Plot hole: When Jessica Tandy walks into Mr. Faucet's house everything is in perfect shape except a couple of broken cups, and there's not a dead bird in sight. (00:59:40)
The Birds (1963)
1 plot hole - chronological order
Directed by: Alfred Hitchcock
Starring: Jessica Tandy, Rod Taylor, Suzanne Pleshette, Tippi Hedren
Continuity mistake: As everyone runs out of the restaurant, the bushes on the left of the building that were there before are gone. (01:26:10)
Mitch Brenner: Aren't those lovebirds?
Melanie Daniels: No, those are, uh, red birds.
Mitch Brenner: Oh, I thought they were strawberry finches.
Melanie Daniels: Oh, yes. We call them that, too.
Trivia: When we see the school children running away from the birds, they were actually running on a treadmill with Bodega Bay footage added in the background.
Question: What's making the birds go insane and attack people?
Answer: None of the characters knew why the birds attacked, or the reason it was confined to the one town of Bodega Bay. The incident was so isolated that the outside news media was barely aware it was occurring or knew its severity, so there was no investigation into it at the time. Alfred Hitchc0ck's explanation was that the birds were rising up against humans to punish them for taking nature for granted.
Answer: It is never explained, probably because it would ruin the mystery of the film to spell it out.
If you are a Hitchcock fan, you pay attention to dialogue. They mention quite often that it started Melanie's arrival, of which she brought 2 caged birds. The rest is implied.