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.
Other mistake: It seems well established that the film takes place over a weekend. However, Melanie drives up on Friday, stays at Annie's apartment that night, goes to Cathy's party the next day, Saturday and stays at Mitch's that night. Nowhere in the movie is it implied that she stays another night and in fact she doesn't. Everything else in the movie after the attack at Mr. Faucet's farm takes place on Sunday and there would be no school in session when the children are attacked.
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.
Continuity mistake: The gas station attendant drops the gas hose as the bird hits him and the first spray of gas wets the gravel to the right of it. The next time the hose is seen it's moved closer to the cement below the pump, it's shooting at a different angle and the gravel to the right is dry.
Continuity mistake: As Melanie and Mitch walk past the side of the restaurant look closely at the windows at the top left of the screen. They have transparent green shades on the rear windows that are pulled down about one-third of the way from the top. In the next shot, when we see the back of the restaurant, no shades are pulled down.