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)
Continuity mistake: The children pass the cream-colored station wagon parked across from the white picket fence as they run down the hill. But you can tell by looking at the rear fins that it's a different car than the one Melanie brings the two children into several shots later. (01:14:00)
Continuity mistake: The end of this scene as the children near the bottom of the hill has been shot at a different time from the beginning, as the sun is now the front of them and their shadows are cast in the opposite direction. (01:14:00)
Continuity mistake: A blond boy in a light brown shirt is attacked by a crow and goes from having no blood on his neck to having a lot of blood smeared on his neck between shots. (01:14:10)
Continuity mistake: As the kids are almost at the bottom of the hill they pass the church, fence and propane tanks again that were back at the top of the hill, on the same block as the school. (01:14:25)
Continuity mistake: As Melanie and the two girls get into the cream colored station wagon parked on the side of the road the upholstery on the driver's side door changes drastically between shots. (01:15:00)
Continuity mistake: Inside the restaurant Melanie talks to her father on the phone at the bar. The phone's chord changes position between shots. At first it's resting on her sleeve then in the next shot it's on her forearm. (01:15:45)
Continuity mistake: When we first see Melanie talking to her father the phone is pushed right up against the edge of the bar. The next time we see it's at a 45-degree angle to the side of the bar and no one has touched it. (01:15:45)
Continuity mistake: As Melanie talks to her father on the phone, Mrs. Bundy walks into the restaurant and an ash tray that Melanie and Deke didn't touch appears right next to the phone between shots. (01:16:10)
Continuity mistake: When Mrs. Bundy tells Melanie that there is a difference between crows and black birds Deke's right arm is leaning on the recessed edge of the bar close to his body. In the next shot his right arm is moved closer to Melanie, placed in the middle of the bar. (01:16:30)
Continuity mistake: Mrs. Bundy's handkerchief in the left pocket of her jacket changes from when we first see her walk into the restaurant. Then when the drunk at the bar starts reciting from the Bible, her handkerchief almost disappears and then reappears several shots later. (01:18:10)
Continuity mistake: The pink flowers behind Deke get rearranged between shots and no one touched them. The second time we see them, one flower in the center sticks out higher than the rest. (01:18:40 - 01:19:20)
Continuity mistake: As Mrs. Bundy tells everybody how many birds there are in the world a lock of hair falls out from under fishermen Sebastian Sholes' hat between shots, disappears and falls out again as the scene progresses. (01:21:00 - 01:24:00)
Continuity mistake: Two gulls attack a man by his car and he falls to the floor, knocking a red bucket and metal can to the floor near him. But in the next shot of Mitch running to help him, the red bucket has now moved further away from the man. (01:21:15)
Continuity mistake: When the birds attack the town, two horses gallop down the street, dropping crates of vegetables. In a later shot the crates are more broken and the vegetables are more spread out and are in a much different position than when they were in the earlier shot of when they were dropped. (01:23:45)
Continuity mistake: When Melanie looks out the window to see the gas station attendant attacked by the birds the coat hanger that was there earlier in the scene, next to the window, is gone. (01:24:30)
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: The gas station attendant is standing on a red hose as the bird hits him. The next time we see him he's out cold but the red hose has been neatly wrapped up even though no one went near it. (01:24:35)
Continuity mistake: The gas station attendant drops the gas hose as the bird hits him and the first spray of gas wets the gravel to the right of it. The next time the hose is seen it's moved closer to the cement below the pump, it's shooting at a different angle and the gravel to the right is dry. (01:24:35)
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.