In my notes I've been referring to these as "nests"
Actually though, the patch system won't create more critters when they wander out of the patch area because it doesn't determine the amount present by counting the number of critters in the area-- it counts the number of critters in the world that have spawned from that patch-- they are identified by an ID number stored in the spawned agent.
In most plants, I have them set to clear that ID number when the fruit hits the ground so they are no longer counted by the patch (as noted in the annotated chilis cos guide thing), but if you don't clear it, there's no reason the spawned critter can't go wherever it wants and still be counted as part of the patch.
The "max population" for any patch is calculated by dividing the height/width of the patch box by the size of the fully grown critter (the size of the image you define in NAME "sprite_file"/"sprite_pose"), and then multiplied by density as a percentage. So while a little complicated, it's still perfectly functional to use the patch system for critters too. In beta I was screwing with an aquamites patch and it functions rather nicely.
That said, while 'nests' are still technically functional as is, it would
make things clearer to the user and less clunky for critters to have their own system. I can see it being annoying if you want a large population but the room has a lot of walls that won't allow you to draw a large patch-box, and a set max-population variable would be a little more definite. It would also be less resource-intensive since the critter-spawning system wouldn't have to worry about placing objects so they don't overlap each other. It's going to be one of those things I'll probably put in if enough people nag me about it
Either way, I would love to see some C2 conversions!