Continuity mistake: When Melanie yells, "Where's Cathy?" Mitch looks up to see her in the window and she is moving the right curtain with her hand as she cries. In the next shot as Mitch runs to the door the curtain is not moving. (01:30:40)
Continuity mistake: When Mitch covers Annie's body with his jacket the back of it is not as dirty as it was a few seconds ago when they walked up to the school. (01:31:05)
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: Mitch runs up and opens Annie's door in order to get Cathy from the house. The door is not wide open when they walk away. When Mitch walks back and puts his coat over Annie's body even though no one went near it, the door has been opened as far as possible and is now flush with the wall behind it on the right. (01:31:30)
Continuity mistake: As Mitch lifts Annie's body to take her inside the house two black rags or shreds of her clothing appear on the porch that weren't there in a previous shot. (01:31:45)
Continuity mistake: Mitch walks out of Annie's house and his shirt gets dirtier between shots. It's cleaner by the time they get inside Melanie's car. (01:32:30)
Continuity mistake: As Melanie, Mitch and Cathy walk to her car, after finding Annie dead, watch the rungs of the steering wheel. As they get into the car it's been turned to the left between shots and Mitch hasn't touched it. (01:33:05)
Continuity mistake: As Melanie, Mitch and Cathy get into her car to leave the school, Annie's red mailbox can be seen out on the road behind them. In the next shot of them inside the car driving away the post is there but the red mailbox is not on it. (01:33:30)
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: As Melanie hands lumber to Mitch the upper window has no curtains on it and a light can be seen shining through. In the next shot there are curtains on the window. (01:34:00)
Continuity mistake: When Rod Taylor is putting boards over the windows of the house, he asks Tippi Hendron for "another plank" and she hands him one, using both hands. The shot cuts to him taking the plank from her, and when it cuts back to her, she is holding another plank. (01:34:10)
Continuity mistake: The last piece of wood that Melanie hands to Mitch is not the same one that he grabs. The end is split and doesn't have a prominent knot in it but when Mitch grabs it there's no split and it has a large knot in it. (01:34:20)
Continuity mistake: As Melanie and Mitch go back into the house after Lydia calls him he grabs for the doorknob and we immediately here the sound of it turning as he walks through the other side of the door in the next shot. The problem is that they left out the screen door he would have had to open first before he could have entered the house. (01:34:55)
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)
Continuity mistake: When we see Melanie sitting with Cathy on the couch, as Mitch checks the windows, the coffee table with the coffee pot on it is between them and the ash tray on the table to the left of the brown chair. Later when Cathy asks, "Mitch, can I bring the lovebirds in here?" we see the ash tray and plenty of space to the right of Cathy but the coffee pot has been moved out of the way for this shot. (01:36:50 - 01:37:20)
Continuity mistake: As Mitch checks the doors and windows look closely at the table to the left of the brown chair on the bottom left of the screen, it has on it an ash tray very close to a plate and a silver box. The shadow of the brown chair falls on the silver box but the next time we see this table, after Lydia straightens up, the brown chair has been moved away to make room for the camera and its shadow no longer falls on the box. (01:36:50 - 01:39:10)
Continuity mistake: As Mitch checks the window behind the TV set we see the picture of father above the piano there's no shadow on it. Later as they all sit waiting for the bird attack even though no one's turned any lights on or off a dark shadow appears above the middle of the picture. (01:37:05 - 01:40:45)
Continuity mistake: As Mitch looks at the birds in the kitchen, the position of the perch changes between shots from in the middle of the cage to right of the door in the close-up. (01:38:00)
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.