Continuity mistake: All through the party scene, the balloons change position, and balloons disappear and appear too. (00:49:45 - 00:51:10)
Continuity mistake: When Lydia comes outside with Cathy's cake, the door behind her is open halfway, but later when the birds attack, you can see the door is now closed completely. (00:49:45 - 00:50:45)
Continuity mistake: In one shot, a boy hides under a table, when the birds attack at Cathy's party. In the wide shot you can see the table is quite far onto the grass, away from the patio. But a few shots later, the table is right beside the patio. (00:50:20)
Continuity mistake: At the children's party, when the birds attack, you can see a child roll under the table with the cake and other party snacks on it. The birthday cake swaps ends with the bowl of drink a few shots later, with nobody re-arranging it. (00:50:20)
Continuity mistake: On the hill as Melanie talks to Mitch about her mother, the back of her collar changes between shots. (00:51:20)
Continuity mistake: In the shot of the party looking down from the top of the hill the barbecue is sitting behind and to the middle of two lounge chairs. In the next shot the barbecue is to the left of the two lounge chairs. At the end of the scene it's moved back. (00:51:35)
Continuity mistake: After Mitch finished pouring out the drinks for Melanie and himself there was still a little bit left in the bottle. He never pours out any more but as they get to the bottom of the hill when they throw away their glasses the bottle is empty. (00:51:35)
Continuity mistake: As Melanie and Mitch walk down the hill Annie and Cathy are standing in the middle of the yard with two other children between them and the lounge chair. In the next shot as we see Lydia carrying the cake, the two children disappear and Annie and Cathy have moved to the left closer to the door. (00:51:35)
Continuity mistake: As Mitch and Melanie walk down the hill he's behind her and they're not touching. In the next shot he has his left hand behind her. (00:51:35)
Other mistake: As Melanie and Mitch come back down from the hill the camera pans to Annie, you can see the stage lighting flickering on her sweater. (00:51:40)
Continuity mistake: When Annie turns to look at Lydia coming out of the house there are no shadows against the side of the house where the door is. In the next shot there's a shadow of a balloon there and the shadows on the back wall are spaced closer together. (00:51:50)
Continuity mistake: As she carries the birthday cake out from the house, the position of Lydia's necklace changes between shots. (00:51:50)
Continuity mistake: As Lydia looks at Melanie and Mitch walking down the hill, the large piece of molding in the corner between the door and window disappears between shots. (00:51:50)
Continuity mistake: As Lydia carries the birthday cake out, the shades of the window behind her are open. When we see this window again a few seconds later as she runs into the house the shades are closed. (00:51:55)
Continuity mistake: As Lydia carries the birthday cake out from the house the position of the vine growing on the pole on the right jumps from right to left between shots. (00:51:55)
Continuity mistake: Annie's sweater and face go from having no sunlight on them to being in the sunlight, before she moves towards Cathy as the birds attack the party. (00:51:55)
Continuity mistake: The doorknob on the door behind Lydia disappears between shots as she brings the cake out. (00:51:55)
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)
Continuity mistake: As Mitch and Melanie walk down the hill to the party the sun is behind them casting the shadows of the children towards the house. Three shots later in the close-up of Cathy the sun is in the opposite direction coming from across the bay. (00:52:00)
Continuity mistake: The house has only one gazebo and when the first bird swoops down at Cathy it flies over a different gazebo from the one we see later in the scene. The top is pointed and higher, and the correct one has a tree above almost touching it and shadows from the tree falling on it. (00:52:00)
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.