Continuity mistake: How does Tippi Hedren (Melanie), every time she is attacked by birds, manage to get to a top-notch hairdresser before the next scene? Melanie's hair changes many times in consecutive shots throughout the film.
Continuity mistake: At the party Melanie grabs a girl from the birds, but when they all run into the house Mitch is now carrying the same girl.
Other mistake: Annie is the only teacher in the town, and there is only one classroom with students, all who are around the ages of 10-12 years old, even though the schoolhouse she teaches at is a two-story, mid-sized building with multiple rooms. There are no administrators, staff, custodians, or other teachers and no students in lower and upper grades at all. Even when Annie and Melanie are evacuating the children during the crow attack, there is only the two women and Annie's students. There is no-one else.
Deliberate mistake: The closing scene contains one of the most famous deliberate omissions. As Mitch, his mother, and Melanie approach the front door to leave the house, there is no front door; it was removed so the camera can see them. Mitch mimes reaching for the door and pulling the handle. As he does this, a light is panned onto the actors' faces to help sell the illusion that a real door was opened. Victoria Cartwright later reported that she asked Hitchcock why people wouldn't see there was no door there. He replied that it was the magic of movies - no one would notice. And no one ever does! Read more in the transcript of the documentary "All About the Birds."
Other mistake: It seems well established that the film takes place over a weekend. However, Melanie drives up on Friday, stays at Annie's apartment that night, goes to Cathy's party the next day, Saturday and stays at Mitch's that night. Nowhere in the movie is it implied that she stays another night and in fact she doesn't. Everything else in the movie after the attack at Mr. Faucet's farm takes place on Sunday and there would be no school in session when the children are attacked.
Continuity mistake: Melanie starts to blow the horn of the cream colored station wagon and we see her left hand go from flailing around to being firmly on the steering wheel between shots.
Audio problem: That is not a Martin outboard motor sound when Melanie is on the bay; it is a two-cylinder OMC outboard motor running (in neutral).
Factual error: The firefighters arrive at the gas station and proceed to spray water on the fire. You cannot put out a gasoline fire with water; it requires firefighting foam. Water would only spread the fire even more.
Continuity mistake: During the gulls' attack on Cathy's birthday party, Melanie sees a gull attacking a girl in a blue dress and takes her suit coat off to wave it at the gull to frighten it away. The next shot, from a distance, shows them running into the house and Melanie is wearing the coat again. A second later there's a closer shot of them running in and Melanie's coat is off again and she's carrying it.
Continuity mistake: When facing the exterior of Annie Hayworth's house, the bay window alcove to the left of the front door isn't deep enough to contain everything that we see in it when we later see it from the inside.
Other mistake: When Melanie is signing the card for Cathy on top of her car there is a brown paper bag underneath the card, she leaves it, when she drives out it's no longer there.
Revealing mistake: In virtually all the outdoor attack scenes (but most noticeably in the phone booth sequence), the rotoscope seagulls and crows are proportioned about 150% to 200% of their natural size, about the size of turkeys.
Revealing mistake: When hundreds of sparrows pour out of the fireplace, filling the house, they do not cast shadows on the furniture, walls or people.
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.