Is that a rare, seldom-seen Madagascan white goose? Perhaps it nests in the baobab in the background...
The enclosures for the black and white ruffed lemurs, red ruffed lemurs and ring-tailed lemurs all have moats at the front, and are shared with various waterfowl. The ring-tailed lemurs even have a black swan and a mute swan in with them (apparently, the mute swan is a wild one who settled there one day).