Attack #3: Annie’s door hit in a gull suicide dive at 47:18.
Melanie has been implored by Cathy not to abandon her as she celebrates turning eleven. Melanie has also witnessed Lydia’s cool disapproval of this invitation. Returning to Annie's, Melanie has the following exchange:
Melanie: Do you think I should go?
Annie: Well, that's up to you.
Melanie: No, it's really up to Lydia, isn't it?
Annie: Never mind Lydia. Do you want to go?
Melanie: Yes.
Annie: Then go.
Melanie: Thank you, Annie.
Immediately after Melanie's reassured "Thank you", a seagull crashes into Annie’s door killing itself. A seagull: an unconscious reaction of Lydia-Mother to a conscious observation - a threat inspired by Annie in Annie’s home.
Yes, Lydia knew Melanie was staying at Annie’s. Lydia knew that Cathy wanted Melanie at her birthday party, but - up until that very second - Lydia did not know Annie would oppose Lydia’s wishes by dismissing Lydia & supporting Melanie.
How did Lydia know the contents of Melanie’s discussion with Annie? Through the eyes of Annie. She, like Mitch has already been dominated by Lydia. Annie explains:
Annie: You know, her attitude nearly drove me crazy. When I got back to San Francisco, I
spent days trying to figure out what I'd done to displease her.
Melanie: Well, what had you done?
Annie: Nothing. I simply existed. So what's the answer? A jealous woman, right? A
clinging, possessive mother? Wrong. With all due respect to Oedipus, I don't
think that was the case.
Melanie: Then what was it?
Annie: Lydia liked me. That's the strange part. Now that I'm no longer a threat, we're very
good friends.
In the vein of Norman Bates, one has to wonder if Annie has not - in fact - gone a little crazy. Leaving her life in San Francisco to Bodega Bay which Annie describes thus:
There's a lot of spare time in Bodega Bay.
and again
I suppose it [Bodega Bay] doesn't offer much to the casual visitor, unless you're thrilled by a collection of shacks on a hillside.
The remarkably strange part is that Annie never explains how she became “no longer a threat” to Lydia. But we do know the when:
Annie: I was seeing a lot of him [Mitch] in San Francisco. One weekend, he invited me
up to meet Lydia.
Melanie: When was this?
Annie: Oh, four years ago, shortly after his father died.
And we do know the final effect on Annie:
Actually, I'm an open book, I'm afraid. Or, rather, a closed one.
In the mind of Lydia, one can see the order of events. Once Frank was gone, Lydia assigned Frank’s role to Mitch. Lydia then has Mitch bring Annie up to subjugate her as Annie would eventually be the reason for Mitch to abandon Lydia.
And in the meantime, Annie can be useful caring for the one person Lydia cannot leech strength from: her daughter Cathy.
Lydia doesn't bother with her kids' needs:
I don't fuss and fret about my children.
Being only a taker, Lydia left the role of giving to Frank.
When Frank died... You see, he understood the children. He really understood them. He had the knack of entering into their world, of becoming part of them.
But with Frank suddenly gone & Mitch assigned to Lydia, Mother needed someone to dump Cathy on. Rival for Mitch's love no more, Annie would enter into Cathy's world in place of Frank.
Now, serving as Lydia’s extra eyes, Lydia learns of Annie’s betrayal through Annie just as she learned of Mitch’s sudden interest in Melanie through Mitch. Lydia’s disapproval was swift and implies a terrible price for Annie. One symbolically & literally brought to Annie’s front door.
Attack #4: Cathy Brenner’s hair clipped during her party at 52:00.
Now comes the party. Melanie & Mitch go off alone and have a bonding conversation about Melanie & her pains over her mother. Mitch even tells her outright:
You need a mother's care, my child.
As though addressing Melanie’s inner-eleven year old, Mitch calls her “child”. (It is creepy to consider what role Mitch may be playing in acquainting Melanie with Lydia.)
Returning to the party, Lydia sees the two, deduces what’s transpired & gazes disapprovingly.
Not a second or so later a seagull swoops down and hits Cathy, almost in the exact spot Melanie was hit on the bay.
Why would Lydia lash out against her own daughter? Technically, she isn’t. Cathy is a symbol for who Melanie truly is: a little girl in need of a mother’s care. A role of giving-mother Lydia wants no part of.
This is a problem as Lydia, with her history of leeching off others' strengths, does not want to be a giver.
Only seagulls attack and no real damage is done except to cause the birthday party - the event facilitating the bonding between Mitch & Melanie that Lydia does not want - to come to a screeching halt. Once the party is cancelled, Melanie has no reason to remain as Lydia points out a bit later:
Hurry up with yours, Mitch. I'm sure Miss Daniels wants to be on her way.
As has been stated, the lovebirds are symbols for Lydia & Melanie. Bearing in mind that Lydia needs the strength of others to sponge off of & the fact that Mitch is gone five days a week, consider the following desires of Mitch at the beginning of the film:
Mitch: Well, these [the lovebirds] are for my sister for her birthday, and as she's
only gonna be 11, I wouldn't want a pair of birds... that were... too
demonstrative.
Melanie: I understand completely.
Mitch: At the same time, I wouldn't want them to be too aloof.
Melanie: No, of course not.
Mitch: Do you happen to have a pair of birds that are... just friendly?
Is Mitch unconsciously recruiting for Lydia's need? A friend neither too involved not too aloof?
It is a screwy proposition to think about if one looks no further than the characters as people. As metaphors, though, a whole new way of perceiving these events emerges quite clearly. So clearly, that to dismiss it entirely as coincidence becomes unreasonable.