I couldn't find any references on the toxicity of mulberries for ducks or birds in general. ASCAP lists them as non-toxic to horses, cats, and dogs.
http://www.aspca.org/Pet-care/poison-control/Plants/mulberry-tree.aspx
Based on the lack of information and the prevalence of mulberries in...
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Onions and garlic are dangerous if birds too eat much of it. A little bit won't hurt them, but should be avoided whenever possible.
Onions and garlic are part of the lily family of plants (includes chives, shallots, etc.) contain sulfur compounds that when metabolized can break down...
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Very good point. This is especially true for galvanized steel which is coated in zinc (toxic to birds) and should not be used at all regardless of whether or not you give them vinegar or any acidic foods like tomatoes.