Revealing mistake: In the first shot of the children running down the hill looking from behind we see several children in the front of the pack (superimposed background) with fake birds tied around their shoulders. (01:14:00)
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)
Revealing mistake: A red headed kid in a blue shirt gets pecked on the cheek by a blackbird as he runs down the street. If you look closely at the bird before he touches the kid the blood he smears on the child's cheek is in a sack hanging off of his upper beak. (01:14:30)
Revealing mistake: The glasses of the girl in the red sweater fall off as she calls for Cathy to help her. If you look closely the glasses are broken before they hit the ground and we hear the sound of them smashing. (01:14:40)
Revealing mistake: As Cathy and Melanie go to help the girl in the red sweater lying in the road look at the poor helpless blackbird on her shoulder. Its beak is attached to her sweater so that it can't get away. (01:14:45)
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)
Revealing mistake: In two of the shots inside the cream colored car right after Melanie blows the horn either wires or a visual artifact from the superimposing process distinctly appear outside of the rear right window. (01:15:20)
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)
Audio problem: When the uptight mom asks how to get to San Francisco the guy at the bar that offers to take her lifts his glass up to take a drink and then we hear the overdub of the ice jangling in the glass. The jangling continues into the next shot after the glass is stationary and no longer near his lips. (01:22:10)
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)
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.