Continuity mistake: As Mitch returns to the living room before the lights go out, there's much more wood in the pile than before when he was putting it into the fire. (01:44:05)
Continuity mistake: After the final attack they're all in the living room in the dark. As the camera pans from Melanie to Mitch the wick of the kerosene lamp faces it. In the next shot of Mitch alone the lamp has been rotated so that the wick still faces the camera. (01:45:15)
Continuity mistake: When Melanie reaches for the flashlight on the coffee table it's been moved closer to and slightly in front of the kerosene lamp and she hasn't touched it. (01:45:20)
Continuity mistake: When Mitch is first seen sleeping on the couch, after the last attack, the red throw pillow is protruding out from behind his right shoulder. The next time he is seen from the front the pillow is totally behind his back. (01:45:25)
Continuity mistake: After the final attack as Mitch sleeps in the chair the tabletop and the three items on it to his left are lit up. They get much darker the next time we see them. (01:45:25)
Continuity mistake: When we first see Mitch sleeping on the couch after the last attack, the corner of the silver box to the left of him is facing Melanie. After she calls to him the box has been rotated so that one complete side is facing her. (01:45:25)
Revealing mistake: When Melanie is brought downstairs and onto the couch, she starts flapping her arms out going 'No! No!' You can see Cathy turn her head and laugh to herself a little, then pretend to cry. (01:45:50)
Continuity mistake: Melanie is sitting and she reaches for the flashlight on the coffee table. In the next shot, as she picks it up, she's standing. (01:45:55)
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: Before Melanie walks up the stairs to the bedroom there's a shadow of a plant on her jacket. This is wrong because in the shot before we can see up the stairs and there's no plant with any light behind it. (01:46:50)
Revealing mistake: As Melanie walks up the stairs she's pointing the flashlight out in front of her but it's able to illuminate her face like a spot light. (01:46:50)
Continuity mistake: As Melanie opens the door to the bedroom there is no light except her flashlight but you can see two distinct reflections in the door knob, one of the flashlight and one of the studio light behind her. (01:47:15)
Continuity mistake: The flashlight is aiming right at the doorknob before Melanie walks into the room. In the next shot, as she walks through the door (before they turn the spotlight on) there's no light because the flashlight is off and isn't pointing at the doorknob. (01:47:30)
Revealing mistake: When Melanie walks into the bedroom and covers her face with their hands something is wrong with the superimposing process. It looks as though there are wires running up and down the screen as soon as the birds start flying at her. (01:47:40)
Continuity mistake: In the upstairs bedroom attack the first blackbird scratches Melanie's face and her left hand goes from pointing upward to pointing across her face between shots. (01:47:50)
Continuity mistake: As she's attacked in the bedroom the first scratch on Melanie's left forearm moves further up two shots later when see it again. (01:48:05)
Continuity mistake: The fourth shot of Melanie's legs in the room with the birds shows ripped stockings around her ankles looking like cob webs. The next time we see her feet, the stockings are intact. (01:48:15)
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)
Deliberate mistake: It's supposed to be a moonlit night when Melanie walks into the bedroom. But when we see the opening where the birds got in there's a blue sky much brighter than it would actually be on such a night. (01:48:35)
Continuity mistake: When Melanie is being attacked by the birds she says, "Oh Mitch," and the cut below her right eye changes between shots. In the first shot it angles up and the second it's straight across. (01:48:45)
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.