One of the challenges of city birding is identifying the mixed up ducks not found in any field guide. These “mutt ducks” are the hybrids of mallards paired with escaped domestic ducks.
It’s easy for domestic ducks to hybridize with mallards because nearly all of them(*) are descended from mallards (Anas platyrhynchos).
Mallards hybridize with wild ducks, too, as shown in this a mallard X gadwall mix.
Some ornithologists worry that mallards will hybridize their closest relatives — American black ducks, Mexican ducks and mottled ducks — out of existence, as in this mallard X Mexican duck mix.
But perhaps they’re forgetting how recently those species evolved from mallards. The Mexican duck (Anas diazi) that occurs in Mexico and the U.S. Southwest was thought to be a subspecies of mallard until 1957.
Mallards are just working on creating new species. 😉
Read more about mixed up ducks in this vintage article: Ugly Ducks
(*) Some domestic ducks are descended from Muscovy ducks.
(photos from Wikimedia Commons; click the captions to see the originals)