How does birds are territorial explain why the birds are attacking?

The birds are acting out the territorial animosity engendered by the other "birds" in the story--the female characters. Melanie comes to the pet shop and, as a joke, poses as the shop clerk when the fluffy, hen-like woman in charge is out of sight. That's when the birds first start massing. Then she travels to Mitch's farm by
boat in order to snag him in her talons and is given a warning to go away by an aggressive seagull. Then she arrives at Annie's house and gets the background on
Mitch from her wary rival, Annie the mourning dove; their talk is interrupted by a seagull smashing into the door.

Melanie uses Cathy's birthday party to get close to Mitch; the birds attack again, with a few swipes at the kids so that the mothers can get the hint that maybe there's some weird connection between the attacks and the arrival of this
outsider who complains about her own unloving mother. Later, Melanie visits the school, presided over by her romantic rival Annie, the sole person in charge of the kids. The birds attack the kids when the two women team together. Melanie goes to the Brenner home to get close to Mitch and starts behaving maternally to Cathy, who has imprinted herself onto Melanie at first sight, as baby birds do; Lydia's unspoken suspicion and resentment of her is manifested by the sudden invasion of a flock of sparrows from the chimney and fireplace.

Lydia leaves Mitch and Melanie to speak to a nearby farmer; she discovers the farmer dead from a bird attack and races home, cutting short yet another conversation between the two. Later, Melanie goes to the diner, where she argues with the wise old owl Mrs. Bundy and attempts to contradict her on her ornithological knowledge. The attentive mother bird, meanwhile, is anxious to
protect her little nestlings, and after the subsequent bird attack, she pecks and squawks at Melanie, claiming that "they" (presumably some of the other women, who gossiped with some of the mothers at the party) said that Melanie
somehow caused the birds to act this way. Melanie strikes out at her with her claws, as birds will do in self-defense.

Annie, meanwhile, is looking after Cathy at her home; when the birds attack, she
sacrifices herself to protect the girl, as mother birds put themselves in the paths of predators to save their young. Melanie barricades herself at the Brenner house, and Melanie subliminally recognizes that this is all about her encroachment on the territory of the other bird life in town. That's why she
makes the foolish move of entering the bedroom; on some level, she recognizes that she has to be punished for her interference with the natural order of things that she's disrupted--the relationship among the other women. Her last words before what she thinks is her death mention Cathy, which reveals that she just wanted to be a good mama bird after all. Mitch helps the wounded Melanie by folding her flapping wings against her chest, and Lydia, in helping to take care of her, regains her chief maternal position in the pecking order and now can feel much more kindly towards her. Now that this is resolved, the birds no longer have to attack, and just perch quietly as the movie ends.


I'm not crying, you fool, I'm laughing!

Hewwo.

There is no explanation for the bird attacks, this is from the IMDb FAQ page:
"There is no explanation. Hitchcock himself answered that question by saying, "If you provide an explanation for the phenomenon then the film becomes science fiction; we're not making science fiction, Birds is a thriller, hence we leave out any explanation." The fact that it is never revealed to the audience why normally peaceful birds suddenly start attacking humans is a technique that Hitchcock used frequently in his movies.
It is called a MacGuffin (or McGuffin), which Hitchcock defines as "The plot device, of little intrinsic interest, such as lost or stolen papers, that triggers the action." (Quotation from Halliwell's Filmgoers Companion).
Just as the audience never finds out what is written on the stolen papers or what the secret formula is for, the audience is never told why the birds started attacking.
Some suggestions in the movie are that the birds are massing to migrate, that they have lost their way in the fog or the dark, that they are panicking, that they are being fed bad chicken feed, that the children have bothered them, or that they are attracted to light."