Is this an existing object? If not....
Now, if you can destroy it, you will have to be able to hit it. Each object that you hit in bf has a thing called a "bounding box". That is the area which, if a projectile is with in that box, the damage system kicks in. Then an object has another mirror of its self (less detailed) called a collision mesh. In order for someone to stand on an object, it has to have this. Do you have a geometry for this circle of birds? even if its invisible, it needs something to detect its been hit.
http://battlefieldsingleplayer.planetba ... h_file.htmscroll down there is a thing on "Collision Meshes."