Window films increase avoidance of collisions by birds but only when applied to external compared with internal surfaces of windows

Window collisions are one of the largest human-caused causes of avian mortality in built environments and, therefore, cause population declines that can be a significant conservation issue. Applications of visibly noticeable films, patterns, and decals on the external surfaces of windows have been associated with reductions in both window collisions and avian mortality. It is often logistically difficult and economically prohibitive to apply these films and decals to external surfaces, especially if the windows are above the first floor of a building. Therefore, there is interest and incentive to apply the products to internal surfaces that are much easier to reach and maintain. However, there is debate as to whether application to the internal surface of windows renders any collision-reduction benefit, as the patterns on the films and decals may not be sufficiently visible to birds. To address this knowledge gap, we performed the first experimental study to compare the effectiveness of two distinct window films when applied to either the internal or external surface of double-glazed windows. Specifically, we assessed whether Haverkamp and BirdShades window film products were effective in promoting the avoidance of window collisions (and by inference, a reduction of collisions) by zebra finches through controlled aviary flight trials employing a repeated-measures design that allowed us to isolate the effect of the window treatments on avoidance flight behaviors. We chose these two products because they engage with different wavelengths of light (and by inference, colors) visible to many songbirds: the BirdShades film is visible in the ultraviolet (shorter wavelength) range, while the Haverkamp film includes signals in the orange (longer wavelength) range. We found consistent evidence that, when applied to the external surface of windows, the BirdShades product resulted in reduced likelihood of collision and there was marginal evidence of this effect with the Haverkamp film. Specifically, in our collision avoidance trials, BirdShades increased window avoidance by 47% and the Haverkamp increased avoidance by 39%. However, neither product was effective when the films were applied to the internal surface of windows. Hence, it is imperative that installers apply these products to exterior surfaces of windows to render their protective benefits and reduce the risk of daytime window collision.

Field-testing effectiveness of window markers in reducing bird-window collisions

Bird-window collisions are a major source of human-caused mortality for which there are multiple mitigation and prevention options available. Despite growing availability of products designed to reduce collisions (e.g., glass with etched patterns or markers and films adhered over existing glass), few replicated field tests have been conducted to assess their effectiveness after installation on glass. We conducted a field study to evaluate the effectiveness of a commercially marketed product (Feather Friendly® markers) in reducing bird-window collisions at glass-walled bus shelters in Stillwater, Oklahoma, USA. This study included a before-after control-impact (BACI) analysis comparing numbers of collisions at 18 bus shelters in both pre-treatment (2016) and post-treatment (2020) periods, and an analysis comparing 18 treated and 18 untreated shelters during 2020. For the BACI analysis, collisions were significantly reduced between 2016 and 2020 at shelters treated with the Feather Friendly® markers even though collisions increased at shelters that remained untreated. For the 2020 analysis, there were significantly fewer collisions at treated than untreated shelters. Relative to a baseline study in 2016, we estimated that treating half of Stillwater's bus shelters resulted in a 64% reduction in total annual bird collisions. Together, these analyses provide a rigorous field test of the effectiveness of this treatment option in reducing bird-window collisions. Our research provides a model for similar studies at both bus shelters and buildings to evaluate and compare products designed to reduce bird-window collisions, and therefore, contribute to reducing this major mortality source affecting bird populations.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11252-022-01304-w.

Bird protection treatments reduce bird-window collision risk at low-rise buildings within a Pacific coastal protected area

Background

In North America, up to one billion birds are estimated to die annually due to collisions with glass. The transparent and reflective properties of glass present the illusion of a clear flight passage or continuous habitat. Approaches to reducing collision risk involve installing visual cues on glass that enable birds to perceive glass as a solid hazard at a sufficient distance to avoid it.

Methods

We monitored for bird-window collisions between 2013 and 2018 to measure response to bird protection window treatments at two low-rise buildings at the Alaksen National Wildlife Area in Delta, British Columbia, Canada. After 2 years of collision monitoring in an untreated state, we retrofitted one building with Feather Friendly^® circular adhesive markers applied in a grid pattern across all windows, enabling a field-based assessment of the relative reduction in collisions in the 2 years of monitoring following treatment. An adjacent building that had been constructed with a bird protective UV-treated glass called ORNILUX^® Mikado, was monitored throughout the two study periods. Carcass persistence trials were conducted to evaluate the likelihood that carcasses were missed due to carcass removal between scheduled searches.

Results and Conclusions

After accounting for differences in area of glass between the two buildings, year, and observer effects, our best-fit model for explaining collision risk included the building's treatment group, when compared to models that included building and season only. We found that the Feather Friendly^® markers reduced collision risk at the retrofitted building by 95%. Collision incidence was also lower at the two monitored façades of the building with ORNILUX^® glass compared to the building with untreated glass. Although more research is needed on the effectiveness of bird-protection products across a range of conditions, our results highlight the benefit of these products for reducing avian mortality due to collisions with glass.

Using citizen science to identify environmental correlates of bird-window collisions in Poland

Bird collisions with windows are among the highest sources of human caused mortality to this group of animals. However, environmental correlates of spatial patterns in collision risk are poorly understood, thus making mitigation measures difficult to implement. We took advantage of Covid-19 lockdown in the spring of 2020, when people were obligated to stay mainly at home, and performed a memory-recall questionnaire survey concerning bird-window collisions in Poland. We received information on bird-window collisions with 1800 buildings across the whole country accompanied by characteristics of each building, its vicinity and resident's behavior (time spent home, window cleaning). We supplemented these data with landscape description and performed statistical models to estimate importance of 13 explanatory variables as predictors of number of bird-window collisions. Reported number of collisions increased with the share of forests and arable land within 2 km of the building, and with proximity to rivers. Number of collisions also increased when single trees were close to buildings. More collisions were reported for houses than for flats and for new buildings than for old ones. Reported number of collisions increased with window cleaning which might suggest that cleaning reduces glass visibility for birds. As bird-window collision risk is highly variable among buildings but can be reduced with several measures improving glass visibility for birds, we recommend to use predictive models to identify collision hotspots for applying these measures. New houses located near rivers, in forests or agricultural landscapes have highest collision risk, and trees near buildings, often planted to benefit birds, can additionally elevate collision rate, thus potentially creating ecological traps. In such collision hotspots, reduction of window cleaning frequency can be considered as a mitigation measure unless the visual markers improving glass visibility for birds are installed on the panels.

Bird-window collisions: Mitigation efficacy and risk factors across two years

Background

Research on bird-window collision mitigation is needed to prevent up to a billion bird fatalities yearly in the U.S. At the University of Utah campus (Salt Lake City, Utah, USA), past research documented collisions, especially for Cedar Waxwings (Bombycilla cedrorum) drawn to fruiting ornamental pears in winter. Mirrored windows, which have a metallic coating that turns window exteriors into mirrors, had frequent collisions, which were mitigated when Feather Friendly®bird deterrent markers were applied. Bird-friendly windows–ORNILUX®ultraviolet (UV) and fritted windows–also reduced collisions when data were collected across fall and winter. Extending this prior research, we evaluated additional mitigation and tested the replicability of effects for pear trees, mirrored windows, and bird-friendly windows across two years.

Methods

Using published data from eight buildings monitored for collisions in year 1 (Fall and Winter, 2019–2020), we added another year of monitoring, Fall and Winter, 2020–2021. Between years, Feather Friendly®mitigation markers were added to collision-prone areas of two buildings, including both mirrored and transparent windows.

Results

The two buildings that received new Feather Friendly®mitigation had significantly fewer collisions post-mitigation. Control areas also had nonsignificant decline in collisions. The interaction of area (mitigation vs. control) by time (year 1 vs. 2) was significant, based on generalized estimating equations (GEE). The total yearly collisions across all eight buildings declined from 39 to 23. A second GEE analysis of all 8 buildings showed that mirrored windows, pear trees, and bird-friendly windows were each significant when analyzed separately. The best-fit model showed more collisions for mirrored windows and fewer collisions for bird-friendly windows. We found pear tree proximity to be related to more collisions in winter than fall. In addition, pear trees showed reduced collisions from year 1 to 2, consistent with new mitigation for two of three buildings near pear trees.

Discussion

Feather Friendly^® markers can mitigate collisions with transparent windows, not only mirrored windows, compared to unmitigated areas over 2 years. Results also underscore the dangers of pear tree proximity and mirrored windows and the efficacy of bird-friendly windows. Thus, bird collisions can be prevented by window mitigation, permanent bird-friendly windows, and landscape designs that avoid creating ecological traps.

Multi-scale temporal variation in bird-window collisions in the central United States

Expansion of urbanization and infrastructure associated with human activities has numerous impacts on wildlife including causing wildlife-structure collisions. Collisions with building windows represent a top bird mortality source, but a lack of research into timing of these collisions hampers efforts to predict them and mitigate effects on avian populations. In Stillwater, Oklahoma, USA, we investigated patterns of bird-window collisions at multiple temporal scales, from within-day to monthly and seasonal variation. We found that collisions peaked during overnight and early morning hours, a pattern that was consistent across seasons. Further, temporal variation in fatal collisions was explained by an interaction between season and avian residency status. This interaction illustrated the expected pattern that more migrant individuals than residents collided in fall, but we also documented unexpected patterns. For example, the highest monthly total of collisions occurred in spring migration during May. We also found similarly high numbers of resident and migrant collisions in spring, and a roughly similar amount of migrant mortality in spring and fall migration. These findings, which provide unprecedented quantitative information regarding temporal variation in bird-window collisions, have important implications for understanding mechanisms by which birds collide and improving timing of measures to reduce this major bird mortality source.

Ultraviolet-reflective film applied to windows reduces the likelihood of collisions for two species of songbird

Perhaps a billion birds die annually from colliding with residential and commercial windows. Therefore, there is a societal need to develop technologies that reduce window collisions by birds. Many current window films that are applied to the external surface of windows have human-visible patterns that are not esthetically preferable. BirdShades have developed a short wavelength (ultraviolet) reflective film that appears as a slight tint to the human eye but should be highly visible to many bird species that see in this spectral range. We performed flight tunnel tests of whether the BirdShades external window film reduced the likelihood that two species of song bird (zebra finch, Taeniopygia guttata and brown-headed cowbird, Molothrus ater) collide with windows during daylight. We paid particular attention to simulate the lighting conditions that birds will experience while flying during the day. Our results indicate a 75-90% reduction in the likelihood of collision with BirdShades-treated compared with control windows, in forced choice trials. In more ecologically relevant comparison between trials where all windows were either treated or control windows, the estimated reduction in probability of collision was 30-50%. Further, both bird species slow their flight by approximately 25% when approaching windows treated with the BirdShades film, thereby reducing the force of collisions if they were to happen. Therefore, we conclude that the BirdShades external window film will be effective in reducing the risk of and damage caused to populations and property by birds' collision with windows. As this ultraviolet-reflective film has no human-visible patterning to it, the product might be an esthetically more acceptable low cost solution to reducing bird-window collisions. Further, we call for testing of other mitigation technologies in lighting and ecological conditions that are more similar to what birds experience in real human-built environments and make suggestions for testing standards to assess collision-reducing technologies.

Bird-window collisions: different fall and winter risk and protective factors

Background

To reduce bird fatalities from millions of window collisions each year in North America, it is important to understand how design and landscape elements relate to collision risk. The current study extends prior research that found that buildings near ornamental pear trees (Prunus calleryana) and buildings with mirrored windows significantly increased odds of collisions among eight buildings on the University of Utah campus in winter. The previous study found bird-friendly glass was not related to collision risk, although only one fatality occurred at two buildings with ORNILUX® ultraviolet (UV) or fritted windows. We reasoned that extending data collection to include fall might provide a better test of efficacy. We tested the following three hypotheses: (1) Buildings with mirrored windows would experience more collisions, replicating the original study; (2) the addition of fall migration data would reveal fewer collisions at the buildings with bird-friendly windows; (3) the danger of pear tree proximity would be heightened in winter, when fruit is ripe enough to appeal to frugivores, especially the Cedar Waxwings (Bombycilla cedrorum) that frequent these trees.

Methods

Trained observers monitored buildings three times per week in Fall (September 12 to October 27, 2019) and Winter (October 29, 2019 to January 24, 2020). Collisions were photographed and documented in the iNaturalist University of Utah Bird Window Collision Project.

Results

There were 39 total collisions, from 0 to 14 per building.Using generalized estimating equations, buildings near pear trees had 3.33-fold increased odds, mirrored windows had 5.92-fold increased odds, and bird-friendly windows had an 84% lower odds (Odds ratio = 0.16) of bird window collisions when analyzed separately; all were statistically significant (p < 0.01). A test of all possible combinations of risk and protective factors revealed that the best fit model included pear trees (odds = 2.31) and mirrored windows (odds = 2.33). A separate analysis tested the pear tree by season interaction model; it yielded the deadliest combination, with 40-fold increased odds for buildings near pear trees in winter season.

Discussion

This research provides the first peer-reviewed evidence found for the efficacy of bird-friendly fritted windows and ORNILUX ® UV windows in buildings. In addition, it replicated a study that established the dangers of mirrored windows and fruiting pear trees near buildings. These risks were especially dangerous to Cedar Waxwings, who constituted 62.2% of the identifiable window collision victims. This research highlights how building risks depend on window design, landscape choices, species, and season. If replicated, analyses of risk factors can help identify buildings that require mitigation to make existing windows less deadly. Results also support the installation of bird-friendly glass in new or renovated buildings to reduce fatalities.

Main causes of bird-window collisions: a review

Bird-window collisions are a major cause of bird mortality in the world; up to one billion birds die each year from collisions with glass panes in North America alone. However, relatively little attention had been given to this issue in the broad scientific literature, despite a recent increase in the number of papers. In this paper, the indexed literature on bird-window collisions was reviewed, specifically addressing the causal factors. The search retrieved 53 papers, mostly from North America. The factors linked to higher collision rates were large areas of continuous glass, the presence of nearby vegetation and feeders, bird migration, abundance, and behavior. Several factors were site-specific, preventing the global extrapolation of these findings. There is a lack of scientific knowledge regarding bird-window collisions in tropical countries. One of the challenges to mitigating this problem is the small amount of information and - frequently - the extrapolation of findings described for temperate regions to other areas. There is a need for a greater and urgent effort to fill this gap.

Winter bird-window collisions: mitigation success, risk factors, and implementation challenges

Millions of birds die in bird-window collisions in the United States each year. In specialized test settings, researchers have developed methods to alter window designs to mitigate collisions. However, few published studies provide pretest and posttest evaluations of mitigation treatment areas and untreated control areas on existing buildings. We initially monitored bird-window collisions at a single building on the University of Utah campus in Salt Lake City, Utah, USA, during winter 1 (November 9, 2017-January 2, 2018). We found 15 bird-window collisions, most under a portion of the building with a mirrored façade. To test a mitigation treatment, we installed Feather Friendly® bird deterrent film on part of the mirrored façade after winter 1. The unmitigated areas of the same building served as a control area. We continued monitoring during the following winter 2 (November 15, 2018-January 12, 2019). The treated area collisions declined from seven before mitigation to two after mitigation, a 71% reduction. The control area had eight collisions at both times. Results of a generalized estimating equation yielded a significant area by season interaction effect (p = 0.03) and fewer collisions in the mitigated area than the control area at winter 2 (p = 0.03), supporting efficacy of the mitigation. In winter 2 we also expanded monitoring to eight total buildings to evaluate the risks of mirrored windows and proximity to fruiting pear trees (Prunus calleryana) and the benefits of bird-friendly glass. Bird-friendly glass, found on two buildings, included windows with permanent fritted dots or embedded ultraviolet patterns. We counted 22 collisions across the eight buildings. Mirrored windows and proximity to fruiting pear trees related to higher odds of bird-window collisions, based on separate generalized estimating equations. The best fit model included mirrored windows and pear trees. The two buildings with bird-friendly glass had only one collision, suggesting that these designs deter collisions, although the difference was not statistically significant. To publicize the study and to receive reports of additional bird collisions or fatalities on campus, we created a citizen science project on iNaturalist and engaged in additional outreach efforts that yielded 22 ad hoc reports. Many previous studies have documented Cedar Waxwing (Bombycilla cedrorum) collisions, but at relatively low numbers. Cedar Waxwings accounted for 31 of 34 identifiable collisions from the monitoring study and 4 of 21 identifiable collisions or fatalities from ad hoc reports.

Using a Sound Field to Reduce the Risks of Bird-Strike: An Experimental Approach

SYNOPSIS

Each year, billions of birds collide with large human-made structures, such as building, towers, and turbines, causing substantial mortality. Such bird-strike, which is projected to increase, poses risks to populations of birds and causes significant economic costs to many industries. Mitigation technologies have been deployed in an attempt to reduce bird-strike, but have been met with limited success. One reason for bird-strike may be that birds fail to pay adequate attention to the space directly in front of them when in level, cruising flight. A warning signal projected in front of a potential strike surface might attract visual attention and reduce the risks of collision. We tested this idea in captive zebra finches (Taeniopygia guttata) that were trained to fly down a long corridor and through an open wooden frame. Once birds were trained, they each experienced three treatments at unpredictable times and in a randomized order: a loud sound field projected immediately in front of the open wooden frame; a mist net (i.e., a benign strike surface) placed inside the wooden frame; and both the loud sound and the mist net. We found that birds slowed their flight approximately 20% more when the sound field was projected in front of the mist net compared with when the mist net was presented alone. This reduction in velocity would equate to a substantial reduction in the force of any collision. In addition to slowing down, birds increased the angle of attack of their body and tail, potentially allowing for more maneuverable flight. Concomitantly, the only cases where birds avoided the mist net occurred in the sound-augmented treatment. Interestingly, the sound field by itself did not demonstrably alter flight. Although our study was conducted in a limited setting, the alterations of flight associated with our sound field has implications for reducing bird-strike in nature and we encourage researchers to test our ideas in field trials.