Continuity mistake: As Melanie signs the card to Cathy there's a guy in the phone booth to the left of the Bodega Bay bar who's not there in the next shot. (00:19:00)
Continuity mistake: After Melanie signs the card to Cathy she never puts her left glove back on. We're able to see her drive down uninterrupted to the boat and she never puts it on, but when she walks out of the car to the boat it's back on. (00:19:05)
Continuity mistake: Melanie's car is parked in front of the Bodega Bay bar as she fills out the card to Cathy then she pulls straight forward. In the next shot we see a car parked in front of her that she should have hit. (00:19:15)
Continuity mistake: When going along the water in her hired boat, Melanie passes a pole bobbing about in the water. In the next wide shot the pole has vanished. (00:19:50)
Continuity mistake: As Melanie approaches Doodles Weaver to get her boat there are no lobster traps where he is standing. Two shots later she climbs down the ladder and there are two traps to the right of the ladder. (00:20:00)
Continuity mistake: As Melanie approaches the boat keeper, there's a guy working on the boat to the right of him. When we first see this guy he's looking down, then in the next shot he's looking up. (00:20:00)
Continuity mistake: As Melanie approaches the boat keeper her pocketbook is hanging around her wrist. In the next shot as she speaks to him the pocketbook is in the crook of her arm. (00:20:00)
Continuity mistake: Doodles Weaver changes from looking at the boat to looking at Melanie's pocketbook between shots before she walks down the ladder. (00:20:05)
Continuity mistake: The wood on the ladder changes between shots. First it's unstained, then it stained green and then it's unstained as she gets to the bottom. (00:20:05)
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)
Continuity mistake: When Melanie starts on her journey across the bay the sun should be on her left as we saw in the way her shadow fell as she walked up to the boat keeper. A considerable amount of time must have gone by because the sun is now in front of her and to the right. (00:20:40)
Continuity mistake: As Melanie starts off across the Bay, we see two boats and a red building to the left of the matte drawing that weren't there when she approached the boat keeper. (00:20:45)
Continuity mistake: At the house Mitch's white car is parked facing the water. When we see it later it's parked at a 45-degree angle to the water but he hasn't touched it. (00:21:20 - 00:23:35)
Continuity mistake: As Melanie approaches Mitch's dock, a yellow pole extends from the far left side of the dock to the brown pail in the front of the dock. In the next shot, as she walks on the dock, the pole is gone. (00:21:35)
Continuity mistake: As Melanie walks up to Mitch's house the boat on the side of the dock is tied closer to the house between shots. (00:21:35)
Continuity mistake: As Melanie approaches Mitch's house for the first time we see one single fence around the yard where the children are attacked by the birds in the party scene. Later in the movie there's a double fence there. (00:21:35 - 00:52:35)
Continuity mistake: As Melanie pulls up to Mitch's dock a piece of rope tied to the post extends above the middle of the yellow buoy. In the next shot it's not there. (00:21:35)
Continuity mistake: Melanie leaves the oar of the boat leaning against the rear seat. Later when she comes back to the boat, the oar is lifted up and sitting on the middle and rear seat. (00:21:50 - 00:23:10)
Continuity mistake: As Melanie climbs out of the boat, she holds her pocketbook around her arm with the latch facing her body. After she enters the house the latch is facing outward and she's never taken it off her arm. (00:21:50)
Continuity mistake: When we see Mitch's red barn up close the sun is not shining and there are clouds behind it. The next time we see it a few seconds later it's a bright sunshiny day with a blue sky behind it. (00:21:55)
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.