Continuity mistake: As Melanie walks over to the cashier in the pet shop the bird in the cage on the right with the greenish beak faces the other way between shots. (00:02:30)
Continuity mistake: As Annie leans up against the middle of the red mailbox she knocks the flag down. When we see her again her right arm is pushed up against the front of the mailbox and some amount of time, longer than the three seconds implied, has gone by as the shadow of the mailbox's handle moves between shots. (00:18:00)
Continuity mistake: So it can be seen better as Melanie looks out the window the stuff inside Mitch's barn is lit differently and moved between shots. (00:22:05 - 00:22:40)
Visible crew/equipment: As they head for the restaurant, a crew member's reflection can be seen on the far left of the screen moving in the lower green section of the phone booth just as Mitch's shadow passes the phone booth. (00:26:25)
Continuity mistake: The pink flowers behind Deke get rearranged between shots and no one touched them. The second time we see them, one flower in the center sticks out higher than the rest. (01:18:40 - 01:19:20)
Revealing mistake: There are two people next to a red and white pick-up truck in the upper right of the screen who are waiting for their cue to enter the scene after the car explodes. (01:25:35)
Continuity mistake: As Mrs. Bundy tells everybody how many birds there are in the world a lock of hair falls out from under fishermen Sebastian Sholes' hat between shots, disappears and falls out again as the scene progresses. (01:21:00 - 01:24:00)
Continuity mistake: As Melanie walks through Mitch's front door she's looking down at the door knob. In the next shot as she walks through she's looking forward. (00:22:05)
Continuity mistake: After Mitch suggests that they get help up at the restaurant, the shadow of Melanie's hand on her forehead changes between shots. (00:26:15)
Audio problem: In the restaurant when Mitch says, "We can get Cathy at Annie's now" his lips are out of sync with what he is saying. (01:28:10)
Continuity mistake: In a close-up of her driving to Bodega Bay Melanie turns the steering wheel to the left and we hear the screeching of her wheels, but the background indicates she's still going straight. (00:11:05)
Continuity mistake: As Mitch checks the doors and windows Melanie sits with Cathy on the couch. Notice the fruit bowl under the lamp. At the beginning of the attack the red apple jumps to the front of the bowl from the back without anyone being near it. (01:36:50)
Visible crew/equipment: Out on the water you can see a lot of stage lights reflected in the engine of Melanie's boat. (00:21:30)
Continuity mistake: When we first see Annie's house shrubs are growing right up to the edge of the right and the left side of her fence. Then when she is lying there dead the shrubs on the left side of the fence are different. (01:30:30)
Continuity mistake: When Mitch is first seen sleeping on the couch, after the last attack, the red throw pillow is protruding out from behind his right shoulder. The next time he is seen from the front the pillow is totally behind his back. (01:45:25)
Visible crew/equipment: The reflection of the spotlight can be seen going before them in the bottles of booze as it leads Melanie and Mitch down the bar. (01:28:00)
Continuity mistake: When we first see Mitch on the ladder it's resting against the 6th plank below the window. Mitch never moves it but after Melanie hands him the last piece of lumber and he climbs down it's resting against the 7th Plank. (01:34:00)
Continuity mistake: After the final attack as Mitch sleeps in the chair the tabletop and the three items on it to his left are lit up. They get much darker the next time we see them. (01:45:25)
Revealing mistake: As Lydia talks on the phone Mitch walks into and out again from the kitchen and you can see his shadow fall on the drapes to the right of the dining room table. Later on Cathy goes into the kitchen and right after Mitch sits on the piano stool look closely at her shadow. Though not as prominent as Mitch's it can be seen in the same place for 20 seconds as she waits for her cue to enter the dining room. (00:34:00)
Continuity mistake: As Mitch checks everything in the kitchen he walks past the birdcage and looks in it. Look closely at the door of the birdcage as he walks around it. At first it's closed then in the close-up it's open. (01:38:00)
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.