I've noticed many young chickens that do not make it to maturity to lay eggs are hatchery stock with vaccination. Of course, some of the chickens from backyard/professional breeders don't make it, but in my experience I lose way more hatchery birds vs non-vaccinated backyard chickens. Is my assumption more or less correct?
I have raised quite a lot of chicks from hatcheries (hundreds of them, spread out over a number of decades.)
I have also raised quite a lot of chicks that I hatched in an incubator (again, hundreds of them, spread out over a number of decades.)
For both groups, after they reached a few days old, if they died before laying age, it was almost always because I killed them to eat. A rare few were caught by predators. There have been no obvious differences between the groups in ability to live to laying age (or equivalent age in males, if I hadn't killed them myself).
To the best of my knowledge, I have never owned a vaccinated chicken. So that is the big missing point in my information. (When I started raising chickens, I never heard of an option to get chicks vaccinated. When it was available later, I already had a history of unvaccinated chicks doing just fine, so I saw no reason to do anything different.)
My theory is that, for the most part, birds from a breeder are going to be more hardy because of genetics. Hatcheries are going to be less selective with their breeding stock because they are breeding huge volumes, we're talking thousands of chicks of the most popular breeds with each weekly hatch. Breeders are hatching on a significantly smaller scale and are choosing each individual bird in their breeding program, they're going to weed out the inferior birds before they get a chance to pass their genes on.
That's interesting, because I have a theory that is almost exactly the opposite: hatcheries will refuse to deal with any bird that needs special care, so they will just cull any bird with any kind of health problem. A breeder will be more motivated to breed from a chicken that is exceptional in some way, even if it is slightly less healthy.
So I would expect hatchery birds to be uniformly resistant to everything they can encounter in the hatchery breeding flock, but birds from private breeders will be variable depending on what things were important to that particular breeder.
But I would expect hatchery breeder flocks to be protected from exposure to some kinds of diseases, and extreme weather conditions, so there would be no selection there. That could leave the birds from private breeders as the only ones with any selection for tolerating those traits.
Breeders are hatching on a significantly smaller scale and are choosing each individual bird in their breeding program, they're going to weed out the inferior birds before they get a chance to pass their genes on.
By the time they weed out the ones with inferior color, body size, body shape, feathering, temperament, and various other traits, they may not have many birds left to choose from. Those other points are commonly criticized in chickens from hatcheries.
