What is the difference between a natural Bat Swarm and one summoned by the spell besides it having a duration and concentration by the caster?
Do the summoned bats not behave as natural bats?
Are their primal instincts altered magically?
The summoned swarm acts as described by the spell:
You summon a swarm of bats, rats, or spiders (your choice), which attacks all other creatures within its area. (You may summon the swarm so that it shares the area of other creatures.) If no living creatures are within its area, the swarm attacks or pursues the nearest creature as best it can. The caster has no control over its target or direction of travel.
They attack all other creatures (including other swarms), and pursue nearby creatures.
A natural bat swarm acts as described by the monster entry:
Hundreds of high-pitched squeaks fill the air as a mass of small, carnivorous bats surges forth, all of them hungry for blood.
Organization: solitary, pair, flight (3–6 swarms), or colony (11–20 swarms)
Bat swarms dwell in large caves, ruins, or even city sewers—anywhere they can find darkness to hide in during the day and a supply of food to feast upon at night. They are only encountered outside in a group at dawn or dusk, or when they have been startled and forced to flee their lairs.
They are usually hostile to the PCs when encountered, but not to other swarms if they are part of the same colony. They may be more interested in escaping than attacking, depending on the situation they're encountered in.