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: Lydia sits up in her bed and weeps, "Forgive me, this business with the birds has upset me," and the white chair moves about a foot to the left of where we saw it before without being touched by them. (01:07:05)
Continuity mistake: On the hill as Melanie talks to Mitch about her mother, the back of her collar changes between shots. (00:51:20)
Continuity mistake: In the restaurant Melanie doesn't have to poke around at all as she opens her pocketbook and pulls out and the compact and tissue. They must have been the only things placed in her prop pocketbook so they would be ready to come out without any fuss. (00:27:00)
Continuity mistake: Melanie fiddles around in her pocketbook looking for the two cards, but in the next shot they're in her hand ready to come out. (00:22:25)
Audio problem: When Mitch says to Lydia in the kitchen, "Yeah, I suppose so. Her father's part owner of one of the big newspapers in San Francisco," his lips don't match up with what he's saying and his voice sounds entirely different from his previous sentence. (00:38:10)
Continuity mistake: As Mitch and Melanie walk down the hill he's behind her and they're not touching. In the next shot he has his left hand behind her. (00:51:35)
Audio problem: After the gull hits her in the head, Melanie says to Mitch, "Yes, I think so," but her mouth does not go along with what she's saying. (00:25:35)
Continuity mistake: When Lydia recalls Mr. Faucet's face, the chair on the left is casting a very dark shadow against the wall that wasn't there before when Melanie brought the tea in. (01:03:35)
Visible crew/equipment: During the final bird attack Lydia and Kathy stand in the kitchen door and the shadow of a crew member in the kitchen can be seen moving on the wall behind them. (01:41:20)
Continuity mistake: The picture frame on the left of Annie's desk has been moved from when we first saw it. It's been turned in slightly and moved away from the lamp at the end of the scene. (00:46:45)
Continuity mistake: When the birds start to attack again Mitch starts to run out of the restaurant and the old drunk at the bar is asleep. In the next shot of Mitch at the door he's suddenly awake. (01:24:40)
Continuity mistake: As they talk to the policeman Lydia rearranges the picture of her father above the piano. In the next shot a black bird appears next to the fallen statue on the left that wasn't there before. (00:57:30)
Revealing mistake: As Mitch drives off in his Galaxy 500, some spectators across the street are watching the crew filming. (00:08:00)
Audio problem: They use a different overdubbed tone for the bell on the door of the general store when the manager opens it. Before as Melanie walked into the store we heard the real sound. (00:13:10)
Continuity mistake: When Melanie shows Annie the nightgown she bought, there's a box directly behind Melanie's brandy glass. Now it's silver, but before it was turquoise. (00:46:55)
Continuity mistake: After she throws up, Melanie brings Cathy back out from the bathroom and the scales on the table and the curtains behind them are very well lit. No one turned off any lights, but a few shots later when the birds start the next attack Melanie stands and the scales and curtain are in the dark. (01:40:10)
Continuity mistake: In the party scene the close-up of Melanie, Annie and Mitch together had to be shot at a totally different time. Before, the structure behind the swing set had a long shadow across its window. Now the window has no shadow on it at all. (00:52:30)
Continuity mistake: After a good night's sleep, Melanie looks out the window to see Lydia drive off in the green pick-up truck. If you look at the light fixture above and to the left of her head you can see by the shadow on the wall that the lighting changes between shots. (00:58:25)
Continuity mistake: As Melanie and Mitch approach the top of his hill he's walking in front. In the next shot she is. (00:48:10)
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.