I found this in the manuel:
Collision with a surface of the level during a MOVE instruction. The entity must not be PASSABLE and it's PUSH value must be 0 or less. On start of the EVENT function, the vector NORMAL is set to a direction perpendicular to that surface, and the vector BOUNCE is set to the direction into which the entity would bounce off.

So double check your push values, my.push for the bird should have a 0 or less value