I guess the point I'm trying to make wasn't perfectly clear. My island base wasn't particularly big, but it had deep water surrounding it. In other words, the area guarded by the workbenches etc was not big enough to cover the entire area where the invasion mobs are spawned. Since no flying mobs showed up, I assume it must have been the water that prevented the spawns. Someone who has studied the source code for modding could probably verify or debunk this observation, but based on my experience it seems true.