Craftland.org

Birds seem to naturally despawn after some time, even right infront of you.
I gave my ravens name tags and they still despawned. The bird spawner i has only ends up spawning one or two birds before the last set despawn and its just fustrating

I think one of the issues is that birds and ravens have a unique ai that can travel anywhere (in terms of height or "Y") - so it can get messy quickly where you have a giant flock of birds high up in the sky roaming around endlessly and unreachable.

So this is most likely a compromise/solution for that.
(So I guess a bird/raven spawner is more like a particle effect)