There is no scientific evidence that suggests a bird striking one's window is an omen. This popular myth has long been associated with foretelling impending death, especially if an ill or injured person is located near the window struck by the bird.
There are several theories that point to why a bird may strike a window. If the window is clean enough to appear as if it is an open space, a bird can collide with the window pane without realizing the glass is a solid barrier. If a bird sees a reflection of itself in the window, the mirrored movements can resemble a rival and the bird can strike the glass as if it were striking a foe. High rise buildings with numerous windows can disorient a bird, potentially causing the bird to strike one or more windows as it attempts to reorient itself.