Continuity mistake: When Mitch reaches out the window to grab the shutter the curtain has been pushed all the way to the left of his shoulder and he isn't touching it. The next time we see the curtain it's covering the entire window to the left of his head. (01:41:50)
Continuity mistake: After calming Lydia in the green chair Mitch holds his bloody left hand with his right as he goes to the bathroom to get a bandage. In the next shot with the front door in the background he switches to holding his right hand with his uninjured left hand. (01:43:15)
Continuity mistake: As they lift Melanie off the couch and walk her to the door her hair and the front right section of the head bandage change between shots. (01:56:15)
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)
Continuity mistake: As she's fighting off of the bird attack in the bedroom Melanie looks like she is turning off the light switch on her right. There's no power in the house but the light on her does go out. It's not a reflection from the flashlight, which is aiming at the ceiling before the light goes out. (01:48:30)
Continuity mistake: At the school, as Melanie sits down to have a smok,e the dark markings on the upper plank of the wooden fence behind her move higher between shots. (01:09:50)
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 the school children start running down the hill Melanie holds her hand out to a little girl in a mustard colored sweater. In the next shot the little girl is running alone in middle of the pack near the left of the screen. (01:13:35)
Continuity mistake: When Mitch first picks Melanie up he swings her around and has his left arm around her neck. In the next shot as he walks down the stairs his arm is around her back. (01:49:55)
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)
Continuity mistake: Mr. Faucet's tiny windows are not wide enough to let in the amount of sunlight that's shining on him from left to right on such an overcast day. (01:00:30)
Continuity mistake: As Melanie climbs out of the boat, she holds her pocketbook around her arm with the latch facing her body. After she enters the house the latch is facing outward and she's never taken it off her arm. (00:21:50)
Continuity mistake: As Mitch takes Melanie to try to get help he tells a guy, "A gull hit her" and a long shadow of a wire appears on the white sliding door directly behind them. In the next shot we see the door in middle of the screen and the shadow is gone. (00:26:05)
Continuity mistake: Just before the lights blowout Mitch comes back into living room and there is a red ottoman to the right of the green chair that Lydia and Cathy are sitting in. A few moments before, all three of them just walked through that spot when they were hiding by the bookcase and it wasn't there. (01:44:05)
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)
Continuity mistake: Before Mitch rescues Melanie from the telephone booth we can see that all the fires are still burning. But in the shot of him rescuing her they forgot to add the flames out in the road behind them. (01:27:40)
Continuity mistake: Melanie is only in Mitch's house for one minute, but the tide has already gone out enough to beach the other boat tied to the dock. (00:23:00)
Continuity mistake: There's a fixture with a wire hanging down from it in the front left corner of the restaurant. When they built the interiors for this scene they forgot to put it there and when Melanie opens the phone booth door trying to get out the wire hanging from the bottom of the roof of the restaurant is missing. In the close-up as Melanie tries to get out the cord that we saw previously sticking out has been tucked back in towards the phone for this shot. (00:26:25 - 01:26:45)
Continuity mistake: As Melanie approaches the boat keeper, there's a guy working on the boat to the right of him. When we first see this guy he's looking down, then in the next shot he's looking up. (00:20:00)
Continuity mistake: Before Melanie sits down at the table in the restaurant there's a little bit of powder on it left from the previous customer. In the next shot it's cleaned up and shiny. (00:26:45)
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.