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Window collisions are a leading cause of bird mortality

Collisions with glass windows on buildings, fences, transportation shelters, and noise barriers are currently a major cause of bird mortality in many parts of the world. Public awareness of bird-window collisions has increased in recent years, due to surveys for dead birds beneath windows. But since collision events are difficult to observe directly, there are still substantial gaps in understanding how and why birds fly into windows, and what happens to them afterwards.

A recent study published in the journal PeerJ has now provided a first glimpse of what happens immediately before a bird collides – or avoids impact – with a window, by analyzing audio and video recordings of 29 collisions and nine near-misses at a residential setting with bird feeders. According to the researchers, birds’ velocity and angle of approach predicted the outcomes following collisions, with faster flights at near-perpendicular angles of approach being the most dangerous for birds.

Of the 29 collisions that were recorded, very few resulted in an immediate fatality and were detected by building inhabitants. Thus, some birds may suffer injuries, but manage to fly away, only to die afterwards far from the window site. This finding has major implications for estimating population-level impacts of bird-window collisions, suggesting that the number of collisions may be highly underestimated by traditional survey methods.

“Bird-window collisions happen all over the world throughout the year, but the frequency and severity of collisions seem to be underestimated by the public, especially at residential settings,” said study lead author Brendon Samuels, a doctoral student in Biology at the University of Western Ontario in Canada. “One reason is that collisions happen suddenly and are difficult for people to observe directly. When birds fly away afterwards, it’s unclear what ultimately happens to them. Our findings highlight how common collisions can be in residential settings, especially where there are bird attractants like feeders.”

Moreover, although previous studies of bird-windows collisions have tended to focus mostly on quantifying the incidence of fatal collisions at larger structures, residential buildings are the most abundant types of structures with windows, and could thus represent the greatest cumulative threat to birds. In addition, since backyard bird feeding has gained in popularity during the Covid-19 pandemic, more birds are now drawn into high-risk settings.

According to the experts, new buildings can be designed based on practices that limit the risk of collisions. Moreover, existing building windows could be retrofitted by using simple materials to add visual markers to the exterior of the glass. Further research is needed though to characterize how birds orient their eyes to detect and avoid collisions with windows, so that collision deterrents may be designed optimally to match bird vision. 

By Andrei Ionescu, Staff Writer

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