Continuity mistake: After Melanie says, "Oh Mitch," during the attack, they have a close-up of the knuckles of her left hand. The first time we see them they're scraped and bloody but the second time it's the palm and wrist and not the knuckles that are bloody. (01:48:50)
Revealing mistake: Just before Mitch comes through the door to save Melanie there's a shot of the birds coming at her with the opening in the roof in the background. The opening in the roof is in color but the superimposed birds are in black and white. (01:49:15)
Other mistake: When Mitch is in the car, after listening to the radio he switches it off and the subtitles read 'Clicks of radio', yet it should be 'Clicks off radio.' (01:49:50)
Continuity mistake: After rescuing her Mitch picks Melanie up and carries her down the stairs. At first her right leg is sticking straight out and her left is bent but in the next shot both her legs are bent. (01:49:50)
Continuity mistake: As Mitch picks up Melanie and swings her around we get a good look at the cut on her left shin. In the next shot, as he carries her down the stairs, the cut is totally different with much more blood there. (01:49:50)
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: 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 Melanie has been bandaged up after her attack, she sits up on the sofa, and you can see two large bloody scratches on her cheek. These scratches disappear when Mitch and Lydia walk Melanie outside to the car. (01:51:25)
Continuity mistake: As Mitch goes out the dining room door to the garage there's a big oily, drippy stain behind and to the right of his head. In the next shot it's gone. (01:52:30)
Continuity mistake: As Mitch leaves the house to get Melanie's car in the garage the large brown wooden chair that has been to the right of the door throughout the movie has disappeared. (01:52:40)
Continuity mistake: As Mitch walks up to the garage door all the birds on the ground that are near him run away and there's nothing to his right except a garbage pail. Then just before he walks through the door two birds appear pecking at his feet. (01:53:00)
Audio problem: When Mitch turns on Melanie's car radio he turns the tuner knob and stops on the station that he eventually tunes in, goes a little bit forward and then back again to the same spot. The over-dub is not correct as we would have heard the announcer speaking the first time he landed on the station. (01:53:55)
Continuity mistake: When Mitch returns from the garage, the blood stain on Melanie's head is about an inch lower that it was before when Lydia put the bandage on it. (01:55:10)
Continuity mistake: As they all prepare to leave the house part of Melanie's bloody dress sticks right up towards her chin, but when Mitch and Lydia swing her around towards the car it's not sticking up anymore. (01:56:10)
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 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)
Audio problem: At the end of the movie when they're walking out of the house you hear a door open, but the doorway is empty - it's just an open space, with no door. (01:56:40)
Continuity mistake: As they all leave the house and walk through the door, there's a distinctive gash in the door post to the right of Lydia's head. When Mitch goes back to get Cathy and the lovebirds it's not there. (01:56:55 - 01:57:50)
Continuity mistake: Lydia's expression as she rests her head on Melanie's changes from a warm smile to a stern look between shots. (01:58:15)
Continuity mistake: When Tippi Hedren is having her head wound cleaned in the restaurant, Rod Taylor is wiping her blood off with a tissue but it does not actually come off; in the next shot, however, there is clearly no blood on her head.
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.