During every autumn migration, the Chintimini Wildlife Rehabilitation Center admits numerous birds that have suffered injuries from colliding with windows. As birds group together and move en masse to warmer climes, they often frequent backyard birdfeeders and concentrate around homes.
Greater concentrations mean more birds are likely to crash into windows.
Rather than perceiving the glass, the birds think they are flying into the reflected vegetation. In houses with large picture windows, the birds may be able to see all the way through the building, which creates the illusion of open space and a clear flight path. They may be fleeing predators or they may even be the predator chasing a prey species, and for all of the above reasons do not perceive the glass in their high-speed maneuvering. Attach black silhouttes of diving hawks or strips of opaque tape to windows. Move houseplants away from the glass and close curtains over windows and sliding glass doors whenever possible.