoo in one day.
It is best to start your tour from the top gate (if you arrive by ferrry, take the gondola to the top), then walk down hill.
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