Bird-brain is flying troop. He can fly over you with a big speed. Bird-brain is quite big, so despite that he fly high above the ground, you shouldn't have any troubles in killing this alien - just strafe to avoid his fire. He is shooting with energy blasts, and he can punch you when you get too close to him. Both methods are harmful, and makes big damage to you, so try to kill him fast.
Sphere of Death
This creature is like a guard. It is very accurate, so it can hit you even from a very long distance. It looks like a little circular flying robot and it is not so easy to kill. The easiest method to destroy it is to use sniper mode, aim, and shoot quickly, to avoid being hit by its energy blast. A few shots should be enough to destroy it. In group Spheres of Death can be dangerous, so don't let them surround you.