Mallard

The bill of a female American Black Duck is yellow with a black tip. My best guess it is a female Mallard. You know it could be a hybrid with an American Black Duck, but since the females are so similar you just can't really tell. Mysteries.

dferris ,could be a mallard.

http://en.wikipedia.org/wiki/Speculum_feathers

I think it is a Mallard, though it could also be a black duck. Would be easier to ID with a clear view of the Speculum

Does not look like a Mallard hen. cute pic!