We might have heard the thud or even seen it happen while at home or at work. It is easy to assume this is a pretty accurate reflection of the frequency or volume of the problem. But, the highest volume of collision related deaths happen without witness during the night or early morning hours.
Night migrating birds are confused and disoriented by artificial light. If the light is strong enough it will cause birds to circle until the point of exhaustion and death. If the light is more diffuse, as is common, it can disorient and confuse them causing them to stray off course and end up in our cities and towns.
If they have managed to survive the landing, they will soon face reflective and transparent glass as the sun rises, offering what appears to them to be opportunities for food and cover. Night or early morning collisions claim many more birds than we may may ever know about because an injured or recently deceased songbird is easy pickings for rats, raccoons, foxes, gulls, raptors and other predators and scavengers who will quickly remove this easy meal before a person encounters it.
Very often, building maintenance crews simply remove and discard deceased birds as part of their early shift work.
Working together in building monitoring programs and utilizing strategies to prevent deaths wherever possible is the only real way to understand the incidence and to have an impact on this problem. Light draws them in more frequently, but ultimately colliding with glass is what injures or more likely, kills them. Seen or unseen, it is there and we can stop it.