Prey not Spawning?

[Oblivion]

So, I have several quests. Most of them, though, are not spawning.
The prey that won't spawn are Bats(all kinds), Hummingbirds, Vultures, Dire Rays and Ravens.
Is there a reason these enemies are not spawning?

[Dutchval]

I know for a fact that Dire Rays are rarer than the typical spawn. Hummingbirds should be relatively common to my knowledge. I can't say anything about the others. I'll be more certain once I can get on my main PC later today xD