Visible crew/equipment: As Melanie drives up to have dinner at Mitch's, the reflection of the cameraman on the inside of the window can be seen moving in the upper-left hand portion of the screen. (00:32:10)
Visible crew/equipment: In a scene where Melanie, Mitch and Cathy are walking to the building on the hill along side Melanie's car, you can see the camera move in the reflection on the side of the car. (01:29:50)
Visible crew/equipment: In the close-up of the orange car there's a perfect shot of the cameraman in the bumper as the gas hits the tire. (01:25:10)
Visible crew/equipment: When Melanie walks out the door carrying the tray with the tea pot on it you can see a reflection of the stage lighting for the entire shot. (01:07:50)
Visible crew/equipment: As Mitch watches Melanie with his binoculars he looks directly into the opening of the camera lens. You can see its dark reflection on the right lens of his binoculars. The next time we see them the reflection is gone and both lenses look the same. (00:24:20)
Visible crew/equipment: As Mitch admires the lovebirds, before he walks out of the kitchen, you can see the shadow of the camera on the cabinet on the far left as it zooms in. (01:38:00)
Visible crew/equipment: As they talk Annie walks over to sit behind her desk and we see a quick shot of her hallway. Look closely at the mirror hanging on the far wall and you can see the quick reflection of the studio equipment in it. (00:42:05 - 00:46:50)
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: When Melanie pulls up to the school to check on Cathy we hear voices of the children singing inside and a crew member's reflection is seen moving in the car door as she opens it. (01:08:30)
Visible crew/equipment: When Cathy brings a pot of coffee into the dining room, you can see several bright studio lights reflected in it throughout this scene, and you can also briefly see the reflection of someone's arm moving about on it. (00:51:55)
Visible crew/equipment: When Melanie asks Annie if she can use her phone the reflection of a crew member behind her can be seen moving in the window of the house across the street. (00:31:55)
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)
Visible crew/equipment: As Melanie walks into the living room to find a place to put the bird cage down the shadow of the camera appears on her coat. (00:22:20)
Visible crew/equipment: You can see a flash as the boom mike operator hits the light above the front door when Mitch runs back with the hammer and nails. (01:43:45)
Visible crew/equipment: As they head for the restaurant, a crew member's reflection can be seen on the far left of the screen moving in the lower green section of the phone booth just as Mitch's shadow passes the phone booth. (00:26:25)
Visible crew/equipment: Out on the water you can see a lot of stage lights reflected in the engine of Melanie's boat. (00:21:30)
Visible crew/equipment: The reflection of the spotlight can be seen going before them in the bottles of booze as it leads Melanie and Mitch down the bar. (01:28:00)
Visible crew/equipment: After Melanie walks through the doorway, the camera moves backwards as she walks down the hallway. Its shadow moves across the doorpost to the left of the scales. (00:22:15)
Visible crew/equipment: As Melanie walks into the living room a moving shadow appears where it shouldn't be, to the left of the fireplace next to the clock. (00:22:20)
Visible crew/equipment: Just after Lydia says, "they are birds aren't they " we see Melanie and Cathy and the shadow of the cameraman moving on the chair at the far left of the screen. (01:37:25)
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.