Full Citation: Riding, C. S., O’Connell, T. J., & Loss, S. R. (2020). Building façade-level correlates of bird-window collisions in a small urban area. The Condor, 122, 1-14. https://doi.org/10.1093/condor/duz065.
The light at the end of the tunnel isn’t always a good one.
Imagine the following scenario: you’re driving late at night on a country road. It’s completely dark except for your headlights and the stars above. Suddenly, you see extremely bright lights on the horizon, and they’re getting closer to you. As you approach, the lights are so bright that they start to blind you, and you become a bit disoriented. The lights keep getting closer and keep getting brighter. You squint, thinking you’ll safely pass the vehicle coming in the other direction. Then at the last second, as you can make out the structure of the vehicle, you realize that you have swerved out of your lane and are about the collide head-on with the vehicle.
That scenario may be very similar to one that billions of birds face annually during migration. Many songbirds migrate at night, when predation risk is lower and temperatures are cooler, reducing energy expenditure. Throughout most of their nocturnal voyage, the birds are flying over mostly dark areas until they come upon a large city. The bright lights from buildings blind birds, and draw them in towards the city, resulting in the birds colliding with the buildings.
A study in 2015 estimated that anywhere from 365-988 million birds are killed annually in the United States from building collisions, which represents the second greatest source human-caused mortality for birds. To reduce collision-related fatalities, it is important to know which characteristics of a building (e.g. amount of glass, height of building) may make it prone to high collision rates. Once these buildings are identified, scientists can work with policymakers and building owners to implement strategies aimed at reducing collision rates. Compared to studies examining how the greater landscape and geographic features influence collision rate, studies focusing specifically on structural characteristics of a building have been limited. Furthermore, it remains unknown if the building characteristics that influence collision rate vary among season or species.
Why do birds collide with some buildings more than others?
Motivated by this research gap, a team led by Corey S. Riding sought to determine building characteristics that influence collision rate across seasons and species. Riding and colleagues monitored 16 buildings in the small urban area of Stillwater, Oklahoma. Each building was monitored at least 6 days per week during spring, summer, and fall in 2015 and 2016, and 14 buildings were monitored during spring 2017. Monitoring efforts consisted of having trained observers slowly walking around the perimeter of each building and searching all windowed walls for signs of bird collisions (e.g. carcass, pile of feathers). Information such as species, location, and date were recorded. Using digitalized photographs, the team measured 8 characteristics hypothesized to influence collision rate for each distinct section of the building: height of building section, length of building section, section type (the general shape of the section), distance to trees, the amount of 3 landcover variables (impervious surfaces, lawn, flowerbeds) within 2-meters of the building, and proportion of glass for the section.
When pooling mortality across all seasons and species, the researchers found that proportion of glass, section height, and section length all positively influenced collision rate. This means that building sections that are tall, wide, and have a high proportion of glass lead to the greatest collisions rates. The team also found that section type was an important predictor, with alcove shapes (see image below) leading to the greatest number of collisions compared to other shapes. The funnel-like nature of alcoves may trap the birds, increasing their vulnerability to window collisions. When breaking collision rates down by season, the team discovered some variable results. The characteristics important for total collisions were the same characteristics that influenced collision rate during the spring, but for summer collisions, section type was no longer important, but the amount of lawn cover had a positive influence on collision rate. The fall collision rate was positively influenced by proportion of glass, section height, mean tree distance, and the amount of lawn cover.
The researchers selected 8 specific species to individually focus on; these 8 species collectively accounted for 47% of the total collisions. Species-specific characteristics that influenced collision rate varied widely between species. Proportion of glass was the characteristic shared among the greatest number of species, with 5 out of the 8 species negatively responding to glass. Some species responded differently to the same characteristic. For example, the collision rate for urban-adapted, non-migratory species such as the American Robin (Turdus migratorious) and House Finch (Haemorhous mexicanus) was negatively related to the amount of impervious surface, but for migratory birds such as the Painted Bunting (Passerina ciris), impervious surface positively influenced the collision rate. The researchers hypothesized that these differences could be due to differences in foraging patterns and habitat selection strategies.
What can be done to help reduce the number of collisions?
With factors such as cost and time limiting the extent that policymakers and building owners can implement bird-friendly building practices, knowing which sections of a building are likely to witness the highest collision rate is essential for effective mitigation efforts. While proportion of glass seemed to consistently have a strong influence on collision rate, other characteristics were also important for different species and in different seasons. A standard set of building design guidelines may not be equally effective for all species, which indicates the need for careful consideration of multiple species when implementing bird-friendly design practices. As green building certification programs (e.g. the LEED program) begin to incorporate bird-friendly design practices into their guidelines, having accurate data on building characteristics that influence collision rate is crucial to implement the most effective collision-reduction practices. The new light that Riding and colleagues shed on this issue can help steer migratory birds back on course.