Birds don’t see the way mammals do, and a reflection in a window might look like another bird. Most birds that are active during the day have eyes on either side of their heads, which gives them a wide field of view but little depth perception. In the springtime, many birds are territorial and when they establish territories they become aggressive and chase off intruders. Unfortunately, they don’t distinguish between their own reflection in a window (or car mirror) and they try to chase that reflection off. People sometimes add awnings and window screens to eliminate the reflection and stop birds from colliding into their homes or office buildings. Although a bird can crash into glass at any time, the behavior is seen less often when nesting season begins.