It is probably best to eat only a bird you processed or had processed. Sure, if there were nothing else to eat and you happened across a dead bird, why not? (LOL. To many survival shows off Discovery Channel
)
You do not want to eat any sick animals. Sometimes commercial producers "push" sick animals through, but they are not suppose to. Sick animals are not on the menu. Even commercial plants for beef and such, do not eat cows that die on their own(even if it is just minutes away from being killed).
You do not want to eat a chicken that has been killed by a predator. You do not know if the predators was healthy or carrying something that could pass onto you. Yes, cooking can kill bacteria and whatnot, but do you
really want to risk eating a chicken killed by a fox or coon or anything else?
A bird that dies of old age, probably isn't going to be very good to eat. LOL. Most birds are considered tough after their first year of life, can you imagine how tough they would be after dying on their own?
If the chicken is dead because of some kind of impact(getting hit by car, stomped by horse, etc) then I could see it being safe to eat. I would not eat it, if you don't know exactly how long it had been dead.
Honestly, I probably wouldn't gamble it. There may be exceptions. Like my friend had some really flighty birds, so flighty that when she entered the coop one day(she's old and doesn't move quick either) that two cockerels flew into a roost and killed themselves. I would have eaten the cockerels had they been a decent size. I would NOT eat a bird that I just found laying in the coop dead.
Just my thoughts on the matter. When it comes to ingesting things, it is better safe than sorry.
-Kim