To do what the phrase says accurately could require a lot of birds, many generations, multiple enclosures, identical conditions, uniform exposure, a control, and a reliable source of the same strain of pathogen for many years. There would be a lot of test mating to demonstrate the heritability of resistance.
Or, just a lot of birds in one flock over enough years and generations, living on the same soil, with disease exposure regularly occurring via exposure to new animals, and carriers who shed remaining present at all times, and all offspring being continually exposed to those birds. No need for 'test matings' per se, as every mating is a test mating under such conditions. (Note that what I've described there is the same sort of mongrel flock I was referring to before).
Resistance also is used interchangeably with immunity, which would be misleading. Just how resistant is resistant?
We're talking about resistant enough to not succumb to it, never mind die from it.
'Remains for all intents and purposes healthy despite infection' kind of resistance. You may have observed if you read earlier posts in this thread (don't know if you did) that we all agreed that birds who became sick but recovered were not really what we consider 'resistant'.
I don't think anyone on this thread intentionally referred to resistance and immunity as being the same thing; to my way of thinking, immunity would be the term used for a bird that never even carried the disease despite being exposed to potential infection, whereas resistant birds could carry it but wouldn't become ill with it.
I used land race breeds to your credit. They best illustrate your point.
I don't believe so. I'm referring to birds being exposed to new diseases regularly in ways that only birds under domesticity can be, whereas landrace birds are not the same thing as they are not domesticated... At least in my sense of the term. Perhaps it means other things to you.
A landrace bird is one completely left to its own devices, which includes not having new birds from other regions regularly introduced to its territory; a mongrel is a domestic bird whose environment is continually subject to human influence. Being left to choose its mates does not make a bird a landrace. Landrace animals are not being exposed regularly to the new strains of disease all domestic birds are, simply because in order to be truly landraces, they must be living outside of human interference, and pretty much as a rule they're found in remote areas.
Feral domestics are somewhat inbetween the two, but as with mongrels, they too must have greater overall resistance to withstand being exposed to the domestic hubs of continually evolving diseases.
They would be more uniformly tolerant of local conditions than other birds would be. I have Thai game fowl mixes in Thailand, birds in tropical Africa etc., in mind. Birds in tropical areas that are barely managed and disease pressures are high.
I believe when you refer to 'landraces' here, the term is actually being used to refer to feral domestics or mongrels, no?
Anyway, this conversation has derailed into confusion over the meaning of the term 'mongrel', which is not really constructive beyond a certain point given the thread topic. Probably not much more to be said here. Best wishes.