Check out the title credit sequence: birds are swooping and crashing into the titles, and the words shatter. Watching it on TV makes it look like your screen is about to shatter into pieces.
This motif continues throughout the film. There's broken glass everywhere: windows, eyeglasses, phone booths, and more windows. Glass is no match for these killer birds; it's often the first sign of the horror to come. Lydia has seen what they've done to her china teacups, and she dreads what she's about to find once she sees the same broken teacups at the farmer's house.
We get some foreshadowing of the broken glass motif in the first scene, when Mitch reminds Melanie of having broken a window during one of her pranks. Images of broken glass amp up the suspense throughout the film—when you see it, you know what's happened or is about to happen. The images add to the overall mood of the film. Not only is life meaningless and inexplicable, but your own little world is fragile and can be unexpectedly shattered in a moment.