Others can probably add to this, but this is what I know regarding purchasing baby chicks in the US:What I don't understand is why buying chicks would pose less of a risk of introducing a disease, than adult chickens. Sure, the chicks will be kept separated from the adults for at least six weeks, which is more or less the same as what many folks on BYC do for quarantine. But if they are carrier of a disease that doesn't reveal itself in that time like Marek's, avian leukosis, IB or IL could do, they would still introduce it just like adults would ? Or am I missing something ?
Since large hatcheries incubate eggs separately from the flock and in controlled environments (i.e. different people work the incubators, so they are physically removed from the flock/isolated, and use biosecurity measures), there isn't the cross contamination with chicks hatched in flock/with a broody or with a backyard flock where they aren't kept separate with biosecurity measures. So, chicks shouldn't transmit/carry flock diseases (unless they are genetic OR transmissible through the egg/shell).
The other reason it is really common to buy/ship day old chicks in the US is convenience. Since chicks absorb the remaining yolk just before hatching, they can live a few days with no need for food or water - so we exploit nature (I assume that this happens so that chicks can survive until all are hatched and Momma can take the whole clutch off the nest to search for food and water.). This allows them to be shipped all across the country with minimal issues - sometimes a heating pack is added - especially in colder months, and they add a gel liquid (with electrolytes) for them to peck at, but otherwise, they generally travel fairly well as long as there are no delays in shipping.
Shipping older fowl (pullets, adults) required express shipping and more thoughtful travel arrangements - plus are more likely to also carry diseases with them that chicks would not yet be exposed to so aren't carriers (yet).
Besides, it means that as soon as chicks hatch, they are processed and shipped, so for many large hatcheries, they don't need much for brooders (only if they keep pullets for replacement or for later sale at a significant price bump). Otherwise, the 'extras' (mostly males) get, unfortunately, disposed of.
