A swarm is usually because the hive has become overcrowded and to make space it splits - the old queen and roughly half of the bees leaving. Those staying behind have raised a new queen or queens. The first queen to hatch will kill those who haven't. This virgin queen will need to make a mating flight at which point she hopefully returns to begin a career of egg laying in the hive. And... the cycle continues.
As mentioned a feral hive that has lasted several or more years is a rarity in this day and time. The varroa mite has been deadly to hives causing the collapse of many colonies.
If the ones you have are true survivors and not different colonies occupying the same space (tree) over time they are a treasure, as that would imply they had a resistance to the conditions that might cause a colony to collapse.
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