Considering the term "Heritage Poultry" is currently under hot debate, I'd say they are if you want them to be.
That's about how much weight the terminology carries these days!
There are a lot of breeds very deserving of the term, IMO, that do not fit the one definition that has been laid forth by the American Livestock Breeds Conservancy, which, though admirable in many ways, is not an authority on poultry terminology or "branding". No more relavant organization has yet granted their approval of a definition, so, IMO, it's fair game.
Do it justice. If you feel Welsummers have a legitimate right to be dubbed heritage, than so be it. Do your research, and defend their title as such.
Heritage is linked to country of origin.
All the breeds mentioned do not have their origin in the USA.
Welsummers & Barnevelders are of Dutch origin, and are heritage breeders in Holland.
Marans are French, same applies.
Thus none of them are American heritage breeds.
Well, that would be a starting point for an American Heritage breed, but that is not what the OP asked.
There are many breeds that fit the ALBC heritage criteria, which most seem to go back to, that do not have their origins in America.
Which is why I go back to the fact that everyone has a different idea of what Heritage poultry means, so just go with whatever it means TO YOU since there is no more concrete definition in this context yet.
If we're going to start yet ANOTHER debate on heritage definition. . . .
The term heritage signifies being far back towards the beginnings/roots. In the beginnings of our country, at least (
) - the ALBC's qualifications that list breeds before 1950 is correct. In fact, anything before 1940 to me is heritage, as before 1940, raising chickens was about living sustainably, not having egg laying machines or miracle-gro meat producers. Regardless of what is considered heritage in terms of looks, lineage, etc - Still, the breed must actually be an "original." Yes, many did not originate in America, but in the old days, what DID?
In another country, yes, Welsummers may easily be considered heritage.