Continuity mistake: Mitch picks up a perfectly polished oval stone from Annie's garden. In the next shot as he gets ready to throw it at the crows on the roof, it changes into a jagged rock half the size. (01:31:10)
Continuity mistake: After finding Annie's dead body on her porch, Mitch finds a stone to throw at the crows up on the roof. As he reaches for the stone there's nothing on his left, but in the next shot a bush with stones around it appears. It disappears again in the next shot. (01:31:10)
Continuity mistake: When we first see Annie's dead body we also see the well groomed and raked gravel of her front walkway with not a pebble out of place. One shot later as Mitch runs up to her we see the impressions of his feet appearing in the gravel from a previous take. (01:30:20)
Continuity mistake: In a close-up of Melanie in the phone booth after a blue and white car crashes into the orange car look at the left side of the screen. You'll see the stop sign that the blue and white car just knocked over back up where it was seconds before. (01:26:40)
Continuity mistake: As Melanie speaks to the pet shop cashier she holds her head to the left. In the next shot she's looking straight at the cashier. (00:02:40)
Continuity mistake: When Mitch puts the mirror up against the front door the rug in front of it becomes bunched up. Mitch never touches it but later it's been set straight again. (01:43:30)
Continuity mistake: The second shot of the background of the school and the children running down the hill is superimposed behind the five live action children in the front. Watch the boy to the right in the red sweater very closely because in the next shot he doesn't change but the five children in the front do. (01:13:40)
Continuity mistake: After Mitch closes the shutter, he sits Lydia down in the green chair. She then grabs his arm and leans forward with Cathy's arm around her back. In the next shot she's grabbing at his arm with her back against the chair. (01:43:00)
Continuity mistake: In the last bird attack Lydia and Cathy run past the drapes behind the dining the room table and they're hanging straight down to the floor. Before when we saw Mitch and Lydia sitting the drapes were crooked. (01:41:20)
Continuity mistake: As Mitch ties up the shutter, he goes over to Lydia. Look at the shades pulled up well above the middle of each window. Later when we see the windows again, just before the lights blow out, the right shade has been pulled down to the middle of the window. (01:42:45)
Continuity mistake: As Mitch asks Melanie how you can tell if birds are molting his head leans to the right, then to the left between shots. (00:05:20)
Continuity mistake: As Lydia looks at Melanie and Mitch walking down the hill, the large piece of molding in the corner between the door and window disappears between shots. (00:51:50)
Continuity mistake: Melanie starts to blow the horn of the cream colored station wagon and we see her left hand go from flailing around to being firmly on the steering wheel between shots.
Continuity mistake: As Melanie pulls up to Mitch's dock a piece of rope tied to the post extends above the middle of the yellow buoy. In the next shot it's not there. (00:21:35)
Continuity mistake: As she sits and listens to Melanie talk to Mitch on the phone Annie's right hand changes positions between shots. She starts to move it up, then in the next shot it's already leaning on her left hand. (00:45:50)
Continuity mistake: When the first bird perches on the monkey bars behind Melanie the sun is behind her as you can see by the long shadow she casts as she walks to the chair. In the close-up she casts a shadow in the opposite direction on the fence. (01:09:55)
Continuity mistake: As Melanie picks up the phone to talk to Mitch the shadow of the cord falls on different areas of her jacket between shots. (00:45:40)
Continuity mistake: After Mitch bandages his hand he looks at the front door. As the birds peck at it look closely at that top right quarter as a hole appears between shots. (01:43:20)
Continuity mistake: As Mitch pulls the window shutters shut and looks at the power cord of the lamp as a possible way to tie it shut there's a candle in a holder to the left of the lamp. In the next shot when he reaches over for the lamp the candle is gone and the next time we see the top of the desk it's not there, or on the floor knocked over. (01:42:10)
Continuity mistake: The wood on the ladder changes between shots. First it's unstained, then it stained green and then it's unstained as she gets to the bottom. (00:20:05)
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.