Unfortunately, this is a common response from them... or that they don't have the capability of caring for a single baby... taking in a mama and her brood is much easier as she does most of the work caring for the babies... whether the mama duck was around or not is moot at this point, this...
Definitely not a mallard... does look like a wood duck to me, but I'm not very familiar with them... maybe @attimus could shed some light on the subject? :)