Mixing duck breeds...?

[KsTornado11]

Is it true that if you mix duck breeds,their offspring will be infertile?

 

[DuckLady]

Any mallard derived duck (all domestics except muscovy) can be mixed and it will not affect fertility.
If you cross a muscovy with a mallard derived duck, if female it will lay eggs, but they will never hatch. Males will be sterile.

 

[KsTornado11]

OK,that's what I thought. But I read someone on another board saying that mixing duck breeds would result in mules,didn't think that sounded right. Thanks!