How birds see, what, when and where they are seeing contribute to the danger. A NYC Audubon study done in 2009 collected data on 5,400 bird collisions in Manhattan over a ten year period. Focusing on factors associated with window collisions, the study found that most victims (two thirds of the collisions resulted in fatalities) were migrating birds from the warbler, thrush and sparrow families. Bird strikes were also found to occur more frequently along "portions of the exterior glass surface that reflect outside vegetation." The study further found that collisions were more like to happen during daylight hours as opposed to evening. The Audubon study concluded that that migrating birds are more at risk of window kills while Klem notes that the more birds around glass the more collisions, with our own misplaced bird feeders being just as likely, if not more so, to place birds at peril. Both statements are no doubt correct as long as glass is present for birds to fly into.
Our human disconnect with failing to appreciate the severity of how dangerous navigating the built environment is for birds may result from our failing to adequately contemplate how very differently birds perceive the world. Birds like humans rely on vision as their primary sense and while bird vision is extraordinary compared to human vision in some respects it is markedly different in how birds process visual information-from the rate at which their brains perceive and process information, to the spectrum of ultraviolet light they are aware of. Structural differences in anatomy add to differences in perception. Avian expert, Graham Martin succinctly compares human vision to bird vision: "The human visual world is "in front" and humans move "into" it" and "The avian world is "around" and birds move "through" it." For a bird, with eyes on either side of the head, the field of vision is mostly along the side or lateral with visual coverage above and behind the head. Frontal vision is employed for specialized tasks, beak control, chick feeding, nest construction and targeting an object right before capture. There is a blind area directly in front of the bill and a limited binocular forward field, this forward field is the bird's peripheral vision.