Mallards are one of the few species that can mate with a different species and produce hybrid offspring that are fertile, and capable of reproducing. Other animals can produce hybrids, such as breeding a horse with a donkey to produce a mule, but those offspring are usually sterile, and cannot reproduce. Mallards have been known to hybridize with American Black Ducks, Northern Pintail Ducks, Pacific Black Ducks, Gadwalls, Spot-Billed Ducks, and a variety of Teal, not to mention a wide assortment of domestic ducks. This mixing of the gene pool can make duck identification quite a challenge in some areas.