Continuity mistake: When Melanie is walking up to the counter in the bird shop she has a black purse in her hands. In one shot she is walking to the counter with her purse in her hands, but in the following shot, her purse is laying on the counter and is now taking off her gloves. (00:02:25)
Continuity mistake: At the party the tablecloth is blowing under the table in the long shot as everyone runs toward the house. In the next shot as they enter the house it's not blowing and there is no wind at all. (00:53:00)
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: As the birds are flying around the house we see in the close-up that the fireplace has three logs. The right half of the log in the middle moves up onto the log in the front between shots. (00:55:05)
Continuity mistake: When everyone begins running down the hill Melanie is in the middle and Annie is in the back. We're watching continuous action but several seconds later Melanie jumps to the rear without ever being passed by Annie. (01:13:45)
Continuity mistake: As Melanie starts off across the bay we see the boat keeper watching her from the ladder. A few seconds ago there were lobster traps piled all around and above the fence behind him. (00:20:40)
Other mistake: Annie is the only teacher in the town, and there is only one classroom with students, all who are around the ages of 10-12 years old, even though the schoolhouse she teaches at is a two-story, mid-sized building with multiple rooms. There are no administrators, staff, custodians, or other teachers and no students in lower and upper grades at all. Even when Annie and Melanie are evacuating the children during the crow attack, there is only the two women and Annie's students. There is no-one else.
Continuity mistake: The gravel walkways and driveways in "The Birds" are immaculately raked and maintained as seen in many shots. When Lydia pulls up to the house after finding Mr. Faucet's body, there's a tire mark in the exact same spot on the gravel from a previous take. (01:01:20)
Other mistake: In the close-up of Melanie holding Cathy Lydia starts to laugh as she tries to fend off all those birds. (00:55:30)
Continuity mistake: As Melanie opens Mitch's front door, her pocketbook is strapped around her wrist. In the next shot, as she comes through the door, the pocketbook is up around her forearm. (00:22:10)
Continuity mistake: As Melanie gets back into the boat she places her pocketbook in the back with the latch facing her. In the next shot as she watches Mitch on the shore the pocketbook is reversed and it stays that way until she gets out of the boat after being hit by the gull. (00:23:25)
Continuity mistake: When Melanie walks over to see the gas station attendant attacked, the green drape is flush with the side of the window. No one touched it but a few shots later it's been pulled out about six inches from the side of the window. (01:24:30)
Continuity mistake: When Lydia first looks into Mr. Faucet's room she sees a dead bird smashed up against his window. If you look at the drawing of the backdrop outside the window it's a bright sunshiny day but when Lydia was just outside, and again when she leaves, the sky was overcast. (01:00:20)
Continuity mistake: The shadow of the statue on the right of the piano changes between shots. When we first see it in this scene the shadow falls right in the center of the picture of father then as they all sit waiting for the attack the shadow falls on the right side of the picture. (01:40:25)
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)
Continuity mistake: As Melanie sooths Kathy she has a towel in her right hand that she drops on the green pillow to the left of the couch. We catch a glimpse of it lying there as Mitch runs to put wood in the fireplace. Melanie never touches the towel again but the next time we see it it's in the middle of the couch. (01:40:50)
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 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 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.