Glass is invisible to both birds and humans. Humans learn to 'see' glass, through a combination of experience, cues and expectation, although every year, people are injured by walking into glass doors. Unfortunately, most birds’ first encounter with glass is fatal. They collide at full speed when they try to fly to sky, trees or other objects seen through glass or reflected on its surface. Light pollution exacerbates the problem, disorienting birds, especially night-flying migrants. Death is frequently not instantaneous, and may occur as a result of internal hemorrhage days after impact, far away from the original collision site. This makes monitoring the problem more difficult. - - No one knows exactly how many birds are killed by glass – the problem exists on too great a scale, both in geography and number, but estimates range from 100 million to one billion birds each year in the United States. Despite the enormity of the problem, however, currently available solutions can reduce bird mortality while retaining the advantages that glass offers as a construction material, without sacrificing architectural standards.
