American Black Duck x Mallard hybrid, Ottawa River, Ottawa (Ontario, Canada), 7th April 2016 - copyright Gordon Johnston(photo ID: 2787)
The first bird shown here is perhaps as obviously intermediate as any on this page though Gordon pointed out the cheeks are darker than on some and it shows a dark crescent in the cheek area. It seems that both characteristics are seen on this hybrid from time to time - a few examples can be found when searching through images online.
Black Ducks are a declining species in North America and at least part of the problem is linked to hybridisation with Mallard. Steve Mlodinow tells us that at Everett (where he photographed the next one below), Black Ducks were introduced in around 1970 and initially the populating grew to hundreds. Subsequently hybridisation with Mallards became so prevalent that in the end there were no pure Black Ducks remaining.
For the bird below the overall dark plumage and somewhat dusky tail indicate the presence of American Black Duck genes.
American Black Duck x Mallard hybrid, Everett (Washington, USA), 18th December 2008 - copyright Steve Mlodinow
(photo IDs: 1415-1416)
American Black Duck x Mallard hybrid, Everett (Washington, USA), 10th May 2009 - copyright Steve Mlodinow
(photo IDs: 1417-1418)
The next one's a female but the darkish tail and overall plumage seems to indicate some Black Duck genes.
American Black Duck x Mallard hybrid, Everett (Washington, USA), 21st October 2010 - copyright Steve Mlodinow
(photo ID: 1419)
Cathy notes on the next bird say,
"The light was dim, but this bird still seemed much darker and less patterned than any of the Mallard x Mexican Ducks that I have seen. The speculum also seems to have very restricted white around it. T ail was a bit 'brighter' than might be expected, but still think most traits point to American Black Duck as one parent."
American Black Duck x Mallard hybrid, South Platte River, Adams County (Colorado, USA), 29th November 2014 - copyright Cathy Sheeter
(photo IDs: 1837-1838)
American Black Duck x Mallard hybrid, Spruce Run Reservoir (New Jersey, USA), 27th May 2015 - copyright Cathy Sheeter
(photo ID: 2324)
Steve writes about the next bird that it could easily be mistaken for a pure Black Duck (but notes partially curled tail feathers and warmer brown sides). He also points out that Mexican Duck might look much like this bird but would also lack curl to the tail feathers, would have a blue (not purple) speculum and would have internal markings on the wings and scapulars. A Mexican Duck x Mallard would also have those internal markings and would likely look paler. Finally, Mexican Ducks usually show a darker/warmer brown chest contrasting with remainder of sides.
Steve also notes that the white edges to the speculum were narrower than it appears on the second photo below.
American Black Duck x Mallard hybrid, Firestone Gravel Pits, Weld County (Colorado, USA), 20th November 2013 - copyright Steve Mlodinow
(photo IDs: 1138-1139)
The next birds are rather close to Black Duck in appearance but there is a hint of green in the heads and the flight shot shows clear white borders to the speculum.
American Black Duck x Mallard hybrids, Swoope (Virginia, USA), 24th December 2013 - copyright Marshall Faintich
(photo IDs: 3168-3171)
American Black Duck Anas rubipres
Mallard Anas platyrhynchos