The life span of ALL animals is increased slightly when in captivity as compared to in nature.. as stated.. predation.. but also parasites, nutrition, and ambient temperature.. all tax the immune system IF out of whack.. Natural food sources depend on the weather and vary by season.. which also impacts breeding AND production of animals.. USUALLY decreasing both during the more sparse nutrition that is MANY places during winter.. (My bug and plant populations both change throughout the seasons.. but my FEED stays the same).. The animals adjust THEIR intake according to their energy/nutrient need.. when things are consistent.. NONE of my birds PREFER lower protein feeds.. THIS is WHY gradual transition is recommended for pet food. Mix 25/75 to start, then 50/50, next 75/25 eventually transition is complete.. .. to adjust the taste buds AND expectations of the creature of habit and NOT because it MIGHT throw of digestion as many suggest despite being ALMOST the exact same ingredients. Obviously gradual transition may be less key if the feed being offered is higher in something excitable like fat, etc.. but when less desirable gradual helps a LOT!Maybe I should re word my post to .................
What is the lifespan of a free range chicken vs the lifespan of a factory fed chicken? [not counting culling for 'stewers']
MOST of us keep animals in better condition than those that are "wild".. ever seen a coyote with mange.. or deer with ticks.. nature is brutal, period! We also keep them BETTER than factory conditions.. which is NOT the same as using a formulated ration or what you are calling "factory fed".. those factory feeds ARE coming from FARMED plants.. they are technically being fed a factory formulated ration according to the avian nutrition and health experts.. "layer" feed.. is the factory MINIMUM for keeping a light bodied layer like Leghorn in laying condition.
To extend longevity.. a domestic chicken offered a formulated ration WILL live longer than a bird trying to live in free range inadequate conditions.. TRUE free range ONLY chickens.. NEED about an acre PER head according to my research.. and will never liver longer. We ALL want to keep chickens in "more natural" conditions.. but chickens as we know them today are NOT natural. They were selectively bred to put out a maximum number of eggs..
Nutrition and condition CAN effect egg production.. BUT unless SERIOUSLY out of whack.. most keepers will never notice the 10% or so reduction. Genetics plays huge key in everything production, longevity, even nutritional requirements!
Final statement.. it's ALWAYS genetics which *may* be (is) influenced by environmental conditions.. we do our best, according to our CURRENT understanding.. along with OUR goals.
Those of us (IN the know) who DO "free range" or "pasture raise" our DOMESTIC birds.. do so in ADDITION to feeding an age/gender/species APPROPRIATE formulated ration.. not instead of. And THAT my chicken friend is HOW you achieve maximum life span.. Which is hugely key to some of us who have invested YEARS and generations of collection and selective breeding not to mention cost of purchase, housing etc.. Giving my birds anything less then the absolute best is doing MYSELF a disservice..
I prefer a flock raiser.. with oyster free choice on the side ALONG with pasture ranging.. it DOES not increase production OR life span.. again, I contend THAT is genetic.
I have NOT lost a bird on lock up using a formulated ration ever.. I have lost several birds free ranging to aerial predators. I accept the risk and the cost most of the time but go on lock up during times of heavy predator load like during early spring/late fall hawk migration. During these times.. I like to sprout or fodder barley, wheat, etc.. as enrichment and a treat but NOT as a nutrition supplement.. idle minds are the Devil's playground and bored chickens are Stew Pidasso's.. reducing crowding and stress certainly impact hormones and stress levels.. increased stress levels ALWAYS decrease longevity. Regardless of which feed or ranging routine you use.. having adequate space, enrichment, and nutrients ALONG with proper husbandry (fresh air, not ammonia smelling) will ALWAYS give the "best" results.
Hope this is helpful, sorry if it rambles on!