Continuity mistake: As Mitch takes Melanie to try to get help he tells a guy, "A gull hit her" and a long shadow of a wire appears on the white sliding door directly behind them. In the next shot we see the door in middle of the screen and the shadow is gone. (00:26:05)
Continuity mistake: When Mitch takes Melanie to try to get help after she is hit by the gull the first door they come upon says, "out to lunch". If you look closely as they walk up to the door they're in bright sunlight but in the next shot as they get to the door it's overcast. (00:26:10)
Continuity mistake: As they go up to the restaurant to get help Melanie says, "I had a booster before I went abroad last May" and the background changes abruptly between shots. (00:26:15)
Continuity mistake: As they start off going up to the restaurant for help two large brown crates appear out of nowhere on the dock behind them to the left of the screen. (00:26:15)
Continuity mistake: After Mitch suggests that they get help up at the restaurant, the shadow of Melanie's hand on her forehead changes between shots. (00:26:15)
Continuity mistake: After Mitch and Melanie walk past the door that says, "Out to Lunch," look at the numbers on the front of the boat behind them you'll see that they change between shots. (00:26:15)
Continuity mistake: As Melanie and Mitch approach the restaurant there's a fence that they pass on the left before they get to the door. When we see this spot later in the movie the fence is not there. (00:26:25 - 01:26:15)
Continuity mistake: As Melanie and Mitch walk past the side of the restaurant look closely at the windows at the top left of the screen. They have transparent green shades on the rear windows that are pulled down about one-third of the way from the top. In the next shot, when we see the back of the restaurant, no shades are pulled down. (00:26:25)
Continuity mistake: There's a fixture with a wire hanging down from it in the front left corner of the restaurant. When they built the interiors for this scene they forgot to put it there and when Melanie opens the phone booth door trying to get out the wire hanging from the bottom of the roof of the restaurant is missing. In the close-up as Melanie tries to get out the cord that we saw previously sticking out has been tucked back in towards the phone for this shot. (00:26:25 - 01:26:45)
Continuity mistake: As Melanie and Mitch walk past the side of the restaurant look closely at the second and third windows from the right. Now we can see right into the place through these but after they walk into the restaurant look behind the bar all three windows have drawn blinds. (00:26:25)
Continuity mistake: No one is sitting inside the window at the left of the restaurant as they walk by. In the next shot inside the restaurant there are people sitting there. (00:26:25)
Continuity mistake: The phone booth changes when Melanie is in it during the bird attack later in the movie. Here the green part on the bottom has three vent slits per row, later when the fire is behind it you can see four vent slits per row. (00:26:25 - 01:26:15)
Continuity mistake: As they get to the restaurant door, Melanie is to the right of Mitch, but in the next shot inside the restaurant she walks around him again to get on his right side. (00:26:30)
Continuity mistake: Before Melanie sits down at the table in the restaurant there's a little bit of powder on it left from the previous customer. In the next shot it's cleaned up and shiny. (00:26:45)
Continuity mistake: After the gull attack Deke, Carter comes around the bar twice as Melanie sits down. (00:26:45)
Continuity mistake: Before Melanie sits down at the table the position of the metal milk container changes from the left to the right side of the table between shots. (00:26:45)
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: As Melanie takes the Kleenex out of her pocketbook we can see the latch facing away from her. She doesn't touch it again but near the middle of this scene, as she holds the cotton on her head, the pocketbook is reversed with the latch facing her. (00:27:10 - 00:28:50)
Continuity mistake: When Mitch unwraps the cotton to put peroxide on Melanie's cut, the ashtray on the table is empty. Mitch never lets the piece of cotton out of his fingers until he has Melanie hold it on her head, and no one else touches the cotton on the table, yet near the end of the scene another used piece of cotton appears in the ashtray. (00:27:15 - 00:28:50)
Continuity mistake: Melanie cleans off all the blood on her forehead and in her hair with the Kleenex, but in the next shot there's more blood that appears in her hair than before. (00:27:20)
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.