Bird Continually Hitting Window
This is a problem that is most common in spring as male birds are establishing and defending territories. The male sees his reflection in the window and thinks it is a rival trying to usurp his territory. He flies at the window to try and make the rival leave. This behavior is most often reported in mockingbirds, robins, and cardinals.
Stopping this behavior can be difficult. Covering the window so that the bird does not see its reflection is one option particularly if there is a specific window you would like to keep them away from. However, in most cases, the bird will just fly to a new window and start again.
If this behavior is occurring in spring, it is probably associated with territorial behavior at the beginning of the breeding season and once a male has found a mate and has a nest with eggs or young, the behavior should stop.
One exception is with male cardinals who will often keep this up all year long as they maintain year-round territories. An option in this case is to take an old mirror or get a car side mirror and put it out in the yard in a place where the male should find it, and let him fight his phantom rival out there away from your windows.