Continuity mistake: Before Melanie runs back into the school to warn them about the birds, the sun was behind the school. Now her shadow on the steps is in the wrong direction, going in front of her. (01:11:45)
Continuity mistake: As Melanie runs to Annie in the classroom and says, "Close that door quickly," all the children's jackets and sweaters and everything on the bench below changes between shots. (01:12:00)
Continuity mistake: As Melanie runs to Annie, the pile of papers on the left of her desk changes between shots and a blue booklet appears on top. (01:12:00)
Continuity mistake: When Melanie runs back into the classroom Annie's hands are by her sides. In the next shot they're up higher. (01:12:00)
Audio problem: As Annie says to the students, "All right children now quiet," her lips do not match what she says. This continues before she says, "Miss Daniels" and her lips move and nothing comes out. (01:12:20)
Audio problem: After Annie tells the students about the fire drill, their lips, especially those of the girl in the front, are not in sync with what they're saying. (01:12:30)
Continuity mistake: As the children leave the school none of them go to the coat rack to get their coats. Later many are wearing their coats while running down the hill. (01:13:05)
Audio problem: As the birds sit on the monkey bars outside the school we hear the sound of many birds flapping their wings for several seconds before any take off. (01:13:20)
Continuity mistake: As the children start running down the hill away from the school the road is wet. In the next shot of the same spot it's dry. (01:13:30)
Continuity mistake: The kid in the purple plaid shirt and white pants is the first down the street way in front of everybody else. But in the next two shots he's in the back of the pack looking back at the school both times. (01:13:30)
Revealing mistake: As the children start running down the hill from the school a girl in a white dress and blue sweater is cracking up. (01:13:35)
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)
Revealing mistake: The birds that attack the schoolchildren have no shadows. (01:13:40)
Continuity mistake: As the school children are running down the hill a white fence appears between shots in the background on the right of the screen. (01:13:40)
Continuity mistake: As Melanie drove past the propane tanks down the road from the school the sun was to her right casting a shadow to the left of the car. As the children pass the exact same spot the sun is over their left shoulders, casting a shadow in front and to the right of them. (01:13:40)
Deliberate mistake: Some of the children that are running in front of the superimposed background were never in the classroom. For example the blonde girl in the blue and white sweater in the middle of the second shot and the girl in the green sweater in the center of the third shot were not there. (01:13:40)
Continuity mistake: The second shot of the background of the school and the children running down the hill is superimposed behind the five live action children in the front. Watch the boy to the right in the red sweater very closely because in the next shot he doesn't change but the five children in the front do. (01:13:40)
Continuity mistake: The blonde girl in the red with the red bows in her hair has a crow on her head that suddenly jumps to her right shoulder between shots. (01:13:45)
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 holds out her right hand to shield the boy in the white jacket we get a really good close-up of his neck. It was bleeding three shots ago as a crow was attacking him but now it's not. (01:13:50)
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.