A mage has cast a 'Beast Friend' spell on a swarm of creatures -- say, a swarm of Razor Wings. The swarm then takes a wound and splits into two swarms. Does the mage now have control of all of the new swarms, or only one of the new swarms?
None of the above. When a swarm takes a Wound, it is Incapacitated (generally meaning that it disperses), just like any other Extra. Wounding a swarm doesn't split it. The description of swarms' "Split" ability reads:
Split: Some swarms are clever enough to split into two smaller swarms (Small Burst Templates) should their foes split up.
It says nothing about them splitting in response to damage.
If the player suggests to the swarm that it should split, then I would allow him to maintain control of both sub-swarms, under the following conditions:
- He spends an action coordinating the split and makes an arcane skill roll as if casting a spell normally.
- He pays 1 PP to make up the difference in cost between controlling a medium swarm (5 PP) and controlling two small swarms (3 PP x 2 = 6 PP total).
- Beast Control is thereafter counted as two maintained spells for purposes of penalties to arcane rolls, etc.
The end result is then identical regardless of whether he used Beast Friend once on a medium swarm and split it or used it twice on two separate small swarms - it costs 6 PP, takes up two actions, and counts as two maintained spells either way.