BIRD-WINDOW COLLISIONS

Collisions of birds with windows were studied by reviewing the literature, collecting data from museums and individuals, monitoring man-made structures, and conducting field experiments. Approximately 25% (225/917) of the avian species in the United States and Canada have been documented striking windows. Sex, age, or residency status have little influence on vulnerability to collision. There is no season, time of day, and almost no weather condition during which birds elude the window hazard. Collisions occur at windows of various sizes, heights, and orientations in urban, suburban, and rural environments. Analyses of experimental results and observations under a multitude of conditions suggest that birds hit windows because they fail to recognize clear or reflective glass panes as barriers. Avian, manmade structural, or environmental features that increase the density of birds near windows can account for strike rates at specific locations. A combination of interacting factors must be considered to explain strike frequency at any particular impact site. Received 28 Oct. 1988, accepted 17 April 1989. The earliest account of a bird hitting a window in North America is by Nuttall (1832:88). He described a Sharp-shinned Hawk (Accipiter striatus) which, in the pursuit of prey, flew through two panes of greenhouse glass only to be stopped by a third. Townsend (1931) described a series of five fatalities of the Yellow-billed Cuckoo (Coccyzus americanus). His paper was the first to suggest that avian vulnerability to windows may be more marked in some species than in others and that specific windows claim a succession of victims. He termed the victims "tragedies" and apparently regarded them as rare, self-destroying incompetents. Picture windows were relatively uncommon through the end of World War II, and there was little reason for concern about their threat to birds. In the postwar period, a building boom stimulated the rapid expansion of the sheet glass industry, and large glass windows were incorporated into the designs of new and remodeled structures. Today, it is not uncommon to find modern buildings that are entirely surfaced with glass. I found 88 papers reporting bird-window collisions, primarily after the mid-1940s (Klein 1979). They document strikes in North America, South America, West Indies, Europe, and Africa, and, with few exceptions are cited in annotated bibliographies on man-caused mortality to birds (Weir 1976, Avery et al. 1980). However, most textbooks and encyclopedia treatments of ornithology present little, if any, description of the fatal hazards that windows pose to birds. The sheet glass industry and its commercial allies appear to be unaware of the problem. On the other hand, I found avian fatalities resulting from window strikes to be common knowledge among the general public. Birds have been reported to strike two general types of windows as classified according to their visual effects on the human eye. These are transparent windows which appear invisible and reflective windows which mirror the facing outside habitat. Two general types of collisions have been described (Wallace and Mahan 1975:456) and both reveal the ability of glass to misinform and misguide at least some birds. One primarily involves birds such as Northern Cardinal (Cardinalis cardinalis) that commonly flutter against picture windows and harmlessly peck the glass during the spring and summer. These birds seldom, if ever, stun or injure themselves or shatter the glass and usually are males defending their territories against their reflected images. In the second type, birds fly into transparent or reflective windows as if unaware of their presence. These collisions often have fatal consequences, and are the subject of this paper. In this paper my objectives are: (1) to propose an explanation for why birds collide with windows, (2) to describe and analyze species, environmental and manmade structural characteristics associated with bird-window collisions in the United States and Canada, and (3) to suggest how these select characteristics account for the differential frequency with which birds strike windows in various man-made structures.