Hybrids between mallard derivatives and graylags occur, and hybrids between mallatd derivatives and swan geese have been reported. Can not find my copy of Gray (1958. Bird Hybrids) right now, but seem to remember swan goose mallard hybrids. Yashima (1953) is cited in McCarthy. Handbook of avian hybrids of the world, as have 38 eggs of swan goosexmallard parentage....24 were fertile, 12 died within 2 days, 1 survived a week, and 1 survived, but had a deformed bill. Incubation was listed as 28 days.
Clint