Anyone consider that the birds being purebred, are just a little weaker genetically?
Like with overbred anything.
No I personally don't. I've read up a lot on line breeding, inbreeding, bringing in new blood for the same breed, pure bred. Crosses result in what they call 'hybrid vigor' ie sexlinks can give a better bird for that particular generation, and then a lesser result after. Some hybrids or crosses have problems of their own, cornishx have all sorts of health problems.
They are chickens, not dogs or horses or humans. In the wild they would have a limited genetic base. I've read pure bred lines of birds such as RIR can be ruined when crossed with someone else's line. I can't remember his name right now but their is a line of RIR that has been line bred for a hundred years and his are tops for quick feathering and growth and egglaying ability.

