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HC wrote:
Are you sure you aren't confusing Muscovies with ducks? Muskies
are
genetically, a goose.

I have heard that before, and certainly Muscovies are very
goose-like
(they hiss rather than quack). But they cross-breed with other
ducks,
so I guess that makes them the same species?

Horses and donkeys interbreed and they are the same species being both
from the equidae family. The offspring (mules and hinnies) are
(almost) always sterile. I'm fairly sure that the same "species" rule
applies to both Muscovies and ducks and I know that the sterility bit
does. I've yet to hear of the offspring of the duck/muscovy mating
being fertile.