Why do birds fly into windows?

Despite what it might look (and sound) like, cardinals and robins are not blindly running into windows this time of year. These territorial species are defending their home ground against their own reflection that appears in the window.

The ability to recognize oneself in a reflected image is rare in songbirds, with the exception of the corvid family (crows, jays, magpies). Breeding season is in full swing for northern cardinals (March-April) and starting up for American robins (April-May). Therefore, laying claim and defending territories is of high priority for males and females of these species. Cardinals and robins tend to be the most common culprits of window attacks. Often, the agitated bird will give a warning call to the reflection in the window -- if it is a cardinal, it will be a high pitched "chip" -- then vibrate its wings and lower its body or open its mouth, all of which are intended to scare the reflection in the window away.

Of course, these reflections do not leave and they match every aggressive posture until the cardinal or robin is forced into full-on attack mode. When this happens, the real bird hits the reflected bird and encounters the unyielding surface of the window, making the real bird even more intent on fighting the reflection. Sometimes, this fight can go on for weeks.

If a cardinal, robin or other songbird repeatedly is attacking a window, simply cover the window with newspaper for a few days or soap the outside of the window to prevent a reflection. Another option is to tape silhouettes of large birds, such as hawks or owls in flight, onto the glass. These serve the dual purpose of disrupting the bird's reflection, as well as eliciting fear.

Read on:  http://www.lancastereaglegazette.com/article/20120427/NEWS01/204270313