Continuity mistake: On top of the hill Melanie spills a drop of her drink on her green skirt. It's on her right leg six inches below the bottom of her jacket. The action doesn't stop, but three minutes later the stain is gone. (00:48:40 - 00:51:25)
Continuity mistake: As the children leave the school none of them go to the coat rack to get their coats. Later many are wearing their coats while running down the hill. (01:13:05)
Continuity mistake: As the birds gather on the monkey bars behind Melanie she looks to her right and straight up and sees one lone bird flying past a television antenna. This is incorrect because first of all she's looking up to high and second we've seen in that direction and there is no house there, just a hill. The house is across the street. (01:11:10 - 01:33:05)
Continuity mistake: Mrs. Bundy's handkerchief in the left pocket of her jacket changes from when we first see her walk into the restaurant. Then when the drunk at the bar starts reciting from the Bible, her handkerchief almost disappears and then reappears several shots later. (01:18:10)
Continuity mistake: As everyone runs into the house there are four neat stacks of paper cups around the punch bowl. When we saw this spot before there were only two cups around the punch bowl and most of the other paper cups were stacked on trays and knocked over as a bird plowed into them. (00:53:05)
Continuity mistake: After Melanie decides to stay for the night Lydia watches the policemen and everyone else leave. Between shots they've rearranged her pearl necklace and it now sits lower on her neckline. (00:57:45)
Continuity mistake: When Annie opens her door and sees the dead bird on the porch we see a good shot of the railing and support beams that go up to the roof. Two shots later they disappear. (00:47:25)
Continuity mistake: Lydia is holding her glove in her right hand and her pocketbook in her left as she walks into Mr. Faucet's room. The pocketbook never switches hands but when she runs out, she drops both of them from her right hand. (01:00:15)
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)
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)
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)
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.