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Certain hybrids do produce more than either parent. Breeding the hybrids to each other produces chicks with various rearrangements of the genes, and some of those will not lay as well as their hybrid mothers did. Inbreeding can cause problems with reproduction (fewer eggs, poor fertility, poor...

At least in chickens, yes that is the main trait. I don't know for sure about ducks, but it seems likely to work with them too.