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)
Continuity mistake: As Mitch checks the doors and windows he returns to the living room and everything on the table has been rearranged and no one moved from where they were sitting. He's not there but his cup moves from the being next to the edge of the tray to being adjacent to the handle. (01:38:15)
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)
Continuity mistake: As Lydia cleans off the coffee table and walks out with the tray the green pillow on the left of the couch is at the center of the screen. No one touches it but a few shots later as Melanie brings Cathy back from the bathroom it's been fluffed up with the right corner now being higher than the left. (01:39:15)
Continuity mistake: As they await the last bird attack Lydia cleans off the coffee table and comes back and sits in the chair next to the piano. Her head looks left and then right between shots. (01:39:20)
Continuity mistake: After Lydia cleans off the coffee table when she returns and sits down there are only two boxes and a bowl on the coffee table. When Melanie brings Cathy back from the bathroom there are more than three items, such as an ashtray, on the table and Mitch and Lydia haven't moved from their seats. (01:39:20)
Continuity mistake: After she throws up, Melanie brings Cathy back out from the bathroom and the scales on the table and the curtains behind them are very well lit. No one turned off any lights, but a few shots later when the birds start the next attack Melanie stands and the scales and curtain are in the dark. (01:40:10)
Continuity mistake: The lamp over the dining room table, when Melanie brings Cathy back from the bathroom, is not this same lamp that was there as they were listening to the television report. Before it had a glass bottom and now the entire bottom is made of metal. (01:40:15)
Continuity mistake: The shadow of the statue on the right of the piano changes between shots. When we first see it in this scene the shadow falls right in the center of the picture of father then as they all sit waiting for the attack the shadow falls on the right side of the picture. (01:40:25)
Continuity mistake: The seat that Mitch sits on is placed right in front of the piano. Later in the scene it moves to the right without being touched. (01:40:25)
Continuity mistake: As Melanie sooths Kathy she has a towel in her right hand that she drops on the green pillow to the left of the couch. We catch a glimpse of it lying there as Mitch runs to put wood in the fireplace. Melanie never touches the towel again but the next time we see it it's in the middle of the couch. (01:40:50)
Continuity mistake: During the bird attack, Lydia and Cathy start running towards the dinner table and the drapes to the left of the door are pitch black. The next time we see these drapes they're lit up. (01:41:00)
Continuity mistake: As Mitch throws logs onto the fire we see the couch behind him and no one is there. In the next shot Melanie appears sitting on the pillows on the edge of the couch. (01:41:05)
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)
Continuity mistake: After Mitch puts the logs in the fireplace he runs without jumping over anything to the window behind the desk. Later when we see that spot just before the lights blow out the logs extend all the way to the brown chair without any space in between for him to run through. (01:41:20)
Continuity mistake: In the shot of Mitch holding the log just before he runs to the window behind the desk, the brown reclining chair that sits by the fireplace has been moved for the camera. We see it there in the scene when Lydia says, "Did you get the windows in the attic Mitch?" and earlier in the movie when Melanie brings the lovebirds in. (01:41:20)
Continuity mistake: In the last bird attack Lydia and Cathy run past the drapes behind the dining the room table and they're hanging straight down to the floor. Before when we saw Mitch and Lydia sitting the drapes were crooked. (01:41:20)
Continuity mistake: During the bird attack Melanie crushes the shade on the lamp in the corner. Several shots later it looks almost like new. (01:41:35)
Continuity mistake: When Mitch reaches out the window to grab the shutter the curtain has been pushed all the way to the left of his shoulder and he isn't touching it. The next time we see the curtain it's covering the entire window to the left of his head. (01:41:50)
Continuity mistake: When Mitch is closing the shutter after the birds have started to fly into the window you can visibly see blood on the shutter, yet when they switch cameras and he shuts the shutter the blood is gone. (01:42:05)
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.