In the seagull attack scene, coming back across the bay by boat, Melanie watches Mitch drive his car around the edge of the bay to meet her. She looks amused and smug—but then, she deliberately changes her expression to one of innocence and confusion. As she does, she tilts her head like a bird. And, at that moment, a seagull flies out of the sky and strikes her forehead, bloodying it.
One critic suggested that this is Hitchcock's twisted version of getting hit by Cupid's arrow. (Source) Because Melanie is so unlikable at this point, she gets pecked by a gull instead. Is this attack retribution for Melanie's attitude and behavior, suggesting that the rest of the violence in the film can be read as just deserts for everyone? Or, is this explanation just Hitch's MacGuffin—the thing that gets the audience's attention but is ultimately meaningless?
CATHY: He has a client now who shot his wife in the head six times. Six times! Can you imagine it? I mean, even twice would be overdoing it, don't you think?
MELANIE: Why did he shoot her?
MITCH: He was watching a ballgame on television.
MITCH: His wife changed the channel.
Birds aren't the only perpetrators of unprovoked violence in this story. Violence in The Birds doesn't have a reason; it just happens, and you have to deal with the trauma (and the feathers).
MOTHER IN DINER: [to Melanie] Why are they doing this? Why are they doing this? They said when you got here, the whole thing started. Who are you? What are you? Where did you come from? I think you're the cause of all of this. I think you're evil. EVIL!
Is Melanie the precipitating cause of violence? The early plot of the film is structured in a way that suggests that the violence does start with her and spreads to the people she's relating to. It implies that her sexual pursuit of Mitch gets things rolling. And, after her attack, the birds seem temporarily pacified. But, there's other evidence to make us discount that theory and conclude that things have been completely random. And that's even scarier.
MELANIE: [watching a man lighting his cigar as gasoline leaks around him] Look at the gas, that man's lighting a cigar!
Melanie is in the position of you, the viewer; she's watching in horror as something horrible unfolds without being able to do anything about it. That's a classic Hitchcock suspense-building move: he gives us info that the victim is unaware of so that we have to watch helplessly to see what happens. As Hitch once said, "There's no terror in the bang, only in the anticipation of it." (Source)
CATHY: Mitch, why are they doing this, the birds?
MITCH: We don't know, honey.
CATHY: Why are they trying to kill people?
MITCH: I wish I could say.
Explanations are comforting, but even big brother Mitch doesn't have any. If they knew why the birds were attacking, they could avoid doing whatever is provoking them, whether it's flirting, wearing fur, or ruining the environment. In Shmoop's opinion, waiting for the violence to occur in this film is scarier than seeing it. The bird attacks are horrible, but watching the crows massing on that school jungle gym? We can't even.