Continuity mistake: As Melanie drives up the main street of Bodega Bay a kid on a bike appears riding behind her between shots. He wasn't there in the previous shot when they show her behind the steering wheel and we can see back behind her car. (00:12:05)
Continuity mistake: When we first see Melanie talking to her father the phone is pushed right up against the edge of the bar. The next time we see it's at a 45-degree angle to the side of the bar and no one has touched it. (01:15:45)
Continuity mistake: At Mr. Faucet's house, both doors that Lydia walks through are blue but after she walks in the door is white. (00:59:35)
Continuity mistake: In an earlier scene as Melanie plays the piano and Cathy stands there look closely at the drapes on her left. In that scene the drapes were right up against the edge of the wall, now as Mitch checks the windows before the last attacks they're pushed back several feet. (00:57:30)
Revealing mistake: The birds on the floorboard of Melanie's car are fake. If you look closely at the one on the right it wobbles like a toy as she's driving. (00:10:00)
Continuity mistake: After finding Mr. Faucet's body, Lydia gets back in her truck through the passenger side door. Notice that the window is rolled up and the side vent window is shut tight. Seconds later we see her inside the truck with the window rolled down and the vent window tilted open. (01:00:55)
Continuity mistake: As Lydia walks up to Mr. Faucet's room, the hallway is dark except for the light coming through the door. The sunlight on the floor is much wider than the window of the door in front of her and there's no evidence of any light fixture above her or her hair would be lit up. (01:00:10)
Continuity mistake: As Lydia carries the birthday cake out from the house the position of the vine growing on the pole on the right jumps from right to left between shots. (00:51:55)
Continuity mistake: In the upstairs bedroom attack the first blackbird scratches Melanie's face and her left hand goes from pointing upward to pointing across her face between shots. (01:47:50)
Continuity mistake: As Mitch leaves the house to get Melanie's car in the garage the large brown wooden chair that has been to the right of the door throughout the movie has disappeared. (01:52:40)
Continuity mistake: When Mitch returns from the garage, the blood stain on Melanie's head is about an inch lower that it was before when Lydia put the bandage on it. (01:55:10)
Audio problem: Before Melanie starts off across the bay we hear the boat keeper start the boat's engine. But this is only an over-dub, the engine's not really running as there's no turbulence behind the boat like in the next two shots. (00:20:30)
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.
Continuity mistake: As Annie and Melanie speak about how they met Mitch, watch the shadow move down the front door. A considerable amount of time has gone by between shots. (00:18:05)
Continuity mistake: The signature on the card that Melanie signs to Cathy in the close-up on the hood of the car is different when she leaves it leaning on the birdcage at Mitch's house. The "Y" at the end of her name is different on the two envelopes. (00:18:55)
Audio problem: When the uptight mom asks how to get to San Francisco the guy at the bar that offers to take her lifts his glass up to take a drink and then we hear the overdub of the ice jangling in the glass. The jangling continues into the next shot after the glass is stationary and no longer near his lips. (01:22:10)
Continuity mistake: As Melanie talks to her father on the phone, Mrs. Bundy walks into the restaurant and an ash tray that Melanie and Deke didn't touch appears right next to the phone between shots. (01:16:10)
Continuity mistake: As Melanie holds out her right hand to shield the boy in the white jacket we get a really good close-up of his neck. It was bleeding three shots ago as a crow was attacking him but now it's not. (01:13:50)
Continuity mistake: As Melanie rings Annie's doorbell, there's a distinct reflection in the door. Later when Annie walks up onto the porch there's no reflection in the window of the door because they've been removed. (00:16:30)
Continuity mistake: When Lydia comes outside with Cathy's cake, the door behind her is open halfway, but later when the birds attack, you can see the door is now closed completely. (00:49:45 - 00:50:45)
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.