Continuity mistake: A blond boy in a light brown shirt is attacked by a crow and goes from having no blood on his neck to having a lot of blood smeared on his neck between shots. (01:14:10)
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)
Visible crew/equipment: As Melanie drives up to have dinner at Mitch's, the reflection of the cameraman on the inside of the window can be seen moving in the upper-left hand portion of the screen. (00:32:10)
Continuity mistake: When Lydia says to Melanie, "Do you think Cathy's alright at the school?" the two pictures hanging above her head disappear the next time we see her. (01:03:15)
Continuity mistake: After Melanie has been bandaged up after her attack, she sits up on the sofa, and you can see two large bloody scratches on her cheek. These scratches disappear when Mitch and Lydia walk Melanie outside to the car. (01:51:25)
Continuity mistake: When Tippi Hedren is having her head wound cleaned in the restaurant, Rod Taylor is wiping her blood off with a tissue but it does not actually come off; in the next shot, however, there is clearly no blood on her head.
Continuity mistake: After Melanie signs the card to Cathy she never puts her left glove back on. We're able to see her drive down uninterrupted to the boat and she never puts it on, but when she walks out of the car to the boat it's back on. (00:19:05)
Continuity mistake: Melanie's car changes positions from the time she parks near the front steps of the school. As Melanie walks out of the school to sit by the monkey bars to when she runs back to the school, and then when the children run out of the school. (01:09:30 - 01:11:40)
Continuity mistake: As Lydia carries the birthday cake out, the shades of the window behind her are open. When we see this window again a few seconds later as she runs into the house the shades are closed. (00:51:55)
Continuity mistake: When Lydia brings out Cathy's birthday cake, in a wide shot the icing is white, then in a closeup the icing is a pink colour. (00:49:45)
Continuity mistake: When Annie tells Melanie that she should go to Cathy's party the belt of her robe draped over her right knee moves between shots. (00:47:15)
Continuity mistake: When Mitch and Melanie are talking in the café, the bottle of peroxide on the table moves from being near Mitch to near Melanie. (00:27:45 - 00:28:35)
Continuity mistake: Inside the restaurant Melanie talks to her father on the phone at the bar. The phone's chord changes position between shots. At first it's resting on her sleeve then in the next shot it's on her forearm. (01:15:45)
Continuity mistake: As Annie opens the door and finds the dead bird on the floor, the background across the street changes from of a very dark house with two lights on to a moonlit road between shots. (00:47:20)
Continuity mistake: After talking to Mitch on the phone, as Melanie sits behind Annie's desk there are two pens on top of the sheets of paper to the left of the pencil holder that were not there before. Neither Annie or Melanie touched a pen or pencil in this scene. (00:46:40)
Continuity mistake: The tall structure on the hill behind the swing set has a lot of clouds around it. When we see it seconds later the sky is clear. (00:52:05)
Continuity mistake: As Melanie approaches Mitch's dock, a yellow pole extends from the far left side of the dock to the brown pail in the front of the dock. In the next shot, as she walks on the dock, the pole is gone. (00:21:35)
Continuity mistake: As Melanie leaves the card at Mitch's apartment door the drapes on the window behind her are still. In the next shot they are blowing in the wind. (00:10:20)
Audio problem: When Annie says, "Did you drive up from San Francisco by the coast road?" her lips are mouthing something different. (00:17:55)
Continuity mistake: Before Lydia walks into Mr. Faucet's room she looks to her left and her face is entirely illuminated by the light coming from his room. In the next shot as she walks in only the left side of her face is illuminated. (01:00:10)
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.