Window collisions are the second biggest killer of birds with just under half of those collisions being with homes and smaller buildings. In a study last year, about 3.5 billion birds died from Bird-Window Collisions. The two biggest factors for why birds fly into windows are transparency and reflectiveness. The birds either see something on the other side of the glass that they want to get to — we call this the pass through effect. Reflectiveness is when they see a reflection of something they want to get to.