Revealing mistake: A fake bird, unable to fly straight, swoops down as Mitch saves Melanie from the phone booth. (01:27:45)
Revealing mistake: The cameraman loses focus on the uptight mom as she asks Melanie where she came from. In the next shot she is sharp as a tack. (01:28:50)
Revealing mistake: As Mitch and Melanie look at the birds perching on the Bodega Bay School they're all fake. The only two that move are to the right of the wire and they're attached so they swivel forward and backward in the wind. (01:29:40)
Revealing mistake: As Mitch is about to pick up Annie's dead body, you can see her left foot move slightly. (01:31:45)
Revealing mistake: As Mitch pulls the cord of the lamp out of the socket, the bulb goes out before the crew turns off the spotlight on him. (01:42:10)
Revealing mistake: When Mitch runs to get the hammer and nails the camera follows him around the corner of the kitchen door. Watch very closely as the door on the far right of the screen moves when the camera hits it. The film also jiggles as this happens. (01:43:40)
Revealing mistake: When Melanie is brought downstairs and onto the couch, she starts flapping her arms out going 'No! No!' You can see Cathy turn her head and laugh to herself a little, then pretend to cry. (01:45:50)
Revealing mistake: As Melanie walks up the stairs she's pointing the flashlight out in front of her but it's able to illuminate her face like a spot light. (01:46:50)
Revealing mistake: When Melanie walks into the bedroom and covers her face with their hands something is wrong with the superimposing process. It looks as though there are wires running up and down the screen as soon as the birds start flying at her. (01:47:40)
Revealing mistake: Just before Mitch comes through the door to save Melanie there's a shot of the birds coming at her with the opening in the roof in the background. The opening in the roof is in color but the superimposed birds are in black and white. (01:49:15)
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.