Glass "skywalks" joining buildings, glass walls around planted atria, windows installed perpendicularly on building corners, and exterior glass handrails or walkway dividers are dangerous because birds perceive an unobstructed route to the other side.
Black Hole or Passage Effect
Birds often fly through small gaps, such as spaces between leaves or branches, nest cavities, or other small openings. In some light, glass can appear black, creating the appearance of just such a cavity or "passage" through which birds try to fly.
Factors Affecting Rates of Bird Collisions for a Particular Building Every site and every building can be characterized as a unique combination of risk factors for collisions. Some, particularly aspects of a building's design, are very buildingspecific. Many negative design features can be readily countered, or, in new construction, avoided. Others, for example a building's location and siting, relate to migration routes, regional ecology, and geography–factors that are difficult if not impossible to modify.
Glass causes virtually all bird collisions with buildings. The relative threat posed by a particular building depends substantially on the amount of exposed glass, as well as the type of glass used, and the presence of glass "design traps". Klem (2009) in a study based on data from Manhattan, New York, found that a 10% increase in the area of reflective and transparent glass on a building facade correlated with a 19% increase in the number of fatal collisions in spring and a 32% increase in fall.
The type of glass used in a building is a significant component of its danger to birds. Mirrored glass is often used to make a building "blend" into an area by reflecting its surroundings. Unfortunately, this makes those buildings especially deadly to birds. Mirrored glass is reflective at all times of day, and birds mistake reflections of sky, trees, and other habitat features for reality.
Non-mirrored glass can be highly reflective at one time, and at others, appear transparent or dark, depending on time of day, weather, angle of view, and other variables, as with the window pictured below. Tinted glass reduces collisions, but only slightly. Low-reflection glass may be less hazardous in some situations, but does not actively deter birds and can create a "passage effect," appearing as a dark void that could be flown through.
As building size increases for a particular design, so usually does the amount of glass, making larger buildings more of a threat. It is generally accepted that the lower stories of buildings are the most dangerous because they are at the same level as trees and other landscape features that attract birds. However, monitoring programs accessing setbacks and roofs of tall buildings are finding that birds also collide with higher levels.
Building orientation in relation to compass direction has not been implicated as a factor in collisions, but siting of a building with respect to surrounding habitat and landscaping can be an issue, especially if glass is positioned so that it reflects vegetation. Physical features such as outcrops or pathways that provide an open flight path through the landscape can channel birds towards or away from glass and should be considered early in the design phase.
Windowed courtyards and open-topped atria can be death traps for birds, especially if they are heavily planted. Birds fly down into such places, and then try to leave by flying directly towards reflections on the walls. Glass skywalks and outdoor handrails, and building corners where glass walls or windows are perpendicular are dangerous because birds can see through them to sky or habitat on the other side.
Glass that reflects shrubs and trees causes more collisions than glass that reflects pavement or grass (Gelb and Delecretaz, 2006). Studies have only quantified vegetation within 15-50 feet of a facade, but reflections can be visible at much greater distances. Vegetation around buildings will bring more birds into the vicinity of the building; the reflection of that vegetation brings more birds into the glass.
Taller trees and shrubs correlate with more collisions. It should be kept in mind that vegetation on slopes near a building will reflect in windows above ground level. Studies with bird feeders (Klem et al., 1991) have shown that fatal collisions result when birds fly towards glass from more than a few feet away.
Green roofs bring habitat elements attractive to birds to higher levels, often near glass. However, recent work shows that well designed green roofs can become functional ecosystems, providing food and nest sites for birds.
Siting of green roofs, as well as green walls and rooftop gardens should therefore be carefully considered, and glass adjacent to these features should have protection for birds.
Areas where fog is common may exacerbate local light pollution. Areas located along migratory pathways or where birds gather prior to migrating across large bodies of water, for example, in Toronto, Chicago, or the southern tip of Florida, expose birds to highly urban environments and have caused large mortality events.
Christine Sheppard is Bird Collisions Campaign Manager for the American Bird Conservancy. She earned a B.A. and Ph.D. at Cornell University, and has held multiple positions with the Wildlife Conservation Society and the American Zoo Association, focusing on ornithology and sustainability. She became interested in the issue of bird collisions because it combines "green practices" and wildlife conservation.
Glass balustrades, as used at the step backs on this building, are a dangerous trend for birds, especially when they front vegetation. Photo: Michael MoranExtra Large Image
Architectural cues show people that only one panel on the face of this shelter is open, while to birds, all the panels appear to be open. Photo: Christine Sheppard/ ABCExtra Large Image
Large panes of glass can appear to be a clear pathway. Photo: Christine Sheppard/ ABCExtra Large Image
Birds flying from a meadow on the left are channeled towards the glass doors of this building by a rocky outcrop to the right of the path. Photo: Christine Sheppard/ ABCExtra Large Image
The same glass can appear transparent or highly reflective, depending on angle, weather, or time of day. Photo: Christine Sheppard/ ABCExtra Large Image
Plantings on setbacks and rooftops can attract birds to glass they might otherwise avoid. Photo: Christine Sheppard/ ABCExtra Large Image
Planted courtyard spaces lure birds down, then prove dangerous when birds fly into surrounding windows trying to leave. For this extensively glazed and open building, birds could be confused by both reflections of plants inside the courtyard and by views of outdoor spaces seen through the building. Photo: David Owen/ Artifice ImagesExtra Large Image