Most people with wooded yards have had birds collide with their glass windows. This is a common but huge problem that takes the lives of millions of birds annually. What to do?
The window collisions occur when a flying bird sees the refection of the yard or sky in the glass and flies into it. Anything that will reduce or eliminate these reflections in the glass will reduce bird collisions. Some people hang shiny streamers or fine screening on the windows during peak migration periods. Others cloud the glass with soap. If the house is under construction, the windows can be installed tilting downward slightly to reduce reflections. Other people paste silhouettes of hawks, owls, or spider webs on the windows, which is effective only around the area where the silhouette is located. Locating feeders on or near the windows will reduce the speed at which birds hit the glass.
It has been my experience that only one out of ten collisions is fatal. Usually the bird is stunned, falls to the ground, and begins a period of recovery that may take up to an hour. During that recovery period, the bird is vulnerable to hawks, house cats, or weather conditions. Some hawks have learned a hunting strategy of swooping down on active bird feeders, causing the birds to panic in all directions, including into windows, where they become easy prey.
To protect a stunned bird that has hit a window, I cover it with a large kitchen sieve. The bird is less visible and is confined, allowing it time to recover. When the bird attempts to leave the sieve, it has recovered enough to be liberated.
-- George H. Harrison