Revealing mistake: As she speaks to the cashier, most of the birds perching in the cage behind and to the right of Melanie are fake. The perches sway but the birds don't move. (00:02:55)
Revealing mistake: The bird that Mitch catches under his hat is fake and doesn't move. However, in the next shot when he takes it out from under his hat the bird is real. There are numerous instances throughout the film where it is obvious that the birds are fake. (00:06:15)
Revealing mistake: As Mitch drives off in his Galaxy 500, some spectators across the street are watching the crew filming. (00:08:00)
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)
Revealing mistake: Melanie has no problem taking the letter for Mitch out of her pocketbook as she places it on top of the bird cage. The crew put adhesive tape on the back of it, so she can push on the letter with her thumb to keep it in place for the close-up. (00:10:20)
Revealing mistake: As Melanie drives up to Bodega Bay the birds are supposed to lean as the car goes around corners but the mechanism for the bird on the right gets stuck and reacts slower than the bird on the left. (00:11:00)
Revealing mistake: When they show the perspective of being in Melanie's driver's seat looking out the windshield we don't see Melanie's legs as you would expect, but a mechanism that allows someone to steer from the passenger seat so the camera has an unobstructed view of the driver's seat. (00:11:05)
Revealing mistake: If you watch closely as she sets sail Melanie's boat nudges the boat that the camera is on causing the film to jiggle slightly as she passes by. (00:20:35)
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)
Revealing mistake: Many times through the movie it is noticeable that the distant background hills and beaches are backdrop paintings. A good example of this is when Mitch and Melanie are talking on the hill at Cathy's birthday party. (00:49:00)
Revealing mistake: As the children start running down the hill from the school a girl in a white dress and blue sweater is cracking up. (01:13:35)
Revealing mistake: The birds that attack the schoolchildren have no shadows. (01:13:40)
Revealing mistake: In the first shot of the children running down the hill looking from behind we see several children in the front of the pack (superimposed background) with fake birds tied around their shoulders. (01:14:00)
Revealing mistake: A red headed kid in a blue shirt gets pecked on the cheek by a blackbird as he runs down the street. If you look closely at the bird before he touches the kid the blood he smears on the child's cheek is in a sack hanging off of his upper beak. (01:14:30)
Revealing mistake: The glasses of the girl in the red sweater fall off as she calls for Cathy to help her. If you look closely the glasses are broken before they hit the ground and we hear the sound of them smashing. (01:14:40)
Revealing mistake: As Cathy and Melanie go to help the girl in the red sweater lying in the road look at the poor helpless blackbird on her shoulder. Its beak is attached to her sweater so that it can't get away. (01:14:45)
Revealing mistake: In two of the shots inside the cream colored car right after Melanie blows the horn either wires or a visual artifact from the superimposing process distinctly appear outside of the rear right window. (01:15:20)
Revealing mistake: The crew rigged the explosion to start from under the car and not from the lit match which isn't thrown anywhere near where the explosion originates. (01:25:30)
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)
Revealing mistake: The wing of a fake bird, unable to fly straight, swoops down and hits Melanie in the head as she runs to the phone booth. (01:26:10)
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.