Taking a Closer Look at Bird–Window Collisions

By Tim O’Connell

Linked paper: Building façade-level correlates of bird–window collisions in a small urban area by C.S. Riding, T.J. O’Connell, and S.R. Loss, The Condor: Ornithological Applications.

You have probably heard the startling impact of a bird hitting a glass pane, or perhaps you’ve come across a dead bird beneath a window. While the effect on the bird was lethal, you were likely affected by that event too. People don’t want birds to die at their windows, and they are increasingly looking for solutions when they discover collision problems at their homes and businesses.

Birds can be at risk wherever they encounter glass in their environment, but that risk varies from place to place. Identifying what causes some areas to be collision hotspots can inform building retrofitting and design approaches that make windows safer for birds. We know, for example, that coastlines and other features that concentrate migrants can put more birds at risk of colliding during their spring and fall migrations. Tall, glassy buildings ––especially those lit from within at night –– can be particularly dangerous for migrating birds. ­ Smaller buildings and homes outside of major flyways are sites of collisions, too, and features that attract birds, such as vegetation or feeders, also can increase those collisions. Beyond that, we still have much to learn about the fine-scale factors that put birds at risk of collision.

I started keeping track of window-killed birds in 1993. One day I stumbled (almost literally) across five collision victims along one façade of a single building in a corporate office park. All five were Nearctic-Neotropical migrants. This wasn’t a coastal skyscraper or a home with bird-attracting feeders. This was a group of unremarkable, three- to five-story buildings surrounded by manicured lawns and small forest patches. You might know of such a place in your own community. I have been searching such places for window-killed birds ever since.

photo of the front of a building with many large windows, with a small dead bird lying on the pavement in front of hte windows
Façade features illustrated where the window-killed Tennessee Warbler (Leiothlypis peregrina) was found at a building on the campus of Oklahoma State University in Stillwater, OK. This façade has a high proportion of reflective glass and forms the terminus of an alcove. Note that this façade is highly reflective with background features visible, provides transparency to planters with vegetation inside the building, and creates a pass-through illusion to a glass alcove on the opposite side of the building. Photo by Tim O’Connell.

In my experience, some things about bird–window collisions in such developments are predictable. Collision rates are seasonal; migrants in passage are disproportionately represented compared to residents. Other things are unpredictable. For example, despite the prevailing directions of travel that birds follow in migration, fine-scale movements, local attractants, and specific architectural features of buildings largely determine the location of collision hotspots.

My Oklahoma State University colleagues Corey Riding and Scott Loss and I recently investigated sources of that fine-scale variability at homes and mid-size buildings in our small city (pop. 46,000) of Stillwater, Oklahoma. Corey and his technicians (and sometimes his children) walked mile upon mile to survey about fifteen building perimeters for window-killed birds over two years. Rather than summarize the data by building, Corey analyzed carcass locations to within two meters of where they were found. This allowed us to examine mortality by individual façades –– by season and by year for at least eight of the 63 species we confirmed as collision victims. The variability in risk factors among species provides direction for future research, but there were some generalities in the results. Long, tall façades with a high proportion of glass surface area were the most deadly for the most species. Indentations in a building’s shape, such as façades at the terminus of an alcove, also accounted for a disproportionate number of collisions compared to flat or curved façades.

Ours is the latest of multiple studies to confirm that a high proportion of glass area is a strong predictor of collision hotspots. Reducing the loss of birds to window collisions depends on a societal commitment to reduce the overall surface area and proportion of reflective glass in both newly designed and existing buildings. Our example of methods and analysis to tease out fine-scale influences on bird-window collisions is highly repeatable and can help guide actions to reduce collisions in other communities. Lessons from this research are already informing collision mitigation strategies and future bird-friendly architectural designs on our campus. We welcome the news of any community to make such commitments; there is much work to be done.

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