We usually see birds flying into windows in summers without any provocation. Window glasses reflect the open outside world, which appears a clear path of flight according to the bird’s point of view and encourages them to explore the wide sky, trees and flowers reflected on the windowpane. This misconception prompts the birds to collide with the windowpane, and procure fatal injuries. According to a survey, around one billion birds are killed annually only in North America from colliding with large windowpanes.
While another theory suggests that birds see their own reflection in the windowpane, or even bumpers of a car, and consider it as a threat to their territory, especially during the nesting or mating season. For birds don’t have the intellect to figure out the concept of reflection on the windowpane. During mating season, many species of birds like, American Robin, Northern Cardinal, Northern Mockingbird, American Goldfinch, Wild Turkey and Ruffed Grouse, have a strong sense of territory and protect their territory aggressively and chase off their rivals from their nesting area to minimize the competition for food and nest sites, so they could mate and grow their offspring in a peaceful and prosperous environment.
When they see their reflection on the windowpane, they perceive it as a potential threat to their territory and attack the windowpane brutally, which ends up in serious injuries, such as internal hemorrhaging and brain swelling, leading to an unnatural death. The most interesting aspect of this territorial behavior is that birds keep repeating their attacks on their own reflection, which appears a territorial or mating rival to them, on the windowpane, until the breeding season ends.