Continuity mistake: As Lydia walks into Mr. Faucet's room her shadow from the window in front of her falls on the wall to the right of the hallway. In the next shot looking at her coming through the door there's no shadow on the wall. (01:00:15)
Continuity mistake: Lydia is holding her glove in her right hand and her pocketbook in her left as she walks into Mr. Faucet's room. The pocketbook never switches hands but when she runs out, she drops both of them from her right hand. (01:00:15)
Continuity mistake: When Lydia first looks into Mr. Faucet's room she sees a dead bird smashed up against his window. If you look at the drawing of the backdrop outside the window it's a bright sunshiny day but when Lydia was just outside, and again when she leaves, the sky was overcast. (01:00:20)
Continuity mistake: In the very last close-up of Mr. Faucet's face the right side of his collar has been changed from the previous shot. It's now tucked under his shirt. (01:00:30)
Continuity mistake: In his bedroom when Lydia sees Mr. Faucet's body with his eyes pecked out. Watch very closely and you can see his left foot move. (01:00:30)
Continuity mistake: The crew moves Mr. Faucet's body between shots. His right foot is moved under the bed and the pattern of the rug underneath his feet is the tip off. (01:00:30)
Continuity mistake: Mr. Faucet's tiny windows are not wide enough to let in the amount of sunlight that's shining on him from left to right on such an overcast day. (01:00:30)
Continuity mistake: After finding Mr. Faucet's body, Lydia gets back in her truck through the passenger side door. Notice that the window is rolled up and the side vent window is shut tight. Seconds later we see her inside the truck with the window rolled down and the vent window tilted open. (01:00:55)
Continuity mistake: The interior of Lydia's truck (especially the roof) changes when they show the close-up of her driving. (01:00:55)
Visible crew/equipment: As Lydia gets into her truck you see a reflection of the stage lights in the steering wheel. (01:00:55)
Visible crew/equipment: After finding Mr. Faucets body Lydia gets back in her truck and you can see the reflection of a crewmember wearing headphones in the lower right part of the windshield. (01:00:55)
Continuity mistake: The gravel walkways and driveways in "The Birds" are immaculately raked and maintained as seen in many shots. When Lydia pulls up to the house after finding Mr. Faucet's body, there's a tire mark in the exact same spot on the gravel from a previous take. (01:01:20)
Continuity mistake: As Lydia runs into the house after arriving back home from the scene at Mr. Faucets house there's a brown shade at the top on the window to her right. Earlier after the party as she opens the curtain and looks outside from the dining room, the window has a drape and a curtain and no brown shade. (01:01:40)
Continuity mistake: In the beginning of the movie when Mitch catches the yellow bird under his hat take a good look at the injury (subungual hematoma) he has under his left thumb nail. It takes weeks for these to go away, but Mitch's is gone when we see his left thumb again as he holds Melanie in the kitchen. (01:02:15)
Continuity mistake: As Melanie takes the tea into Lydia's room, the tea pot changes positions between shots. First the handle is to Melanie's right, then as she walks into the room it's facing her. (01:02:35)
Continuity mistake: When Lydia says to Melanie, "Do you think Cathy's alright at the school?" the two pictures hanging above her head disappear the next time we see her. (01:03:15)
Continuity mistake: As they talk in her bedroom and Lydia says, "Do I sound very foolish to you?", the pillows behind her are propped up differently between shots. (01:03:15)
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)
Continuity mistake: You can tell by the shadows on the wall that the headboard of Lydia's bed is pushed up further against the wall in the overhead shot. (01:04:00)
Continuity mistake: There's a bright reflection of a studio light on the wall to the right of the clock on the mantle that appears between shots. (01:04:25)
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.