Chicken or quail have black or dark red spots on the meat

Antibiotics might be a reason, but it's not the only cause. Other significant reasons are:

• exercise
• feed
• post-processing

Most of the chicken available in grocery stores in the U.S. is factory raised ... they're bulked up as quickly as possible without threat of predators. They're fed corn, rather than their acting as pest control on farms, where they'd be eating insects, moving about (in their search for insects and other things to eat), and possibly running away from predators (getting more exercise). That's not to say that free roaming chickens wouldn't be fed corn or other processed feed (or even antiobiotics), but that they'd still have an opportunity for other food. The lack of predators means that chickens never have to fly, so they don't need fast-twitch breast muscle, and they can grow to a size where they'd never be able to fly, even for short flight to excape predators.

Also not common in the U.S. are old chickens for stewing ... we have large chickens, but not necessarily the old ones, such as formerly egg-laying chickens that are no longer producing eggs. I have no idea what's done with those ... they're not in grocery stores, so I assume that they're used in some other way (dog food?).

One other possibility is the gender of the chicken. I don't believe that males are raised for meat (or anything else, really, other than in token amounts to sustain the species) in the U.S.. I assume that a higher percentage of roosters would be produced in other countries, but I have no idea if they might be culled early (if there are agression issues, etc, that would make them difficult to raise), like male cows are.

And as for the post-processing comment ... much of the U.S. chicken is sold cut up, possibly with a brine solution injected, rather than being sold whole. This doesn't seem like a big deal, other than the possibility of the brining, but it also means that chickens have been selectively bred for breast meat, rather than whole carcass weight. It also means that most of what we're eating is white meat, rather than a mix of white & dark meat.

Your chicken is fully cooked after simmering for an hour. The red bits are from the bone marrow and don't indicate that it's undercooked. You'll see the same thing if you roast a chicken, and you'll notice that the red bits are always in the meat surrounding bone joints.