The problem is ranging or free ranging birds have to deal with a greater threat from predators and this as one might expect reduces the average lifespan considerably.
Do you have any sensible stats for that expectation? For what it's worth, now into my 7th year of free ranging, the birds that get or are assumed predated here these days (and there are not many) are youngsters, not the older birds. Averages can be very misleading.
Inexperienced birds - which youngsters are by definition - are more likely to get predated than experienced ones, and this is not sufficiently recognized in most studies I've seen. Most experiments I've come across are far too short term, and often incapable by design of giving realistic results. A chick hatched in an incubator and raised by machines in a sterile environment does not behave like a chick raised by a broody free-ranging from hatch.
Anyone who starts free-ranging experiences losses from predation. Some keepers respond by promptly bringing their birds back into the protection of captivity, and assume that the losses they suffered are typical and that they would continue at that rate until all birds are gone. Many posters on BYC are exemplars of that. I lost my first 6 birds to the foxes within the year.
But keepers who persevere, and who analyse the losses - letting the chickens roost in trees was my mistake - and fix the issue(s) identified - make them sleep in the coops in my case, together with some research on foxes and methods of their deterrence - find that, with experience, the birds get better at predator detection and evasion, just as they get better at foraging and other natural behaviours. And they pass this knowledge from experience on to the chicks raised amongst them.
A proper study of the life expectancy of free-range versus confined chickens should compare the data of a long-established multi-generational free ranging flock - not one of identical clones put together ex nihilo for the purpose of the experiment - with those from a variety of confined type flocks. I don't bet, but I'd wager the losses from disease among the latter would outnumber the losses from predation among the former. And there may be some very old birds among the latter, but I'd wager the general health of the flock was better among the former. Further, it might not happen during the experiment, but it does happen in real life, that some flocks in confinement face catastrophic losses when a predator gets into their 'secure' coop. Perhaps an actuary's approach would be appropriate?
I'm in full agreement with Perris on the feeding of whole grains and forage. Where we diverge is the matter of fermentaion. There are circumstances where fermentation of whole grains may be of benefit, but there are others where it could be downright unhealthy. More about this topic in my almost completed article.
