Anyone who kills a bird whether intentionally or otherwise. For example; a friend of mine hit a bird today.. and instead of calling her a "bird murderer" in the text that I was typing to her, I consolidated and threw "birderer" at her. I actually think it cheered her bird-killing self up a bit.
girl driving car: oh crap i think i just hit that bird
guy passenger: wow it's not moving, I think you killed it... YOU BIRDERER