Audio problem: The overdub of Melanie playing piano in the living room is the same volume when the scene changes to a different room as Mitch and Lydia talk inside the kitchen. (00:37:55)
Audio problem: As Lydia talks on the phone about the chicken feed she says, "Well I'll try to go over and see him, maybe he ...". The next time she says something, her lips are moving differently. (00:35:15)
Audio problem: That is not a Martin outboard motor sound when Melanie is on the bay; it is a two-cylinder OMC outboard motor running (in neutral).
Audio problem: As Melanie pulls up in front of the general store a woman with bags of groceries exits the store. They dub in the sound of the door closing but they didn't dub the sound of the bell on top of the door that we hear later as Melanie walks in. (00:12:20)
Audio problem: As Melanie sits and smokes at the piano, Cathy says, "Of course they're mostly hoods" but her lips say something different. (00:35:55)
Audio problem: As the cameraman moves in on Mitch and the birds in the kitchen, listen very closely and you can hear his wheels squeaking on the floor. (01:38:00)
Audio problem: As Mitch hammers the mirrored cabinet to the front door they overdub a second hammer sound over the actual sound to make it more intense. If you listen closely they don't get them all right and they miss the very first swing. (01:43:50)
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.