Continuity mistake: Mitch puts the glass of brandy up to Melanie's lips and we see the brandy touch her upper lip and slosh around but then as he pulls it back there's not a trace of liquid on her lips. (01:50:45)
Continuity mistake: After Lydia gets off the phone the wires of the light fixture hanging over the dining room table are pushed around to the sides of the bulb so you can barely see them. In the next shot it's been rotated and you can see them very well. (00:35:25)
Continuity mistake: In the wide shot of the party scene as everyone is running into the house a girl in a bright red sweater and white skirt pulls her mother in the blue dress by the hand. In the next shot they've changed positions and are together with the mother's arm around the girl's back. (00:53:00)
Continuity mistake: The orange car that catches fire, when the guy throws the match, moves between shots. When he lights up the cigarette there's a white parking line to left of the tire. In the previous shot it wasn't there. (01:25:15)
Continuity mistake: As they all walk straight through the dining room to get out of the house the light fixture above the dining room table has been removed for this shot. (01:56:35)
Continuity mistake: As Melanie walks around with her flashlight she goes into the kitchen and looks at the lovebirds. This cage has 11 rungs on the side and is not the same one that she walked into the house with when she brought the lovebirds up from San Francisco, which had 19 rungs. (01:46:15)
Continuity mistake: As she attempts to get the bird out of the cage for Mitch, Melanie places the pencil over her ear and a lock of her hair drapes over it. Then when she says to Mitch, "What did you say?" the pencil is now pushed into her hair but she never went near it. (00:05:45 - 00:06:25)
Continuity mistake: In the shot of Mitch holding the log just before he runs to the window behind the desk, the brown reclining chair that sits by the fireplace has been moved for the camera. We see it there in the scene when Lydia says, "Did you get the windows in the attic Mitch?" and earlier in the movie when Melanie brings the lovebirds in. (01:41:20)
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)
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)
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)
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.