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: 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: As Melanie parks in front of the general store stage lighting and the reflection of a crew member can be seen in her car door as she opens it. (00:12:20)
Visible crew/equipment: As Melanie sits down on Annie's couch you can see the shadow of the boom mike move across the books in the bookcase to her left. (00:41:20)
Visible crew/equipment: As the birds fly around the house there's a close-up of Melanie holding Cathy with Lydia in the background. If you look closely near the picture on the left wall you can see the shadow of the wires holding up some of the birds. (00:55:30)
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)
Visible crew/equipment: After Mitch puts the bird back in its cage as he walks towards Melanie the shadow of the camera appears on his jacket. (00:06:30)
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.