Each year, millions of migratory birds travel through the Salt Lake valley, a region that contains 85% of Utah’s human population and the highest levels of light pollution in the state. Studies in other areas have documented impacts of light pollution on birds. For example, nocturnal migrants can be drawn into areas with light pollution and collide with buildings before finding their way out of the city. However, there is little information about bird-building collisions in the western U.S., and no other study has examined this phenomenon in Utah’s urban center. We investigated the following research questions: 1) How do collisions rates and species composition in Salt Lake City compare to other urban areas?, 2) Do building or landscape characteristics influence collision rates? Staff and community scientists conducted early morning collision surveys of 20 downtown blocks in Salt Lake City during spring (April-May) and fall (September-October) 2017-2019. We measured a suite of building, landscape, and light-level characteristics at collision sites (n=60) and an equal number of randomly selected buildings within the study area. We compared our findings to other collision studies, and used logistic regression analysis to evaluate whether any building characteristics impacted the likelihood of a collision.
Results/Conclusions
During 2017-2019, we conducted 569 surveys and found 126 bird-building collisions representing 38 species. New world sparrows, wood warblers, and hummingbirds were documented most often (58.3% of collisions). A vast majority of collisions were migratory (81%), and 69% were nocturnal migrants. Similar to other studies, a higher proportion of birds were found in the fall (87.6%) than in the spring (63.8%). Out of the 20 blocks surveyed, 80-90% of collisions occurred in 9 blocks, and 7 buildings had 54.4% of the collisions. Our logistic regression analysis revealed high support for models that included nearby trees, reflective glass, windows lighted at night, glass surface area, and building height as factors that influence the likelihood that a building would have one or more bird collisions. Compared to random buildings, collision buildings were 1.6x taller, had 3x more glass surface area, 1.7x reflective glass area, and 2.7x more windows lighted at night. Our findings contribute new information to growing literature about bird-building collisions and the impacts of light pollution on collision rates and patterns. We are also using this information to encourage businesses and private residents to turn off unnecessary lights during migration periods as part of a local Lights Out initiative.