When you encounter this bird it is passive unless you take its egg. It's battle attacks are a spin attack and it does the most damage. The other one is a bite attack at it's not as effective. So when in battle do a spin attack to beat your foes.
These things are vicious when you steal their eggs. One time, I did so, and I just see this oversized death seagull flying at me and it pecked at me until I succumbed to my wounds. If you have the proper equipment these things are fairly easy to kill.