Blueprint "bird" actors get deleted in builded game

Hi everyone!

In my game I created birds flying around in my level. In the editor everything works fine.
However when I build the game the birds are not there anymore. Even in “Standalone-game” they work fine.
When I launch the level they are not there either.

The bird blueprint is just an actor blueprint with a CustomMesh and my own “flying AI”.
The birds choose a point (actor aswell) to fly to and when they reach it they choose another random actor of the class AI_Node.
So there is no real way they flew out of the level or anything because all those actors are visible. They don’t have collision so they can’t get stuck behind somewhere.
If I look into my actor blueprint to the defaults there is nothing that I changed, and I can’t see something strange about it. (mesh is set to visible, etc.)
I am still working in the 4.5 engine.

Did anybody have this problem before? Am I doing something wrong while building?
How can I maybe check if the birds are in my build but are just bugged out or something?

Thank you,
Elias Jacobs