Lydia is kind of a mess.
Her husband died several years before our story gets started, and she hasn't gotten over it (obviously). Because she's now clinging to Mitch, she's unhappy when there's another woman in his life. Annie believes that Lydia ruined her relationship with Mitch because she feared Mitch would leave her if he got married. Lydia is just as leery of Melanie; she makes sure to bring up Melanie's bad-girl history:
LYDIA: Forgive me. I suppose I'm just naturally curious about a girl like that. She's very rich, isn't she?
MITCH: I suppose so. Her father's part-owner of one of the big newspapers in San Francisco.
LYDIA: You'd think he could manage to keep her name out of print. She's always mentioned in the columns, Mitch.
MITCH: Yes, I know.
LYDIA: She is the one who jumped into that fountain in Rome last summer, isn't she?
MITCH: Yes, mother.
LYDIA: Perhaps I'm old-fashioned. I know it was supposed to be very warm there, Mitch, but ... well ... actually ... well, the newspaper said she was naked.
MITCH: Yes, I know, dear.
LYDIA: It's none of my business, of course, but when you bring a girl like that to—
MITCH: Darling? […] I think I can handle Melanie Daniels by myself.
After Lydia is traumatized by seeing the farmer dead and missing his eyes, Melanie tries to comfort her and bring her tea. By this point, Melanie has been drawn into the family by the bird attacks, and Lydia confides in her:
LYDIA: Mitch is important to me. I want to like whatever girl he chooses ... Well, I don't think it's going to matter very much to anyone but me ... Mitch has always done exactly what he wanted to do. But, you see, I don't want to be left alone. I don't think I could bear to be left alone. Oh, forgive me ... This business with the birds has upset me. I don't know what I'd do if Mitch weren't here ... I wish I was stronger.
We're more sympathetic to Lydia now that she's fessed up to her fears.
If Looks Could Kill
Remember the movie Carrie? When Carrie gets angry, bad stuff happens. Things crash around or spontaneously burst into flame. It's like her unconscious wishes cause this stuff to happen without her meaning them to.
Some film critics think the same about Lydia Brenner: Lydia's unnatural attachment to Mitch has somehow caused the bird attacks.
There seem to be two different views on the matter:
One targets Mitch's girlfriends as the object of Lydia's destructive wrath. Melanie gets attacked for the first time after sneaking into the Brenner house; Cathy's party is attacked because Cathy is the one who invited Melanie to stick around Bodega Bay for the party. As Melanie and Mitch get closer, the attacks increase in ferocity, and they only stop (or, uh, pause) when Melanie is totally destroyed. Even Annie gets killed.
By the end of the film, Melanie is more like a helpless child. Now, Lydia doesn't have to worry about losing Mitch. Once the birds have rendered Melanie senseless, Lydia can now be back in control. (Source)
The second view is a little more, well, Greek. Mitch and Lydia's relationship is too close—he calls her "darling," and she's very possessive. It has what psychoanalysts would call "Oedipal" overtones. As in Oedipus, the mythical Theban king who accidentally, and unbeknownst to him, killed his father and married his mother. This unnatural event disturbed the universe and brought plagues raining down on the country while Oedipus and his mother reigned as king and queen.
Applying this thinking to the birds, we see that it's not just Annie and Melanie who pay the price for Lydia's attachment to Mitch. It's not that the bird attacks are manifestations of Lydia's Carrie-ish anger. It's that the natural order of things has been disturbed—and everybody pays. (Source)