Dave is correct with his identification, though another common name for this species is Australian white-eye and that how the St. Louis Zoo's Bird Department has them inventoried. There are a total of three males of the species on the Zoo's south lake; each part of the Zoo's collection and not escapees/wild fly-ins.
The male Ring-necked duck (a wild bird) does prefer the company of the white-eyes. In fact, the Ring-necked duck remains at the Zoo all year and has for the past three years, though he could fly away at any time, and occassionally does spend parts of some days on other lakes at the Zoo via flight. Free food is too good to pass up, I suppose.
Matt Schamberger

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