Urban environments are rife with hazards that wild animals are not well equipped to deal with. Windows on houses and other structures are one such example, especially where birds are concerned.
Under the right conditions windows can become very reflective. Depending upon what is reflected in these windows, they can create the illusion of a legitimate pass through to wide open spaces. These reflected illusions can confuse fast moving birds into commiting to flying through, often a high speeds. When this happens a window collision will result. High speed impacts with glass windows can be devastating to birds, often causing severe injury or even death.
This series of photographs is of an American Kestrel that made a miscalculation of this type with fatal results.