Visible crew/equipment: A large equipment shadow can be seen on Melanie and Mitch as they stop to look at the people in the restaurant hiding in the corner. (01:28:05)
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: 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)
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 it sits on the table you can see the reflection of a crew member moving in the coffee pot at the center of the screen when Cathy asks Mitch why the birds are attacking. (01:38:20)
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: 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)
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.