So i'm wondering are you guys just guessing your flocks are disease resistant when you are culling for only visual symptoms, or if you get your flocks tested to find out of they are truly resistant
No, I don't have birds tested. For one thing I don't know if there are tests available for all common illnesses. For another I'm sure it would be prohibitively costly.
At the same time I'm not guessing. My data is I don't often get sick birds & I do lots of things people here tell me I shouldn't. I go to shows & don't isolate birds on their return. I visit other farms & always welcome visitors to mine. On those rare occasions when I do bring a new bird or birds in they go directly in the existing flocks.
This is not a one generation process. When I first began this practice I culled a number of birds. Each year the number became fewer & now a sick bird is a rare thing.