The Singapore Zoological gardens take the open zoo concept about at far as it can go. Lots of animals appear to be perfectly free to go wherever they want; signs indicate that they are "conditioned" to stay within appropriate parts of the zoo. This means we're walking down a concrete path, gawking at a troop of chimpanzees doing amusing chimp schtick, when ZOOP! down the path goes a little brown furry thing that turns out to be a lemur, swiming from the shrubbery. Then ZOOP! there goes another. ZOOP! ZOOP! ZOOP! ZOOP! go the rest of the gang. Way cool.
Presumably, the free-flying peafowl know that it's OK to hang out with the gibbons, but not so cool to bother the lions. How they learn this is a mystery.